why can't heat be used to create energy?

[meksta]

heat is a release of energy...

why can't the heat released from the cpu in my laptop be used to recharge my battery?

not looking for free energy...but will it work in theory?

 

[CycloWizard]

Theoretically, you could reclaim 50% of the energy lost as heat. However, to do so, you would need what is known as a Carnot engine. Such an engine does not exist in reality because it requires physically unrealizable systems. A more realistic solution is to use a peltier to turn the temperature of your CPU into current, which could be fed back to your battery. However, the electronics involved are so expensive that it would take about 10^18 years to pay for itself with the minimal savings in electricity.

 

[spikespiegal]

Actually what you'd need is a thermocouple, which can directly convert heat energy into electricity. Someobdy else would need to do the math anf figure out how much energy dissipated by a computer processor could be efficiently turned into electricity. Given the size of the generators on space probes, I'm guessing not very much.

http://en.wikipedia.org/wiki/Thermocouple

Dumping the heat energy released from your laptop into a water heater or other device would be more efficient. Or, using the heat energy to make a bowl of Mac -n- Cheese, etc.

This is why current Intel dual Core P4's might be better measured in terms of 'Kraft' units rather than clock cycles.

 

[CycloWizard]

Originally posted by: spikespiegal
Actually what you'd need is a thermocouple, which can directly convert heat energy into electricity. Someobdy else would need to do the math anf figure out how much energy dissipated by a computer processor could be efficiently turned into electricity. Given the size of the generators on space probes, I'm guessing not very much.

http://en.wikipedia.org/wiki/Thermocouple

Dumping the heat energy released from your laptop into a water heater or other device would be more efficient. Or, using the heat energy to make a bowl of Mac -n- Cheese, etc.

This is why current Intel dual Core P4's might be better measured in terms of 'Kraft' units rather than clock cycles.

Actually, that would be what I said, only a peltier would be a more efficient way to feed power back to the system than a thermocouple.

 

[lexxmac]

Can it be done? Yes. Is it practical? Not really (with current technologies). You'd be better off building/designing components that put out less heat and are more efficient to begin with than retrofitting an inefficient machine with a recovery system.

 

[HVAC]

Lexxmac gets it right when he says that it is better to reduce inefficiencies than to try to recover lost energy. This is due to the physical observation that natural processes only tend to increase entropy. This is a fancy way of saying that once you let the smoke out of something, you cannot put it back in....... or that the electrical energy, once converted to heat, requires effort to convert it back to electrical energy.

With that said, recovery of waste heat is usually not economically viable. We have all sorts of different machines for doing such things including those mentioned above (peltier devices, carnot engines, water baths feeding who knows what kind of converter or the like).

My personal favorite right now would have to be a micromachined Stirling engine or series of engines. They could be fashioned in such a way as to generate power from very little temperature differential. This approach would probably still suffer from economic infeasibility, but it could be as geometrically flexible as a peltier device.

A neat beginning spot for more info on Stirling engines....
http://www.keveney.com/

(I apologize for use of the word "neat". I am sure it was passe before I was in diap.... I apologize for not accenting pass...... oh...nevermind...)

 

[Gibsons]

Originally posted by: HVAC

A neat beginning spot for more info on Stirling engines....
http://www.keveney.com/

That website is neat.

Thanks for the link.

 

[bendixG15]

Topic Title: why can't heat be used to create energy?
Topic Summary: i.e laptops
----------------------------------------------------------------
Simple answer is that nobody has figured out how to do it..

 

[jagec]

Originally posted by: bendixG15
Topic Title: why can't heat be used to create energy?
Topic Summary: i.e laptops
----------------------------------------------------------------
Simple answer is that nobody has figured out how to do it..

No, there are tons of ways of getting energy from heat. They're just not practical for laptops.

 

[joshd]

How much electricity is created by these peltiers? How mcuh would it extend the battery life by? I can't see it doing a HUGE amount, the system would be very inefficient.

 

[msparish]

Originally posted by: CycloWizard
Theoretically, you could reclaim 50% of the energy lost as heat. However, to do so, you would need what is known as a Carnot engine. Such an engine does not exist in reality because it requires physically unrealizable systems. A more realistic solution is to use a peltier to turn the temperature of your CPU into current, which could be fed back to your battery. However, the electronics involved are so expensive that it would take about 10^18 years to pay for itself with the minimal savings in electricity.

You can't even get that efficiency theoretically. More realistically, it would be more like 15% for the maximum theoretical effeciency. Assuming ambient temperature of 298K, and a CPU temperature of 353K (80 degrees Celsius...smoking hot for a CPU), your max efficiency (with a Carnot engine) is 15.6%.

 

[CycloWizard]

Originally posted by: msparish
You can't even get that efficiency theoretically. More realistically, it would be more like 15% for the maximum theoretical effeciency. Assuming ambient temperature of 298K, and a CPU temperature of 353K (80 degrees Celsius...smoking hot for a CPU), your max efficiency (with a Carnot engine) is 15.6%.

