| All Electric Home?

All Electric Home?

asked:


If a certain amount of fuel (oil, gas or coal) is burned in your house stove it would produce X amount of heat. Now, if that same amount of fuel is burned in an electric generating plant, and all the electricity so generated is used to heat your house by means of an electric stove, the electric stove would:

a) produce more than X amount of heat, because electricity is more efficient than gas.

b) produce exactly X amount of heat, because of conservation of energy.

c) produce much less than X amount of heat, because heat can never be completely converted to electricity.
The answer is: c. Near most elecric power generating plants you see coolng towers or warm water being discharged into a river, lake or bay. The reason for this is that heat energy cannot be competely converted to electric energy. Some of the heat energy must go to waste. (At hydroelectric power generating plants, however, there is negligible waste because except for small friction losses the mechanical energy of falling water is completely converted into electric energy.) Why cannot the heat that is discharged in the towers or rivers by recycled and put back into the power plant’s boiler? Because heat, by itself, will not flow from a cool place to a hot place — and the boiler is always much hotter than the waste heat. Well, why not use a heat pump to force the wase heat back into theboiler? Because the pump requires energy to operate. How much energy? At least as much energy as the power generatng plant produced while making the waste heat!
So, there would be absolutely no electric power left to sell.

Well, why does there have to be waste heat in the first place? Because in a steam engine or turbine the gas must expand as it pushes on the engine’s piston or turbine’s blades. As it expands it cools. If it could be expanded until its temperature dropped to absolute zero all the heat energy could be converted to work. But in reality, it can get no cooler than the rest of the outside world, which is about 300 degrees above absolute zero. So, you cannot get all of the energy out of the heat.

Well, how about this idea? You can expand the steam until it turns to water and then just put the hot water back into the boiler. How can there be any waste doing that? You may think there is no waste because you think you have a closed cycle, but you don’t. First, some energy comes out as the steam does work pushing on the piston as it expands, but that is just what you want, so that is okay. Now comes the waste.
The steam expands until its temperature drps to 100 degrees C — then the pressure inside the engine is the same as the external atmospheric pressure. It can expand no more, but it is not water yet. It is still 100 degrees C steam and you cannot simply put a large volume of low pressure steam back into a high pressure boiler. You must first reduce te volme of the steam by turning it into water. But to condense the 100 degree C steam into 100 degree C water you must remove its latent heat of condensation. As steam turns into water its temperature does not change, but heat — a lot of heat — must come out of it. That heat cannot be returned to the boiler because the temperature of that heat is only 100 degrees C and the temperature of the boiler is much higher. The latent heat of condensation becomes waste heat. Too bad.
Why must the temperature of the boiler be above 100 degrees C? Because the pressure of 100 degree C steam does not exceed atmospheric pressure.

When you pay for all electric heating you pay both to heat your house and the rivers, sea and sky.

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One Response to “All Electric Home?”

  1. Remo Ω Aviron on June 2nd, 2009 3:02 am

    c. The amount of electricity generated by the plant is limited by thermodynmics, more particularly, the carnot cycle, which states that the limit on efficiency is

    = 1 - Tcold/Thot

    This means that the efficiency of the electric plant will be limited by the temperature differential between the hot boiler and the cold sink. Typical efficiencies range from 30 to nearly 50% for steam turbine plants. Gas turbines have made it up to 60% efficiencies. See citation below.

    The electric heater may be 100% effective in converting electricity into heat, but you will never make up for the loss of 60% of the heat in generating the electricity.

    *********
    Going slightly beyond your original question, it is possible using a heat pump (basically an air conditioner run backwards) to extract heat from the outside environment. Here, you can get efficiencies well over 100%, which can make up for the heat loss at the power plant.