[zeke3000]

Maybe a stupid question but let´s try.

Is it theoretically possible to convert heat into electricity ? Could it be possible to convert excess heat generated by fossil fuel burning "back" into usable energy (electricity,mechanical or somethin else maybe) ? Ie. using the greenhouse effect to generate energy. E.g. Iceland is using heat (steam?) somehow to produce electricity I understood..

[frankthetank]

Yes...buts its very inefficient... i think they use it in Iceland, but nothing large scale...actually very small scale. I also think they may use something like that in the space program.

[diemos]

It's not stupid at all, but it does reveal that you've never taken a basic physics class.

Physics may be one of the "hard" classes but its worth the effort because it covers everything we know about how the universe actually works, which is pretty handy stuff to know.

[MD]

First attempt to increase efficiencies in order to reduce waste heat. Once you've done that try and capture the rest of the waste heat for other useful purposes such as drying, greenhouse growing, building heat, etc.

It would certainly be very cool to have a film coating that would convert heat passing though it into electricity directly. I know a number of people that have pushed that idea around. No one has gotten anywhere with it though, to my knowledge. It's a bit of a reach (which is a bit of an understatement). It's pretty much in the realm of sci-fi/fantasy dreams like gravity and/or warp drives.

[zeke3000]

The problem has something to do with entropy ? I was thinking about the term when writing, but yeah I have not taken more than 1 or 2 physics classes back in high school, which was years ago. Heat is some kind of an end state of energy ?

[diemos]

heat is disordered energy, there's no way to convert it to useful work without violating the laws of thermodynamics.

To get useful work you have to put a machine in between a hot area and a cold area. Then as heat moves from hot to cold you can convert some of that heat flow into work.
The greater the temperature difference the higher efficiency for converting heat flow to work.

So a fraction of the exhaust heat from an engine or a power plant could in theory be converted to work if you had a colder area to discharge it to but that's rarely feasible. The best thing you can do with waste heat is to use it to heat water or a living area. Applications where you just want to make something warmer without trying to extract work.

[zeke3000]

So the potential for wind power is on the rise as climate gets hotter? At least that´s what some people hope for, I have read. A wind turbine is such a machine, right ? Or maybe the extra heat will be evenly distributed and not create any more winds&waves. So we should put wind turbines between the earth and cold space instead.

[dsula]

The best thing you can do with waste heat is to use it to heat water or a living area. Applications where you just want to make something warmer without trying to extract work.

There's plenty of industrial applications where waste heat is used to drive a sterling motor. Easy, cheap, pretty efficient. And of course there's always the old and trusted peltier/seebeck effects to directly convert heat into electricity and vice-versa.

[PrestonSturges]

http://www.greendiary.com/entry/mit-res ... te-energy/

Peter Hagelstein, an associate professor of electrical engineering at MIT, suggests how close realistic technology could achieve the theoretical limits for energy conversion efficiency called the Carnot Limit. Since the surplus energy from gadgets and automobiles usually dissipates in the form of heat, his collaborative research with graduate student Dennis Wu aims at achieving 90 percent of this limit by harnessing this wasted heat.

[diemos]

Carnot efficiency is 1 - Tc/Th where temperature is measured in Kelvin.
If Tc is room temp and Th is 30 degrees C hotter then the maximum efficiency
for conversion is 10%.

Power plants run hotter and get up to 33% efficiency.

Your link is for a device that exploits quantum effects and almost certainly can't be scaled up to a useful device. It's a laboratory curiosity only.

[Gerben]

This topic made me look at steam engine developments. I came upon an interesting development by Cyclone Power. They are developing a steam engine that can compete with ICEs and can run on just about any fuel. They are also working on a version that can use the waste heat of a combustion engine.

[yesplease]

Power plants run hotter and get up to 33% efficiency.

It's up to almost 50% in terms of converting heat from combustion into electricity last I checked. The average efficiency for all electricity production from coal was somewhere in the low thirties, and for NG it was in the low forties as of 2007.

[zeke3000]

Maybe a stupid question but let´s try.

