A REVIEW ON: GEOTHERMAL ENERGY TECHNOLOGY

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1 A REVIEW ON: GEOTHERMAL ENERGY TECHNOLOGY Rajat Nag ( ) School of Biosystems and Food Engineering, University College Dublin, Belfield, Dublin 4, Ireland. Abstract This paper investigates the current state of geothermal energy technology and its possible improvement. To meet this criteria the help of present establishments were studied mentioning its strength and weakness both. Furthermore the comparison of the policy towards renewable energy and people s feedback are illustrated. Introduction Renewable energy is the only option to minimize the CO2 emissions from the power industries. Solar, Wind, Ocean including tidal and wave, Hydropower are the different resources of renewable or green energy. However there is a lot of use of fossil fuel in the time of the construction of a power plant and the machinery used during operations. Hence the net CO2 emission per unit of energy production defines which renewable energy is better for the future generations. Hydropower and Ocean energy could not be used in small scale uses however wind, solar and geothermal can be scaled down to desired requirement. Solar and geothermal can be used directly other than electricity generations such as direct heating. Table 1: Savings in energy, carbon and greenhouse gases using geothermal energy Use Fuel oil Carbon CO2 SOx NOx bbl. TOE TOE TOE TOE TOE As electricity As direct heat Note: figures in millions and TOE = tons of oil equivalent (Zheng et al. 2015). According to Celiktas and Kocar (2013) within the renewables its current growth is only steady but rather slow whereas wind and solar PV reelects exponential growth. The choice of renewable energy is based on certain factors such as the availability of resources and technology, change of climate throughout the year, government encouragement, public acceptability, duration of the need and environmental impacts. The geothermal energy eliminates the problem with climate change during the year and colonization of the energy resources. The spontaneous flow of heat is spreading radially from the core of the earth by conduction and convection processes and the energy resource is available for billions of years to come. However it is easier to get access to the unlimited heat source near active volcanoes. The technology is divided into two main aspects. One is deep geothermal (more than 1500 meter from the earth crust) and the shallow geothermal energy (limited to mostly 200m). The deep geothermal energy is majorly used for electricity production whereas shallow geothermal is used for direct use and heat pumps. Initial investment, reluctance of government, motivation factor of public interest are the major challenges towards geothermal plant procurements. The objective of this study was to review geothermal energy in terms of current state of the technology, energy conversion principles, strengths and weakness of the energy system, energy production capacity, recent technological developments, and relevant energy policy and sustainability attributes of the energy system.

2 Materials and methods The technology The technology uses the concepts of geothermal springs for power plants and it is based on hydrothermal conversion system where power plants drill their own holes into the rock to more effectively capture the heat in the form of steam. There are three basic designs namely dry steam, flash steam, and binary cycle. All of those falls under deep geothermal energy. In dry steam technology, the steam goes directly through the turbine, then into a condenser where the steam is condensed into water. In a second approach, very hot water is depressurized or "flashed" into steam which can then be used to drive the turbine. In the third approach, called a binary cycle system, the hot water is passed through a heat exchanger, where it heats a second liquid such as isobutane in a closed loop. Isobutane boils at a lower temperature than water, so it is more easily converted into steam to run the turbine. These three systems are shown in the figure below. Figure 1. The three basic designs for geothermal power plants: dry steam, flash steam, and binary cycle (Source: U.S. Department of Energy). The choice of which design to use is determined by the resource. If the water comes out of the well as steam, it can be used directly, as in the first design. If it is hot water of a high enough temperature, a flash system can be used; otherwise it must go through a heat exchanger. Since there are more hot water resources than pure steam or high-temperature water sources, there is more growth potential in the binary cycle, heat exchanger design. Geothermal direct use means where geothermal heat energy is used in applications other than the generation of electricity. The processes which require input of heat can successfully utilize geothermal heat energy directly, instead of generating heat from other fuel sources. Installations can be standalone (i.e. one heat source, one use) or cascaded where applications are set up in series, each using fluid sand heat from the upstream process. In other option after extracting heat from the geothermal fluid it goes to a group of apartments (say 2000) with 55 C temperature and the heat can be utilized as floor heating. It exits from the system as 35 C and enters another process. There the heat exhausted from refrigeration, heat pump re heat the fluid to 58 C and it reenters the loop and heats approx apartment radiators and returns with 55 C temperature. Finally from the intermediate junction after extracting the heat it returns to injection well at 10 C. Geothermal reservoirs of low to medium temperature (usually C) can provide heat for industrial, commercial or residential applications. The hot fluid is allowed to flow through pipes to heat a certain number of apartments and it comes back with a lower temperature. Similar to this it can serve as the heat pump. A heat pump transfers the energy from the source to a sink where compression / expansion cycle is used to elevate or lower fluid

