Analysis of Lifecycle Water Requirements of Energy &
Transportation Fuels: Electricity from Geothermal
Resources
- Model Description
By
Gouri Shankar Mishra ( gouri. mishra@ gmail. com)
William Glassley ( geobg@ nf. au. dk)
Sonia Yeh ( slyeh@ ucdavis. edu)
September, 2010
Version 1.0
Report Number: UCD- ITS- RR- 10- 20A
Institute of Transportation Studies
University of California, Davis  
 
A B S T R A C T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1  
A C K N O W L E D G E M E N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2  
N O T A T I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3  
M O D E L   O B J E C T I V E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4  
D E F I N I N G   T H E   S Y S T E M   B O U N D A R Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5  
W A T E R   R E Q U I R E M E N T S   C O N S I D E R E D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5  
F r e s h w a t e r   r e q u i r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5  
D e g r a d e d   w a t e r   r e q u i r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7  
G e o t h e r m a l   f l u i d   r e q u i r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8  
F U N C T I O N A L   U N I T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9  
T Y P E S   O F   G E O T H E R M A L   R E S O U R C E S   A N D   P O W E R   P L A N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0  
G E O T H E R M A L   R E S O U R C E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0  
H y d r o t h e r m a l   R e s o u r c e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0  
E G S   R e s o u r c e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1  
P O W E R   P L A N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2  
C O O L I N G   T E C H N O L O G I E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3  
S U M M A R Y   O F   W A T E R   R E Q U I R E M E N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5  
I .   E L E C T R I C I T Y   F R O M   H Y D R O T H E R M A L   R E S O U R C E   U S I N G   O R C   P L A N T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5  
I I .   E L E C T R I C I T Y   F R O M   E G S   R E S O U R C E   U S I N G   O R C   P L A N T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6  
I I I .   E L E C T R I C I T Y   F R O M   H Y D R O T H E R M A L   R E S O U R C E   U S I N G   S I N G L E   F L A S H   P O W E R   P L A N T . . . . . . . . . . . . . . . . . . . . . . . . 1 7  
I V .   E L E C T R I C I T Y   F R O M   E G S   R E S O U R C E   U S I N G   S I N G L E   F L A S H   P O W E R   P L A N T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8  
W A T E R   R E Q U I R E D   B Y   B I N A R Y   P O W E R   P L A N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9  
T H E R M O D Y N A M I C S   B E H I N D   O R C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9  
I m p a c t   o n   e f f i c i e n c y   –   c o o l i n g   s y s t e m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2  
D r y   C o o l i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2  
W A T E R   R E Q U I R E D   B Y   F L A S H   P O W E R   P L A N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2  
T H E R M O D Y N A M I C S   B E H I N D   A   S I N G L E   F L A S H   P O W E R   P L A N T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2  
M E T H O D O L O G Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5  
E x e r g y   e f f i c i e n c y   o f   f l a s h   p o w e r   p l a n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6  
P a r a s i t i c   p o w e r   c o n s u m p t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 7  
W A T E R   R E Q U I R E D   T O   R E C H A R G E   H Y D R O T H E R M A L   R E S O U R C E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8  
W A T E R   R E Q U I R E D   F O R   H E A T   M I N I N G   O F   E G S   R E S O U R C E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8  
P O W E R   R E Q U I R E M E N T S   F O R   G E O T H E R M A L   F L U I D   P R O D U C T I O N   A N D   I N J E C T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9  
 
  1  
Abstract
The document describes the methodology and data sources for the model “ Analysis of
lifecycle water requirements of energy and transportation fuels: electricity from geothermal
resources”. The model estimates water requirements for electricity from various forms of
geothermal resources. It considers two types of hydrothermal resources – wet steam and hot
water; as well as enhanced geothermal systems ( EGS). Electricity can be generated using
flash or binary ( organic Rankine cycle) technology depending upon the temperature and
pressure of geothermal fluid. Power plants can use three different types of cooling
technologies – wet re- circulating, dry systems, and hybrid cooling systems. Requirements are
calculated separately for freshwater, degraded water and geothermal fluid.
The spreadsheet- based model ( Report Number: UCD- ITS- RR- 10- 20B) is available at
h t t p : / / p u b s . i t s . u c d a v i s . e d u /  
 
The model is part of a series exploring the water footprint of future transportation fuels
including bio- fuels and electricity. Other models currently under development examine the
lifecycle water requirements of ethanol from corn grain and crop residue, and electricity
from concentrated solar power, and biodiesel from soybean.
 
  2  
Acknowledgements
We thank all those who have offered ideas, data, information, and comments on the model
including John Maulbetsch, Maulbetsch Consulting; Ron DiPippo, Professor Emeritus,
University of Massachusetts Dartmouth, Alissa Kendall, University of California, Davis, and
Chuck Kutscher, National Renewable Energy Laboratory.
The research effort is funded by the California Air Resources Board, the Energy Foundation
and the David & Lucile Packard Foundation. Funding was also provided by Chevron
Corporation Graduate Fellowship.
 
  3  
Notations
ACC Air cooled condensers ( sometimes also referred to as direct dry cooling
systems)
BEV Battery electric vehicle
EGS Enhanced geothermal systems ( sometimes also referred to as engineered
geothermal systems)
IHE Internal heat exchanger
NCG Non- condensible gases
ORC Organic Rankine Cycle ( also called Binary cycle)
OTC Once through cooling system
PHEV Plug- in hybrid electric vehicle
TDS Total dissolved solids ( in mg/ L)
T& D Transmission and Distribution
ZLD Zero liquid discharge
 
  4  
Model objectives
The model estimates water requirements for electricity from various forms of geothermal
resources. It considers two types of hydrothermal resources – wet steam and hot water; as
well as enhanced geothermal resources ( EGS). The model considers multiple power
generation technologies including single flash and various configurations of binary or organic
Rankine cycle ( ORC). Requirements for freshwater, degraded water and geothermal fluid are
reported separately.
Based on inputs regarding temperature and form of a geothermal resource, configuration of
the power plant and technology of the cooling system, the model estimates “ average” water
intensity for the electricity produced. The model depends upon an extensive literature review
to determine various relationships necessary to determine water usage – for example the
relationship between thermal efficiency of a binary power plant and temperature of inlet
geothermal fluid, differences in efficiency between various ORC configurations, or different
efficiency of a plant with a wet re- circulating cooling system versus one with a dry cooling
system.
Since there are wide variations in the environmental conditions, input variations, plant
design, and operation conditions, the relationships identified in this study were specified
under specific parametric conditions but were generalized here to a broader set of conditions
or scenarios. Further, water consumption of power plants will depend upon a large number
of factors that the model currently does not consider – mineral content of the geothermal
fluid, ambient temperature and humidity, specific design parameters of the power plant, and
dissolved solids and chemical composition of freshwater withdrawn from ground or surface
sources. Even for a specific plant, water intensity will vary over time due to fluctuations in
ambient temperature and humidity, and in temperature of the geothermal fluid.
For these reasons, the model’s water usage estimates should be treated as first- order
estimates.
 
  5  
Defining the system boundary
Establishing a quantitative model of a system requires that the boundaries of the system be
rigorously established. For the purposes of this study, the model’s boundary includes the
geothermal field, the power generation unit, and the transmission and distribution ( T& D)
infrastructure to get electricity to the end user. The geothermal field consists of those
systems used for geothermal fluid production and transfer to the energy conversion system,
and then reinjection back to the ground. The geothermal field and power generation unit are
nearly always collocated and jointly called the geothermal power plant.
Figure 1: System boundary for lifecycle analysis of electricity from geothermal
In our model, analysis has been undertaken for two types of end user – electricity end user
and plug- in hybrid ( PHEV) or battery electric vehicle ( BEV) end user. Water requirements
are represented using different functional units for each of the two types of end users.
Further, to assess water requirements of electricity for charging of PHEV or BEV, two
additional factors need to be considered – battery charger efficiency and battery efficiency.
Water requirements considered
The model considers withdrawal and consumption of three types of water – freshwater,
degraded water, and geothermal fluids.
Freshwater requirements
The key focus of this study is freshwater use – water with low concentrations of dissolved
solids. Per US Geological Survey ( USGS 2010), freshwater has concentration of total
dissolved solids ( TDS) of less than 1,000 mg/ L. Water with increasing levels of dissolved
solids are classified as lightly saline ( 1,000 - 2,000 mg/ L), medium saline ( 3,000 - 10,000
mg/ L), and highly saline ( 10,000 – 35,000 mg/ L). Our focus on freshwater is due to its
scarcity and lack of substitutability at reasonable economic costs; saline water on the other
hand is relatively abundant ( Gleick 1996).
We adopt the following indicators of freshwater usage – withdrawal, consumptive use and
degradative use ( Owens 2001; NETL 2008; Bayart, Bulle et al. 2010). Freshwater withdrawal is
the removal from a natural water body or groundwater aquifer for industrial, agricultural or
domestic usage. Freshwater consumptive use denotes the use of freshwater when it is not released
into the same watershed because of evaporation, product integration, or evapotranspiration
by crops. Discharge into different watersheds or the sea, and sinking to a deep salt sink is
also counted under consumptive use. Water released is the difference between withdrawal and
G e o t h e r m a l  
f i e l d
P o w e r  
g e n e r a t i o n  
u n i t
E n d   u s e r
T r a n s m i s s i o n  
&   D i s t r i b u t i o n
 
  6  
consumptive use. If the quality of water is “ significantly” altered within the system, water
released is referred to as degradative use.
Table 1: Water requirements considered for geothermal electricity
Freshwater Degraded Water Geothermal Fluid
Requirements considered
- Power plant cooling
- Injection to maintain pressure
of hydrothermal plants
- Withdrawal for power
generation
- Injection for heat mining of
EGS resources
- Steam condensate used
for power plant cooling
Requirements not
considered
- Resource exploration &
development ( hydraulic
fracturing)
- Resource exploration &
development ( hydraulic
fracturing)
- Energy inputs for water
supply and treatment
- Energy inputs for water
supply and treatment
- Power plant construction
- Power plant cleaning
- Power plant cooling using
degraded water
Freshwater is required by geothermal power plants primarily for cooling and dissipating
waste thermal energy. There are multiple types of cooling systems and their impact on plant
efficiency and water consumption differ. Freshwater is also required for a plant’s “ hotel”
load i. e. for cleaning, drinking and sanitary use.
While the key differentiator for freshwater is the overall concentration of dissolved solids,
concentration of any individual component affects its usability. For example, EPRI ( 2003)
lists the maximum allowable concentration of various constituents in the freshwater used for
cooling systems. Similarly, the potable water standards by US Environmental Protection
Agency ( EPA) has the maximum allowable limits for various components like inorganic and
organic chemicals, radionuclides, disinfection byproducts, and microorganisms. In this
model, we do not analyze the concentration of individual constituents. When we refer to
freshwater, we simply imply a low TDS and an “ acceptable” level of concentration of
individual chemical constituents.
In the current version of the model, we do not consider the energy consumption and
corresponding “ embodied water” requirements for supply and conveyance, treatment and
distribution of freshwater to geothermal plants. In California around 5% of the state’s
electricity is consumed to supply treated freshwater for industrial, agricultural and residential
purposes ( Klein, Krebs et al. 2005). Nationally, the average energy intensity of freshwater
supply is 1.94 kJ/ L ( Klein, Krebs et al. 2005). Given the average water intensity of electricity
produced in the US ( Mishra and Yeh 2010), the embodied freshwater requirements to supply
freshwater to a geothermal plant would be around 0.033 liters of fresh water withdrawn and
0.001 liters of fresh water consumed per liter of freshwater supplied. Since these numbers
are small, we ignore the embodied water from energy use for water treatment and supply in
our analysis.
 
  7  
The energy consumption for water supply, treatment and distribution varies significantly by
locations. For example, the value is around 1.38 kJ/ L in Northern California and around 9.7
kJ/ L in Southern California ( Klein, Krebs et al. 2005). The differences are largely due to
conveyance distances and the need for extensive pumping to transport water over mountain
ranges.
Degraded water requirements
“ Degraded” water usually refers to water that cannot be referred to as freshwater and
includes contaminated groundwater, treated municipal effluent, industrial process water or
wastewater, irrigation return water, brackish water, and other types of water impacted by
humans or naturally- occurring minerals ( EPRI 2003). This category does not include ocean
water ( TDS of around 35,000 mg/ L).
In the context of power generation from geothermal resources, degraded water may be used
for injection and heat mining. Under normal operating conditions, geothermal fluids that are
obtained from a hydrothermal system are re- injected into the geothermal reservoir. This is
done to prevent declines in pressure and, hence, production. However, loss of fluid during
power generation and cooling commonly results in a net reduction of fluid that is re- injected
into the reservoir. For this reason additional water is often required from an external source.
In the dry steam geothermal resources at Geysers, California, storm water runoff from
power plant sites and treated municipal effluent are injected to maintain pressure ( EPA 1999;
City of Santa Rosa 2007). Sea water is injected for enhancement of dry steam resources at
Larderello, Italy ( Kaltschmitt 2007). Blowdown from cooling towers of power plants may
also be injected to the geothermal resource.
For EGS resources, which do not have sufficient naturally occurring geothermal fluids,
water is required to draw thermal energy to the surface for conversion to electrical energy.
Degraded water may also be used for such purposes. Degraded water is used at the EGS
resources at Cooper Basin in Australia ( based on personal communications with Chris
Mathews, Australian Geothermal Energy Association, May 2010).
Thus a wide range of degraded water sources, with significantly different chemical
constituents both in type and quantity, may be used to maintain pressure in hydrothermal
resources and mine heat from EGS resources. The only stipulation about the water quality is
that it should not result in deposition of minerals that would reduce permeability, or result in
excessive dissolution of rock that would ultimately result in mineral deposits on turbines or
other infrastructure. During injection, the water will be heated as it passes through the
thermal reservoir, so the only deposition that might happen would be related to minerals
that have retrograde solubility - minerals like carbonates for which the solubility goes down
as the temperature goes up. This implies that water with high levels of calcium, magnesium
or carbon dioxide need to be treated before use.
 
