7 The pros and cons, and future of geothermal energy
Geothermal energy is renewable but the fluids emit gases such as CO2, H2S, SO2, H2, CH4 and N2 when used for electricity generation. However, geothermal power plants are usually sited in areas of natural geothermal activity, where such emissions occur anyway. Other potential pollutants are various ions dissolved in the geothermal fluids, but these are almost always returned to the reservoir when the spent fluids are re-injected.
As regards safety, accidents are rare, although in 1991 a well failure at a geothermal plant on the flanks of a volcano in Guatemala vented hundreds of tons of rock, mud and steam. Protracted geothermal exploitation can induce ground subsidence, similar to that which occurs when groundwater is extracted, although this is minimized by re-injecting spent fluids. Small earthquakes sometimes result from large-scale exploitation. However, most high-enthalpy geothermal areas are naturally prone to seismicity.
The main disadvantage of geothermal power generation is that suitable high-enthalpy areas are geographically very restricted, many being in areas of low population density (or under the sea). Conversely, the very low-enthalpy potential of normal heat flow is universal, and potentially useful for heating and even air conditioning, given the necessary investment.
In 2003, global electricity generating capacity from geothermal energy was 8.4 GW (Table 1), equivalent to just eight power stations using fossil fuels. Existing technology is potentially capable of providing global electricity generating capacity up to 72 GW, rising to 138 GW with HDR exploitation. Yet that falls well short of the 2004 globally installed nuclear power capacity of 362 GW and would only provide about 9% of current world electrical generating capacity (1.5 TW). Moreover, as with all energy resources, there is an absolute upper limit to geothermal power.
The Earth's surface area is about 5 × 1014 m2, and its average surface heat flow is 87 mW m−2 . What is the total geothermal power of the Earth compared with present total power use in TW?
Total geothermal power = 5 × 1014 m2 × 87 mW m−2 = 4.35 × 1016 mW = 4.35 × 1013 W = 43.5 TW. The total power use world-wide in 2002 was 14.3 TW, or about one-third of total geothermal power.
However, only 13.5 TW of this total global heat flow passes through the continental crust, where it can be used most conveniently. Converted into annual emission of energy, continental heat flow totals 425 EJ globally — about the same as global annual energy use (451 EJ in 2002). Only a tiny amount of this continental heat flow gives rise to high-enthalpy geothermal conditions, so is clear that heat flow is destined to play only a minor role in future electricity generation. Using low-enthalpy sources for heating, however, has far greater potential.
The vast amount of heat (4.3 × 107 EJ) stored in the top 3 km of the Earth's crust, might suggest that geothermal energy could have a much larger role to play. However, exploiting heat stored by rocks would be akin to mining, once use exceeded the annual heat flow; it is then a non-renewable resource, however vast.
Table 3 shows estimates by the International Geothermal Association of the ultimate geothermal potential, assuming advances in geothermal technology, on a continent-by-continent basis (including some use of stored geothermal heat). The total electricity-generating potential of 2.24 × 104 TWh yr−1 is 45% more than total world electricity production in 2001 (1.55 × 104 TWh). Note also that low-temperature resources are more than three times current primary energy use globally. To achieve such output from current levels clearly requires enormous growth in the geothermal industry.
Figure 6 showed the proportion of energy delivered as electricity in the UK. Around 12% of the total energy is 'delivered' as electricity to UK users, the rest being by burning coal and petroleum products. It is about 1 EJ yr−1, i.e. about five times current global electricity production from geothermal sources and around 1.25% of the total in Table 3. The UK is not well-known for its active volcanoes and geysers, and although it may have some HDR potential, the UK's geothermal electrical energy cannot supplant more conventional supplies. However, the UK could probably extract sufficient low-enthalpy energy to cater for a significant proportion of space heating.
In the short term, even twenty- to forty-fold growth in geothermal capacity will not result in it being a significant contributor to global energy needs by 2020. Installing capacity takes time (note the average annual changes in capacity in Table 1) and of course money, whatever the benefits. You should recall that geothermal power plants are generally much smaller than fossil-fuel and nuclear generators; tens of MW, rather than the tens of GW for the biggest 'conventional' plants. To replace a single 1 GW fossil fuel power station requires between 20 to 33 geothermal stations. The seeming benefit of local and 'environmentally friendly' geothermal power generation, in the economic context of their construction, delays its adoption at national to international scales. Direct-use geothermal projects suffer from slow growth also, because of economic factors. Even though installation of domestic ground-source heat pumps is growing at 10% per annum in a few countries, that growth rate would need to be sustained globally for a great deal longer than 20 years to make any real difference to energy use patterns. That issue characterizes all alternative energy sources.