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Example:
We wish to determine how much coal is needed to produce 1 kWh of
electricity. Assume the power plant is 33% efficient, with 85% of
waste heat to cooling tower, and 15% to stack. Assume you can get
24 kJ of energy from 1 gram of coal. Note that 1 kW of electricity
is equivalent to 1 KJ/s of electricity. What are the environmental
issues?
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Figure
11: Coal Fired Power Plant.
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1
KJ/s kWh * 3600 s/hr = 3600 KJ per hour
If
the power plant is 33% efficient, then need 3600 * 3 = 10,800 KJ
10,800
KJ * 1/24 KJ/gram = 450 grams coal = approximately 1 lb = 454 grams
Therefore,
we need 450 grams coal for 1 KWh of electricity.
How
much electrical energy do we use worldwide?
10
EJ (1990), 1 ExaJoules = 1018 Joules, or 27.8 * 1015
KWh.
A home in the US may average 500 kWh over a month.
What
happens to non useful energy in this example?
0.85*7200
= 6120 KJ to cooling water (note that we use vast amounts of water
to cool waste heat; this cooling water produces thermal pollution
of water bodies)
Also,
0.15*7200 = 1080 KJ goes to stack as waste heat, which carries
impurities in the form of air pollution.
Environmental
issues for this example are nonrenewable natural resource consumption,
air pollution, water pollution, and solid waste.
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