  8  
Even though degraded water may also be used for cooling purposes, we do not consider the
use of water with such high levels of TDS for cooling purposes given paucity of data 1 .
Maulbetsch and DiFilippo ( 2006) analyzed the use of brackish water from agricultural return
flows or low- TDS oil- field produced water ( 2,000 to 5,000 mg/ L), and saline water from
high- TDS agricultural return or high- TDS oil- field produced water ( 5,000 mg/ L) for cooling
purposes in power plants. Power plants with degraded water supply for re- circulating cooling
tower need water treatment facilities with high capital and operating costs, primarily parasitic
power consumption and chemicals ( EPRI 2003).
As in case of freshwater, we do not consider the energy consumption and corresponding
“ embodied water” requirements for supply and conveyance, treatment and distribution of
degraded water to the geothermal fields. These requirements can be significant – for example
9 MW of power is required to convey 42 million liters of water daily for recharging of the
Geysers Steamfield in California ( City of Santa Rosa 2010); this represents 1% of the total
operating capacity of around 1,000 MW.
It should be noted that although significant differences in quality exist between the various
sources of degraded water, the model treats all under a single category.
Geothermal fluid requirements
Power generation from geothermal resources involves withdrawal of large volumes of
geothermal fluids in form of steam or hot water or some combination of both. Power plant
technologies convert thermal energy from these fluids to electrical energy, and re- inject most
of the fluids through an injection well. Steam condensate at the outlet of the turbine is used
for cooling water requirements and cooling tower blowdown is usually injected back to the
ground. The volume of geothermal fluids withdrawn per unit of electricity generated
primarily depends upon the temperature of the fluids, and to a small extent on the ambient
temperature and efficiency of the power plant technology.
The quality of geothermal fluids can vary significantly from one location to another; and
over time from the same location. In geothermal resources with naturally occurring
geothermal fluids ( hydrothermal resources), concentration of solids especially sodium
chloride in the water can be very high. Chloride- bearing fluid is commonly discharged from
hot springs and from most geysers ( Barbier 2002). High concentrations of sulfate and
bicarbonate may also exist making the geothermal fluid highly corrosive. Additionally,
dissolved gases like ammonia, hydrogen sulfide, and methane may also be present.
Concentration of TDS in the hydrothermal resources in the Imperial Valley of California can
range from 240,000 mg/ L in Salton Sea to 7,600 mg/ L in East Mesa ( Layton and Morris
1980).
                                                                                                 
1     O c e a n   w a t e r   i s   u s e d   i n   p o w e r   p l a n t s   w i t h   o n c e - t h r o u g h   c o o l i n g   s y s t e m s .   S u c h   p o w e r   p l a n t s  
a c c o u n t   f o r   a r o u n d   3 0 %   o f   t o t a l   e l e c t r i c i t y   g e n e r a t e d   i n   t h e   U n i t e d   S t a t e s .   O c e a n   w a t e r   r e q u i r e m e n t s   f o r  
s u c h   c o o l i n g   s y s t e m s   a r e   w e l l   r e s e a r c h e d .   H o w e v e r ,   i n f o r m a t i o n   i s   l i m i t e d   a b o u t   r e q u i r e m e n t s   o f   s a l i n e  
w a t e r   a n d   o t h e r   f o r m s   o f   d e g r a d e d   w a t e r   f o r   u s e   i n   w e t   r e - c i r c u l a t i n g   c o o l i n g   t o w e r s   a n d   w a t e r - a u g m e n t e d  
d r y   c o o l i n g   t o w e r s .      
 
  9  
The model reports the amount of geothermal fluid that needs to be withdrawn for power
generation. In flash power plants ( which will be elaborated later), some of the fluid in vapor
state condenses to form high quality water with very low mineral content, which is then used
for cooling purposes. The evaporated portion of this condensate is counted as geothermal
fluid consumed by the model. It is not considered freshwater consumption because it is not
sourced from a body of freshwater. This portion also represents the amount of supplemental
degraded water that needs to be injected to the geothermal resource to maintain pressure.
Functional units
Estimate of water requirements of electricity end user is represented in form of liters of
water ( withdrawn and/ or consumed) per kJ of power consumed by the end user.
For PHEV and BEV end users, the model also presents estimates of water intensity in form
of liters of water ( withdrawn and/ or consumed) per vehicle kilometer traveled ( VKT). This
representation takes into account energy efficiency of electric vehicles, battery charger
efficiency, and battery efficiency. Energy efficiency of electric vehicles is assumed to be
937.3 MJ/ VKT or 1,429.7 BTU/ mile ( GREET 2010) 2 . The model defaults to a battery
charger efficiency of 87% ( for a 240 V charging system), and a battery efficiency of 85%
( King and Webber 2008).
The model assumes transmission and distribution ( T& D) losses of 6.14% which is the
national average for 2008 ( EIA 2010). However, there is significant regional variability in
T& D losses; it ranges from less than 1% for Rhode Island to 21% for Montana ( Appendix
C).
Inclusion of T& D losses increases water intensity of electricity consumed by an electric end
user by around 6.2% relative to water intensity when measured at the power plant level.
Subsequent inclusion of battery charger efficiency and battery efficiency increases water
intensity of electricity consumed by an electric vehicle end user by around 44% relative to
water intensity when measured at the power plant level, and 35% relative to water intensity
of electricity consumed by an electricity end user.
                                                                                                 
2     E n e r g y   e f f i c i e n c y   o f   c o n v e n t i o n a l   g a s o l i n e   v e h i c l e s   i s   3 2 8 0 . 4 5   M J / V K T   o r   5 0 0 3 . 8 8   B T U / V M T  
( G R E E T   2 0 1 0 ) .    
 
  1 0  
Types of geothermal resources and power plants
The following table summarizes the various geothermal resources, and types of power plants
and cooling technologies considered in the current version of the model.
Table 2: Resource types and power plant technologies considered
Geothermal resources Power plants technologies Cooling technologies
Within scope
Hydrothermal – Wet
steam
Organic Rankine Cycle ( ORC,
various configurations).
Dry cooling tower / Air cooled
condenser
Hydrothermal – Hot
water
Flash power plant ( single flash
only)
Wet re- circulating cooling tower ( with
and without zero liquid discharge
( ZLD))
Enhanced Geothermal
Systems ( EGS)
Hybrid cooling system ( with and
without ZLD)
Outside scope
Hydrothermal – Dry
steam or Vapor
Double flash power plant Once- through cooling system
Geo- pressured
resources
Flash- Binary combined plant Cooling ponds
Magmatic resources
Geothermal Resources
There are four types of geothermal resources: hydrothermal, enhanced geothermal systems
( EGS), geopressured, and magmatic. Nearly all geothermal power generated today is from
hydrothermal resources. EGS creation and exploitation has been demonstrated extensively at
various experimental sites in the US, Europe, and Japan ( EPRI 1997; Tester, Anderson et al.
2006; DiPippo 2008); hence it is included in the current version of the model. There have
been a few pilot projects to demonstrate the technical viability of geopressured and
magmatic resources ( EPRI 1997; Tester, Anderson et al. 2006; DiPippo 2008); however
significant technological developments are necessary before these can be exploited on a
commercial basis. Further, due to lack of data on water use by these resources, we have
excluded these resources from further analysis.
Hydrothermal Resources
Hydrothermal resources can be classified into dry steam ( or vapor) dominated fields and
water- dominated fields ( Barbier 2002). Water dominated fields are further classified into hot
water fields and wet steam fields.
Vapor- dominated reserves play a very significant role in geothermal power generation today;
about half of geothermal electricity in the world today comes from six steam fields including
the Geysers in California ( Barbier 2002). Such reservoirs are either at a temperature in excess
of the critical point ( 373.946 ° C), or at a high enough pressure such that the fluid will not
 
  1 1  
intersect the two- phase region of “ Liquid + Vapor” on its rise to the energy conversion
facility. From a thermodynamic point of view, dry steam systems provide the greatest
amount of energy per kilogram of fluid extracted ( Glassley 2010). This results from the fact
that there is much less separation of liquid from the steam and the resulting partitioning of
the enthalpy between those two phases. Instead, most of the enthalpy of the fluid remains
with the steam as it enters the turbine, and becomes available for energy conversion
( Glassley 2010).
Wet steam fields contain pressurized water at temperatures exceeding 100 ° C and small
quantities of steam. An impermeable cap- rock generally exists to prevent the fluid from
escaping to the surface, thus keeping it under pressure. When the fluid is brought to the
surface, as in case of producing well, and its pressure decreases, a fraction of the fluid flashes
to steam while the greater part remains as boiling water. The water- steam ratio varies from
field to field, and even from one well to the next in the same field ( Barbier 2002). Hot water
fields produce hot water at the surface at temperatures of around 100 ° C indicating that the
heat source is not sufficient to generate steam.
Vapor and hot steam fields manifest at the surface in form of geysers and hot springs, while
hot water fields appear only as hot springs. Today, hot water fields play a very limited role in
electricity generation but a significant role in direct- use heat applications - for example state
of Oregon uses geothermal energy in the form of direct use applications to the extent of
around 500 million BTUs, which is equivalent to around 20 MWt ( thermal megawatts of
power).
Coproduced hot water from oil and gas operations also represents a potential source of
geothermal power; Potential electricity generation from such fluids in the US has been
estimated at 6,000 MW ( Tester, Anderson et al. 2006). Some of these resources exist at
pressures greater than hydrostatic, thus making them " geopressured". Such resources often
also contain significant methane associated with the geothermal fluid. Hence, the
combination of thermal, chemical and kinetic energy inherent in such systems makes them
potentially attractive geothermal energy resources.
EGS Resources
EGS resources have high temperature but contain little or no geothermal fluid, and are not
very permeable. To exploit such resources, a permeable reservoir must be created by
hydraulic fracturing, and fluid from the surface must be pumped through the fractures to
extract heat from the rocks. A pair of wells is drilled into the rocks terminating several
hundred feet apart. Fluid, which is usually water, is injected under high pressure through the
injection well which creates an artificial reservoir. The fluid then returns to the surface
through the production well, and thus transfers the heat to the surface as steam or hot water.
The injection pump provides the sole motive force for moving the water continuously
around the loop to mine energy from the reservoir and deliver it to a power plant on the
surface.
 
  1 2  
EGS resources are sometimes referred to as engineered geothermal systems or hot dry rock
( HDR). Over the long term, it is anticipated that most of the future growth in electricity
from geothermal resources may come from EGS resources ( Tester, Anderson et al. 2006;
Chandrasekharam and Bundschuh 2008).
Power plants
There are three broad geothermal power plant technologies used to convert thermal energy
in geothermal fluids to electricity: dry steam, flash, and binary or Organic Rankine Cycle
( ORC). The type of conversion technology used depends on the state of the geothermal
fluid ( whether vapor or liquid) and its temperature.
Dry steam systems are best suited for producing electricity from vapor- dominated
hydrothermal resources. Flash steam plants are appropriate when geothermal fluids are
about 200C. Fluid is sprayed into a tank held at a much lower pressure than the fluid,
causing some of the fluid to rapidly vaporize, or " flash." The vapor then drives a turbine,
which drives a generator. If any liquid remains in the tank, it can be flashed again in a second
tank to extract even more energy and thus increase conversion efficiency.
ORC, also called binary cycle, geothermal plants are the closest in thermodynamic principle
to conventional Rankine cycle fossil power plant in that the heat transfer fluid undergoes an
actual closed cycle. Hot geothermal fluid and a secondary ( binary) fluid with a much lower
boiling point than water pass through a heat exchanger. Heat from the geothermal fluid
causes the secondary fluid to flash to vapor, which then drives the turbines. The binary fluid 3
is condensed at the other side of turbine and returned to the evaporator by means of a
feedpump. Since no steam condensate is formed, the entire geothermal fluid can be injected
back into the reservoir thereby doing away with the need for external reservoir recharge.
Although binary power plants constitute 32% of all geothermal power plants in operation
globally, they generate only 4% of total geothermal power indicating that the average power
rating is quite low at 2.3 MW per unit ( DiPippo 2008). DiPippo ( 2008) also indicates that
binary cycle power plants are most appropriate for generating power from low temperature
( enthalpy) geothermal resources.
                                                                                                 
3     Organic liquids such as n- butane ( Normal boiling point NBP: - 0.5C), isobutane
( NBP: - 11C) and toluene ( NBP: 110C) are most commonly used binary fluids
( Chandrasekharam and Bundschuh 2008)
 
 
  1 3  
The following table maps the preferred energy conversion systems to geothermal fluid
temperature.
Table 3: Mapping of power plant technology to resource temperature
Geofluid
temperature, ° C
Options for Energy conversion system
100 Basic ORC cycle or Kalina cycle
150 Advanced ORC for e. g. regenerative ORC, ORC with
IHE, regenerative ORC with IHE, Dual pressure
ORC; Kalina Cycle
200 Advanced ORC as above; or Single Flash systems
250 Multiple- flash; Flash- Binary combined plants
300 Multiple- flash; Flash- Binary combined plants
Source: DiPippo ( 2008); Franco and Villani ( 2009); Tester, Anderson et al. ( 2006)
Cooling Technologies
In both flash and ORC power plants, the vapor exiting the turbine ( steam for flash plants
and binary fluid vapor for ORC plants) has to be condensed in a condenser. The following
section briefly introduces the four types of cooling systems considered in our model.
In wet re- circulating systems, the vapor exiting the turbine is condensed in a shell- and- tube
condenser. The vapor condenses on the shell side by transfer of heat to cooling water
flowing through tubes in the condenser ( NETL 2008). The warm cooling water is pumped
from the steam condenser to a cooling tower, where the heat from the warm water is
transferred to ambient air flowing through the cooling tower. In the process, a portion of the
warm water evaporates from the cooling tower – this is the principal method of heat
dissipation and is referred to as latent heat transfer. The cooled water is then recycled back
to the condenser. Because of evaporative losses, a portion of the cooling water needs to be
discharged from the system – known as blowdown − to prevent the buildup of minerals and
sediment in the water. Make- up water is required to compensate for evaporative losses and
blowdown.
Dry cooling systems use air for condensing the vapor and dissipating heat. The type of dry
cooling system considered in our model is also referred as “ direct” dry cooling system or
alternatively air- cooled condenser ( ACC) system. In such systems, the vapor exiting the
turbine flows through tubes of an air- cooled condenser; heat is dissipated via conductive
heat transfer to ambient air blown by fans across the outside surface of the tubes ( NETL
2008). We do not consider “ indirect” dry cooling system where the vapor is condensed by
flow of cooling water across the outside surface of the tubes. Subsequently, a dry cooling
tower is used to conductively transfer the heat from the water to the ambient air. As a result,
there is no evaporative loss of cooling water with an indirect dry cooling system ( NETL
2008). Such systems are usually considered for retrofitting of power plants with once
through cooling system.
 
