digesters running on crop and food residues and human and animal wastes are used in
rural areas of China to provide methane for cooking and lighting.
Geothermal Energy
Underground heat reaching the surface from Earth’s hot, molten rock interior has
been tapped as a source of energy for approximately a century. This geothermal energy
was first used to generate electricity at Larderello, Italy, in 1904. It has since been
developed in Iceland, Japan, Russia, New Zealand, the Phillipines, and at the Geysers
in northern California. The most desirable form of geothermal energy coming from the
ground is in the form of dry steam. Such steam is relatively rare and superheated water
mixed with steam is the most common source of geothermal energy. The water byproduct
is sometimes relatively pure, but often contains salts and gases that can cause problems.
Air pollutant hydrogen sulfide is also evolved with some geothermal steam. (The odor
of hydrogen sulfide is very noticeable around steam vents in Yellowstone National Park.
If the park were an industrial plant, it would be closed for violating safety standards
because of the toxic H 2 S gas emitted by geothermal features in the park.) Experiments
have been conducted with injecting water into hot, dry underground water formations
to produce steam. Success with this technology could increase resources of geothermal
energy ten-fold.
6.9. Nuclear Energy: Will it Rise Again?
The only source of nuclear energy currently available depends upon the neutron-
induced fissioning of heavy atomic nuclei, most commonly those of the uranium isotope
with a mass number of 235, to produce radioactive fission products, an average of 2.5
more neutrons and an astounding amount of energy compared to an ordinary chemical
reaction. A typical example of such a fission reaction is
92 U + 0 n 51 Sb + 41 Nb + 4 0 n (2.9.1)
235 1 133 99 1→
A nuclear reactor operating at a constant power level is controlled such that on
average 1 neutron from each fission reaction is absorbed to cause another fission reaction,
thus sustaining a chain reaction. The excess neutrons are absorbed by nonfissionable
material. In order to cause the desired fission, the neutrons, initially released as rapidly-
moving, high energy particles must be slowed down, which is done by a moderator,
such as water, in the reactor.
The basic function of a nuclear power reactor is to serve as a heat source to produce
steam used to generate mechanical energy. The basic components of a nuclear power
reactor are shown in Figure 6.11. Pressurized superheated water circulates through the
hot reactor core in an enclosed loop (to prevent escape of radioactive contaminants).
Heat from this water is used to convert water to steam in a heat exchanger. The rest of
the power plant is like a conventional fossil-fueled plant with a steam turbine coupled to
a generator and the steam from the steam turbine being cooled to provide water for the
heat exchanger.
Chap. 6. Energy Relationships 155