"Donna Ohl" wrote in message
. net...
On Wed, 19 Mar 2008 04:32:02 -0700 (PDT), ransley wrote:
So, I think the fact that all none of the heat energy that went into the
electric coils goes up any flue - it's all absorbed by the water - is what
makes the electric water heater 98% efficiency.

Correct.

But, as someone stated, I suspect the power generation is about 70%
efficiency,

so, the true efficiency of electric water heating must be
vastly lower than 98% taking distribution into account.

But, how can we account for that true efficiency?

Donna

Average power generation isn't anything like 70% efficient. Typical
efficiency for a coal fired station is 30-40% with only the latest
generation achieving 60%+. Gas fired around 47% and nuclear around 38%. Then
another 5-6% is lost in transmission. The average depends on what mix your
country has but I can't see it being much above 40-50% overall by the time
it reaches your house.

This web site compares the cost of different fuel sources in the UK. It's
the only site I've seen that takes into account boiler efficiency. The key
figure is the middle one "Pence per kWh after boiler efficiency". The actual
boiler efficiency is in brackets...

http://www.nottenergy.com/energy-costs-comparison2

Note that electric heating is indeed 100% efficient but the cost of that
electricity makes it expensive to run.

Heat Pumps have efficiencies of over 100% and in the case of a ground source
heat pump (GSHP) around 350%. This more than compensates for the loss of
efficiency producing the electricity needed to power. Overall a GSHP is the
cheapest system to run (ignoring capital costs). It would be interesting to
know if anyone makes a small scale gas or oil powered GSHP and how the
efficiency of those compare.

In theory it would _just_ be possible to use the heat from a GSHP to power a
Stirling engine to power the GSHP. This would not violate COE because there
is a heat source (the sun) providing power into the system. However most of
the heat produced by the GSHP would go into the stirling engine with very
little left over to heat your house. Stirling engines that big would also be
rather big physically. Overall such a system would be too big and expensive
to be practical - but it would be free to run.