  1 4  
In addition to the above two cooling systems, we consider two other systems which reduce
the energy penalty associated with dry cooling systems and high water consumption
associated with wet re- circulating cooling systems. In water augmented dry cooling systems like
inlet air cooling, water is introduced into the inlet air stream of the air- cooled condenser to
lower the temperature of the ambient air leading to more effective cooling. In hybrid cooling
systems, heat is rejected through two separate cooling systems — a dry system which carries
the cooling load during most of the year and the wet system picking up a portion of the load
during the hotter periods when the performance of the dry system is limited.
 
  1 5  
Summary of water requirements
I. Electricity from hydrothermal resource using ORC plant
   Freshwater: Cooling water makeup accounts for freshwater withdrawal. Evaporation
from cooling towers and blowdown account for freshwater consumptive and degradative
use respectively. In water augmented dry cooling system, the entire water is assumed to
evaporate and hence there is no blowdown.
   Degraded water: Degraded water is not required for electricity produced from
hydrothermal resources using ORC plants.
   Geothermal Fluid: All fluid withdrawn for heat mining is injected back. No consumptive
use of geothermal fluid
C o o l i n g   t o w e r   –
W e t   r e - c i r c u l a t i n g  
a n d   H y b r i d
E v a p o r a t i o n   ( F )
B l o w   d o w n   ( F )  
M a k e - u p   w a t e r   ( F )
D r y   c o o l i n g   s y s t e m  
– W a t e r   a u g m e n t e d  
( s p r a y e d )
E v a p o r a t i o n   ( F )
M a k e - u p   w a t e r   ( F )
Figure 2.1: Water required for cooling
G e o t h e r m a l  
R e s o u r c e
F l u i d   E x t r a c t i o n   ( G ) F l u i d   i n j e c t i o n   ( G )
O R C   P o w e r   P l a n t
C o n s u m p t i v e   u s e
D e g r a d a t i v e u s e
W i t h d r a w a l
Figure 2.2: Water required for heat mining of geothermal resource
L e g e n d
( F )   :   F r e s h w a t e r   w a t e r  
( D )   :   D e g r a d e d   w a t e r  
( G )   :   G e o t h e r m a l   f l u i d  
 
  1 6  
II. Electricity from EGS resource using ORC plant
   Freshwater: Same as before.
   Degraded water: Degraded water is required to make up for fluid losses from the
geothermal resource resulting from leakage from the fracture system to the surrounding
rocks. We do not consider the initial injection of water to set up a loop for heat mining;
only incremental water injections during the operational phase of the project are
considered. The initial water injection requirement is likely to be small when it is
normalized to the electricity generated over the lifetime of the project.
   Geothermal Fluid: All fluid withdrawn for heat mining is injected back. No consumptive
use of geothermal fluid.
G e o t h e r m a l  
R e s o u r c e
F l u i d   E x t r a c t i o n   ( G ) F l u i d   i n j e c t i o n   ( G )
O R C   P o w e r   P l a n t
F l u i d   L o s s e s   ( D )
D e g r a d e d   w a t e r  
I n j e c t i o n   ( D )
Figure 3.1: Water required for cooling
C o o l i n g   t o w e r   –
W e t   r e - c i r c u l a t i n g  
a n d   H y b r i d
E v a p o r a t i o n   ( F )
B l o w   d o w n   ( F )  
M a k e - u p   w a t e r   ( F )
D r y   c o o l i n g   s y s t e m  
– W a t e r   a u g m e n t e d  
( s p r a y e d )
E v a p o r a t i o n   ( F )
M a k e - u p   w a t e r   ( F )
Figure 3.2: Water required for heat mining of geothermal resource
C o n s u m p t i v e   u s e
D e g r a d a t i v e u s e
W i t h d r a w a l
L e g e n d
( F )   :   F r e s h w a t e r   w a t e r  
( D )   :   D e g r a d e d   w a t e r  
( G )   :   G e o t h e r m a l   f l u i d  
 
  1 7  
III. Electricity from hydrothermal resource using single flash power plant
   Geothermal Fluid: Geothermal fluid is withdrawn for electricity generation. Steam
condensate evaporated in the cooling tower is accounted for under geothermal fluid
consumption. The liquid portion of the fluid after flashing and the cooling tower
blowdown is injected back.
   Degraded water: Degraded water has to be sourced from external sources and injected to
compensate for loss of geothermal fluid through evaporation of steam condensate in the
cooling tower. This represents degraded water withdrawal and consumption.
   Freshwater: As mentioned before, most or nearly all the makeup water for cooling is
provided by steam condensate. Additional water may be withdrawn during summer due
to increase in water evaporation to dissipate heat to an environment with higher ambient
temperature. If freshwater is withdrawn, then it is assumed that it is completely
evaporated ( consumed).
C o o l i n g   t o w e r   –
W e t   r e -
c i r c u l a t i n g
E v a p o r a t i o n   ( G ) ( F )
B l o w   d o w n   ( G )  
M a k e - u p   – S t e a m  
C o n d e n s a t e   ( G )
M a k e - u p   ( S u m m e r )  
– F r e s h w a t e r   ( F )
G e o t h e r m a l  
R e s o u r c e
F l u i d   E x t r a c t i o n   ( G ) F l u i d   i n j e c t i o n   ( G )
F l a s h   P o w e r  
P l a n t
D e g r a d e d   w a t e r  
I n j e c t i o n   ( D )
Figure 4.1: Water required for cooling
Figure 4.2: Water required for heat mining of geothermal resource
C o n s u m p t i v e   u s e
D e g r a d a t i v e u s e
W i t h d r a w a l
L e g e n d
( F )   :   F r e s h w a t e r   w a t e r  
( D )   :   D e g r a d e d   w a t e r  
( G )   :   G e o t h e r m a l   f l u i d  
 
  1 8  
IV. Electricity from EGS resource using single flash power plant
   Geothermal Fluid: Same as before.
   Degraded water: Degraded water is required to be injected not only to compensate for
loss of geothermal fluid through evaporation of steam condensate in the cooling tower,
but also to account for fluid losses from the geothermal resource resulting from leakage
from the fracture system to the surrounding rocks. This represents degraded water
withdrawal and consumption. We do not consider the initial injection of water to set up
a loop for heat mining; only incremental water injections during the operational phase of
the project are considered.
   Freshwater: Same as before.
C o o l i n g   t o w e r   –
W e t   r e -
c i r c u l a t i n g
E v a p o r a t i o n   ( G ) ( F )
B l o w   d o w n   ( G )  
M a k e - u p   – S t e a m  
C o n d e n s a t e   ( G )
M a k e - u p   ( S u m m e r )  
– F r e s h w a t e r   ( F )
G e o t h e r m a l  
R e s o u r c e
F l u i d   E x t r a c t i o n   ( G ) F l u i d   i n j e c t i o n   ( G )
F l a s h   P o w e r  
P l a n t
F l u i d   L o s s e s   ( D )
D e g r a d e d   w a t e r  
I n j e c t i o n   ( D )
C o n s u m p t i v e   u s e
D e g r a d a t i v e u s e
W i t h d r a w a l
L e g e n d
( F )   :   F r e s h w a t e r   w a t e r  
( D )   :   D e g r a d e d   w a t e r  
( G )   :   G e o t h e r m a l   f l u i d  
Figure 5.1: Water required for cooling
Figure 5.2: Water required for heat mining of geothermal resource
 
  1 9  
Water required by binary power plants
In this chapter, we study the water requirements of binary power plants. These include water
requirements indicated in Figures 2.1 and 3.1.
Thermodynamics behind ORC
The following section describes the thermodynamics behind a basic ORC plant. Condenser
duty is determined by first estimating the thermal input from the geothermal fluid, from
which the gross power output of the plant ( equal to the sum of net power output and
parasitic power load) is subtracted to determine the heat load on the condenser and hence
cooling tower. The quantity of water required to dissipate heat from a wet cooling tower
system can be determine by taking into account the heat capacity and latent heat of
vaporization of water.
Note: Figure reproduced with permission from Yari ( 2010)
Figure 6: Schematic diagram of a basic ORC cycle
 
  2 0  
Figure 7: Heat flow in a Binary cycle power plant
   Thermal energy input
The thermal input to the ORC cycle ( or the heat transfer in the evaporator) can be calculated
by the enthalpy drop for the geothermal fluid. This enthalpy loss for geothermal fluid is
equal to the enthalpy gain by the working fluid ( assuming no losses associated with the heat
transfer process).
( ) ( ) Q i n = m g e o h 5 − h 6 = m w f h 3 − h 2
• • •
( Equation 1)
where,
•
i n Q is the heat input in terms of kJ/ s
m g e o m w f
• •
& are the mass flow rates in kg/ s of geothermal and working fluid
respectively
h 5 & h 6 are enthalpies in kJ/ kg of the geofluid entering and exiting the evaporator
respectively
h 2 & h 1 are enthalpies in kJ/ kg of the working fluid entering and exiting the
evaporator respectively
The thermal input can also be represented as:
( ) i n g e o p g e o , i n g e o , o u t Q = m C T − T
• •
( Equation 2)
where,
C p is the specific heat capacity in kJ/ kg-° C of the geothermal fluid
T geo, in and T geo, out are the geothermal fluid temperatures in ° C at the inlet and outlet of
the evaporator respectively.
Frick, Kaltschmitt et al. ( 2010) suggested a range of 3.5 to 4.2 kJ/ kg-° C for the specific heat
capacity of geothermal fluid - lower and upper values correspond to a very high and a very
low mineral content of the fluid. The model assumes a default value of 3.85 kJ/( kg ° C) and
can be changed by the user.
T h e r m a l   e n e r g y  
i n p u t   f r o m  
g e o t h e r m a l   f l u i d  
t o   O R C   c y c l e   Q i n
N e t   e l e c t r i c i t y  
p r o d u c e d   W n e t
( =    t h Q i n )
C o n d e n s e r  
D u t y Q o u t
P a r a s i t i c   l o a d   W a u x
 
  2 1  
A lower outlet or rejection temperature of the geothermal fluid will increase the thermal
energy extracted and power produced. However, the rejection temperature should be high
enough to avoid silica oversaturation, which could lead to silica scaling and fouling problems
in the evaporators, and in mineral deposition in pipes and valves. The model defaults to a
value of 70 ° C for T geo, out ( Franco and Villani 2009). Silica deposition is not a universal
problem - hence, this approach provides a conservative estimate.
   Power Output
The net power output is represented as
• • • •
= − − n e t g r o s s a u x p l a u x r e s W W W W , , ( Equation 3)
where,
•
n e t W and
•
g r o s s W are the net & gross electricity produced by the plant respectively in
kJ/ s or kW
•
a u x p l W , is the total parasitic power consumed by various equipment within the plant -
working fluid feed pump; circulating water pump and cooling tower fans for wet
cooling tower; and ACC fans for dry cooling system in kJ/ s or kW.
•
a u x r e s W , is the total parasitic power consumed at the reservoir - primarily geothermal
fluid pump in kJ/ s or kW
We define the first law net thermal efficiency as
•
• • •
•
•
− −
= =
i n
g r o s s a u x p l a u x r e s
i n
n e t
t h
Q
W W W
Q
W , ,
h ( Equation 4)
Further detail regarding the thermal efficiency of an ORC plant will be provided later.
   Condenser Duty
The heat load on the condenser is calculated by the model as follows
• • • • •
= − − − o u t i n n e t a u x p l a u x r e s Q Q W W W , , ( Equation 5)
where,
•
o u t Q is the heat load on the condenser; and equivalently represents the heat that
needs to be rejected by the cooling tower
In terms of enthalpy, the heat rejected by the condenser is given by:
( ) ( ) 4 1 7 8 Q o u t m g e o h h m c o o l a n t h h = − = −
• • •
( Equation 6)
•
o u t Q is the heat rejected by the condenser in kJ/ s
 
  2 2  
m c o o l a n t
•
is the mass flow rate of either air or water used to cool the working fluid in
kg/ s
   Cooling Tower Analysis
Wet re- circulating cooling towers rely on the latent heat of water evaporation to exchange
heat between the system and the air passing through the cooling tower. The rate of
evaporation of water from the tower is related to the heat load on the tower, and is given by:
e v p o u t l a t e n t v a p w = Q × f / h
• •
( Equation 7)
where,
•
e v p w is the evaporation rate of water in kg/ s ( or liter/ s assuming water density to be
1 kg/ liter), and sometimes referred to as water consumption
f latent is the fraction of total heat rejected by latent heat transfer i. e. evaporation of
water
v a p h is the latent heat of vaporization of water and equal to 2,270 kJ/ kg
f latent depends largely upon ambient conditions and to some extent on design choice. The
fraction can range from 0.65 to 0.9 ( based on personal communication with Dr. John
Maulbetsch, Maulbetsch Consulting May 2010). The fraction is higher for higher wet bulb
temperatures. EPRI ( 2002) assumed a f latent value of 0.9 to analyze cooling tower
performance in California. Our model defaults to a value of 0.8. The remaining heat is
dissipated through sensible heat i. e. increase in temperature of water which is a function of
the specific heat capacity and mass flow rate of water.
Operation of wet cooling towers necessitate regular discharge of water ( blowdown) from the
cooling system in order to control the buildup of dissolved and suspended materials that
concentrate in the system as a result of the evaporation cycles. Technical considerations may
impose limits on the maximum allowable concentration of any particular chemical
constituent either in the circulating water inside the tower or in the blowdown water; and
this constituent will usually define the concentration limit for the cooling system. Regulatory
criteria may also apply, e. g. limits are set on concentration of certain constituents such as
copper or ammonia in the discharged water. EPRI ( 2003) has summarized limits of 21 key
chemical constituents or constituent pairs. For each constituent of concern, cycles of
concentration ( N) can be calculated based on the constituents’ concentration in intake or
make- up water.
M U i
i t i
i C
C
N
,
l i m ,
= ( Equation 8)
where,
N i is the cycles of concentration applicable for constituent i
C Limit, i is the water quality limit for constituent i in mg/ liter
C MU, i is the concentration of constituent i in the make- up water in mg/ liter
 
  2 3  
Thus, N i needs to be calculated for each of the constituents of concern, and the constituent
with the lowest calculated N i value will be the limiting parameter for that source of water.
This value of N will be the maximum cycles of concentration achievable. In addition to
defining the maximum concentration for a limiting chemical constituent, N is also used to
determine the blowdown rate, and consequently the make- up rate.
( − 1 )
=
•
•
N
w
w e v p
B D , ( Equation 9)
and
• • •
= + M U e v p B D w w w ( Equation 10)
where,
•
B D w is the blowdown rate in kg/ s ( or liter/ s)
•
M U w is the make- up rate in kg/ s ( or liter/ s) and sometimes referred to as the
withdrawal rate
The above equations indicate that smaller the value of N, the larger will be the blowdown
rate. The following graph shows the impact of cycles of concentration on makeup and
blowdown. It assumes an evaporation rate of 100 liters/ second.
 
