The economic and industrial potential of geothermal energy as well as its environmental risks have been pointed out in several studies [1
]. Lund [4
] noted that recorded accounts show uses of geothermal water for bathing, cooking and space heating by Romans, Japanese, Turks, Icelanders, Central Europeans and the Maori of New Zealand. The first use of geothermal energy for electric power production occurred in Italy a century ago with the commissioning of a commercial power plant (250 kWe). This was followed by similar plants at Wairakei in New Zealand in 1958; an experimental plant at Pathe, Mexico in 1959; the first commercial plant at The Geysers in the United States in 1960, and at Matsukawa, Japan, in 1966 [5
]. All of these early plants used steam obtained directly from the earth (i.e.
dry steam fields), except for New Zealand, which was the first to use flashed or separated steam for running the turbines. An assessment of geothermal resources in the United States has also been reported by White and Williams [8
]. The former USSR produced power (680 kWe) from the first true binary power plant, using 81 °C water at Paratunka on the Kamchatka peninsula, the lowest temperature at that time. Iceland first produced power at Namafjall in northern Iceland, from a 3 MWe non-condensing turbine. These were followed by plants in El Salvador, China, Indonesia, Kenya, Turkey, Philippines, Portugal (Azores), Greece and Nicaragua in the 1970s and 80s. Later plants were installed in Thailand, Argentina, Taiwan, Australia, Costa Rica, Austria, Guatemala, Ethiopia, with the latest installations in Germany and Papua New Guinea [5
]. Bouchekima [9
] reported on the use of brackish underground geothermal water to feed a solar still installed in the south of Algeria. Iceland is widely considered as the most successful state in the geothermal community. The country of just over 300,000 people is fully (i.e.
100%) powered by renewable forms of energy in terms of electricity production (Figure 1
), ranking the highest in the 15 top countries that generate electricity from geothermal resources.
Share of total electricity generation from geothermal resources in the top 15 countries [10
Share of total electricity generation from geothermal resources in the top 15 countries [10
Geothermal energy comes from the natural generation of heat, primarily due to magma, as well as the decay of the naturally occurring radioactive isotopes of uranium, thorium and potassium in the earth. The total thermal energy to a depth of 10 km is estimated at 1.3 × 1027
J, equivalent to burning 3.0 × 1017
barrels of oil. Wright [11
] has reported that worldwide energy utilization is equal to about 100 million barrels of oil per day. Based on this he has estimated that the Earth’s high temperature hydrothermal reservoir to a depth of 10 kilometers could theoretically supply all of mankind’s power needs for several million years (Figure 2
). On average, the temperature of the Earth increases about 30 °C/km of depth. Thus, the temperature at 10 km would be over 300 °C. However, most geothermal exploration occurs where the gradient is higher, and thus where drilling can be shallower and less costly.
Geothermal resources may therefore be classified by type of rock formation/form of water and temperature, ranging from 20 °C to over 300 °C [4
]. Resources above 150 °C are normally used for electric power generation, although power has recently been generated at Chena Hot Springs Resort in Alaska using a 74 °C geothermal resource [12
]. Resources below 150 °C are typically used in direct-use projects for heating and cooling.
Fridleifsson et al.
] has reported that electricity is produced by geothermal means in 24 countries. Furthermore, direct application of geothermal energy for heating and bathing has been reported by 72 countries. By the end of 2004, the worldwide use of geothermal energy was 57 TWh/yr of electricity and 76 TWh/yr for direct use. Six developing countries are among the top fifteen states reporting direct use with China on the top of the list. Fridleifsson et al.
] goes on to argue that it is considered possible to increase the installed world geothermal electricity capacity from the current 10 GW to 70 GW with present technology, and to 140 GW with enhanced technology.
