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Water for Energy and Fuel Production
process heating for commercial and industrial applications. For example, 50% of
process heating, air-conditioning, and electrical requirement for a clothing factory in
Shenandoah, Georgia, is provided by a solar energy project [21–25].
Finally, power generated by solar energy using photovoltaic (PV) systems needs
to be stored. Off-grid PV systems have traditionally used rechargeable batteries to
store excess electricity. Another approach is the use of pumped storage of hydroelectricity that stores energy in the form of water pumped when the energy is available
from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water to run through a hydroelectric
power generator [25–35]. Solar energy can also be stored by producing solar fuels
such as hydrogen using numerous techniques described in Chapter 11. The production of solar fuels mostly involves dissociation of water. Hydrogen can also be
produced using solar reforming of fossil and biofuels using steam. Different techniques required to accumulate concentrated solar power are described in numerous
references [25–35].
3.5 steam tUrBine
A steam turbine is a device that extracts thermal energy from pressurized steam and
uses it to do mechanical work on a rotating output shaft [36]. Because turbine generates
rotary motion, it is particularly suitable to drive an electrical generator. In 1996, about
90% of all electricity generation in the United States used steam turbine [36]. The
steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of steam. Steam
turbines are made in a variety of sizes ranging from <0.75 kW used for mechanical
drives for pumps and compressors to 1.5 million kW used for electricity generators.
Basically five types of steam turbines are used: condensing, noncondensing,
reheat, extraction, and induction [36]. Condensing turbines are most commonly
found in the electric power plants. In this type, steam coming out of turbine is condensed (about 90%). Process steam applications mostly use back-pressure noncondensing steam turbine (commonly used in paper and pulp operations, refineries and
desalination plants, etc.) in which exhaust pressure is controlled to suit the needs of
the steam pressure. Reheat turbines are exclusively used in the electric power plants.
Here, steam returns to the boiler from turbine, picks up more superheated steam, and
returns back to turbine to continue its expansion. Extraction turbines are common in
all applications. In this case, steam is released from the various stages of the turbine
and used for industrial process needs and sent to boiler feedwater heaters to improve
an overall cycle efficiency. Induction turbines introduce low-pressure steam at an
intermediate stage to produce additional power.
Steam turbines are very valuable because they can be used for any fuel. For
example, in a nuclear reactor, nuclear energy is converted to thermal energy by generating steam, and the steam can then be used to generate power by steam turbines.
In combustion processes using coal, waste, biomass, or other fuels, heat generated
by combustion is absorbed by water to generate steam, and the steam is then used to
generate power via the use of a steam turbine. Steam is thus a very benign vehicle for
energy conversion and heat and power generation.