  2 4  
Figure 8: Impact of cycles of concentration ( N) on cooling tower make- up &
blowdown
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
C y c l e s   o f   C o n c e n t r a t i o n
B l o w d o w n   ( l i t r e s / s )
0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
M a k e u p   ( l i t r e s / s )
B l o w d o w n
M a k e u p
The graph indicates that below 5- 6 cycles of concentration, blowdown rates increase
dramatically. Water treatment systems allow plants to de- mineralize water so that higher
cycles of concentration could be adopted and waste water blowdown and freshwater makeup
rates reduced.
The model assumes freshwater supply and defaults to N= 8.
 
  2 5  
Methodology
The model follows a four- step process to determine water requirements of an ORC plant.
Figure 9: Methodology to determine water requirements of ORC cycle
   Step 1
The starting point of the model is the electricity consumed by the end customer, which in
turn gives the electricity required to be produced by the ORC plant after accounting for
transmission and distribution losses.
( 1 ) n e t c o n s u m e d T & D W = W + l
• • ( Equation 11)
where,
c o n s u m e d W
•
is the electricity in kJ/ s or kW required by end consumer
l T& D represents transmission and distribution losses.
   Step 2
To estimate the required thermal input from geothermal field, the net efficiency of the ORC
plant is estimated in steps ( 2a) and ( 2b). In ( 2a), we first estimate the “ base” net thermal
efficiency level of a particular ORC plant configuration ( e. g. ORC with Internal Heat
Exchanger and wet cooling towers) based on a linear relationship between temperature of
the geothermal fluid and thermal efficiency. The particular ORC configuration was chosen
because the relationship between efficiency and temperature of geothermal fluid for this
configuration was available in the literature.
To account for more advanced ORC configurations or different cooling systems, correct
factors could be used.
( 1 ) , c f t h t h b a s e h = h + ( Equation 12)
where
S th is the net efficiency of the ORC configuration being examined
S th, base is the net efficiency of base ORC configuration for the given inlet
geothermal fluid temperature
1. Net electricity
to be produced by
ORC cycle
3. Heat load of the
condenser
1a. Account for
T& D losses.
4. Estimate water
requirements
1. Water required by
wet cooling towers
2. Water required by
hybrid systems
3. Impact of ZLD
system
2. Estimate
thermal input to
the ORC cycle
2a. Estimate base
efficiency
2b. Estimate
correction factor
2c. Estimate
parasitic load
Electricity
requirements
 
  2 6  
cf is the correct factor
In Step ( 2c) we determine parasitic load. The total thermal input required by the ORC plant
given W c o n s u m e d
•
is:
t h
n e t
i n
W
Q
h
•
•
= ( Equation 13)
In Step ( 2c) we determine parasitic load which is the sum of parasitic load at the plant and at
the geothermal field.
   Step 3
The heat load on the condenser is given by equation 5 which can be rewritten based on
above equation as
• • •
−
−
= n e t a u x
t h
t h
o u t Q W W
h
( 1 h )
( Equation 14)
   Step 4
The water consumption for a wet re- circulating cooling system can be estimated based on
equations 7 to 10. Hybrid cooling systems will consume a certain percentage of the water
consumed by wet re- circulating cooling systems as detailed in later sections.
 
  2 7  
Thermal Efficiency of OC plants
   Ideal thermal efficiency for geothermal binary plants
DiPippo ( 2007) argues that the ideal thermodynamic cycle appropriate for binary power
plants is the triangular ( or trilateral) cycle. The triangular efficiency imposes a lower upper-bound
on the thermal efficiency in comparison to the Carnot cycle which is used to define
the upper bound of traditional Rankine cycle power plants fueled by coal and nuclear. The
efficiency of the ideal Carnot cycle can be expressed as 4 :
H
H L
T
T   - T
  =  
C
t h h ( Equation 15)
where,
C
t h h is the thermal efficiency of the ideal Carnot cycle
T H is absolute temperature in Kelvin of the heat source. In this case, it is the
temperature of the inlet geothermal fluid
T L is absolute temperature in Kelvin of the heat sink ( condensing temperature)
DiPippo ( 2007) indicates that since the heating medium in a geothermal binary plant is not
an isothermal source, but rather a fluid that cools as it transfers heat to the cycle working
fluid, the Canot cycle is not appropriate. Rather, the triangular cycle with the following
thermal efficiency is more applicable:
H L
H L
T   T
T   - T
   
+
=
T R I
t h h ( Equation 16)
where,
T R I
t h h is the thermal efficiency of the triangular cycle
If the geothermal fluid could be cooled down to the dead- state temperature, T 0 , thus
allowing all of the waste heat to be discharged at the lowest possible temperature, then the
maximum thermal efficiency is defined as
H 0
H 0
, m a x T   T
T   - T
   
+
=
T R I
t h h ( Equation 17)
where,
T R I
h t h , m a x is the thermal efficiency of the triangular cycle
T 0 is the dead state or ambient temperature in Kelvin
DiPippo ( 2007) analyzed six ORCs, and finds that on average the thermal efficiency is
around 55% of the maximum triangular cycle thermal efficiency
T R I
h t h , m a x .
                                                                                                 
4     I n   e q u a t i o n s   1 5 ,   1 6   a n d   1 7 ,   t h e   t e m p e r a t u r e s   a r e   i n   K e l v i n .   I n   a l l   o t h e r   e q u a t i o n s   p e r t a i n i n g   t o   t h e  
O R C   c y c l e ,   t h e   t e m p e r a t u r e s   a r e   i n   ° C.
 
  2 8  
   Base- case thermal efficiency of ORCs
Heberle and Brüggemann ( 2010) simulated the impact of inlet geothermal fluid temperature,
and selection of working fluid on the thermal efficiency of ORCs. They configured the ORC
to include an internal heat exchanger 5 ( IHE) and wet cooling system; and made parametric
assumptions regarding pinch point temperature ( 5 ° C), turbine and feed pump isentropic
efficiency ( 0.75) and cooling water temperature ( 15 ° C). The following equations summarize
the linear relationship between the geothermal fluid’s inlet temperature and thermal
efficiency:
1 0 0       0 . 0 9 2 5 T - 2 . 0 1 7 8 = g e o ,   i n t h h ( fluid: iso- pentane) ( Equation 18)
1 0 0       0 . 1 0 0 2 T - 2 . 8 8 = g e o ,   i n t h h ( fluid: R245fa) ( Equation 19)
where,
) th is the thermal efficiency of the ORC
T geo, in is the inlet geothermal fluid temperature in ° C
The relationship is shown to hold for geothermal fluid inlet temperatures ranging from
around 75 to 180 ° C.
Note: Figure reproduced with permission from Yari ( 2010)
The MIT study ( Tester, Anderson et al. 2006) carried out a statistical analysis based on data
from ten ORCs across the world, and came up with the following relationship
1 0 0       0 . 0 9 3 5 T - 2 . 3 2 6 6 = g e o ,   i n t h h ( Equation 20)
                                                                                                 
5     I m p a c t   o f   a n   i n t e r n a l   h e a t   e x c h a n g e r   o n   e f f i c i e n c y   r e l a t i v e   t o   a   b a s i c   O R C   w i l l   b e   d i s c u s s e d   l a t e r .  
S u c h   a   c o n f i g u r a t i o n   i s   a l s o   r e f e r r e d   t o   a s   “ r e c u p e r a t e d   c y c l e ” .   H o w e v e r ,   w e   w i l l   u s e   t h e   t e r m   “ O R C   w i t h  
I H E ”   t o   d i s t i n g u i s h   i t   f r o m   t h e   “ r e g e n e r a t i v e   O R C ”   w h i c h   i s   d e f i n e d   l a t e r .    
Figure 10: Schematic diagram of an ORC cycle with IHE ( recuperated cycle)
 
  2 9  
The dataset included plants with inlet fluid temperatures ranging from 103 to 166 ° C. It
includes ORCs with both wet and dry cooling; and both basic and advanced ORC
configurations – for example the Húsavík plant in Iceland is a Kalina cycle ORC
Figure 11: Comparison of thermal efficiencies of Carnot and Triangular cycles
0 . 0 5
0 . 1 0
0 . 1 5
0 . 2 0
0 . 2 5
0 . 3 0
0 . 3 5
0 . 4 0
1 0 0
1 1 0
1 2 0
1 3 0
1 4 0
1 5 0
1 6 0
1 7 0
1 8 0
1 9 0
2 0 0
T ( H )   /   T ( g e o ,   i n )   i n   C e l s i u s
T h e r m a l   E f f i c i e n c y
C a r n o t T r i a n g u l a r H e b e r l e   &   B r ü g g e m a n n   ( 2 0 1 0 )
 
Assumptions: ( i) Heat sink or condensing temperature ( TL) is equal to the dead state or ambient
temperature ( T0) of 20 ° C
For our analysis, we have adopted the thermal efficiency estimates for iso- pentane by
Heberle and Brüggemann ( 2010). These estimates give us a base case scenario – impact of
dry cooling and advanced ORC configurations like Kalina cycle or regenerative ORC will be
included in the correction factors discussed in the next section. We considered iso- pentane
instead of R245fa because organic liquids are the most commonly used working fluids
( Chandrasekharam and Bundschuh 2008).
 
  3 0  
   Impact on efficiency – ORC configurations
Yari ( 2010) analyzed and compared four different ORC configurations – the basic ORC,
ORC with an Internal Heat Exchanger ( IHE), regenerative ORC, and finally regenerative
ORC with an IHE 6 .
Both the IHE and regenerative process aim to increase the temperature of the working fluid
on its way from the condenser to the boiler. This increases the average temperature at which
heat is added to the working fluid from the geothermal fluid in the evaporator ( boiler); and
thus increases the thermal efficiency.
The IHE relies on the fact that the expansion in the turbine ends for most organic fluids not
in the wet steam regime ( as in traditional Rankine cycle), but in the gas phase above the
condenser temperature. An IHE is used to capture remaining enthalpy and thus improve
efficiency. In regenerative ORC, a “ bleed” vapor is extracted from a suitable intermediate
point in the turbine to pre- heat the working fluid on its way from for the condenser to the
boiler. Based on the schematic diagrams, it can be inferred that an open working fluid heater
system has been modeled where the extracted vapor and the condensate are physically
mixed.
Yari ( 2010) assumed an inlet geothermal fluid temperature of 180 ° C; turbine and feed pump
isentropic efficiencies of 0.8 and 0.9 respectively; and air cooled condensers. Efficiencies
were calculated for three different working fluids - R113, R123, and n- Pentane.
Table 4: Thermal efficiencies of various ORC configurations
Basic
ORC
ORC with IHE
( recuperator cycles)
Regenerative ( bleed
feed heaters) ORC
Regenerative
ORC with IHE
R113 0.131 0.145 0.143 0.153
R123 0.133 0.142 0.145 0.154
n-
Pentane
0.126 0.141 0.141 0.150
Average 0.130 0.143 0.143 0.152
Source: Based on Yari ( 2010)
From the above results, our model defaults to 8.8% fall in efficiency of a basic ORC relative
to an ORC with IHE, and 0.5% and 6.9% rise in efficiency of Regenerative ORC and
Regenerative ORC with IHE respectively relative to an ORC with IHE configuration.
Kalina Cycles are those binary cycle plants where a mixture of ammonia and water is used as
the working fluid. Chandrasekharam and Bundschuh ( 2008) indicate that these plants have
20- 40% higher efficiency than ORC largely because of more efficient heat transfer from
                                                                                                 
6     A s   m e n t i o n e d   b e f o r e ,   t h e   O R C   w i t h   I H E   a r e   a l s o   r e f e r r e d   t o   a s   t h e   “ r e c u p e r a t e d   c y c l e ”   o r   “ O R C  
w i t h   a   r e c u p e r a t o r ” .   S u c h   c o n f i g u r a t i o n s   a r e   f a i r l y   c o m m o n .   H o w e v e r ,   n o   b i n a r y   g e o t h e r m a l   p l a n t s   u s e   t h e  
r e g e n e r a t i v e   c y c l e s   ( b l e e d   f e e d   h e a t e r s ) ;   t h e y   a r e   i m p r a c t i c a l   g i v e n   t h e   s m a l l   s i z e   o f   t h e   t u r b i n e s   i n   g e o -
b i n a r y   p l a n t s   ( b a s e d   o n   p e r s o n a l   c o m m u n i c a t i o n s   w i t h   D r .   R o n   D i P i p p o ,   A u g u s t   2 0 1 0 ) .      
     
 
  3 1  
geothermal fluid to the secondary fluid. However, DiPippo ( 2004) believes such conclusions
are based on insufficient data; superior performance of Húsavık Kalina plant in Iceland is
largely due to use of a very cold Icelandic stream ( 5 ° C) in a once- through cooling
arrangement. He concludes that the difference in performance is about 3% in favor of a
Kalina cycle. In absence of any further literature on the subject, our model defaults to a 10%
difference in performance in favor of Kalina cycle relative to ORC with IHE configuration.
A dual pressure ORC cycle has a two- stage heating/ boiling process – this helps to reduce
the average temperature difference between the hotter geothermal fluid and cooler working
fluid and thus increases the thermodynamic efficiency. Franco and Villani ( 2009) analyzed
thermal efficiencies for six different working fluids and compared thermal efficiencies of
various advanced configurations with basic ORC configuration. We took those scenarios
where the optimized solution was the dual pressure ORC cycle and compared the average
increase in efficiency relative to that of a basic ORC. The average efficiency difference was
18% in favor of dual pressure ORC. Given that ORC with IHE is 8.8% more efficient than
basic ORC ( Table 4 above, based on Yari 2010); Franco and Villani’s ( 2009) result translates
to a 8.2% efficiency difference between ORC with IHE and dual pressure ORC in favor of
the later.
Our model defaults to the following differences in thermal efficiencies relative to the base
case scenario of ORC with IHE
Table 5: Thermal efficiencies of ORC configurations relative to ORC with IHE
ORC Configuration Change in thermal efficiency
relative to an ORC with IHE
Based on
Basic ORC ( single pressure) - 8.8% Yari ( 2010)
Regenerative ORC ( single
pressure)
0.5% Yari ( 2010)
Regenerative ORC with IHE
( single pressure)
6.9% Yari ( 2010)
Dual pressure ORC 8.8% Franco and Villani
( 2009), Yari ( 2010)
Kalina Cycle ( single pressure) 10.0% Chandrasekharam and
Bundschuh ( 2008),
DiPippo ( 2004)
Franco and Villani ( 2009) note that the advantages related to the use of complex technical
solutions ( e. g. dual- pressure level cycles or regenerative cycles) may be important for high
geothermal fluid inlet temperature ( 140– 160 X C). At lower inlet temperatures ( 120– 130 X C),
efficiency gains are negligible. They also indicate that advanced ORC configurations are
sensitive to variations in operating conditions ( e. g. a decrease in geothermal fluid inlet
temperature during the lifecycle of the plant); and hence not always desirable.
For simplicity, our model defaults to differences in efficiencies summarized in the above
table over the entire range of inlet geothermal fluid temperature. Users should choose
advanced configurations only for high inlet temperatures.
 
