Page 275 - Geothermal Energy Renewable Energy and The Environment

266 Geothermal Energy: Renewable Energy and the Environment

solution varies with the amount of salt added. Figure 14.3 shows how the heat of solution changes
as a function of the number of moles of NaCl added to the water. Complicating this analysis is the
fact that natural geothermal solutions contain many different solutes, each with their own heats of
solution, which also are influenced by the concentration of other species. Hence, analysis of the heat
budget at a particular site requires thorough analysis of the solution composition.
Removal of heat from such solutions can result in the solution becoming saturated in one or more
of the dissolved salts, which can lead to their precipitation. If that happens, it will have the effect
of reducing flow in the piping system of the geothermal plant. If the salt is removed from the solu-
tion prior to passing through the heat exchanger of a binary plant, the heat management of the fluid
becomes a significant challenge, since removal of the salt also will result in the release of the heat
of solution. Hence, when considering the temperature of the geothermal fluid, it must be recognized
that some of the heat in that fluid is actually maintaining the salt in solution and is not necessarily
available for power generation.
There is the potential of compensating for the lost energy resulting from resource extraction by recov-
ery metals and other commodities that have economic value. As discussed in Chapter 12, the ability to
economically recover essential metals and other compounds is a rapidly evolving field. Brines associ-
ated with geopressured reservoirs may provide recovery opportunities as the technology improves.
reinjection
Regardless of the strategy developed for management of these fluids during power generation,
they are generally too saline for surface disposal. As a result, reinjection of the fluids will be
required. Although this may add additional costs, the resulting environmental protection justifies
this strategy.
In addition to the environmental benefits obtained by reinjection, improving the sustainabil-
ity of the resource is likely to be an additional benefit. The presence of a sealed horizon in the
subsurface makes it likely that many geopressured zones are receiving little natural recharge. This
is emphasized by the fact that flow tests on some wells in the Gulf Coast showed a significant
drop in pressure when high rates of flow were maintained for relatively short periods (Garg 2007).
Reinjection can makeup some of the fluid volume extracted, potentially mitigating the reduction
in pressure.
In summary, the primary challenges that are faced when trying to develop an economically
viable power generation system for a geopressured resource are:

• Separate the dissolved solute load from the aqueous phase while minimizing the loss of
thermal energy.
• Separate and capture the dissolved methane gas phase from the aqueous phase.
• Efficiently extract the thermal energy and kinetic energy from the fluid while maintaining
sufficient pressure and flow rates.

These scientific and engineering challenges are significant, but are not insurmountable. Current
support for research and development in this area from a variety of agencies suggests their geopres-
sured resources may become a significant contributor to geothermally produced electric power.

enhanced GeoThermal sysTems (eGs)

maGniTude of The resource
The systems we have discussed for generating electricity thus far have been developed in regions
where temperature greater than about 130°C are within a few kilometers of the Earth’s surface.
Such resources are not common, although they are more abundant than the existing distribution
of geothermal generating facilities would suggest. However, as was pointed out in Chapter 2, any