How many nuclear power plants would we need to get us to 100%

[Altair_IV]

23. Students at Stanford?

madokie,

So *students* at Stanford are now considered an authoritative source? Really????

I haven't the time to go through his homework; but if this student says the difference between water requirements of nuclear vs coal power plants is large; I know what his error is.

The student is correct that the steam temperature in nuclear power plants is marginally lower than the steam temperature in coal-fired boilers. Because of the 2nd Law of Thermodynamics; the efficiency of the Rankine steam cycle at the nuclear plant is marginally less efficient than a coal fired plant.That much is true; but it is only marginally so. The common *error* that students make is that when they plug temperatures into the formula for the Carnot Efficiency, they forget to convert the temperature to a temperature on an *absolute* scale. As a Physics Professor, I've seen that mistake many, many times. Students plug the temperatures they are given in degrees Fahrenheit or degrees Celsius into the Carnot Efficiency formula.

The problem is that the zero point on both the Fahrenheit and Celsius scales were arbitrarily chosen and don't correspond to anything that is thermodynamically meaningful. The zero on the Celsius scale is the freezing temperature of water. The zero on the Fahrenheit scale was the temperature on the coldest day of the year plotted by Fahrenheit. In order to get an accurate efficiency difference; the temperature has to be converted to an absolute temperature scale where the zero point corresponds to absolute zero. The Fahrenheit temperature needs to be converted to degrees Rankine, or the Celsius temperature has to be converted to degrees Kelvin. Only by using either a Rankine or Kelvin temperature ( they differ by a multiplicative constant that cancels in the formula ) can one get an accurate efficiency calculation.

So I do agree; the efficiency of the Rankine steam cycle on a nuclear power plant is marginally less than a coal power plant; so the amount of water it uses per Megawatt-hour generated is lower than the coal power plant; but certainly *not* by large factors which is the *erroneous* claim you made.

The lower steam temperature on the nuclear power plant is not an insurmountable problem. The BONUS reactor in Puerto Rico was a reactor that featured nuclear superheating of the steam. The steam from the BONUS reactor was superheated like the steam from a fossil-fueled boiler.

http://en.wikipedia.org/wiki/Boiling_Nuclear_Superheater_%28BONUS%29_Reactor_Facility

The other tack one can take was demonstrated by Consolidated Edison's Indian Point Unit 1 ( now shutdown / decommissioned ). If you look at an aerial view of the Indian Point site, you will see the dome of Unit 1 between the domes of the still operating Units 2 and 3. If you look close, you will also see a tall stack like a fossil fuel plant would have. That's because Indian Point Unit 1 had fossil-fired superheater on it.

So if we choose; and water use per unit energy generated is important enough to us so that we put a premium on that figure of merit; there are ways to get the nuclear plant to perform to levels that rival fossil fuel plants. It doesn't have to be the way it is; marginally inferior that it is.

Altair_IV