Page 117 - Sustainable Cities and Communities Design Handbook

94 Sustainable Cities and Communities Design Handbook

and dishdalthough one (trough) has dominated the US CSP market to date.
These systems employ parabolic mirrors that concentrate solar heat on
fluid-filled receiver that runs the length of each trough. There is currently
1800 MW installed CSP capacity in the United States, 1300 MW of which is
located in California (Concentrating Solar Power).
The operation of concentrated solar plants requires both large tracts of
contiguous land for siting and substantial volumes of water to provide a
cooling reservoir for the steam turbine. Many of the prime locations suitable
for CSP siting are remote, requiring additional transmission to connect to the
grid, and arid, placing a burden on scarce local water resources. These factors,
combined with long construction times and higher cost relative to PV projects,
mean that CSP systems are unlikely to achieve greater penetration market than
photovoltaics. However, unlike PV, CSP systems can be readily and
economically fitted with thermal storage systems such as molten salt. Adding
storage allows for operating at night or during cloudy conditions and turns
CSP into a dispatchable power source. CSP systems can also be paired with
other thermal energy-based systems, such as natural gas power plants, to
increase reliability and efficiency.
On a megawattage basis, small-scale distributed applications of solar
thermal energy have the biggest market share. As of 2013, US consumers had
installed nearly 17,000 MW-th (thermal equivalent) of solar thermal systems,
which include pool heaters and water heating systems (Solar Heat Worldwide,
2013). In 2015, the investment tax credit for solar water heaters was extended,
which may foster continued investment in these systems. However, cost
reductions for PV systems and natural gas may still harm the economic
viability of solar hot water systems.

GEOTHERMAL
Geothermal energy systems tap into underground heat reservoirs, utilizing the
stored thermal energy directly or as a feedstock for electricity production.
Hydrothermal resources exist where magma comes close enough to the
surface to transfer heat to groundwater reservoirs, producing steam or high-
pressure hot water. When hydrothermal resources are sufficiently hot
(several hundred degrees Fahrenheit) and close to the surface (within a few
miles), it can be economically sensible to drill a well and use the steam or hot
water either as a direct power input into a turbine (as with dry and flash steam
plants) or as a heat source to produce steam with a secondary fluid (as with
binary-cycle plants). Shallow hydrothermal resources of more moderate
temperature, which in the United States are located primarily in Alaska,
Hawaii, and many western states, are commonly used directly to provide heat
for buildings, agriculture, and industrial processes.
Even in the absence of hydrothermal resources, geothermal energy can be
harnessed for use. Geothermal heat pumps for buildings take advantage of the