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Power to the people: cars as generators, houses truly efficient |
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Tuesday, 23 January 2007 |
 | | Carl S. Milsted, Jr. |
In my previous column, I half-jokingly suggested that it might make more sense to plug your house into your hybrid car than to plug your hybrid car into your house. This was assuming a plug-in hybrid with an engine that is quieter, cleaner and more efficient than a gasoline engine — a Stirling engine, perhaps.
The idea is rather silly, but is worth contemplating nonetheless. Consider a suburban homeowner with a solar powered home. Solar may work fine when the sun is out, but fails when it is dark outside. Then, the homeowner either needs expensive batteries or a connection to the grid.
Solar
homes selling energy to the grid during the day and buying from the
grid at night does make some sense since overall electrical demand is
higher during the daytime. However, there are significant inverter
losses when you mix solar power with alternating current. And then
there is the possibly more important emotional issue: where is the fun
in going solar if you still have to be hooked up to the grid?
Enter the
automobile as generator. A quick check of my home‘s breaker box
indicates a theoretical 30 kilowatts of peak use before all the
breakers go off. Average power consumption is considerably lower, of
course. The Chevrolet Volt has a 53 kilowatt generator. If our
imaginary quiet-engined hybrid can run its engine efficiently at a
significantly lower than peak level, then it could be used to power a
typical home with peak load headroom to spare. Then, battery power
would only be needed for those times when the sun is out and the owner
is not at home.
OK, so it would
be rather inconvenient, plugging your car into your house every time
you get home and what if you want to run your dishwasher while you are
out on the town at night? But this thought experiment does reveal
something very important: should GM develop an ultra
clean/quiet/efficient engine to run its hybrid cars, it could also use
the same technology to sell engines for home power generation.
But does home
power generation make sense? Due to the high duty cycle and large scale
of operation, it can pay to build a centralized power plant that
maximizes efficiency at the cost of huge capital outlay. It is
unrealistic to expect mass-produced home generators to achieve the 60
percent efficiency achieved at the better generating stations. This
holds even when you take into account transmission losses (roughly 7
percent on average).
The answer is:
home power generation does indeed make sense, even if the efficiency is
only 40 percent (in the diesel range). That‘s because the other 60
percent can be used as heat. For those who have electric hot water
heaters, or electric heat (in the winter), a significant fraction of
electricity usage is for heat. Suppose we could recover 70 percent of
the waste heat from our generator. We would then have an overall
efficiency of .4 + .6 * .7 = 40% + 42% = 82 percent efficiency! No more
grid! No more ugly power lines!
Right?
Maybe not.
Consider all those homes in the north that use fuel oil or natural gas
for heat. Place a heat engine between their fires and heat exchangers
and these homes may well generate a surplus of electricity to sell
through the grid. We might well turn off some natural gas-fired
generating stations in the winter.
So, message to GM: develop that quiet/clean engine for your hybrid cars and you can sell more than cars.
P.S. WhisperGen Limited of New Zealand is already building such home generators using a Stirling engine.
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Carl S. Milsted Jr., chairman of the Libertarian Party of Buncombe County, may be contacted at cmilsted-at-holisticpolitics.org.
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