The coupling of renewable energies such as wind, solar and geothermal with desalination systems holds great promise for increasing water supplies in water scarce regions [2
]. It can be argued that an effective integration of these technologies would allow countries to address water shortage problems with a domestic energy source that does not produce air pollution or contribute to the global problem of climate change. Furthermore this approach will help to bypass the problems of rising fuel prices and decreasing fossil fuel supplies. Desalination plants, for example, may be run with geothermal energy being employed directly to heat the saline or brackish water in multiple effect distillation units and/or it could be used indirectly to generate electricity for operating reverse osmosis units [16
]. Ophir [17
] presented an economic study of desalination powered by a geothermal resource of 110–130 °C. Another technical and economic study was conducted by Karytsas [18
] to analyze the feasibility of using geothermal resources between 75 and 90 °C to power a multiple effect boiling system (MEB). Bourouni et al.
] reported on installations using humidification dehumidification processes in the form of evaporators and condensers made of polypropylene and operated at a temperature between 60 and 90 °C. Furthermore, with the recent progress in membrane distillation technology, the utilization of direct geothermal brine with temperature up to 60 °C has shown promise [22
The aim of this paper is to provide a critical overview of seawater and brackish water desalination using geothermal resources. Specific case studies are presented as well as an assessment of environmental risks and market potentials. The availability and suitability of geothermal energy in comparison to other renewable energy resources for desalination are also discussed.
2. Desalination Using Renewable Energies
The combination of a renewable energy, such as wind, solar and geothermal, with desalination systems holds immense promise for improving potable water supplies in arid regions [2
]. It can be argued that an efficient amalgamation of these technologies will allow nations to deal with water shortage problems with a domestic energy source that does not produce air pollution or contribute to the global crisis of climate change. Repetitive! Furthermore, while fuel prices are rising and fossil fuel supplies are decreasing, the fiscal outlay for desalination and renewable energy systems is steadily decreasing. The latter is due in part to a variety of possible arrangements that can be envisaged between renewable power supplies and desalination technologies [25
One of the more successful solar desalination devices is the multiple-effect still [27
]. Latent heat of condensation is recovered, in two or more stages (generally referred to as multi-effects), so as to increase production of distillate water and improve system efficiency. A key feature in improving overall thermal efficiency is the need to gain a better understanding of the thermodynamics behind the multiple use of the latent heat of condensation within a multi-effect humidification-dehumidification solar still [27
]. In addition, while a system may be technically very efficient it may not be economic (i.e.
, the cost of water production may be too high) [28
]. Therefore, both efficiency and economics need to be considered when choosing a desalination system. It can be further argued that desalination units powered by renewable energy systems are uniquely suited to provide water and electricity in remote areas where water and electricity infrastructures are currently lacking.
Considering that the energy requirements for desalination continues to be a highly influential factor in system costs, the integration of renewable energy systems with desalination seems to be a natural and strategic coupling of technologies [29
]. As an example of the potential, the southern part of the country of Algeria consists almost entirely (i.e.
90%) of the great expanse of the Sahara Desert. This district has fresh water shortages but also has plenty of solar energy [9
], wind energy [23
] and important geothermal reservoirs [2
]. The amalgamation of renewable resources with desalination and water purification is thus very attractive for this district (Table 1
). This will be discussed in more detail in the Case Study section.
Renewable energy sources (RES) desalination combinations [2
Renewable energy sources (RES) desalination combinations .
|Renewable energy sources technology||Feed water salinity||Desalting technologies|
|Multiple effect boiling (MEB)||Multi-stage flash (MSF)||Reverse osmosis (RO)||Electrodialysis (ED)||Compression (MVC)|
|Solar thermal||Seawater||✓||✓|| || || |
|Photovoltaic||Seawater|| || ||✓|| || |
|Brackish water|| || ||✓||✓|| |
|Wind||Seawater|| || ||✓|| ||✓|
|Brackish water|| || ||✓|| || |
|Geothermal||Seawater||✓|| || || || |
The two most successful commercial water desalination techniques involve thermal and membrane separation methods [2
]. The first method involves heating saline or brackish water to produce water vapor and then condensing the vapor to give pure water. The second method is based on size exclusion where the smaller water molecules can pass through a semi permeable membrane but the larger salt molecules cannot. Thermal separation processes include multi stage flash (MSF), multi effect evaporation (MEE)/multi effect distillation (MED), vapor compression (VC) and solar desalination. Membrane separation processes include reverses osmosis (RO) and electro-dialysis (ED). Reverses osmosis desalination will become increasingly more competitive with thermal desalination processes in the next decade.
