09-05-2014, 01:00 PM
Emerging Geothermal Energy TechnologiesIntroduction
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INTRODUCTION
Our planet, from its centre to its surface, is a massive storehouse of heat energy. This exists because the Earth was formed with a core of molten metal which is slowly transferring heat to its outside crust. Additional heat is generated by the decay of naturally occurring radioactive materials beneath its surface. The Earth also acts as a very large solar collector absorbing the sun’s energy. Humans have made use of this source of energy since prehistoric times with thermal pools and hot springs (“hydrothermal” energy) being used for heating, cooking, bathing and therapeutic purposes. There are many other modern examples of this application around the world where hot water taken from depths of up to about 1 kilometer is piped to heat a wide range of domestic, commercial and industrial buildings and other infrastructure. Another system known as EGS (Enhanced geothermal system) comprises drilling a deep injection well and a production well within the same hot fractured rock mass. Water is forced down the injection well, passing through the hot fractured rock mass to pick up heat and then returning up to the surface via the production well. This hot water is then used to produce steam or another more volatile gas to drive turbines to produce electricity.
Current and Predicted Use of Geothermal Energy
It is comparatively difficult to accurately assess the capacity that is available and the amount that is actually used for the different forms of geothermal energy. Given the very large changes that have taken place in recent years and when considering the major driving political, economic, environmental and technical factors that are omnipresent, it is even more difficult to make reliable predictions of what will happen in 5 years time let alone in the next 40 years. Based on the data available, Table 2.1 presents a summary of the apparent commercial capacity and actual use for 2010. It may be of interest to note that for hydrothermal electricity generation, the capacity factor(Ratio of energy used to the energy available from existing installations) is relatively high at 0.72 whereas for the direct use form, capacity factors are lower and much lower in the case of direct use with GSHPs. This suggests that for direct use installations, there is considerably more potential energy use available even with existing installations. It is quite possible that the capacity and annual use for direct use applications is significantly underestimated. This is presumably because of the very large, and ever growing, number of relatively small applications using GSHPs which simply cannot be tracked in detail. Even if all current geothermal energy sources were working at full capacity, it would only provide about 0.4% of the world’s current annual energy requirement. However, despite this, attempts have been made on the basis of the very limited experience to date. One of the major unknowns in quantifying future energy balances is the potential contribution from EGS. According to predictions based on a wide range of what appear to be reasonable assumptions, Table 2.2 is also given.
Well Drilling
A major cost of any EGS is the cost of drilling. Although there has not been a great deal of EGS drilling experience to date, what little is available seems to correlate quite well with the enormous amount of experience which has been gained from the oil and gas industry. However, it must be emphasized that although the aim of both industries is to advance wells to some considerable depths, there are a number of differentiating characteristics between the two industries, one of which is the need for geothermal wells to have a larger diameter than oil and gas wells and another is the need to cement in place the whole length of the casings for geothermal use. There are a number of factors which will affect this cost including well diameter, casing design and particularly the number of strings, geological materials penetrated, drill bit life, rate of bit penetration, and methods of stimulation and completion.