Posted on 11/14/2006 12:32:02 PM PST by shrinkermd
Actually, it's probably right. Remember--you're now "capturing" heat that would normally just be vented "up the flue" to do space heating and heat water. The latter two can use the "lower quality" energy in the gensets exhaust to excellent advantage.
What they are computing is the percentage of heat that doesn't go up the chimney. Some systems do get up into the 80% and 90% and their exhaust is just warm. Where this system gets its boost in efficiency is it utilizes the expansion of the gasses to generate a little bit of electricity too.
Then it's deceptive because they are not calculating generation losses through friction, absorption, internal energy losses etc. Hell, block your chimney, and you can get 100% efficiency using that standard. ;~))
As I said on an earlier post, even the high-tech multi million dollar utility co-generation systems are struggling to get a 60% efficiency.
That is precisely what combined cycle and co-generation power plants do --- use the exhaust to heat water. And after many billions of investment over the last 50 years the best efficiency they can reach is 60% +- a few depending on ambient air/water temps.
I don't buy 89% unless it's simply turning fuel into heat as opposed to turning it into delivered energy.
I worked for years at a major chemical plant which generated its own power with some of the most efficient co-gen units available, so I know all about the tech.
What your estimate is missing is that use of the normal "waste heat" from home power generation units uses heat of "lower quality" than such a co-gen plant can use, thus attaining higher efficiency. A co-gen plant's (gas turbine, steam turbine, process steam) lowest useful temperature is 212F. A home heating units lowest useful temp is less than 100F. Therefore the higher efficiency number.
You asked how many people would spend more to feel green. I thought "Prius" answered your question quite well. I didn't say it was a good decision.
you are, of course correct that a lot of people spend a lot of money because they are told that something is green.
a fool and his money....
The 89% was a figure quoted for a furnace. Which I assume means that 89% of the heat generated from combustion goes into heating the house, rather than up the chimney (or, more likely at those efficiencies, out a house-side vent). That's quite believable.
But, and it's a large but, not all of us have a ranch.
I admit to having gas once in a while though. ;^)
Exactly, in a home the heat that comes out of the generation unit is a valuable commodity on its own, since we are otherwise burning NG just for the heat, anyway. Turning fuel into heat is easy and can be done with very good efficiencies, into the 90s. Its turning that heat energy back into another useful (low-entropy) from of energy that has, and will always have, relatively low efficiency. Hmmm, what kind of efficiencies do fuel cells have? Apparently 50-60%, but they do have rather exacting fuel requirements.
Actually, there is one type of fuel cell (solid oxide), which can potentially compete directly with this kind of co-gen system. They "burn" natural gas directly, and run at a high enough temperature that the "exhaust gas" is hot enough to do water and space heating, as well. This pushes the overall efficiency quite high.
Do they really use natural gas directly? No splitting the hydrogen from the carbon and then tossing the carbon molecules, first?
Well, it gets into a situaion of "splitting hairs" about "what is reformation". The "reformation reaction" occurs at/on the same catalytic surface that drives the conversion of hydrogen with oxygen to form water and electricity. The carbon content is converted into CO2, and serves to keept the "oxide stack" hot.
So, yes---a "reformation step" does happen, but it's not taking place in a separate "reformer", and the carbon bond energy that would normally be lost there is converted into usable high temperature heat.
In industrial scale size installations, that heat is used to generate steam, that then is used in a downstream steam cycle to generate more electricity--but in a home scale system would be used for hot water and space heating.
You are right they are a little deceptive. The electrical generation is probably in the 20% range compared to the 50%+ range of the utilities. What they are doing is simply adding that 20% to the normal home heating percentage say 70% and coming up with 90%. They are adding two entirely different percentages, but they do have some justification because simply providing heat doesn't use all the energy potential even if it is 100%, like in the case you suggested where there is no exhaust.
The other part of the deception is the question of how much of the potential energy in the fuel is being used? Simple things like altitude can reduce the energy utilized by 7% per thousand feet.
I am sure your neighbors won't mind a pump jack in the front yard. Children like to play around them.
Back yard, BACK yard!
I need it to heat my hot tub. ;^P
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