  3 2  
Impact on efficiency – cooling system
The model considers three types of cooling systems: wet re- circulating cooling towers, dry
cooling systems, and hybrid cooling systems. We also consider the impact of Zero Liquid
Discharge ( ZLD) systems on volume of water withdrawals We do not consider once-through
cooling ( OTC) systems – US EPA's Section 316( b) strongly discourage OTC
systems and NETL ( 2008) expects that most new power plants will have to use closed- loop,
re- circulating systems or dry air- cooled systems.
Geothermal plants are sensitive to temperature of the heat sink. This is evident if one
considers Carnot cycle efficiency ( Equation 15). If the heat source temperature is 500 ° C, as
is the case with thermoelectric Rankine cycle power plants, an increase in heat sink
temperature from 5 ° C ( around 40 ° F) to 40 ° C ( around 105 ° F) will reduce the Carnot
efficiency by around 9%. However, for a heat source temperature of 200 ° C, a similar
increase in heat sink temperature decreases efficiency by 18%. For a low enthalpy
geothermal resource at 100 ° C, the fall in efficiency is as high as 37%.
Figure 12: Decrease in efficiency if heat sink temperature increases from 5 to 40° C
0 %
1 0 %
2 0 %
3 0 %
4 0 %
5 0 %
6 0 %
1 0 0
1 3 0
1 6 0
1 9 0
2 2 0
2 5 0
2 8 0
3 1 0
3 4 0
3 7 0
4 0 0
T ( H )   i n   ° C
%   d e c r e a s e   i n   e f f i c i e n c y
C a r n o t   C y c l e
T r i a n g u l a r   C y c l e
As the above graph shows, Triangular cycles are even more sensitive to the temperature of
the heat sink. In case of geothermal resources, efficiency is more sensitive to heat sink
temperature ( T L ) than to the heat source temperature ( T H ) which is the geothermal resource
temperature T geo, in .
Dry Cooling
Dry cooling systems cool by transferring heat to the ambient air without evaporation
( sensible cooling). As a result they can cool the working fluid vapor only to a temperature
that approaches the dry- bulb temperature or the ambient air temperature. On the other
hand, wet re- circulating cooling systems cool primarily through evaporation; thus the
temperature of the cooled working fluid approaches wet- bulb temperature.
 
  3 3  
Wet- bulb temperature is always lower than dry bulb or ambient air temperature except at
100% humidity. Further, the dry bulb temperature is more variable. The following graph
shows the daily average wet and dry bulb temperatures measured at Brownsville, Nevada
County in California over an entire year. The graph indicates that the difference between the
two temperatures is higher in summer and that the dry bulb temperature is more variable.
Figure 13: Daily average wet and dry bulb temperature at Brownsville, California.
- 5
0
5
1 0
1 5
2 0
2 5
3 0
3 5
J a n - 8 6
F e b - 8 6
M a r - 8 6
A p r - 8 6
M a y - 8 6
J u n - 8 6
J u l - 8 6
A u g - 8 6
S e p - 8 6
O c t - 8 6
N o v - 8 6
D e c - 8 6
T e m p e r a t u r e   i n   ° C
D r y   B u l b   T e m p e r a t u r e
( d a i l y   a v e r a g e )
W e t   B u l b   T e m p e r a t u r e
( d a i l y   a v e r a g e )
Source: UC IPM ( 2010)
As a result, the following may be stated about the difference in performance between a wet
cooled and dry cooled ORC plant:
   Lower efficiency of dry cooled plants
The performance of a power plant is directly related to the pressure drop across a turbine;
which in turn is related to the performance of the cooling system. As a result, dry cooling
systems lead to a greater reduction in power plant’s net energy production than a wet re-circulating
system. In other words, efficiency of power plants with wet re- circulating systems
is always higher.
Since the parasitic energy consumption of both wet and dry cooling systems are the same
( EC 2001; Maulbetsch and DiFilippo 2006), the efficiency penalty can be attributable entirely
to lower plant operating efficiency resulting from lesser cooling.
   Higher variability in operations of dry cooled plants
Power output for an air cooled geothermal plant can decrease by up to 50% from winter to
summer ( Michaelides and Ryder 1992; Kanoglu and Cengel 1999). However, air- cooled
thermoelectric plants suffer a much lower drop in performance in summer ( DOE 2002;
Maulbetsch and DiFilippo 2006); usually around 10% or less. Diurnal fluctuations may also
be significant; DiPippo ( 2004) reports that power output of the air cooled bottoming binary
cycle in Brady was 33% lower at 6PM than at 6AM in the morning, based on observations
 
  3 4  
over 10 days in September 2002. Average ambient temperatures were 30.1 ° C and 16.8 ° C at
6PM and 6AM respectively.
To estimate an annual average energy penalty of implementing a dry cooling system relative
to a wet re- circulating cooling tower, we adopted the following three steps:
1.   Establish an upper lower- bound by estimating the energy penalty for thermoelectric and solar
thermal power plants
The following table summarizes the potential efficiency penalty ( equal to energy penalty) of a
power plant with dry cooled systems relative to one with wet re- circulating cooling system.
Table 6: Efficiency penalty of dry cooling systems relative to wet re- circulating
cooling systems
Source Plant details Site condition &
location
Efficiency
penalty-( 1)( 2)
Notes
WorleyPars
ons ( 2008)
250MW parabolic
trough solar.
Rankine Cycle
Hot & arid ( Mojave
Desert)
5.90% Study was undertaken for
Beacon Solar Energy Project
Maulbetsch
and
DiFilippo
( 2006)
500 MW NG
Combined Cycle
Hot & arid ( Desert;
Riverside, CA)
9.12% Change in gross efficiency of
Rankine cycle only ( design).
Change in overall plant gross
efficiency is 2.98%
350 MW Brayton Hot & humid ( Valley; 4.81% Change in overall plant gross
Observed relationship between power output and heat sink temperature
For a 27MW dry- cooled binary power plant in Reno, Northern Nevada, Konaglu ( 1999)
found that the gross power output was related to the ambient dry bulb temperature by
the following polynomial equation:
W 2 2 , 0 8 9 . 2 0 1 1 9 . 7 8 4 1 T 4 . 1 7 1 0 1 5 T 2 g r o s s = − −
•
where W gross is the gross power output measured in kW
T is the ambient air temperature measured in ° C.
We analyzed the relationship between gross power output and first- law thermal efficiency
of the bottoming binary cycle of the Brady power plant in Nevada based on observations
made by DiPippo ( 2004) during August and September 2002. The following relationships
were established:
W T g r o s s = 7 , 3 8 4 . 5 5 − 1 2 2 . 6 6
•
, and
T t h h = 0 . 1 4 1 6 3 − 0 . 0 0 2 8
The above relationships confirm the negative relationship between power output and
ambient temperature.
 
  3 5  
cycle ( gas turbine) Bakersfield CA) efficiency is 1.20% ( design)
& 150 MW
Rankine cycle
( steam turbine)
Cool & humid ( Coast;
San Francisco, CA)
1.80% Change in overall plant gross
efficiency is 0.3% ( design)
Variable ( Mountain;
Redding, CA)
8.58% Change in overall plant gross
efficiency is 1.9% ( design)
DOE
( 2008)
274MW parabolic
trough solar
( Rankine)
Hot & arid ( Desert) 4.90%
DOE
( 2002)
400 MW
supercritical coal
plant
Delaware River Basin 3.57% ( 6.53%) Retrofit of a once- through
plant to either wet- or a
( indirect) dry- cooling tower
Michigan / Great
Lakes
3.29% ( 7.20%)
Ohio River Valley 3.39% ( 6.60%)
South ( Georgia) 4.41% ( 7.30%)
Southwest 6.96% ( 10.45%)
( 1)   Annual average efficiency penalty
( 2)   Figures in brackets gives efficiency penalty during summer - One percent highest
temperature conditions
The DOE study ( 2002) considered indirect dry cooling systems instead of direct dry cooling
( also referred to as Air cooled Condensers or ACC). In ACC, the heat from the working
fluid vapor, after exiting the turbine, is directly transferred to the surrounding. In an indirect
dry cooling system, the working fluid vapor condenses in the surface condenser, which
utilizes a secondary cooling water loop to reject the heat from the cooling water to the
ambient air via the cooling towers. Indirect dry cooling was considered by DOE ( 2002)
because the scope included retrofitting of existing coal plants with OTC. GAO ( 2009)
indicates that the DOE ( 2002) energy penalty estimates are higher than when a dry cooled
system is designed according to the unique specifications of a newly built plant.
Maulbetsch and DiFilippo ( 2006) compared the impacts of dry cooling and wet re- circulating
cooling systems on energy efficiency of a combined cycle power plant. In the table above,
we have summarized the impact on the Rankine cycle only. It should be noted that power
plants' designs have been optimized in the study for total cost or total levelized cost of
electricity and not for lowest water use or most efficient power production. Thus, for
example, a dry cooled plant could be designed to operate at a higher design ambient
temperature and equipped with a larger air cooled condenser ( ACC). This will reduce the
energy penalty and improve hot day capacity, but might lead to higher capital and operating
costs and lower financial viability. The implication of this is that energy penalties of dry
cooling system have been over estimated in this study.
2.   Estimate change in ideal Triangular cycle efficiency when the heat sink temperature ( T L )
changes from wet bulb to dry bulb ambient temperature.
Kutscher ( 2002) provides the dry bulb ( DB) temperature and wet bulb ( WB) temperature for
each hour of a typical day of each month at Empire, NV ( hot and dry climate). Based on this
data, we calculated the monthly and annual average penalty of dry cooling relative to wet
 
  3 6  
cooling for a range of geothermal resource inlet temperatures. For the calculations we
assumed an approach temperature of 10 ° F.
Table 7: Change in ideal Triangular cycle efficiency when T L changes from WB to
DB temperature – monthly and annual average
Month Energy Penalty
T( H) = 100° C T( H) = 125° C T( H) = 150° C T( H) = 175° C
January 6.3% 5.2% 4.3% 3.9%
February 4.9% 4.0% 3.3% 3.0%
March 5.6% 4.5% 3.7% 3.3%
April 9.2% 7.4% 6.1% 5.4%
May 12.2% 9.7% 8.0% 7.0%
June 14.5% 11.5% 9.4% 8.3%
July 15.2% 12.0% 9.8% 8.6%
August 15.2% 12.0% 9.8% 8.6%
September 13.5% 10.7% 8.8% 7.7%
October 9.8% 7.8% 6.4% 5.7%
November 7.9% 6.4% 5.3% 4.7%
December 6.2% 5.1% 4.2% 3.8%
Average 10.1% 8.1% 6.6% 5.9%
Notes: ( 1) Based on weather data from Kutscher ( 2002)
EPRI ( 2004) provide typical distribution of WB and DB temperature over a year based on
data from the National Climate Data Centre, National Oceanic and Atmospheric
Administration. The data are based on observations made between the years 1972 and 1996
and are available for four stations. As before, we assumed an approach temperature of 10 ° F.
Table 8: Change in ideal Triangular cycle efficiency when T L changes from WB to
DB temperature – annual average for different locations
Energy Penalty
City
Climate
Conditions
T( H) =
100° C
T( H) =
125° C
T( H) =
150° C
T( H) =
175° C
El Paso, TX
Hot & arid
( desert)
10.4% 8.3% 6.9% 5.8%
Jacksonville,
FL
Hot & humid 4.0% 3.1% 2.6% 2.2%
Portland, OR Moderate 7.3% 5.8% 4.8% 4.2%
Bismark, ND
Extreme summers
& winters
6.2% 5.0% 4.3% 3.7%
Notes: ( 1) Based on weather data from EPRI ( 2004)
The above table indicates that energy penalty varies between sites. In Jacksonville, FL, the
difference between WB and DB is small due to high relative humidity. Alternatively, in El
Paso, TX, high ambient temperatures and low relative humidity in summer and in the
afternoons makes use of evaporative cooling quite attractive.
 
  3 7  
3.   Literature review
There is limited literature on energy penalty associated with dry cooling system for a
geothermal binary ORC plant. Two studies reviewed in this section compare dry cooling
with water augmented dry cooling – a system that involves spraying a small amount of water
into the inlet air stream of an air- cooled condenser ( ACC) where it evaporates and cools the
air. Spray enhancement are particularly effective when ambient temperatures are greater than
90° F and relative humidity is less than 40 percent ( EPRI 2003).
Kutscher ( 2002) have developed a model to analyze the impact of water augmented dry
cooling on performance of a 1 MW dry- cooled ORC plant at Empire, NV ( hot and dry
desert climate) with an geothermal fluid inlet temperature of around 120 ° C. The model
results may be used to compare the annual average energy penalty of a dry cooling system
relative to four different types of water augmented dry cooling systems. It should be noted
that the energy penalty is not with respect to a wet re- circulating cooling system but with
respect to various “ water enhanced air cooled systems”.
Table 9: Comparative performance of various water enhanced spray cooling system
Type of cooling
system
Energy penalty of dry cooling
relative to evaporative cooling
Dry cooling
Spray cooling 11.4%
Munters cooling 2.9%
Deluge cooling 15.3%
Hybrid cooling 7.9%
Source: based on Kutscher ( 2002).
Notes: Figures in brackets indicate water requirements as a percentage of water required by a wet re-circulating
cooling system as calculated by our model.
Imroz Sohel, Sellier et al. ( 2009) simulated the impact of retrofitting spray cooling at the 35
MW Rotokawa ORC with dry cooling system in Taupo, New Zealand. The authors estimate
a 1% increase in power generated over the entire year and 6.8% during summer. The paper
does not calculate the amount of water requirements; enough water is used to get 80% of the
potential evaporative cooling effect ( saturation).
The difference in estimated annual average performance of spray cooling by Kutscher
( 2002) and Imroz Sohel, Sellier et al. ( 2009) is partly explained by the differences in climatic
conditions – peak summer DB temperatures are around 28 ° C and 35 ° C at Taupo and
Empire respectively, and differences between WB and DB could touch 15 ° C at Empire
while they are below 10 ° C at Taupo. It is also partly explained by differences in assumed
saturation level of outgoing air under evaporative cooling – 92% assumed for the Empire
study and 80% for the Taupo study.
The above discussion indicates that the user has to consider two key factors while selecting
the annual average energy penalty of dry cooling system relative to wet re- circulating cooling
system. The system defaults to a level of 10%.
 
