A geothermal system uses ground water to either produce electricity or is used as thermal exchanger that when used in homes maintains the temperature needed at particular season. Although the geothermal system is well adopted among all other renewable sources still its growth is slow so an alternative as to how to increase its efficiency, various research has been carried out and one of such is Enhanced geothermal system, that uses advance techniques such as drilling or fluid injection to increase the availability of geothermal resources which can be used to generate electricity using resources from earth origin also known as "engineered geothermal system". It can provide a continuous output which makes this technology a renewable sources of energy, these are eco-friendly as they do not burn any fossil fuel and the geothermal water is safely injected back to geothermal reservoir so that geothermal water do not get mixed with other ground water (Pruess 2005). There are many limitations for traditional geothermal systems as they uses naturally occurring hydrothermal reservoirs which has problem of limited size and location. Enhanced geothermal system solves this problem of traditional geothermal system by using the technology of creation of hydrothermal reservoir in hot rocks (David 2008).
Enhanced Geothermal System Concept and working
At shallow depths the combination of heat, water and permeable rocks forms a natural hot reservoir the fluid contained in these reservoirs can be used to produce electricity or for heat process. In enhanced geothermal system water is injected through injection well to deep into through permeable hot rocks and then the heated water returns to the surface through production well (Ronald 2008 & John 2009).
EGS uses the heat from the deep core of the earth to drive the turbine and generate electricity. The overview of the EGS technology is, drilling impermeable hot rocks at depth, pumping fluid into the newly porous area made and then pumping the heated fluid to drive a turbine to generate electricity. Drilling is therefore an important operation in creating and EGS reservoirs. Drilling costs remains linear for geothermal wells whereas it rises exponentially for drilling of oil and gas wells, but beyond 5km depth the drilling cost changes due to change in geothermal environment (Kohl 2005).
For more details, please visit the GTP website at www.geothermal.energy.gov for more information on the program, EGS and existing EGS sites.
Initially water with very high pressure is pumped into underground so as to forms fracture in the hard rocks that makes a new hydrothermal reservoir various acids are injected into the reservoir for the debris to corrode and fracture to form (Baria Michelet 2005).
As the drilling is done the very next step is the formation of reservoir which is associated with that a large quantity of rock is fractured so that the permeability increases and hence more heat can be drawn. Number of technologies from" seismic imaging" to "radioactive tracers", are used to structure goods injection as well as production well. The injection well should be placed at the center or near about the centre of the reservoir, with many production wells coming out from either edge of the reservoir. This allows water from the injection well to flow outward in all possible directions; this helps to optimize flow rate and reducing loss of fluid. However, it has been not yet been proven, that the reservoir volume will have sufficient "interconnectivity" or "permeability" same at commercial scales also.
After drilling and reservoir establishment the next step is operation of the reservoir and its maintenance, but unfortunately at present there is no experimental proof to state the level of productivity, it is only speculated that," flow rate of 80 kg/sec at 200°C" from each production well takes place which shows that this is advance technology of traditional hydrothermal reservoir. Productivity from the well is the greatest challenge to the technology for its commercialization, apart from the productivity the fracture system provide enough thermal stability for longer period of production the lifetime is assumed for EGS is defined as, " 10°C decline in fluid production temperature", after which the reservoir is re-drilled and have to be re-stimulated, since greater cooling has been observed in the reservoir that are in commercial use that is why the commercial form of EGS is still unknown. In few systems, the cost of water couldis more than the stimulation cost , water loss during operation are also a important cost, there is still no adequate knowledge of amount of water required in various EGS systems, the differences between fluids produced from hydrothermal and EGS reservoirs becomes minimal as soon as the chemical stability is established during circulation (Kohl 2005).
Cost factor
Every new technology is only acceptable only when it is cost effective because every part of the economy for the plant setup has its own risk and requirements, but since always some experiments goes on with this new technology so it makes difficult to estimate the cost of setting up EGS plant at a commercial level initially when estimated it is found that the capital costs of an EGS plant would be around twice from that of a traditional geothermal plant (John 2009). The total expenses of an EGS plan are mainly based on the investments made at the beginning of the project. These investments mainly includes costs for the following
Exploration of reservoir
total cost for drilling an dborehole formation
set up of geothermal fluid cycle
the overall construction of the EGS plant unit on above the surface
Schedule of major HDR and EGS projects worldwide.