3. Geothermal Energy and Desalination
When using geothermal energy to power systems such as desalination plants we avoid the need for thermal storage. In addition, the energy output of this supply is generally stable compared to other renewable resources such as solar and wind power [31
]. Kalogirou [16
] has shown that the ground temperature below a certain depth remains relatively constant throughout the year. Popiel et al.
] reported that one can distinguish three ground zones; surface, shallow and deep, with geothermal energy sources being classified in terms of their measured temperatures as low (<100 °C), medium (100–150 °C) and high temperature (>150 °C), respectively.
Geothermal wells deeper than 100 m can reasonably be used to power desalination plants [16
]. The utilization of geothermal power directly as a stream power in thermal desalination plants can also be envisaged. Furthermore, with the recent progress on membranes distillation technology, the utilization of direct geothermal brine with temperature up to 60 °C has become a promising solution [22
Improved heat exploitation technologies, which are still at the trial stage, have huge potential for primary energy recovery of the Earth’s stored thermal energy [11
]. Direct use of geothermal energy for heating is also commercially competitive with conventional energy sources. An exponential increase is foreseen in the geothermal heat pump sector, for heating and/or cooling. There is an environmental advantage in that geothermal heat pumps driven by fossil fuelled electricity reduce the CO2
emission by at least 50% compared with fossil fuel fired boilers. This will be discussed in more detail in the section on Environmental Considerations
. Furthermore, we support Bertani’s [33
] view that renewable energy sources can contribute significantly to the mitigation of climate change by reducing the use of fossil fuels.
Geothermal energy is accessible day and night every day of the year and can thus serve as an add-on to energy sources which are only available intermittently. An MIT-study indicates a potential of more than 100 GW for USA and 35 GW for Germany [34
]. It is likely that up to 8% of the total world electricity may be produced with geothermal resources, serving 17% of the world population [36
]. Thirty nine countries, situated by and large in Africa, Central/ South America, and the Pacific, can potentially obtain 100% of their electricity from geothermal resources [37
5. Environmental Considerations and Sustainability
Desalination of sea and brackish water requires large quantities of energy which normally results in a significant environmental impact if fossil fuels are used (e.g., CO2
emissions, thermal pollution of seawater). The operating cost of different desalination techniques is also very closely linked to the price of energy. This makes the use of renewable energies associated with the growth of desalination technologies very attractive. We can argue that the ready availability of inexpensive oil and natural gas reserves in such areas of the world as the Arabian Gulf may reduce the need for using renewable energy for desalination. However, looking at this more closely we see that this is non-sustainable since fossil fuels are non-renewable, and with a continually growing population there is an ever increasing demand on the use of fossil fuels for desalination. Take Saudi Arabia as a specific example; in 2008 total petroleum (i.e.
oil and gas) production was 10.8 million bbl/d with internal oil consumption at 2.4 million bbl/d (i.e.
about 25%) [47
]. Most of the internal consumption was used for electricity generation and water desalination. The population is expected to increase from 30 million in 2010 to approximately 100 million by 2050 [48
]. It has been estimated that by then 50% of the fossil fuel production will be used internally in the country for seawater desalination in order to provide fresh water for the people. This will reduce the state’s income, increase pollution and is clearly non-sustainable. There are also concerns about the resulting political instability which could arise due to these effects [49
]. A possible solution to the environmental and sustainability problems is the increased use of renewable, including nuclear, energy sources for desalination [4
Let us take a closer look at the environmental impacts that must be considered during utilization of geothermal resources as outlined by Rybach [50
], Lund [4
]; Kagel, et al.
] and Fridleifsson, et al.
]. These include emission of harmful gases, noise pollution, water use and quality, land use, and impact on natural phenomena, as well as on wildlife and vegetation. The environmental advantages of renewable energy can be seen when comparing, for instance, a coal-fired power plant to a geothermal power plant; the former produces about 25 times as much carbon dioxide (CO2
) and sulfur dioxide (SO2
) emissions per MWh (i.e.