  3 8  
   Hybrid cooling systems
Many different solutions are available to mitigate the energy penalty imposed by dry cooling
systems during the hotter periods. In hybrid or wet/ dry systems, heat is rejected through two
separate cooling systems— a dry system which carries the cooling load during most of the
year and the wet system picking up a portion of the load during the hotter periods when the
performance of the dry system is limited. While these systems can achieve significant water
conservation and still maintain good hot- day performance, their implementation is limited
due to high initial costs as a result of the need for two cooling towers ( or a more complex
integrated single structure), parallel circulating water loop components, more complex
controls, and other requirements associated with providing two, nearly independent cooling
systems EPRI ( 2003). In water augmented dry cooling systems like inlet air cooling, water is
introduced into the inlet air stream of the air- cooled condenser to bring the temperature of
ambient air closer to the wet bulb temperature.
Maulbetsch and DiFilippo ( 2006) indicate that most systems in the United States are
intended for plume abatement and are essentially all wet systems with a small amount of dry
cooling to heat the tower exhaust plume above saturation conditions during cold, high
humidity periods when the wet tower plume is likely to be visible and risks causing winter
icing in nearby roads.
DOE ( 2008) simulated and compared the performance of various cooling systems for a 274
MW gross Rankine cycle solar power plant located in a typical Southwest desert site ( hot and
dry). They modeled a hybrid system that uses an air cooled condenser in parallel with a wet
cooling tower. The wet system was put into service during high ambient temperatures when
a portion of the steam leaving the turbine is routed to a wet cooling system which is only
rejecting a portion of the total waste heat. The simulation of the different hybrid systems
were undertaken with a target condenser pressure ( turbine backpressure) as indicated in
column 2 of the table below.
Table 10: Design and performance of Hybrid cooling systems
Cooling System
( 1)
Target Condenser /
Turbine Back
Pressure in KPa
( inches HgA)
Power ( net efficiency)
penalty relative to plant
with wet re- circulating
cooling system ( 2)
Water consumption -
fraction of wet
cooling tower
Hybrid 1 ( 116%) 8.47 ( 2.5) 0.8% 44.6%
Hybrid 2 ( 71%) 13.55 ( 4.0) 2.2% 13.3%
Hybrid 3 ( 40%) 20.32 ( 6.0) 3.6% 0.9%
Hybrid 4 ( 13%) 27.09 ( 8.0) 3.9% 0.1%
Dry Cooling 5.7% 0.0%
( 1)   Wet cooling tower design capacity ( condenser duty) as a percentage of the capacity of the
tower in wet cooling system ( 224 MWt)
( 2)   Annual average
Based on DOE ( 2008)
The model adjusts the energy penalty for each of the four hybrid systems based on energy
penalty assumed for dry cooling tower system.
 
  3 9  
   ZLD systems
Traditionally power plants have discharged their waste water to a surface water source which
is economically very attractive. Weiss ( 1996) has identified multiple factors that determine
the ability of a power plant to discharge to a surface water source - quality and quantity of
the waste water, waste water treatment concept applied, the quality and quantity ( flow or
volume) of the receiving water body, and the environmental sensitivity of the receiving water
body. Power plants have to treat waste water to meet various federal, state or local norms
before discharge. The waste water may also be sent to the sanitary sewer / municipality
treatment plants for treatment and disposal. Other options available to plants are ground
water discharge including evaporation ponds and trucking waste to offsite disposal sites.
None of these options offer the potential for reuse of water.
Zero Liquid Discharge systems based on mechanical evaporators or reverse osmosis
membranes allow power plants to recover some of the waste water. Such systems have
historically been applied in areas that are deficient in water supply, remote from suitable
receiving streams for wastewater discharge, and/ or at projects seeking to streamline their
licensing schedule. ZLD systems usually consist of two components: a mechanical
evaporator or reverse osmosis membrane that concentrates wastewater into a liquid stream
containing concentrated constituents of the wastewater being treated, and a crystallizer to
further concentrate wastewater to solids which can then be shipped offsite. Maulbetsch and
DiFillipo ( 2006) indicate that these processes produce high quality water - less than 100- 500
mg/ L TDS for reverse osmosis and 10mg/ L TDS for a mechanical evaporator. 90- 98% of
the waste water could be recovered depending upon concentration in the input stream.
However such systems
consume significant
parasitic load –
evaporators consume
80- 90 kJ to evaporate
1 liter of waste water
( blowdown).
Subsequently,
crystallizers consume
190- 285 kJ to
crystallize 1 liter of
reject to solids for
eventual disposal
( Maulbetsch and
DiFilippo 2006). Thus
a 1 liter waste water
stream will generate
0.9 – 0.98 L of treated water for make- up; and consume 90- 120 kJ of energy.
E v a p o r a t o r   /  
R e v e r s e   O s m o s i s  
( E n e r g y   c o n s u m e d   =  
8 0 - 9 0 k J )
C r y s t a l l i z e r s  
( E n e r g y   c o n s u m e d   =  
4 - 2 9 k J )
1   L   o f   c o o l i n g  
t o w e r   b l o w d o w n
0 . 9 - 0 . 9 8   L   o f   t r e a t e d  
w a t e r   - R e c y c l e d
0 . 0 2 - 0 . 1   L   o f  
r e j e c t s
S o l i d s   f o r  
d i s p o s a l
Figure 14: ZLD system - mass flows & energy consumption
 
  4 0  
Our model assumes a 90% recovery rate and parasitic power consumption of 110 kJ per 1
liter of waste water treated.
Some studies also include evaporation ponds as part of a ZLD system. Evaporation ponds
are preferred when there is sufficient space on site and if local meteorological conditions are
favorable for evaporation. However, water cannot be recovered from evaporation ponds. In
our model, ZLD systems will not refer to such evaporation ponds.
 
  4 1  
Estimating parasitic power requirements
Within a power plant – the two key areas of parasitic power consumption are the working
fluid feed pump, and the cooling tower equipment.
For the cooling towers, energy consumption depends upon the configuration of the cooling
system – approach temperatures and range; local conditions – average temperature and
humidity; and seasonal variations – winter versus summer. The major energy users in a
cooling system are pumps and fans. Pumps are used for circulating water in wet cooling
towers. Their energy use is determined by, inter alia, the flow rate or the amount of water
that has to be pumped. Fans are used for ventilation in cooling towers and condensers. Their
energy use is determined by the number, size and the type of fans; and the amount of air to
be ventilated.
Per EC ( 2001), both dry and wet cooling systems require around 0.02kW per kJ/ s of heat to
be dissipated ( i. e. condenser duty). Maulbetsch and DiFilippo ( 2006) analysis indicates
around 0.01kW per kJ/ s of condenser duty.
To account for boiler feedpump and other auxiliary equipment at the power plant, we
assume a requirement of 0.01kW per kJ/ s of condenser duty. This seems reasonable based
on Maulbetsch and DiFilippo ( 2006).
Overall, for power plant related parasitic power requirements, our model defaults to a value
of 0.02 kW per kJ/ s of heat to be dissipated ( i. e. condenser duty). Assuming a 10% net
thermal efficiency, this would imply approximately 20% of the gross capacity of the power
plant.
 
  4 2  
Water required by flash power plants
In this chapter, we estimate water requirements indicated in Figures 4.1 and 5.1.
In flash power plants, geothermal fluid above around 180° C ( EERE 2010) is sprayed into a
tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly
vaporize, or " flash." The vapor ( steam) then drives a turbine, which drives a generator. The
liquid remaining in the tank can be flashed again in a second tank to extract even more
energy ( double flash power plants). In such plants, the steam is condensed to lower the
turbine back pressure and increase overall efficiency. Most or nearly all of the makeup water
for cooling towers is provided by the steam condensate.
We model a single flash power plant with a wet re- circulating cooling system. Double flash
plants and flash- binary power plants will be modeled in later versions of our model.
Thermodynamics behind a single flash power plant
 
The thermodynamic analysis is also important to calculate the required flow rate of
geothermal fluid - fluid losses from EGS resources is directly related to the flow rate. This
will be addressed in the following chapter.
Figure 15: Schematic diagram of a single flash power plant
Source: Figure reproduced with permission from Yari ( 2010)
The following thermodynamic analysis of a single flash plant is based largely on DiPippo
( 2008). Unlike an ORC cycle, the first law thermal efficiency cannot be applied to a flash
power plant because it is not a closed cycle. We use the second law efficiency, alternatively
referred to as exergetic or utilization efficiency to calculate geothermal fluid flow rates and
condenser duty.
   State 1
 
  4 3  
The geothermal fluid will be assumed to be a saturated liquid at an temperature of T geo, in
( user input). With the help of tables regarding water saturation properties ( steam tables,
( NIST 2010), the pressure, entropy, and enthalpy can be obtained. The specific exergy of
incoming geothermal fluid can be calculated based on the following:
( ) e 1 = h 1 − h 0 − T 0 s 1 − s 0 ( Equation 21)
where
e 1 is specific exergy
T 0 is the dead state or ambient temperature
s 0 and h 0 are the specific entropy in kJ/( kg. K) and specific enthalpy in kJ/ kg
respectively at given ambient conditions T 0 ; and can be obtained from steam tables.
s 1 and h 1 are the entropy in kJ/( kg. K) and enthalpy in kJ/ kg respectively in state 1
given temperature T geo, in and corresponding pressure P 1
The total exergy of the incoming geothermal fluid can be calculated by
E i n m g e o e 1
• •
= ( Equation 22)
where,
E i n
•
is total exergy in kJ/ s
m g e o
•
represents the mass flow rate in kg/ s of the geothermal fluid extracted
The exergetic efficiency ( also called second- law or utilization efficiency) of the flash power
plant is defined as
1
•
•
=
E
W n e t
e x h ( Equation 23)
   State 2
The throttle valve ( TV) creates a flow restriction which maintains a pressure drop from state
1 ( saturated fluid at inlet geothermal fluid temperature T geo, in ) to state 2 ( two- phase mixture
of saturated liquid and saturated vapor). After the flash chamber, there is a stream of
saturated liquid at state 3 and saturated vapor at state 4.
The mass fraction of the geothermal fluid which flashes to saturated steam can be
determined by the “ lever rule” which is given as
3 6
2 6
2
h h
h h
x
−
−
= ( Equation 24)
where,
h 2 , h 3 , and h 6 are the specific enthalpies in kJ/ kg at states 2, 3 and 6 respectively and
are obtained from the steam tables.
 
  4 4  
The enthalpy at state 2 is assumed to be the same as state 1 because there is no heat or work
input to the fluid. As a result, the above equation can be written as
3 6
1 6
3 6
2 6
2
h h
h h
h h
h h
x
−
−
=
−
−
= ( Equation 25)
x m g e o
•
2 represents the total mass of steam in kg/ s that goes to the turbine.
Based on DiPippo ( 2008), the optimum flash temperature T 2 ( and hence pressure P 2 ) equals
the average temperature of the inlet geofluid temperature and the condenser temperature:
2
( ) , 5
2
T T
T g e o i n +
= ( Equation 26)
where,
T 2 is the temperatures in the flash chamber
T 5 is the temperatures of the liquid exiting the condenser. The model defaults to a
value of 50 ° C for T 5 .
   States 3 and 4
Here the fluid is assumed to be in the saturated vapor state; and at the same pressure and
temperature as in state 2. For a double flash power plant, it is possible that the fluid at the
inlet of the turbine is wet.
The vapor coming out of the turbine has lower enthalpy ( h 4 ) due to conversion to
mechanical/ electrical energy. The difference in enthalpy between states 3 and 4 is equal to
the gross electric output of the plant after taking into consideration generator efficiency. The
vapor is at a lower temperature in 4 than at 3.
g r o s s g e n t u r g e o W h h x m
• •
= 3 − 4 2 h h ( ) ( Equation 27)
where,
•
g r o s s W is the gross electricity output in kJ/ s
S gen is the turbine efficiency. Model defaults to 0.85 based on MIT ( 2006)
S tur is the generator efficiency. Model defaults to 0.95
   State 5
The saturated vapor exiting the turbine is condensed in the condenser. The fluid in state 5 is
assumed to be in the saturated liquid state and at an outlet temperature of T cond with a
corresponding enthalpy of h 4 . The total condenser duty can be calculated as
( ) 2 4 5 Q o u t x m g e o h h = −
• •
( Equation 28)
where,
 
  4 5  
•
o u t Q is the heat load on the condenser; and equivalently represents the heat that
needs to be rejected by the cooling tower
   State 6
Here the fluid is in the saturated liquid state; and at the same pressure and temperature as in
state 2. The fluid is either injected back, flashed again for additional power generation
( double flash), or run through a heat exchanger to generate power in an ORC cycle ( flash-binary).
In some cases, it may be discharged to the surface as in case of the Denizli- Kızıldere
geothermal power plant in Turkey where the geothermal fluid is discharged to the river
( Dagdas, Öztürk et al. 2005).
   Cooling Tower Analysis
As in case of the ORC cycle, the water required in the cooling tower is given by
e v p o u t l a t e n t v a p w = Q × f / h
• •
( Equation 29)
The blowdown rate can be given as before
( − 1 )
=
•
•
N
w
w e v p
B D , and ( Equation 30)
The blowdown is assumed to be injected back. Makeup water requirement that needs to be
sourced externally is given by
M U E e v p B D g e o w w w x m
• • • •
, = + − 2 ( Equation 31)
where,
•
M U E w , is the makeup rate of water in kg/ s ( or liter/ s assuming water density to be 1
kg/ liter) that needs to be sourced externally.
Methodology
The model estimates two quantities for flash power plants. First, the geothermal fluid flow
rate which is essential to estimate fluid losses for EGS resources. Second, the makeup water
requirements for cooling system of the plant.
 
