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re: controls as important as thermodynamic efficiency
28 jul 1997
john mccarthy wrote:
>nick@ufo.ee.vill.edu (nick pine) writes:
>>suppose you were a utility with a 1 gw nuclear plant sending 2 kwh of heat
>>and a gallon of water up the cooling tower for every kwh of electrical energy
>>produced. an alternative is to pipe 2 gw of hot water through 200,000 houses
>>(10 kw each) to heat them in wintertime, without consuming any water at all.
>cogeneration is economical on a small scale, because there will always
>be some demand for hot water.
the ratio of hot water to electrical energy demand might not depend much
on scale, even though the ease of heat distribution does.
>...if you build a big one, what do you do with the hot water in the summer?
avoid building big ones, store heat for winter, use it for some sort of
dessicant ac, eg licl concentration, sell hot water to a cannery, lose the
heat in a pond, etc...
>do you build the system large enough to provide enough heat on the coldest
>winter days?
i'd guess not, from a balanced economic point of view.
>it looks to me like you have to build the cooling tower anyway for summer
>use and the city requires auxiliary heating facilities for the coldest
>days of winter.
i'm thinking more on the scale of 1 or 10 or 100 houses, as in the intelligen
system. maybe a cogen unit should be sized to provide enough hot water for
showers in summer, and not enough to heat all the houses in winter. i suppose
it depends on local need patterns and economics. seems to me electrical energy
will have a lower cost/kwh as more people sell cogen power to transport grids,
if they can find a way to use the waste heat (2/3 of the fuel value) that
utilities now discard with cooling towers, and consume less water as well. the
need for large centralized electrical power generation may go away entirely,
with enough smaller scale cogeneration, pv panels that plug into wall sockets,
and so on.
>incidentally, a kwh is not quite enough energy to vaporize a gallon of
>water, so i figure that nick pine's gallon per kwh is a rhetorical figure.
no, it's based on the need to lose about 2 kwh (or more, in the case of
typical nuclear plants?) of heat for every kwh of electrical energy produced,
in these typical heat engines.
he who refuses to do thermodynamics is doomed to talk nonsense :-)
nick
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