Jed, et al.
At a typical electric power plant, 66% of the heat is wasted,
producing billowing clouds of steam from the cooling towers.
That would be the typical nuclear plant. Newer gas-fired plants do
better, but there is still plenty of room for improvement - and
perhaps that "improvement" is the best way to implement wind
energy in the short term - as instead of competing with the "Cons"
of the energy world (i.e. Con-Ed) you are actually working with
them.
Let's start with a gas turbine electric power plant, and use
rounded-off approximations and a very simple verbalization of what
can be accomplished in a compound system, using a "bottoming
cycle" of liquid air - which has been produced by an offshore wind
farm.
A typical modern high compression gas-turbine is 40% efficient in
converting the heat content of methane into electricity. This is a
Brayton cycle heat engine - Wiki entry:
http://en.wikipedia.org/wiki/Brayton_cycle
and like the Carnot cycle, it is determined by a temperature
"spread" and then the efficiency within the "spread". The Wiki
entry for Carnot is here:
http://en.wikipedia.org/wiki/Carnot_heat_engine
At any rate there is a high temperature, in the combustor, and
there is a low temperature at the exhaust, giving the "spread". In
theory, you can increase the net efficiency by either raising the
high end or lowering the low end - as the efficiency within the
spread has already been maximized by the design of the turbine.
The high end however - has for many years already been maximized
by the demands of metallurgy. At the low end, then - you find the
only chance for getting more electricity out of your wasted heat -
and you can use "reheat" and "regeneration" to get some increase
by utilizing the waste heat again together with some lesser "new"
heat. The waste heat is usually limited to about 150 degrees C.
over ambient, but this represents 60% of the total original heat.
If one has a ready source of a very cold liquid, which becomes
volatile at less than ambient temperatures, then two things can be
accomplished by using this waste heat. First it can be used to
expand the liquid which is then passed through another (much
cheaper) turbine, and then secondly - just doing this, the
operator can widen the original "spread" of the gas-fired plant.
This low end manipulation can even be done with water - by adding
a steam bottoming cycle to a gas turbine... which will result in
capturing an additional 15-20% of formerly wasted energy, as the
Carnot spread is only going to 150 degrees plus the added new
heat. It gets more complicated than that, since reheat is used,
but for these purposes generalization is more important than
precision.
If, instead of water/steam, you are able to use a cold liquid at
150 degrees below ambient, not requiring addition new heat, then
essentially you can double the available "spread" at the low end
and garner another 10-15% more than before - but even that nice
bump is not the end-of story.
By using a liquid below ambient you have at the same time been
able to increase the "spread" in the first case - somewhat, so
instead of the improved 40+15 =55% efficiency, which results from
using a steam bottoming cycle, you can get something like 42+30 =
72% efficiency - which high figure ignores the wind energy put
into achieving the cold liquid state - so it is not
apples-to-apples. At any rate there is a very desirable niche for
this kind of optimized system in a world of rising methane prices.
In this idealized case, it can be seen that using liquid air gives
a significant net increase in efficiency, which would directly
result in a decrease in methane usage, and the added capital cost
of a cold bottoming cycle is actually less than for a steam
bottoming cycle (which is, in effect a steam plant) - except of
course that this comparison excludes the capital cost of the wind
farm, making the liquid air.
It is a complicated situation - involving competing special
interests, and that is probably why you have not seen this
presented in this way before. Wind energy has previously been
adversarial to natural gas - when in truth, there is a middle
ground, benefiting all concerned.
All and all... this must be an integrated package - planned from
the start - which looks, at first blush, like the best way to use
wind energy since it combines the needed storage capability with
the present day distribution infrastructure - while at the same
time allowing the power company to reduce its methane purchases
and reduce its carbon releases.
Jones