3 temperatures to a required level. The reverse action can be achieved in summers when the outside temperature is higher than the underground temperature the flow cools down the inside temperature of a house. Ground source energy system or shallow geothermal allows us to choose open and closed loop systems. In case of open loop system, hot water is pumped from aquifer, lakes or wells and discharge them to river after heat utilization in the apartments. Closed loop systems involve the circulation of a heat transfer fluid within pipes which are installed into some sub-surface medium (e.g. a pond, energy piles, horizontal trenches, vertical boreholes etc.). Closed loop system is always more sustainable than the open one as the former allows to return the used water into its origin and does not increase the surface water temperature which could lead to further escape of CO2 in the atmosphere from the surface water. b) Horizontal loop piping in series a) Vertical closed loop heat exchange system c) Horizontal loop with piping in parallel Figure 2. Basic designs of closed loop system for heat pump technology (Self et al. 2013). In closed loop systems, which are commonly utilized, the heat transfer fluid is enclosed in a circulating loop and has no direct contact with the ground; heat transfer with the ground occurs through the piping material. A vertical closed loop system (Figure 2) includes a loop field consisting of vertically oriented heat exchange pipes. To enhance heat transfer, the gap between the pipes and the borehole wall are filled with a pumpable grout material. In horizontal closed loop systems, which are common where ample ground area is available, the ground loop is laid out horizontally slightly below the earth s surface in backfilled trenches. Closed pond loop and energy piles are other options to capture energy. Discussions Comparison of different types of technology The following table compares the depth of boring, temperature of fluid at maximum depth, the capacity of the plant where they are suitable to be used. The first dry steam technology was built in 1962 at the Geysers dry steam field in California and it is still the largest producing geothermal field in the world with 1517 MW of active installed capacity with an average production factor of 63% (955 MW).

4 Table 2. Comparison of different aspects of the technology used in geothermal energy Name of technology Depth range (kilometer) Temperature range (degree C) Capacity of the plant where they are suitable to be used Dry steam or more MW Flash steam MW Binary cycle MW Direct use < Not applicable Heat pumps < Not applicable Recent Energy policy Table 3 reflects the interest and initiative of governments to treat geothermal as the renewable energy resource for future generations however a large number of developing countries are dependent on coal, hydropower and nuclear energy. Table 3. Geothermal plants in operation, installed capacity, and annual electricity generated by country and world total in 2010 (Chamorro et al. 2012) Recent technologies Thermal modeling: The geothermal mapping is very important to get an accurate overview of the location of water table, vulnerable soft clay or lime layers (Herbert et al. 2013). 3D grid along with triangulation or finite element modeling develops the mapping. So according to base load the suitable depth can be ensured to achieve certain amount of temperature without hampering the geological strata. As water is injected to the same layer it fulfills the criteria of not to deplete the resource completely.

