25% is a reasonable number. Operating steady state at max efficency power and rpm setting, a good gas engine can hit about 30% and a good diesel about 40%. That's not going to happen under real world automotive operating conditions though.
Solar Cell Efficiency
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You have it right. In simple terms:quote:
Originally posted by Last_Z:
That's what I don't understand. Why is the amount of heat used or wasted the factor which determines engine efficiency? Shouldn't it be fuel used vs amount of work?
On the other hand, I never claimed to be an engineer!
(Work done at the flywheel/potential fuel energy)*100 = thermal efficency
Heat wasted tells you a lot, but in the end, you don't need to know anything about the heat wasted to determine efficency.
You can measure the efficiency of any heat engine by determining the ignition temperature versus the exhaust temperature. One of the reasons why gas turbine engines can achieve fairly high efficiencies is because of the high temperature of ignition.
Of course, solar cells are not heat engines. But it has been very hard to achieve high efficiency with solar cells (on Earth). In space not too far from the Sun solar cells can have higher efficiency. But far from the Sun nuclear generators have been used to power unmanned spacecraft.
Of course, solar cells are not heat engines. But it has been very hard to achieve high efficiency with solar cells (on Earth). In space not too far from the Sun solar cells can have higher efficiency. But far from the Sun nuclear generators have been used to power unmanned spacecraft.
I did not check my old college physics book but I believe the ideal or Carnot efficiency is determined by heat of ignition of the fuel versus exhaust temperature.
But anyway, cars, trucks, and SUVs with gasoline engines waste about 75% of the energy put into them.
But anyway, cars, trucks, and SUVs with gasoline engines waste about 75% of the energy put into them.
There is another way of looking at all of this, however.
It took a tremendous amount of solar energy and heat generated by the Earth to produce the fossil fuels that we largely waste in those cars and trucks and SUVs. The energy we obtain from solar cells is more directly utilizing the solar energy. So I guess we could say that the solar cells are in reality much more efficient.
It took a tremendous amount of solar energy and heat generated by the Earth to produce the fossil fuels that we largely waste in those cars and trucks and SUVs. The energy we obtain from solar cells is more directly utilizing the solar energy. So I guess we could say that the solar cells are in reality much more efficient.
Ok...you're gonna have to dumb it down even further for me to understand.
I can understand an SUV being energy inefficient.....but a WV TDI get 45MPG or more. You can travel 45 miles in a single gallon of diesel. How is that inefficient? There are non-hybrid cars that get even better mileage than that?
I need to go back to school!
You're on the right track.
It's (Th -Tl)/Th
Temperatures are on the absolute scale, Rankine or Kelvin.
Th = Temp of the high temp resevoir
Tl = Temp of the low temp resevoir.
There is no combustion implied.
Maximum theoretical efficency of Otto and Diesel cycle engines can also be calculated if certain characteristics are defined. They are lower than Carnot cycle effciencies but they will never be achieved either.
Interestingly, the Otto cycle (spark ignition) is theoretically more slightly more efficent than the diesel cycle (compression ignition). However, little annoying details like practical limits to compression ratios more than make up for that.
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Getting back on topic: I still don't think PV cells can produce more energy than it takes to manufacture them. GE made them at one time and stopped due to this realization. If this wasn't so ...then why wouldn't you subsidize the production of PV to the point where PV could supply the power to manufacture PV with enough surplus energy production to cover the costs of the energy put into the various composite strata and substrata? That would seem to be a no brainer. You can't that's why, IMHO.
They're applicable, as was mentioned, where it would cost $50k to run powerlines to a cabin in Montana or Wyoming or Colorado and you can do it with PV for $30k.
I like the mirror focused solar steam turbine. Great use of solar power.
..and I had such high hopes for "cold fusion"
[ May 30, 2004, 12:53 PM: Message edited by: Gary Allan ]
They're applicable, as was mentioned, where it would cost $50k to run powerlines to a cabin in Montana or Wyoming or Colorado and you can do it with PV for $30k.
I like the mirror focused solar steam turbine. Great use of solar power.
..and I had such high hopes for "cold fusion"
[ May 30, 2004, 12:53 PM: Message edited by: Gary Allan ]
I had forgotten about that mirror focused solar steam turbine. That may be a possibility that needs to be strongly looked into.
Some people are making a combination of solar cells, wind turbines, solar heating of water and air, and water from windmill pump equipped wells work in remote areas of Colorado. It is expensive but makes sense where it would cost a fortune to build a power line going to a remote site.
Any way you look at it, we are going to need fossil fuels for a while longer. The supply from the Middle East is uncertain. We need to drill in Alaska. Even many of the people in alaska want it.
Some people are making a combination of solar cells, wind turbines, solar heating of water and air, and water from windmill pump equipped wells work in remote areas of Colorado. It is expensive but makes sense where it would cost a fortune to build a power line going to a remote site.
Any way you look at it, we are going to need fossil fuels for a while longer. The supply from the Middle East is uncertain. We need to drill in Alaska. Even many of the people in alaska want it.
Google "solar one" "solar two" daggett and you will find a boatload of info on some good sized mirror array concentrating solar power systems.
Solar Two uses a salt as a heat tranfer liquid.
[ June 01, 2004, 01:10 AM: Message edited by: XS650 ]
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