Question

# A local coal fired 200 megawatt electrical generating plant has a thermal efficiency of 30% and...

A local coal fired 200 megawatt electrical generating plant has a thermal efficiency of 30% and a nuclear 150 megawatt electrical generating plant has a thermal efficiency of 25%.

a. Calculate the waste heat emitted by each (in megawatts)

b. If the cooling water used for the nuclear plant were required to only rise in temperature by 2o C after it absorbs the waste heat, how much water per hour would be required to cool the nuclear plant (in cubic meters/hr) Heat capacity of water = 4186 kJ/m3oC.

c. If the coal fired power plant increased the temperature of the steam which drives the turbines from 500oC to 600oC while the output temperature of the steam remained at 30oC, by how much (in megawatts) would the waste heat emitted by the power plant change (and would it increase or decrease)? (For part c. assume that the plant operates at the maximum possible (Carnot) efficiency.)

#2. Suppose you want to reduce your carbon dioxide ’foot print’ by reducing the amount of CO2 your activities release to the atmosphere. You consider buying an electric car. The car converts 95% of the electrical energy from the battery to mechanical energy driving the vehicle, whereas your current car converts 25% of the thermal energy obtained by burning gasoline into mechanical energy while emitting CO2 On the other hand, the battery for your car would be recharged from the electrical outlet in your garage which is fed by the local power plant. The power plant has a thermal efficiency of 30% for converting thermal energy from burning coal into electrical energy while releasing CO2. By what percent (if at all) will you reduce the amount of CO2 you are causing to go into the atmosphere if you substitute the electric car for the gasoline powered one? (Assume that the amount of CO2 released per joule of thermal energy produced is the same whether you burn gasoline or coal.)

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