Now when it is that the production of electricity from fossil fuel is cheaper than that of EGS so commercial implementation of EGS plants are still awaited, but on the other hand EGS do not require to purchase fuel to generate electricity as it is required with fossil fuels but only a suitable site for drilling is required at first step so estimation cost becomes uncertain as the drilling cost depends on the site, it has been roughly estimate that EGS could be able to produce electricity at a cost of "74.7 cents per kWh.27". Many parameters are taken into account for each EGS system element, such as reservoir productivity, its drilling, and cost of the plant, resource depth, interest rates, and many more. The wide range in cost estimation is due to the risks and variability present in the drilling and reservoir development stages of enhanced geothermal systems (David 2008). It has been observed that drilling alone is found to be more than one third of the total capital costs of an enhanced geothermal system plant. The risk with associated with drilling increase with increase in each feet of the bore as the bore holes require more materials and high risks of failure is also associated and this causes the drilling costs to increase nonlinearly with increasing depth. "Exploratory well drilling", is a necessary and more costly step in development of a geothermal site that increases both cost and uncertainty as well (Pruess 2005).
To setup the surface plant from the entire EGS project the investment cost includes the cost behind the geothermal fluid and the overall cost for the entire plant. The investments involved in the geothermal fluid cycle are the cost for the equipments that are required to generate and circulate the geothermal fluid, those includes pumps, filters, slops, pipes .most of the time in the EGS plants , the production pump becomes the essential cost factor due to some technical essentials which production pump has to fulfill. Heat exchanging stuffs forms the main cost factor in the unit of heat. Manual cost is reduced as these plants can operate even without continuous manually supervision. Few other additional expenses also happened to occur and that depends on the design of the project (Ronald 2008 & Baria Michelet 2005). Other investment costs also takes place when these projects are carried out close to housing areas, noise is the main concern so investments happens to protect noise. Since EGS needs more deep boreholes than
Traditional geothermal plant and EGS system works in a closed system so no gas release takes place so the cost for measures for this is saved.
Source: Tester, Jefferson, 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.
Impact of EGS on the environment
The impact on environment is much lower in EGs in comparison to usual fossil fuels and nuclear power plants even they have lower impact than renewable sources of energy such as solar energy, energy from biomass and wind energy. This happens because the either setup is underground and the surface setup is very compact, this compact system of working also reduces risk of greenhouse gas effect since EGS need not to be processed to distant region as fossil fuels and bio fuels so the discharge of nitrogen oxide or sulfur oxide is much reduced. Although EGs does not have impact on above surface matters but can be significant effect in contaminating ground water, other issues such as noise, of safety, , and use of land regarding drilling and production operations are also there, but they are however manageable (David 2008).
Today environment faces challenges from various resources that are used for the comfort of society, but these resources are producing or effecting the environment in such a way that are directly or indirectly affecting society.
Effect of fossil fuels are most as all fossil fuels are used by combustion that leaves behind ashes and produces harmful gases such as carbon dioxide that contributes to main reason for pollution becoming one of the reason for global warming and adversely affecting the environment moreover too much dependency on these fossil fuels will lead to empty of fossil fuel reservoir and ending up to its crisis, so an alternative to these fuels are renewable resources of energy such as bio-fuels, biogas, wind energy and geothermal energy (Pruess 2005 & Baria Michelet 2005).
Among from these the hot area of interest for today's world is geothermal energy, which uses hot ground water to generate electricity or used as thermal exchanger that when used in homes maintains the temperature needed at particular season. Although the geothermal system is well adopted among all other renewable sources still its growth is slow so an alternative as to how to increase its efficiency and the answer is 'Enhanced Geothermal System'. that uses advance techniques such as drilling or fluid injection to increase the availability of geothermal resources which can be used to generate electricity using resources from earth origin, by drilling permeability is created among rocks in the deep core of the earth and then water in injected through injection pump to the reservoir made artificially and then water injected are heated when comes in contact with hot rocks and then the heated water in extracted back by production pumps that are then used to produce electricity or as thermal exchanger (David 2008 & Ronald 2008 &
John 2009).