5 Figure 3. Typical cross-section through the finite-element grid (Herbert et al. 2013). With the help of above finite element modelling following images can be developed. It is called heat contour mapping. Figure 4 indicates some similar mapping in Canada, two images are formed with the data collected from 3.5km and 6.5km from the earth s surface respectively where blue zone hold minimum temperature and red stands for 300 degree Celsius. Figure 4. Heat flow: depth temperature variations, Majorowicz and Grasby (2010). According to Shulyupin and Chermoshentseva (2013) Pioneering steam water flow calculations were aimed at determining the depth of vaporization, which was associated with the water level in a usual flowing well. eqn. (1)

6 Where L is the depth of vaporization. pm and p0 are, respectively, the wellhead pressure and the pressure at the depth of vaporization, R is the radius of the tube,τw is the shear stress at the wall of the tube, ρ is the density of the mixture, g is the free fall acceleration, and A is a function characterizing acceleration (the fraction of the acceleration component in the total pressure drop). The integral in equation (1) can be taken analytically in some cases. Desalination is another recent option which accelerated the electricity production by geothermal energy. In the islands where rain is the only source of drinkable water desalination is the only option for the production of drinking water. However the reverse osmosis technology is also energy consuming. In a geothermal plant, after generating electricity by transferring the energy to the turbine blades, the steam may be condensed to drinking water. Manenti (2013) presented the relationship vs geothermal temperature vs desalinated water quantity (Figure 5). Figure 5. Temperature pressure relationship for the geothermal loop (Manenti et al. 2013). Energy policy The more welcome the renewable energy the more a nation will be benefitted. For example, according to Apergis and Tsoumas (2011), in United States the Energy Act, declared in 2005 boosted the consumption of geothermal energy over the Solar, Wind, Waste, and Geothermal Power Production Incentives Act of 1990 and the Energy Policy Act in The power generated by geothermal plants are used as a tax credit, while this was previously given for wind energy and biomass projects only. The Energy Independence and Security Act of 2007 and its extension in 2008 declared that the U.S. Senate supported programs of research, development, demonstration, and commercial application in advanced geothermal energy technologies to expand its use. Sustainibility Generating electricity from geothermal energy is sustainable because the amount of steam or hot water extracted from the production well is compensated fully with the amount of cold water injected through injection well. Again the operating cost of different desalination techniques is also very closely related to the price of energy. We can argue that in Arabian Gulf region the ready availability of inexpensive oil and natural gas serve the purpose of fuel however, looking at this more closely we see that this is non-sustainable since fossil fuels are

7 non-renewable, and with a continually growing population will result completely resource depletion. Goosen et al. (2010) expressed that in Saudi Arabia 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%) in Most of the internal consumption was used for electricity generation and water desalination. The population is expected to increase approximately by 100 million by 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. Geothermal energy may serve the deficit in terms of energy and desalination problem. Public response The exploratory research was performed in Australia by Carr-Cornish and Romanach (2014) among similar age gender and location. Online focus groups and a mixed methods approach were adopted. The mixed method approach afforded both in-depth explorations of participants perceptions through typed dialogue, as well as questionnaires which allowed comparison of participants responses. The result summarized in Table 4 suggest that 74% people are agree toward geothermal energy technology being used in Australia. Table 4. Attitudes toward geothermal energy technology being used in Australia, Carr Cornish and Romanach (2014). Geothermal electricity barriers Due to the lack of a systematic analytical framework, much of the research in geothermal energy utilization has had little integrity or universality. There are some limitations of this technology. Such as, there are a few locations around the world where the geothermal can be established without involving deep boring and maintenance cost. Geothermal supplies are located far from the cities where actually the output is required causing a huge transmission loss. Most of the world's high-temperature geothermal resources have already been exploited for electricity generation. Of those geothermal resources which are above 90 C, only a quarter are at 150 C or above (Zheng et al. 2015). For most moderate-temperature geothermal resources, it causes low thermal efficiencies. Therefore, it is no surprise that the electricity generated by a geothermal power plant is only about one thirteenth of the heat which can be harvested and sold to prospective users for space, water, and process heating. Geothermal power plants are noted for its high initial cost and low operating cost. Hence for long term profit it has success and it is not good option for quick profit. The cost involves land survey of a vast area, test borehole drilling, power plant installation, electricity tower construction and need for specialized staff. An example provided by Romanach et al. (2015), suggests that economic feasibility, technology and seismicity holds 48%, 31% and 7% respectively of risk associated with geothermal power as it is majorly located to active volcano zones which is susceptible to high magnitude of earthquake.