Theoretically, you can get that efficiency. You just need a infinitely hot high-temperature reservoir and a cold reservoir at absolute zero. However, in practice, you can't achieve this and you recover your result. That's why I said 'theoretically'.

 

[wacki]

I like this thread

http://en.wikipedia.org/wiki/Carnot_heat_engine

 

[wwswimming]

http://www.stirlingenergy.com/

these guys just got a big contract to build a large pilot plant using their technology, about $500 million from Lockheed and another company for a 250 MegaWatt plant (or is it $250 Mill. for a 500 Megawatt plant ?)

anyway, they're doing big things with using heat energy to create electricity, efficiency about 30%.

Stirling energy technology, not photovoltaics.

 

[silverpig]

Stirling engines can work very well. One that might be applicable and highly efficient will run you a LOT of money though.

Peltiers are not really an option. They're maybe 3-4% efficient for the good ones, and you have to keep the cool side cool.

 

[Liviathan]

Not sure if the discussion is only limited to electronics. But I just read that BMW is working on a hybrid design that uses gasoline and steam. They plan to use the heat that's generated through a byproduct to power a steam engine.

 

[f95toli]

Stirling engines are neat
They are among other thing used in Swedish submarines (Gotland Class).

http://www.kockums.se/Submarines/aipstirling.html

And they are very silent . Gotland is currently leased to the US Navy.
The US navy became interested when,during joint naval exercises, they were unable to track the Swedish submarine.

 

[Evadman]

Originally posted by: Liviathan
Not sure if the discussion is only limited to electronics. But I just read that BMW is working on a hybrid design that uses gasoline and steam. They plan to use the heat that's generated through a byproduct to power a steam engine.

The 6 stroke engine. If you are into automotives and physics, then this 6 stroke is really neat. Basicly, after the exaust stroke water is injected into the cyl, which flash boils using the latent heat left in the piston, cyl, and the little amount of air. Then, the cyl is driven down in basicly a 'free' power stroke.

This has 2 main advantages, first, free power. Second, excess heat is taken up by the water-steam boiling, so the engine as a whole runs much cooler, so the cooling system can be lighter weight.

there are disadvantages too. first, the steam stroke is really hard on the engine. The engine block, pistons, and such all need to be beefier. The face of the block is going to go from ~2500 degrees (flamefront) to ~400 in a few microseconds. First tests were done with a converted diesel block, which weighs in > 100% more than an equivelent gas engine.

Second, Now you need to carry around the water along with the fuel. The amount of water used is > the amount of fuel. so you would need 15 gallons of water and 10 gallons of gasoline, more than doubling the amount of weight to carry around.

Last but not least, there are corrosion issues with all that water flowing over everything, and out the exaust. This will have an interesting impact on catalitic converters, since they run > 1000 degrees to convert the unburned fuel into CO2. I don't know of any testing on cats to see whata ll the extra water vapor would do. anyway, a stainless exaust would be all but required.

 

[kpb]

I don't beleive that it is what he's refering to. A quick search shows several hits to things like http://www.autoblog.com/2005/12/09/bmw-turbosteamer-gets-hot-and-goes/ which is very different. It seems to be a closed system and uses the exhaust heat and not be injected into the cylinder as a 5th/6th stroke.

I'd imagine the 6th stroke idea could be modified to recycle the steam altho it would probably require a seperate exhaust routing for the steam. You could at the very least cool it back off using a radiator to reuse water so the consumption was much less and might even be able to generate some additional power from it or electricity using a turbine or sterling engine. Of corse thats getting pretty complicated.

 

[msparish]

Originally posted by: CycloWizard

Originally posted by: msparish
You can't even get that efficiency theoretically. More realistically, it would be more like 15% for the maximum theoretical effeciency. Assuming ambient temperature of 298K, and a CPU temperature of 353K (80 degrees Celsius...smoking hot for a CPU), your max efficiency (with a Carnot engine) is 15.6%.

Theoretically, you can get that efficiency. You just need a infinitely hot high-temperature reservoir and a cold reservoir at absolute zero. However, in practice, you can't achieve this and you recover your result. That's why I said 'theoretically'.

With an infinitely hot high-temperature reservoir, you can theoretically get 100% efficiency, not 50%. In this case you don't have that, the high-temp reservoir is the CPU. So, max theoretical efficiency is around 15%, depending on CPU temperature. You won't achieve this, but theoretically it is the best you can do. 50% means nothing in this disucssion, theoretical or in the real world.

 

[imported_Hamzter]

Originally posted by: Liviathan
Last but not least, there are corrosion issues with all that water flowing over everything, and out the exaust. This will have an interesting impact on catalitic converters, since they run > 1000 degrees to convert the unburned fuel into CO2. I don't know of any testing on cats to see whata ll the extra water vapor would do. anyway, a stainless exaust would be all but required.

Isn't there a simple solution to this, by keeping the water in a vapour phase? Burning fuel creates water anyway, and that doesn't do much damage because it's superheated...

 

[theMan]

hook up a steam engine.