WATER INTENSITY OF ELECTRICITY FROM GEOTHERMAL RESOURCES
Gouri Shankar Mishra, University of California Davis, +1 (510)378 3187, gouri.mishra@gmail.com
William E. Glassley, Univeristy of California, Davis, +1 (505)795 6709, geobg@nf.au.dk
Sonia Yeh, University of California Davis, +1 (925)551 8059, slyeh@ucdavis.edu
Overview
Electricity from geothermal resources could play a significant role in the United States over the next few decades; A 2006 study
by MIT (Tester, Anderson et al. 2006) considers a capacity of 100GWe by 2050 as feasible; approximately 10% of the total
U.S. electricity generating capacity. The growth will come largely from enhanced geothermal resources (EGS) and across a
range of temperature conditions. Electricity from conventional hydrothermal resources will constitute a small portion of total
generation - around 0.6% by 2035 from 0.06% today (EIA 2010).
However, there is limited research on the water requirements and impacts of generating electricity from geothermal resources conventional as well as enhanced. Water (freshwater) is required for cooling and dissipation of waste heat in the power plant.
Degraded water1 is required to compensate for fluid lost to fractures during heat mining of EGS resources as well as condensate
use for evaporative cooling of flash power plants. Water is also required during project construction especially for hydraulic
fracturing of resources. Factors that affect water intensity of electricity generated (liters/kWh) are the efficiency of energy
conversion, the extent of parasitic power requirement which in turn depends upon the mineral content of the geothermal fluid,
pumping of fluid, and feed and cooling water pumps. We have developed a water intensity model (Mishra, Glassley and Yeh
2010) to account for these factors. Our model also considers factors that influence cooling water requirements; these include the
temperature and chemical composition of geothermal resource, the installed power generation technology (flash, organic
rankine cycle and the various configurations of these technologies), cooling technologies (e.g., air cooled condensers, wet
recirculating cooling, and hybrid cooling) and finally water treatment and recycling installations.
This paper is part of a series exploring the water footprint of renewable energy resource use. The paper's broader objective is to
identify constraints imposed by water limitations to achieving greater market penetration of alternative energy sources.
Methods
The model described here relies upon an extensive literature review to determine various empirical relationships necessary to
determine water usage. Included in this model are empirical descriptions of the relationship between thermal efficiency of a
binary power plant and temperature of inlet geothermal fluid, differences in efficiency between various ORC configurations,
and different efficiency of a plant with a wet re-circulating cooling system versus one with a dry cooling system. This is the first
published model to account for these effects using braod-based, empirically derived relationships. It should be noted that the
exact magnitude of these effects are very location specific – for example the energy penalty associated with dry cooling in a hot
dry desert climate will be significantly higher than that in a cold and humid location. The magnitudes are also subject to
seasonal and diurinal variations. Thus the model’s water usage estimates should be treated as “first order” and “annual
averages” for the requirements of freshwater, degraded water and geothermal fluid, all of which are explicitly estimated. The
methodology is detailed in Mishra, Glassley and Yeh (2010).
This study does not undertake a detailed systems analysis that consider specific local conditions like local meteorology (wet
bulb and ambient temperature, relative humidity, etc.), geothermal fluid composition, plant and cooling system design, and
cooling water composition. Neither does this study undertake a statistical analysis based on reported water consumption data by
existing plants.
“Degraded” water refers to water which cannot be referred to as freshwater (total dissolved solids less than 1,000 mg/L) 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.
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Results
Our analysis indicates that:
(a) There is wide variation in water intensity for generation of geothermal electricity. This is due to a large range of geothermal
resource temperatures - from lower than 70 °C to higher than 300 °C. For ORC plants, an increase in temperature and enthalpy
of the geothermal resource leads to an increase in thermal efficiency and consequently decrease in freshwater requirements for
cooling and geothermal fluid withdrawn for heat mining.
(b) Water intensity is also a function of cooling system and ORC power plant configuration. In comparison to wet re-circulating
cooling towers, dry cooling systems or air cooled condensers (ACC) and hybrid cooling systems consume less freshwater.
However, reduced thermal efficiency leads to increase in geofluid withdrawal and degraded water consumption. Wet recirculating with Zero Liquid Discharge (ZLD) systems have a limited impact on freshwater consumption , but a significant
decrease in freshwater withdrawal and corresponding increase in degraded water requirements. ZLD systems reduce cooling
tower blowdown which could be used for injection into the geothermal reserve. In absence of cooling tower blowdown due to
ZLD systems, degraded water has to be sourced externally.
(c) Different ORC configurations have different levels of thermal efficiency. Higher (lower) thermal efficiency leads to lower
(higher) water requirements and this applies for freshwater, degraded water and geothermal fluid. Thus a basic ORC is around
9% less efficient than an ORC with IHE (recuperated cycle ORC); correspondingly freshwater, degraded water and geothermal
fluid requirements are also around 9% higher. Similarly, a Kalina cycle plant, which is around 10% more efficient than
recuperated cycle ORC, requires 8-10% less water.
(d) When average expected geothermal fluid temperatures (around 200 °C) and evaporative cooling are considered, freshwater
water intensity of geothermal electricity is 2-4 times higher than conventional thermoelectric power plants even after
considering lifecycle water requirements2 and process and cooling water requirements due to carbon capturing. This is because
thermodynamic efficiency of such geothermal power plants are commonly less than 20% while thermoelectric plants can have
efficiencies between 30 to 50%. Similar magnitude of differences were observed for electricity from solar thermal.
Conclusions
Various policy approaches to encouraging renewable energy use, such as the Renewable Portfolio Standard (RPS) promote the
use of renewable resources to generate electricity. However, the impact of these technologies on water availability and quality
is only beginning to be quantified. Our study found that conventional geothermal electricity generation requires more water
than conventional thermoelectric power plants. However, implementation of certain specific design elements and technologies
can reduce water intensity of geothermal electricity. Advanced cooling technologies like hybrid cooling, and water treatment &
recycling technologies can significantly reduce water intensity relative to wet recirculating cooling systems; while minimizing
energy penalties during peak summer relative to dry cooling system. Similarly, advanced power plant configurations like
recuperated cycle ORC or dual pressure ORC increase efficiency of energy conversion which in turn reduces consumption
intensity of fresh and degraded water and withdrawal intensity of geofluid.
Results of this analysis highlight the importance of considering water resource availability to achieve higher penetration of
geothermal electricity. Ultimately, we expect our research will provide important baseline values that are currently missing but
which are necessary to efficiently identify key topics for further research. This approach will contribute to developing a more
rigorous capability for evaluating the many tradeoffs inherent in transitioning from a fossil fuel energy economy.
References
EIA (2010). Annual Energy Outlook 2010. Washington DC, US Department of Energy - Energy Information Administration.
Mishra, G. S., W. Glassley, et al. (2010). Analysis of Lifecycle Water Requirements of Energy & Transportation Fuels:
Electricity from Geothermal Resources: Model Description. Davis, University of California.
Tester, J. W., B. J. Anderson, et al. (2006). The Future of Geothermal Energy - Impact of Enhanced Geothermal Systems (EGS)
on the United States in the 21st Century, Massachusetts Institute of Technology (MIT).
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Lifecycle requirements include water required for coal mining & beneficiation, uranium mining & enrichment.