Advantage for environment
The impact on environment is much lower in EGs in comparison to usual fossil fuels and nuclear power plants even they have lower impact than renewable sources of energy such as solar energy, energy from biomass and wind energy. this happens because the eiter setup is underground and the surface setup is very compact, this compact system of working also reduces risk of greenhouse gas effect since EGS need not to be processed to distant region as fossil fuels and bio fuels so the discharge of nitrogen oxide or sulfur oxide is much reduced, as the entire set up is very compact from delivery of water to underground to extraction of water to the surface so there is very less chance of leakage of any kind of gases that causes environmental pollution.
Disadvantages for environment
Although many advantages have been shown for enhanced geothermal system but with that many disadvantages are also associated first of all extracting fluids from deep crust of the earth may happen to decrease the pressure in underground reservoirs and thus this might cause the land to sink, not only sinking of the ground is the problem but also the surface might tilt that may damage buildings and can put pressure on to the pumps and other equipment that are placed underground, since geothermal fluids are extracted from deep into the ground so it contains high level of many chemicals such as arsenics, mercury, lithium and boron, if the waste is not properly treated and are released into aquatic bodies then it will be very harmful for the flora and fauna of that water body and also polluting the water body and making it unsafe for drinking purpose and even that water cannot be. If it happens that the geothermal fluid comes into contact with atmosphere due to leakage then the harmful dissolved gases present in it are released into the atmosphere, one such harmful gas is hydrogen sulfide that causes acid rain when accumulated in the atmosphere, carbon dioxide present if released is one of the potent greenhouse gas that causes global warming. Drilling causes lots of sound which causes sound pollution.
Various challenges
One of the core challenges in the EGS is the installation of equipment known as heat exchanger a number, "the total volume, the total heat exchange at surface, the flow impedance, and the thermal properties", since the heat exchanger is located so deep that there is no possible way to either detect or change its properties if needed, therefore there is need of remote sensing technologies that can detect and a remote control to control the operations when needed (Baria Michelet 2005).
Long term operational condition of the EGS plants is another challenge, as the heat output generally decreases with the time so to handle the problem only assumed calculations are available, since the heat present in a place changes as the time passes by so increasing this factor is a big challenge but without commercial application of the EGS system it is hard to find real solutions for these challenges, even upgrading or up scaling is also a challenge so as to meet global electricity requirements.
Conclusion
Although the Enhanced Geothermal Systems has proved great potential on papers but its output in the field is yet to be seen, it still in research and developmental stage, the European union is putting lot of efforts to towards EGS, its research and project plans even Australian government is also putting efforts towards development of EGS, but the exploratory phases of a EGS project is also costly that hinders proper and all kind of research to be carried out but the promises of EGS is increasing project investments from government sector as well as private sector, but taking into considerations the principles of enhanced geothermal systems reaching the complete potential of EGS is going to take a decade, or longer. Although if drilling problems and the economic limits of drill depth are solved then also it is not going to happen in a short period as it takes for other plants to set up as the search for energy sources that don't create more greenhouse gases into the atmosphere are in demand and enhanced geothermal system is now looked upon as pollution free energy sources and as it is available day and night so it makes EGS more sort after resource but still main problem associated with is cost and development of sophisticated pumps and other equipments. Enhanced geothermal systems is like boon it those areas where heat and water flow is a problem, this speed up the already existing geothermal projects, EGS is still in "proof of concept" stage so the more efforts are to be put in so as to get this boon in real field. Today, Enhanced Geothermal Systems, e exploits the heat from deep into the earth for producing electricity, enhanced geothermal system are boon to new era for providing energy but at the same time if not properly taken care at each of its step then it poses equal danger to society and the environment as other fossil fuels.
