Environmental Engineering Reference
In-Depth Information
Table 6.2 Advantages and drawbacks in utilizing underground geothermal resources
Advantage Drawback
Constant and stable Complexity and heterogeneity
Isolation properties- barriers to gases Uncertainty and limited information
Renewable and untapped resource Geological faults (instability, leakage)
Low emissions of greenhouse gases Cost (construction, maintenance, maintenance of
flows)
Safe and secure from threats (terrorism) Chemical contaminants and scaling
Table 6.3 Life cycle greenhouse gas emissions by electricity source. (Adapted from IPCC 2011 )
Electricity generating technology CO 2 per KWh electricity (mean of available estimates (g))
Hydroelectric
4
Wind (onshore)
12
Nuclear
16
Solar PV
46
Geothermal
45
Natural gas
469
Coal
1001
Further advantages are that geothermal energy provides base load power with no
seasonal variation and is not weather dependent. Its resource is also isolated under-
ground which makes it secure and also imparts resilience against earthquakes (as
shown in the 2011 Great Eastern Japan earthquake when no geothermal facilities were
damaged). In recognition of these advantages, geothermal energy is increasingly in-
cluded as a desirable renewable energy in some country's renewable energy policies;
for instance, it is eligible for Feed-in-tariff (FIT) subsidies in Japan and Germany.
The potential size of the geothermal heat resource can be estimated from the
mean temperature depth distribution shown in Fig. 6.3 (Armstead and Tester 1987 ).
The mean surface temperature is 15 ºC, with a thermal gradient of 25 ºC/km near
the surface and 11 ºC/km near the 'Moho' where temperature is ~ 600 ºC at a crustal
depth of 35 km (all figures approximate). If we set a maximum depth up to which
the underground heat may be extracted (d in Fig. 6.3 ) as the current depth limit for
directional drillings (6 km), the potential geothermal energy resource can then be
estimated by calculating the mean crustal temperature up to 6 km, and then calculat-
ing the volume that would be required to supply global energy demands. A simple
calculation shows that the heat stored in a rock mass of approximately 96 km 2 by
6 km deep is equivalent to the world energy demand forecast for 2030 of 170 × 10 8 t
oil equivalent. We thus see that the potential of geothermal energy is huge and the
challenge is to extract it efficiently and economically.
However to realise this large potential, a number of barriers need to be over-
come. Those cited by the IEA include public awareness and acceptance, and
logistical problems such as insufficient numbers of trained geothermal scientists and
engineers. There are also barriers and uncertainties which influence the economics
of geothermal energy. In particular these include the rate of failure of drilling to
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