Page 273 - Managing Global Warming

230 Managing Global Warming

For low-grade geothermal heat, however, published values for global potential are
as high as 300,000EJ [9]. Since, as discussed, only about 280EJ is produced annually
at the Earth’s crust, clearly such large estimates rely on drawing down cumulative res-
ervoirs of heat, and are not sustainable in the long run. Low-temperature geothermal
heat can be used in two ways. First hot water from the field can be carried for a few km
in heat-insulated pipes for use in homes, greenhouses, and swimming pools. One dif-
ficulty here is the potential mismatch between geothermal fields and population
centers—particularly in the United States, where geothermal resources are concen-
trated in the Rocky Mountains.
Second, more recently, geothermal heat pumps are being increasingly used. By
2004, over a million were installed worldwide [36]. As with passive solar energy,
it is difficult to classify geothermal heat pump energy. Useful energy for heating
(or cooling) can be extracted from any large heat sink, which is above (or below)
the ambient temperature. Even large lakes, which because of their thermal capacity
are slow to heat or cool, can be used for cooling or heating. Much of what is considered
geothermal energy is really solar energy stored in the ground [36].

6.6.2 Geothermal energy in 2050
Global growth in geothermal electricity output has been linear for the last 3–4
decades, and if this trend were to continue, geothermal installed capacity will rise
from 13GW in 2015 to roughly 20GW by 2050 [5]. At the present average capac-
ity factor output would only be about 115TWh, a minor fraction of even the 2015
global electricity output of 24,100TWh [5]. Such high rates of utilization evidently
are not sustainable in the long run. Even for electricity production from high tem-
perature sources, it is found economic to “mine” the heat source, resulting in deple-
tion of the geothermal field, and the need for perhaps decades of recovery time
before it can be used again.
The energy conversion devices for geothermal electric production, are like wind
turbines, a mature technology. Just as some wind energy researchers are looking to
go higher, to capture the stronger and more reliable winds, so geothermal
researchers are considering utilizing the higher temperatures that progressively
occur with depth below the surface—enhanced geothermal systems (EGS). In
2006 the Massachusetts Institute of Technology published a detailed study on
EGS in the United States context, and gave a resource base for the United States
in the millions of exajoules [37]. To assess its feasibility, two parameters are
important to consider: the temperature and the depth of the geothermal heat source.
The study showed that no heat sources above 200°C existed at depths above 4km.
Since the study also demonstrated that well monetary cost rises exponentially with
depth, it is probable that input energy costs and final electricity costs would also
rise nonlinearly with borehole depth. Disappointingly, no energy analysis of EGS
was undertaken in this otherwise detailed report [38].