Is it theoretically possible to convert heat into electricity ? Could it be possible to convert excess heat generated by fossil fuel burning "back" into usable energy (electricity,mechanical or somethin else maybe) ? Ie. using the greenhouse effect to generate energy. E.g. Iceland is using heat (steam?) somehow to produce electricity I understood..

I believe I should rephrase my question. Could we use BOTH the extra heat generated by fossil fuel burning AND the extra heat trapped becaused of the resulting green house effect and convert both to more usable energy.

First I was thinking mostly about the climate warming (which is being debated, I´m not sure were people here stand on that), but then I realised the physical process of burning fuels also create direct heat). I guess these two "heats" differ widely in character&scale, so you may omit either one. Corrections are welcome.

[diemos]

The earth intercepts 1.27x10^17 W of solar energy of which a total of 1.27x10^17 W is either reflected or absorbed and reradiated as infrared.

The albedo of the earth and the IR properties of the atmosphere determine the surface temperature at which this balance is attained.

Global energy production and consumption is 1.5x10^13 W or about 1/10000th of the energy being continually absorbed and reradiated which is too small to directly affect the temperature.

If you want to cool the earth you can either decrease the amount of sunlight reaching the earth with space based reflectors or by increasing the earth's albedo or you can change the composition of the atmosphere to change its IR behavior.

The atmospheric chemistry and IR behavior is fairly well understood. Where the models become guesswork is how various feedback mechanisms will change the albedo of the earth as greenhouse gases increase.

[Gerben]

It has even been suggested to paint all our rooftops white to reduce global warming.

Note that global warming will make the energy production on Iceland slightly less economic. They use the temperature difference between water heated by the earth's core and surface water. If surface water temperatures rise, that process becomes slightly less efficient.

[vtsnowedin]

Maybe a stupid question but let´s try.

Is it theoretically possible to convert heat into electricity ? Could it be possible to convert excess heat generated by fossil fuel burning "back" into usable energy (electricity,mechanical or somethin else maybe) ? Ie. using the greenhouse effect to generate energy. E.g. Iceland is using heat (steam?) somehow to produce electricity I understood..

I believe I should rephrase my question. Could we use BOTH the extra heat generated by fossil fuel burning AND the extra heat trapped becaused of the resulting green house effect and convert both to more usable energy.

First I was thinking mostly about the climate warming (which is being debated, I´m not sure were people here stand on that), but then I realised the physical process of burning fuels also create direct heat). I guess these two "heats" differ widely in character&scale, so you may omit either one. Corrections are welcome.

Darn you got that in before I could say that yes you can make electricity by building a fire,boiling water and running the steam through a turbine. Use any fuel you want , oil,coal ,NG, or nuclear fusion any heat source will do. But now you want to limit it to waste heat. If they could have figured that out, and sometimes they have, they wouldn't have let it go to waste to begin with.
here is a question for you. If a wind farm extracts X kilowatts of energy from an air mass is the total energy contained by the air mass reduced by X kilowatts or by a larger number based on an efficiency factor. And could that be measured by the cooling of the air mass or only by the reduction in wind speed the mass exhibits downwind of the wind farm. The wind got its velocity from sunlight heating air expanding it and creating a pressure differential between two air masses. If we stick enough windmills in the way can we change the weather?

[Gerben]

If we stick enough windmills in the way can we change the weather?

Yes, we can.

[vtsnowedin]

If we stick enough windmills in the way can we change the weather?

Yes, we can.

Example?

[Gerben]

If we stick enough windmills in the way can we change the weather?

Yes, we can.

Example?

Any object can distort winds. Hurricanes lose power when they reach land. In cities the many obstacles play a role in creating city climates.

[whereagles]

thermal engines that are close to Carnot efficiency aren't that hard to build. Just that they don't have the 1000 horsepower of a power plant turbine. Not even close, actually

Also, you can't convert heat into usable energy, but you CAN convert a heat flux (from hot source to cold sink) into usable energy. That's called a.. thermal engine