  4 6  
Figure 16: Methodology adopted for analyzing Flash power plants
   Step 1
As with ORC plants, the starting point of the model is the electricity consumed by the end
customer, which in turn gives the electricity required to be produced by the flash power
plant after accounting for transmission and distribution losses ( refer to equation 11).
   Step 2
The model defaults to a net exergetic efficiency of 0.32 for single flash power plants; and a
parasitic power consumption of 5.7% of net power generated. An additional modification to
parasitic power is made based on non- condensable gases ( NCG) content of the steam.
Based on the above relationships, total mass of the incoming geofluid can be calculated
based on Equation 22.
   Step 3
In Step 3, the fraction of fluid flashed is calculated based on Equation 24. The enthalpy of
the steam as it exits the turbine can be calculated based on Equation 27, which then allows
us to calculate condenser duty based on Equation 28.
   Step 4
The model assumes that the flash power plant has a wet re- circulating cooling tower. In later
versions of the model, we will analyze dry and hybrid cooling systems.
Exergy efficiency of flash power plants
DiPippo ( 2004) analyzed the relationship between exergetic efficiency and incoming specific
exergy of the geofluid ( which in turn is directly related to the temperature). The analysis was
undertaken for a wide variety of geothermal power plants including plants of flash, binary,
and hybrid technology. The author did not find any trend and concluded that exergectic
efficiency is determined by the sophistication of the plant design irrespective of the
incoming exergy. This is in contrast to the first law thermal efficiency of ORC cycle where
there is a positive linear relationship with temperature of the geofluid.
Based on the data given in the following table, the model defaults to net exergetic efficiency
of 0.32 for single flash power plant.
1. Net electricity to
be produced by
Flash power plant
3. Estimate
condenser duty
1a. Account for T& D
losses.
3a. Estimate fraction of
fluid flashed
3b. Estimate condenser
duty
2. Estimate mass
flow rate of geofluid
2a. Estimate exergy
efficiency
2b. Account for
parasitic load
2c. Estimate total
exergy requirements
Electricity requirements
4. Estimate external
cooling water
required
3a. Estimate water
requirements based on
cooling system
 
  4 7  
Table 11: Exergetic efficiency of various flash power plants
Plant Brine Inlet
Temperature
Net
Exergetic
Efficiency
Cooling
System
Sources
Single Flash
Miravalles, Unit 1; Costa Rica 240 0.28 WC DiPippo ( 2008)
Miravalles, Unit 2 240 0.28 WC DiPippo ( 2008)
Miravalles, Unit 3; 240 0.28 WC DiPippo ( 2008)
Denizli Kızıldere, Turkey( 1) 200 0.20 WC- OT Dagdas, Öztürk et al.
( 2005) and Gokcen,
Kemal Ozturk et al.
( 2004)
Simulation 230 0.35 DC Yari ( 2010)
Cerro Prieto I Units 1- 4,
Mexico
250 0.35 WC DiPippo c
Double Flash
Cerro Prieto I Unit 5 169.5 0.32 WC DiPippo ( 2008)
Cerro Prieto II & III 320 0.49 WC DiPippo ( 2008)
Beowawe, NV 215 0.47 WC DiPippo ( 1999)
Notes: 1. High fraction of NCG ( 10– 21% mass) leading to high parasitic load.
WC: Wet cooling system; DC: Dry cooling system; OT: Once through cooling
Parasitic power consumption
The average parasitic power consumption for many of the flash plants listed in the above
table is 5.1% of gross power generated ( 5.7% of net power generated). The model defaults
to a parasitic power consumption of 5.7% of net power generated.
One of the key determinants of parasitic power consumption is fraction of NCG in the
geofluid. NCG concentrations in steam are typically 0.5- 1.0% ( wt) of steam with CO 2 and
H 2 S constituting about 95% and 1- 2% of the NCG ( DiPippo 2008). NCG could significantly
reduce turbine power output in a dry- or flash- steam power plant by raising the backpressure
on the turbine and thus reducing the enthalpy drop across the turbine. This necessitates the
removal of NCG by some means – steam jet ejectors, vacuum pumps, or turbo- compressors
which increases parasitic power. However, DiPippo ( 2008) indicates that the resulting net
power output is higher than likely output where NCG were left to accumulate. The need for
NCG capture and removal may also be driven by local pollution norms.
At the Denizli Kızıldere power plant in Turkey, NCG constitutes 10- 21% weight of the
steam. As a result, the parasitic power consumption is almost 23%; much of it accounted for
by the compressor ( Gokcen, Kemal Ozturk et al. 2004).
The model defaults to a NCG level of 0.5- 1.0% ( wt) of steam. However, if a higher level of
NCG is selected, then additional compressor duty of 100 kW for every 1 %( wt) of NCG is
assumed based on Gokcen, Kemal Ozturk et al. ( 2004) and Yildirim Ozcan and Gokcen
( 2009).
 
  4 8  
Geothermal Resources
Water required to recharge hydrothermal resources
In this section, we estimate water requirements indicated in Figures 2.2 and 4.2.
Hydrothermal resources can witness pressure and production declines if the volume of
geothermal fluid injected back to the ground is significantly less than volume withdrawn.
This is especially true for steam- dominated resources; the steam condensate available for
reinjection constitutes only 10- 15% of the mass of dry stream withdrawn for power
generation ( DiPippo 2008). In the Geysers steamfield in California, around 42 million liters
of highly treated municipal effluent from the city of Santa Rosa is injected daily ( City of
Santa Rosa 2007; City of Santa Rosa 2010) to maintain steam pressure.
In case of flash power plants, which are used for energy conversion at wet steam dominated
geothermal fields, geothermal fluid and steam condensate available for reinjection constitute
around 70- 80% of the mass withdrawn ( EPRI 1997; DiPippo 2008). The difference of 20-
30% is lost primarily through evaporation of the steam condensate in the cooling towers.
Our model estimates the steam condensate lost due to evaporation. To compensate for this
loss, degraded water has to be sourced externally for reinjection.
In case of binary power plants, which are used for energy conversion of hot water fields, the
entire mass of geothermal fluid produced is available for reinjection. Hence, additional water
is not required for injection.
Water required for heat mining of EGS resources
In this section, we estimate water requirements indicated in Figures 3.2 and 5.2.
EGS resources have high temperature but contain little or no geothermal fluid. Water from
an external source is injected under high pressure through the injection well. The fluid then
returns to the surface through the production well, and thus transfers the heat to the surface
as steam or hot water. The injection pump provides the sole motive force for moving the
water continuously around the loop to mine energy from the reservoir and deliver it to a
power plant on the surface ( Duchane 1996).
Ideally, a closed loop is created whereby cold water is pumped down the injection well and
returned to the surface through the production well after passing through the hot, artificially
fractured formation. However, losses may occur due to permeation and leakage from the
fracture system to the surrounding rocks. The extent of losses will depend upon site specific
conditions like permeability of rocks, depth of the reservoir, as well as age. Losses are also a
function of injection pressure - Tester, Anderson et al. ( 2006) notes that high- injection
pressures extends the fractures and increases permeability which in turn increase flow losses.
Murphy, Drake et al. ( 1985) indicated a loss of 5% of EGS reservoir circulation flow rate
based on experience from the EGS resource at Fenton Hill, New Mexico where losses
during start of operation were greater than 10% but decreased after that. Duchane ( 1992)
 
  4 9  
indicated water losses in the order of 1- 2% of EGS reservoir circulation flow rate. EPRI
( 1997) have calculated water consumption by a EGS resource based on water losses of 5%
and 15%.
Fluid losses should be below 10% for long term viability of the resource ( Barbier 2002;
Kaltschmitt 2007). This is not only true for the very large volumes of freshwater lost in that
way, but also for the additional pumping power required for the makeup water. The MIT
study ( Tester, Anderson et al. 2006) study assumed that fluid losses of 2%.
Fluid losses in an EGS resource present a trade- off decision to planners. Higher efficiency of
energy conversion may require a wet or hybrid cooling tower, but will require lower
geothermal mass flow rates. Water required for cooling purposes may be compensated by
lower fluid losses during heat mining. Another area of trade- off is between flow losses and
electricity generated – higher injection pressures increase flow rates and hence electricity
produced but also increases flow losses ( Tester, Anderson et al. 2006).
The model defaults to a water loss of 5% of the circulation flow rate.
Power requirements for geothermal fluid production and injection
In vapor or steam dominated resources, the geothermal fluid originating from the boiling of
the geothermal fluid in the production wells was brought to the surface via buoyancy .
Kaltschmitt ( 2007) lists several reasons for the need to pump geothermal fluid by using
centrifugal pumps in production wells: buoyancy may be too small to establish a self-pumping
production; or produces an economically justifiable flow rate. Further, a certain
overpressure has to be maintained in the geothermal loop to prevent infiltration of oxygen
causing corrosion in the surface installations and in the casing of the re- injection well and to
avoid plugging of the formation by iron oxide precipitation near the re- injection interval.
Pumping demands are influenced by the above ground horizontal distances over which fluid
has to be pumped. Kaltschmitt ( 2007) indicates that production and injection wells must at
least be located at a distance of approximately one km to avoid a fast arrival of the re-injected
cold water, and to ensure a long technical lifetime of the system. In sites using
directional instead of vertical drilling, the wells could be located in a single well head and this
would save pumping demands.
Frick, Kaltschmitt et al. ( 2010) indicate that pump power requirements range from 1.8 to
10.8 kW/( kg/ s). Heidinger, Dornstädter et al. ( 2006) indicate that parasitic power demand of
the pumps consumes a great deal of the electricity produced from EGS resources. Based on
a graph given for energy consumption by pumps at the Soultz- sous- Forets EGS reservoir in
France, power consumption is in the range of 5- 10 kW/( kg/ s). However, Heidinger,
Dornstädter et al. ( 2006) indicate that pump power requirements is more likely to have an
exponential, rather than linear, relationship with flow rate.
Our model defaults to a linear relationship of 10.8 kW/( kg/ s) for EGS resources and 2.5
kW/( kg/ s) for water dominated hydrothermal resources. Steam dominated resources are
 
  5 0  
assumed to not require pumping. These numbers and relationship can be further refined in
later versions of the model.
 
  5 1  
References
B a r b i e r ,   E .   ( 2 0 0 2 ) .   " G e o t h e r m a l   e n e r g y   t e c h n o l o g y   a n d   c u r r e n t   s t a t u s :   a n   o v e r v i e w . "  
R e n e w a b l e   a n d   S u s t a i n a b l e   E n e r g y   R e v i e w s   6 ( 1 - 2 ) :   3 - 6 5 .  
B a y a r t ,   J . - B . ,   C .   B u l l e ,   e t   a l .   ( 2 0 1 0 ) .   " A   f r a m e w o r k   f o r   a s s e s s i n g   o f f - s t r e a m   f r e s h w a t e r  
u s e   i n   L C A . "   T h e   I n t e r n a t i o n a l   J o u r n a l   o f   L i f e   C y c l e   A s s e s s m e n t   1 5 ( 5 ) :   4 3 9 - 4 5 3 .  
C h a n d r a s e k h a r a m ,   D .   a n d   J .   B u n d s c h u h   ( 2 0 0 8 ) .   L o w - E n t h a l p y   G e o t h e r m a l   R e s o u r c e s   f o r  
P o w e r   G e n e r a t i o n   B o c a   R a t o n ,   T a y l o r   &   F r a n c i s .  
C i t y   o f   S a n t a   R o s a   ( 2 0 0 7 ) .   I n c r e m e n t a l   R e c y c l e d   W a t e r   P r o g r a m   -   A u g u s t   2 0 0 7  
A d d e n d u m   t o   P r o g r a m   E I R   a n d   G e y s e r s   E x p a n s i o n   P r o j e c t   C E Q A   C h e c k l i s t .  
S a n t a   R o s a ,   C A .  
C i t y   o f   S a n t a   R o s a .   ( 2 0 1 0 ) .   " G e y s e r s   P r o j e c t   -   F a c t s   a n d   S t a t s . "       R e t r i e v e d   J u n e   2 0 1 0 ,  
f r o m   h t t p : / / c i . s a n t a -
r o s a . c a . u s / d e p a r t m e n t s / u t i l i t i e s / i r w p / g e y s e r s / P a g e s / F a c t S t a t . a s p x .  
D a g d a s ,   A . ,   R .   Ö z t ü r k ,   e t   a l .   ( 2 0 0 5 ) .   " T h e r m o d y n a m i c   e v a l u a t i o n   o f   D e n i z l i   K I z I l d e r e  
g e o t h e r m a l   p o w e r   p l a n t   a n d   i t s   p e r f o r m a n c e   i m p r o v e m e n t . "   E n e r g y   C o n v e r s i o n  
a n d   M a n a g e m e n t   4 6 ( 2 ) :   2 4 5 - 2 5 6 .  
D i P i p p o ,   R .   ( 1 9 9 9 ) .   S m a l l   g e o t h e r m a l   p o w e r   p l a n t s :   D e s i g n ,   p e r f o r m a n c e   a n d  
e c o n o m i c s .   G H C   B u l l e t i n .   K l a m a t h   F a l l s ,   O R ,   O r e g o n   I n s t i t u t e   o f   T e c h n o l o g y :  
G e o - H e a t   C e n t e r :   8 .  
D i P i p p o ,   R .   ( 2 0 0 4 ) .   " S e c o n d   L a w   a s s e s s m e n t   o f   b i n a r y   p l a n t s   g e n e r a t i n g   p o w e r   f r o m  
l o w - t e m p e r a t u r e   g e o t h e r m a l   f l u i d s . "   G e o t h e r m i c s   3 3 ( 5 ) :   5 6 5 - 5 8 6 .  
D i P i p p o ,   R .   ( 2 0 0 7 ) .   " I d e a l   t h e r m a l   e f f i c i e n c y   f o r   g e o t h e r m a l   b i n a r y   p l a n t s . "   G e o t h e r m i c s  
3 6 ( 3 ) :   2 7 6 - 2 8 5 .  
D i P i p p o ,   R .   ( 2 0 0 8 ) .   G e o t h e r m a l   p o w e r   p l a n t s :   p r i n c i p l e s ,   a p p l i c a t i o n s ,   c a s e   s t u d i e s   a n d  
e n v i r o n m e n t a l   i m p a c t .   O x f o r d ,   U K ,   B u t t e r w o r t h - H e i n e m a n n .  
D O E   ( 2 0 0 2 ) .   E n e r g y   P e n a l t y   A n a l y s i s   o f   P o s s i b l e   C o o l i n g   W a t e r   I n t a k e   S t r u c t u r e  
R e q u i r e m e n t s   o n   E x i s t i n g   C o a l - F i r e d   P o w e r   P l a n t s ,   U S   D e p a r m e n t   o f   E n e r g y   -  
N a t i o n a l   E n e r g y   T e c h n o l o g y   L a b o r a t o r y   ( N E T L )   a n d   A r g o n n e   N a t i o n a l  
L a b o r a t o r y   ( A N L ) .  
D O E   ( 2 0 0 8 ) .   C o n c e n t r a t i n g   S o l a r   P o w e r   C o m m e r c i a l   A p p l i c a t i o n   S t u d y :   R e d u c i n g  
W a t e r   C o n s u m p t i o n   o f   C o n c e n t r a t i n g   S o l a r   P o w e r   E l e c t r i c i t y   G e n e r a t i o n   -   R e p o r t  
t o   C o n g r e s s .   W a s h i n g t o n   D . C . ,   U . S .   D e p a r t m e n t   o f   E n e r g y :   3 5 .  
D u c h a n e ,   D .   ( 1 9 9 6 ) .   P r o g r e s s   i n   m a k i n g   H o t   D r y   R o c k   g e o t h e r m a l   e n e r g y   a   v i a b l e  
r e n e w a b l e   e n e r g y   r e s o u r c e   f o r   A m e r i c a   i n   t h e   2 1 s t   c e n t u r y .   L o s   A l a m o s ,   N M ,   L o s  
A l a m o s   N a t i o n a l   L a b o r a t o r y .  
E C   ( 2 0 0 1 ) .   I n t e g r a t e d   P o l l u t i o n   P r e v e n t i o n   a n d   C o n t r o l   ( I P P C ) :   R e f e r e n c e   d o c u m e n t   o n  
t h e   a p p l i c a t i o n   o f   b e s t   a v a i l a b l e   t e c h n i q u e s   i n   i n d u s t r i a l   c o o l i n g   s y s t e m s .   ,  
E u r o p e a n   C o m m i s s i o n .  
 
