Environmental Engineering Reference
In-Depth Information
to buy electricity from an efficient central power plant and generate process heat in an efficient
in-plant boiler or furnace.
Cogeneration is most useful when process or space heat is required at a low temperature. Then
Q
ex
, instead of being dumped in a condenser and cooling tower, is heat exchanged with the device
that needs the relatively low temperature heat, say for drying or space heating. When a central
power station (or incinerator) is located in a densely populated or commercial downtown area,
Q
ex
may be piped directly into the buildings for space and water heating. This is called
district heating
.
5.3.4
Fuel Cell
The functioning of a fuel cell was described in Section 3.12. A fuel cell is not a heat engine. In
a fuel cell, some of the chemical energy of the fuel is directly converted into electrical energy,
with the rest appearing as heat rejected to the environment. Its theoretical thermal efficiency in
terms of electrical energy generated versus fuel chemical energy input can be close to 100% when
producing a low output of power. However, because of parasitic heat losses (e.g., ohmic resistance)
and because of the power requirements of auxiliary equipment (e.g., pumps, fans), current fuel cells
using natural gas or hydrogen and air (instead of oxygen) have a much lesser thermal efficiency
when operating at maximum power, in the 45-50% range. Furthermore, if hydrogen is used as
a fuel, it has to be generated separately in some fashion, which requires energy. For example,
hydrogen can be generated from water by electrolysis. But, the splitting of water into hydrogen and
oxygen requires about 18 MJ of electric energy per kilogram of water, more than is generated in
the fuel cell by the electrolytic hydrogen. Fuel cell hydrogen is usually generated by the reforming
of methane (see Section 3.14.1).
5.4
CONCLUSION
Fossil-fueled power plants consume 55.5% of the world's fossil energy. Worldwide about two-
thirds of the electrical energy is generated by fossil energy, 80% of which is coal. Almost all
fossil-fueled power plants work on the principle of heat engines where 25-40% of the fossil energy
input is converted into electrical energy, the rest is wasted in the form of heat rejection to a cold
reservoir and parasitic heat losses. Furthermore, fossil-fueled power plants, especially coal-fired
ones, produce a host of pollutants, including particulate matter, sulfur and nitrogen oxides, and
toxic organic and inorganic byproducts of combustion. The emissions of these pollutants must be
controlled in order to safeguard public health and the environment. Emission control devices sub-
tract from the thermal efficiency of power plants and add to the electricity generating cost. Power
plants also produce large amounts of solid and liquid waste, consume significant amounts of fresh
water, and last, but not least, emit large quantities of CO
2
, a greenhouse gas that contributes to
global warming.
However, life in an urban-industrial society cannot be imagined without electricity. We only can
strive toward increasing the thermal efficiency of power plants, improving their emission control
technologies, and gradually replacing them with fuel cells and solar, wind, and other renewable
energy power plants. Eventually, with the depletion of fossil fuels and the looming crisis of global
warming, nuclear-fueled power plants also may have to be utilized in greater proportion than is
currently the case.