994 kg vs.
up to 40 kg for CO2
, 4.71 kg vs.
up to 0.16 kg for SO2
, respectively) [4
] (Table 2
). However, in a geothermal power plant hydrogen sulfide (H2
S) also needs to be routinely treated and converted to elemental sulfur since about 0.08 kg H2
S may be produced per MWh electricity generated. It can be argued that this is still much better than oil-fired power plants and natural gas fired plants which produce 814 kg and 550 kg of H2
S per MWh, respectively. The environmental advantages and sustainability of geothermal energy for electricity production in comparison with oil and coal fired power plants is therefore clearly demonstrated by a significant reduction in emissions of CO2
, and H2
S as well as having a very low fresh water usage (Table 2
Carbon dioxide (CO2
) emission from geothermal power plants in high-temperature fields is about 120 g/kWh (weighted average of 85% of the world power plant capacity). As explained above, there is an environmental advantage in that geothermal heat pumps driven by fossil fuelled electricity reduce the CO2
emission by at least 50% compared with fossil fuel fired boilers. If the electricity that drives the geothermal heat pump can be produced from a renewable energy source like hydropower or geothermal energy, then the emission savings will increase up to 100%. The total CO2
emission reduction potential of geothermal heat pumps has been estimated to be 1.2 billion tons per year or about 6% of the global emission [4
Comparison of CO2
S emissions and fresh water usage from electricity generation (MWh) from different energy sources (adapted in part from Fridleifsson, et al.
Comparison of CO2, SO2, H2S emissions and fresh water usage from electricity generation (MWh) from different energy sources (adapted in part from Fridleifsson, et al. ).
|Energy Source||Coal||Oil (& Gas)||Geothermal|
|CO2 (Kg/MWh)||994||893 (599)||40–120|
|H2S (Kg/MWh)||−||814 (550)||0.08|
|Amount fresh water used (L/MWh)||1,370||1,170||20|
Geothermal plants use about 20 liters of freshwater per MWh, while binary air-cooled plants use no fresh water, as compared to a coal plant that uses 1,370 liters per MWh [13
]. The only change in the fluid during use is to cool it, and usually the fluid is returned to the same aquifer so it does not mix with the shallow groundwater. At The Geysers facility in northern California, for example, 42 million liters of treated wastewater are pumped daily for injection into the geothermal reservoir, reducing surface water pollution in the community and increasing the production of the geothermal field. A similar project supplies waste water from the Clear Lake area on the northeast side of the The Geysers. These projects have augmented the capacity of the field by over 100 MWe.
Geothermal power plants are designed to blend-in with the adjacent landscape, and can for instance be located near recreational areas with the least amount ground and visual impacts. They generally consist of small modular plants under 100 MWe as compared to coal or nuclear plants of around 1,000 MWe, with a geothermal facility normally using 400 square meters of land per GWh compared to a coal facility that uses almost ten times that much area per GWh and a wind farm that uses three times the area for the same power generation [4
]. On the negative side, subsidence and induced seismicity (i.e.
earthquakes) are two land use issues that must be considered when withdrawing fluids from the ground. These are usually mitigated by injecting the spent fluid back into the same reservoir. Problems with subsidence at the Wairakei geothermal field in New Zealand have been reported; however, this has been minimized by injection. Neither of these potential problems is associated with direct-use projects, as the fluid use is small. In addition, utilizing geothermal resources eliminates the mining, processing and transporting required for electricity generation from fossil fuel and nuclear resources.
With regards to impact on natural phenomena, wildlife and vegetation, geothermal plants are usually prohibited from being located near geysers, fumaroles (i.e. vents in the earth’s crust from which volcanic gas escapes into the atmosphere) and hot springs, as the extraction of fluids to run the turbines, might affect these natural thermal phenomena. Most plants are located in areas with no natural surface discharges. If geothermal plants must be located near these natural phenomena, then the fluid extraction depth is planned from a different reservoir to prevent any impact. Any site considered for a geothermal power plant, must be reviewed and considered for the impact on wildlife and vegetation, and if significant, provide a mitigation plan. Direct use projects are usually small and thus have no significant impact on natural features. In summary, the use of geothermal energy is reliable, is renewable; has minimum air emission and offsets the high air emissions of fossil fuel fired plants; has minimum environmental impacts; is combustion free; and is a domestic fuel source. Economic aspects will now be discussed in the next section.