  5 2  
E E R E .   ( 2 0 1 0 ) .   " H y d r o t h e r m a l   P o w e r   S y s t e m s . "   J u n e   2 0 1 0 ,   f r o m  
h t t p : / / w w w 1 . e e r e . e n e r g y . g o v / g e o t h e r m a l / p o w e r p l a n t s . h t m l .  
E I A   ( 2 0 1 0 ) .   A n n u a l   E n e r g y   O u t l o o k   2 0 1 0 .   W a s h i n g t o n   D C ,   U S   D e p a r t m e n t   o f   E n e r g y   -  
E n e r g y   I n f o r m a t i o n   A d m i n i s t r a t i o n .  
E P A   ( 1 9 9 9 ) .   E l e c t r i c   P o w e r   G e o t h e r m a l   I n j e c t i o n   W e l l s .   T h e   C l a s s   V   U n d e r g r o u n d  
I n j e c t i o n   C o n t r o l   S t u d y ,   U S   E n v i r o n m e n t a l   P r o t e c t i o n   A g e n c y .   1 7 .  
E P R I   ( 1 9 9 7 ) .   R e n e w a b l e   E n e r g y   T e c h n o l o g y   C h a r a c t e r i z a t i o n s ,   E l e c t r i c   P o w e r   R e s e a r c h  
I n s t i t u t e   ( E P R I )   a n d   U S   D e p a r t m e n t   o f   E n e r g y   ( D O E ) .  
E P R I   ( 2 0 0 2 ) .   C o m p a r i s o n   o f   A l t e r n a t e   C o o l i n g   T e c h n o l o g i e s   f o r   C a l i f o r n i a   P o w e r   P l a n t s  
-   E c o n o m i c ,   E n v i r o n m e n t a l   a n d   O t h e r   T r a d e o f f s .   P u b l i c   I n t e r e s t   E n e r g y   R e s e a r c h  
P r o g r a m   ( P I E R ) .   S a c r a m e n t o ,   C A ,   E l e c t r i c   P o w e r   R e s e a r c h   I n s t i t u t e   a n d  
C a l i f o r n i a   E n e r g y   C o m m i s s i o n .  
E P R I   ( 2 0 0 3 ) .   S p r a y   E n h a n c e m e n t   o f   A i r   C o o l e d   C o n d e n s e r s .   P u b l i c   I n t e r e s t   E n e r g y  
R e s e a r c h   P r o g r a m   ( P I E R ) .   S a c r a m e n t o ,   C A ,   C a l i f o r n i a   E n e r g y   C o m m i s s i o n ,  
E l e c t r i c   P o w e r   R e s e a r c h   I n s t i t u t e   a n d   C r o c k e t t   C o g e n e r a t i o n .  
E P R I   ( 2 0 0 3 ) .   U s e   o f   D e g r a d e d   W a t e r   S o u r c e s   a s   C o o l i n g   W a t e r   i n   P o w e r   P l a n t s .   P u b l i c  
I n t e r e s t   E n e r g y   R e s e a r c h   P r o g r a m   ( P I E R ) .   S a c r a m e n t o ,   C A ,   E l e c t r i c   P o w e r  
R e s e a r c h   I n s t i t u t e   a n d   C a l i f o r n i a   E n e r g y   C o m m i s s i o n .  
E P R I   ( 2 0 0 4 ) .   C o m p a r i s o n   o f   A l t e r n a t e   C o o l i n g   T e c h n o l o g i e s   f o r   U .   S .   P o w e r   P l a n t s :  
E c o n o m i c ,   E n v i r o n m e n t a l ,   a n d   O t h e r   T r a d e o f f s .   P a l o   A l t o ,   C A ,   E l e c t r i c   P o w e r  
R e s e a r c h   I n s t i t u t e   ( E P R I ) .  
F r a n c o ,   A .   a n d   M .   V i l l a n i   ( 2 0 0 9 ) .   " O p t i m a l   d e s i g n   o f   b i n a r y   c y c l e   p o w e r   p l a n t s   f o r  
w a t e r - d o m i n a t e d ,   m e d i u m - t e m p e r a t u r e   g e o t h e r m a l   f i e l d s . "   G e o t h e r m i c s   3 8 ( 4 ) :  
3 7 9 - 3 9 1 .  
F r i c k ,   S . ,   M .   K a l t s c h m i t t ,   e t   a l .   ( 2 0 1 0 ) .   " L i f e   c y c l e   a s s e s s m e n t   o f   g e o t h e r m a l   b i n a r y  
p o w e r   p l a n t s   u s i n g   e n h a n c e d   l o w - t e m p e r a t u r e   r e s e r v o i r s . "   E n e r g y   3 5 ( 5 ) :   2 2 8 1 -
2 2 9 4 .  
G A O   ( 2 0 0 9 ) .   E n e r g y - W a t e r   N e x u s :   I m p r o v e m e n t s   t o   F e d e r a l   W a t e r   U s e   D a t a   W o u l d  
I n c r e a s e   U n d e r s t a n d i n g   o f   T r e n d s   i n   P o w e r   P l a n t   W a t e r   U s e .   W a s h i n g t o n   D . C . ,  
U n i t e d   S t a t e s   G o v e r n m e n t   A c c o u n t a b i l i t y   O f f i c e :   7 8 .  
G l a s s l e y ,   W .   E .   ( 2 0 1 0 ) .   G e o t h e r m a l   E n e r g y :   R e n e w a b l e   E n e r g y   a n d   t h e   E n v i r o n m e n t .  
B o c a   R a t o n ,   T a y l o r   a n d   F r a n c i s .  
G o k c e n ,   G . ,   H .   K e m a l   O z t u r k ,   e t   a l .   ( 2 0 0 4 ) .   " O v e r v i e w   o f   K i z i l d e r e   G e o t h e r m a l   P o w e r  
P l a n t   i n   T u r k e y . "   E n e r g y   C o n v e r s i o n   a n d   M a n a g e m e n t   4 5 ( 1 ) :   8 3 - 9 8 .  
G R E E T   ( 2 0 1 0 ) .   G r e e n h o u s e   G a s e s ,   R e g u l a t e d   E m i s s i o n s ,   a n d   E n e r g y   U s e   i n  
T r a n s p o r t a t i o n   ( G R E E T )   M o d e l .   A r g o n n e ,   I L ,   A r g o n n e   N a t i o n a l   L a b o r a t o r y .  
H e b e r l e ,   F .   a n d   D .   B r ü g g e m a n n   ( 2 0 1 0 ) .   " E x e r g y   b a s e d   f l u i d   s e l e c t i o n   f o r   a   g e o t h e r m a l  
O r g a n i c   R a n k i n e   C y c l e   f o r   c o m b i n e d   h e a t   a n d   p o w e r   g e n e r a t i o n . "   A p p l i e d  
T h e r m a l   E n g i n e e r i n g   3 0 ( 1 1 - 1 2 ) :   1 3 2 6 - 1 3 3 2 .  
 
  5 3  
H e i d i n g e r ,   P . ,   J .   D o r n s t ä d t e r ,   e t   a l .   ( 2 0 0 6 ) .   " H D R   e c o n o m i c   m o d e l l i n g :   H D R e c  
s o f t w a r e . "   G e o t h e r m i c s   3 5 ( 5 - 6 ) :   6 8 3 - 7 1 0 .  
I m r o z   S o h e l ,   M . ,   M .   S e l l i e r ,   e t   a l .   ( 2 0 0 9 ) .   " E f f i c i e n c y   i m p r o v e m e n t   f o r   g e o t h e r m a l   p o w e r  
g e n e r a t i o n   t o   m e e t   s u m m e r   p e a k   d e m a n d . "   E n e r g y   P o l i c y   3 7 ( 9 ) :   3 3 7 0 - 3 3 7 6 .  
K a l t s c h m i t t ,   M .   ( 2 0 0 7 ) .   U t i l i s a t i o n   o f   G e o t h e r m a l   E n e r g y .   R e n e w a b l e   E n e r g y   -  
T e c h n o l o g y ,   E c o n o m i c s   a n d   E n v i r o n m e n t   M .   K a l t s c h m i t t ,   W .   S t r e i c h e r   a n d   A .  
W i e s e ,   S p r i n g e r :   4 3 7 - 4 9 6 .  
K a n o g l u ,   M .   a n d   Y .   A .   C e n g e l   ( 1 9 9 9 ) .   " I m p r o v i n g   t h e   p e r f o r m a n c e   o f   a n   e x i s t i n g   b i n a r y  
g e o t h e r m a l   p o w e r   p l a n t :   a   c a s e   s t u d y .   T r a n s a c t i o n s   o f   t h e   A S M E .   . "   J o u r n a l   o f  
E n e r g y   R e s o u r c e s   T e c h n o l o g y   1 2 1 ( 3 ) :   1 9 6 - 2 0 2 .  
K i n g ,   C .   W .   a n d   M .   E .   W e b b e r   ( 2 0 0 8 ) .   " T h e   W a t e r   I n t e n s i t y   o f   t h e   P l u g g e d - I n  
A u t o m o t i v e   E c o n o m y . "   E n v i r o n m e n t a l   S c i e n c e   &   T e c h n o l o g y   4 2 ( 1 2 ) :   4 3 0 5 -
4 3 1 1 .  
K l e i n ,   G . ,   M .   K r e b s ,   e t   a l .   ( 2 0 0 5 ) .   C a l i f o r n i a ' s   W a t e r   -   E n e r g y   R e l a t i o n s h i p .   S a c r a m e n t o ,  
C A ,   C a l i f o r n i a   E n e r g y   C o m m i s s i o n   ( C E C ) .  
K u t s c h e r ,   C .   a n d   D .   C o s t e n a r o   ( 2 0 0 2 ) .   A s s e s s m e n t   o f   E v a p o r a t i v e   C o o l i n g   E n h a n c e m e n t  
M e t h o d s   f o r   A i r - C o o l e d   G e o t h e r m a l   P o w e r   P l a n t s   G e o t h e r m a l   R e s o u r c e s   C o u n c i l  
( G R C )   A n n u a l   M e e t i n g   R e n o ,   N e v a d a ,   N a t i o n a l   R e n e w a b l e   E n e r g y   L a b o r a t o r y    
L a y t o n ,   D .   a n d   W .   M o r r i s   ( 1 9 8 0 ) .   G e o t h e r m a l   P o w e r   P r o d u c t i o n :   A c c i d e n t a l   F l u i d  
R e l e a s e s ,   W a s t e   D i s p o s a l ,   A n d   W a t e r   U s e .   L i v e r m o r e ,   C A ,   L a w r e n c e   L i v e r m o r e  
L a b o r a t o r y .  
M a u l b e t s c h ,   J .   S .   a n d   M .   N .   D i F i l i p p o   ( 2 0 0 6 ) .   C o s t   a n d   V a l u e   o f   W a t e r   U s e   a t  
C o m b i n e d - C y c l e   P o w e r   P l a n t s .   P u b l i c   I n t e r e s t   E n e r g y   R e s e a r c h   ( P I E R )   P r o g r a m .  
S a c r a m e n t o ,   C A ,   C a l i f o r n i a   E n e r g y   C o m m i s s i o n .  
M i c h a e l i d e s ,   E .   E .   a n d   J .   K .   R y d e r   ( 1 9 9 2 ) .   " T h e   i n f l u e n c e   o f   s e a s o n a l   a n d   d a i l y  
t e m p e r a t u r e   f l u c t u a t i o n s   o n   t h e   w o r k   p r o d u c e d   b y   g e o t h e r m a l   p o w e r   p l a n t s . "  
I n t e r n a t i o n a l   J o u r n a l   o f   E n e r g y   S y s t e m s   1 2 ( 2 ) :   6 8 - 7 2 .  
M i s h r a ,   G .   S .   a n d   S .   Y e h   ( 2 0 1 0 ) .   A n a l y s i s   o f   L i f e c y c l e   W a t e r   R e q u i r e m e n t s   o f  
T r a n s p o r t a t i o n   F u e l s :   C o r n - b a s e d   E t h a n o l .   D a v i s ,   I n s t i t u t e   o f   T r a n s p o r t a t i o n  
S t u d i e s ,   U n i v e r s i t y   o f   C a l i f o r n i a .  
M u r p h y ,   H . ,   R