8 Financial outcome One of the life cycle assessment performed by Buonocore et al. (2015), indicates that the geothermal power plant resulted to be able to generate electricity driven by locally available renewable resources and only moderately supported by non-renewable resources imported from outside the system. This approach makes the geothermal source eligible to produce renewable electricity being mainly run on local and renewable resources. Figure 6 compares the operational output and construction cost of geothermal energy to others. Form the figure it is understandable that it is the most profitable energy resource to generate electricity. Decommissioning and disposal impacts are so small that they are not shown in the figure. Figure 6. Percentage contribution of the different process steps to cumulative energy demand (CED) (Buonocore et al. 2015). Future geothermal High enthalpy hydro-geothermal power system increases efficiency of electricity generation. Chamorro et al. (2012) presented that at 200 C two flash system has 25.64% more efficiency than single flash system. In a flash system the liquid expand suddenly in low pressure and it allows it to convert into gas and rotate the turbine more efficiently. And he proposed further addition of third flash unit (Graph 1) however in dry steam system there is no such possibility to increase the efficiency. Graph 1. Energy efficiency Ψ for 1F (single flash), 2F (double flash), 3F (triple flash) and DS (dry steam) are presented with a function of water temperature (Chamorro et al. 2012). The investment to a geothermal power plant is very sensitive to location specific characteristics. Such as surface cost depends on the power capacity and the technology used however the reservoir characteristics is dependent on the chemistry used. Hence the survey and geological strata modeling has huge impact on the overall cost of the infrastructure. If the installation depth is varied from the proposed depth to achieve certain fixed temperature the investment would face a significant financial impact both in positive and negative manner.

9 Hence in future planning this criteria is to be treated with care. Sub surface cost generally holds 20-50% in high range of temperature zones. The capital cost is proportional to the $/ kw (Chamorro et al., 2012). Conclusion Geothermal energy technology is well established technology and it has been practiced for long to produce electricity in numerous countries (Chamorro et al. 2012). The Graph 2 shows the importance and possibility in the future for geothermal energy. The spread of the use of geothermal energy will dramatically minimize the emission of greenhouse gasses from conventional thermal power plants driven by fossil fuels. Graph 2. Evolution of installed geothermal power capacity from 1975 to 2010 and planed capacity for 2015 (Chamorro et al. 2012) In case of dry steam system there is no such problem related to resource depletion of minerals and salts of rock strata however in case of other systems for example flash system where water is used with steam the quantity of salts and minerals should be monitored in the injection well so that it can meet sustainability criteria. Government should take ladder of participation principles for liberating people to avoid not in my backyard problems (Carr- Cornish and Romanach, 2014). Acknowledgement The author of this review would like to thank Prof. Kevin McDonnell for his valuable guidance and suggestions. References Apergis, N. and Tsoumas, C. (2011) 'Integration properties of disaggregated solar, geothermal and biomass energy consumption in the U.S', Energy Policy, 39(9), Beckers, K. F., Lukawski, M. Z., Anderson, B. J., Moore, M. C. and Tester, J. W. (2014) 'Levelized costs of electricity and direct-use heat from Enhanced Geothermal Systems', Journal of Renewable and Sustainable Energy, 6(1). Buonocore, E., Vanoli, L., Carotenuto, A. and Ulgiati, S. (2015) 'Integrating life cycle assessment and emergy synthesis for the evaluation of a dry steam geothermal power plant in Italy', ENERGY, 86,

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