6. Market Potential, Barriers to Growth and Risk Management
The capital and operating costs for desalination plants have tended to decrease over the years due primary to improvements in technical efficiency [53
]. At the same time that desalting costs have been decreasing, the price of obtaining and treating water from conventional sources has tended to increase because of the increased levels of treatment being required to meet more stringent water quality standards. This rise in cost for conventionally treated water also is the result of an increased demand for water, leading to the need to develop more expensive conventional supplies, since the readily accessible water sources have already been used up [26
Many factors enter into the capital and operating costs for desalination: capacity and type of plants, plant location, feed water, labor, energy, financing, concentrate disposal, and plant reliability [54
]. For example, the price of desalted seawater is about three to five times the cost of desalting brackish water from the same size plant, due primarily to the higher salt content of the former. In any state or district, the economics of using desalination is not just the number of dollars per cubic meter of fresh water produced, but the cost of desalted water versus
the other alternatives (e.g., superior water management by reducing consumption and improving water transportation). In many arid areas, the cost of alternative sources of water (i.e.
groundwater, lakes and rivers) is already very high and often above the cost of desalting. Any economic evaluation of the total cost of water delivered to a customer must include all the costs involved. This includes the costs for environmental protection (such as brine or concentrate disposal), and losses in the storage and distribution system.
Let us consider for a moment the barriers to growth (Table 3
). Although we can recognize the potential of geothermal energy for seawater desalination, the process has not as yet been significantly developed at commercial level [53
]. The main reason for this is that the existing technology cannot presently compete on produced water cost basis with conventional distillation and reverse osmosis technologies. On the other hand, it is also recognized that there is still important room to improve desalination systems based on geothermal energy. Technical design problems and high investment costs associated with indirect desalination still need to be overcome (e.g., converting thermal energy to electrical energy and using this in RO desalination). Thus the thermal distillation systems directly heated from geothermal sources will be the method of choice. The ongoing Milos low enthalpy geothermal power operation scheme [45
] has confirmed that through innovation, sustainable utilization geothermal energy may be employed for electricity generation and seawater desalination in order to meet local water needs.
Summary of market potential, barriers to growth and risk management of geothermal projects.
Summary of market potential, barriers to growth and risk management of geothermal projects.
Thermal distillation system directly heated from ground well is currently preferred geothermal method due to fewer technical risks
In many arid areas, cost of groundwater, lakes and rivers is very high and often above cost of conventional desalting. This makes geothermal desalination commercially attractive
Geothermal projects benefited from increased prices of oil and gas and from public concern over dependence on these fossil fuel resources
|Barriers to growth|
Rise in cost for conventionally treated water due to increased demand for water, leading to need to develop more expensive conventional supplies, since readily accessible water sources have been used up
Price of desalting seawater is 3 to 5 times cost of desalting brackish water from same size plant, due to higher salt content of former
Geothermal desalination process has not as yet been significantly developed at commercial level
Technical design problems and high investment costs associated with indirect desalination still need to be overcome
Electricity generation using geothermal power is only marginally profitable
Manage budget over-runs, increases in interest rates, and delays.
Run profitability simulations in order to analyze varying scenarios before implementing project
Reserves must always be planned for in financing of project
Limit business risks by suitable structuring of contracts with partners in project (e.g., share risks with drilling companies, power-plant supplier, and civil-engineering companies)
Stakeholders bear drilling risk, (i.e. that drilling company may or may not find something, or not within time predicted, and thus within budget, or that well proves not to be usable for pumping thermal water)
Pass part of risk on to drilling company in contract
Geological risk (i.e. non-discovery) is main hazard; it can be reduced by reprocessing old seismic analyses and preparing new ones.
Non-geological risk covered by equity capital and/or comprehensive insurance that covers both thermal potential of wells, and also absorption capacity of injection wells
There must be a focus on legal and economic aspects from start of any large scale commercial project
Projects must keep standby pumps ready in order to deal with financial risks of a failure of delivery or injection pumps