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Author Topic: Can heat be converted into electricity?  (Read 19104 times)

Dr Graham

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Can heat be converted into electricity?
« on: 02/12/2011 03:30:03 »
Dr Graham asked the Naked Scientists:
   
Is there a device which can convert heat energy into electricity ?

Photo cells can do it with light, Batteries do it chemically, and there are a variety of ways to covert mechanical energy into electricity, so why not heat energy ?

What do you think?
« Last Edit: 02/12/2011 03:30:03 by _system »


 

Offline CliffordK

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Can heat be converted into electricity?
« Reply #1 on: 01/12/2011 09:45:30 »
Certainly a variety of types of steam engines and turbines use heat to produce electricity. 

Photo cells can be tuned to some extent to pick up certain wavelengths of light such as would be emitted by objects on earth.

I'm seeing a lot of notes of an ENECO semiconductor device that is supposed to convert heat directly into electricity.

http://web.mit.edu/newsoffice/2001/electricity-1205.html
http://pesn.com/Radio/Free_Energy_Now/shows/2007/01/06/9700221_Eneco_thermal_electric/

It appears to be about a decade old technology, based on older vacuum tube technology.  But, it hasn't gotten to commercial production yet.  Time will tell if it is eventually developed into a viable commercial technology, or if it just fizzles.

Keep in mind that your typical silicon solar cell is more efficient at cooler temperatures.
 

Offline MikeS

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Can heat be converted into electricity?
« Reply #2 on: 01/12/2011 10:18:38 »
As CliffordK said steam engines and turbines. Also petrol and diesel engines use fuel to produce heat to do work.
A thermocouple directly converts heat into electricity but not a lot.
 

Offline imatfaal

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Can heat be converted into electricity?
« Reply #3 on: 01/12/2011 10:25:35 »
As CliffordK said steam engines and turbines. Also petrol and diesel engines use fuel to produce heat to do work.
A thermocouple directly converts heat into electricity but not a lot.

I never really understood electricity etc - but I thought a thermocouple worked on temperature differences between two conductors.  So even when acting as a thermopile that it was the difference in temperature and not the presence of heat that caused a current to flow
 

Offline MikeS

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Can heat be converted into electricity?
« Reply #4 on: 01/12/2011 10:36:21 »
imatfaal

"A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert a heat gradient into electricity."
http://en.wikipedia.org/wiki/Thermocouple

Thermocouples are obviously not very efficient at converting heat into electricity.  I mentioned them only because they directly convert heat into electricity.
 

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Offline CliffordK

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Can heat be converted into electricity?
« Reply #6 on: 01/12/2011 20:49:43 »
How hot does this have to be?

Think of an air conditioning unit that would actually generate electricity...   :)  Or, at least use an AC heat pump to limit the energy cost.

If you could cheaply capture thermal energy down to about 0°C, you could likely have an end to global warming.  In fact, there would be the risk of it being used to excess...  and suddenly creating the next ice age.
 

Offline Geezer

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Can heat be converted into electricity?
« Reply #7 on: 01/12/2011 21:24:42 »
How hot does this have to be?

Think of an air conditioning unit that would actually generate electricity...   :)  Or, at least use an AC heat pump to limit the energy cost.

If you could cheaply capture thermal energy down to about 0°C, you could likely have an end to global warming.  In fact, there would be the risk of it being used to excess...  and suddenly creating the next ice age.

It's still a heat engine. If we are to believe the French geezer by the name of Carnot, its efficiency is limited by the temperature difference between its hot end and its cold end.
 

Offline techmind

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Can heat be converted into electricity?
« Reply #8 on: 03/12/2011 22:19:32 »
Energy can be extracted from the flow of heat from a hot place to a colder place. Steam engines/turbines are one example of this.

Seebeck-effect thermoelectric devices convert the heat flow directly into electricity, and for the past 50 years have generally been based on bismuth telluride technology (They're mostly made in Russia, although China has been getting in on the act recently). As with all heat-engines, the efficiency gets better the greater the thermal gradient - but for practical Seebeck devices and realistic/compatible temperatue differences) the efficiency is really rather poor (maybe 8% at most).
It's not much use for "energy saving" devices, but can develop enough power to run remote telemetry devices off of hot pipes, or deep-space-probes containing a radioactive source (which keeps warm). I did hear of a scheme to use thermoelectric devices to harvest heat from the catalyser in a car to help power the car electrics. No idea whether it was ever going ot be commercially viable though.

For more info see http://en.wikipedia.org/wiki/Thermoelectric_effect
 

Offline Geezer

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Can heat be converted into electricity?
« Reply #9 on: 03/12/2011 22:31:57 »
Energy can be extracted from the flow of heat from a hot place to a colder place. Steam engines/turbines are one example of this.

Seebeck-effect thermoelectric devices convert the heat flow directly into electricity, and for the past 50 years have generally been based on bismuth telluride technology (They're mostly made in Russia, although China has been getting in on the act recently). As with all heat-engines, the efficiency gets better the greater the thermal gradient - but for practical Seebeck devices and realistic/compatible temperatue differences) the efficiency is really rather poor (maybe 8% at most).
It's not much use for "energy saving" devices, but can develop enough power to run remote telemetry devices off of hot pipes, or deep-space-probes containing a radioactive source (which keeps warm). I did hear of a scheme to use thermoelectric devices to harvest heat from the catalyser in a car to help power the car electrics. No idea whether it was ever going ot be commercially viable though.

For more info see http://en.wikipedia.org/wiki/Thermoelectric_effect


Of course, you could always use a thermoelectric device to drive a heatpump to increase the thermal gradient to increase the efficiency of the thermoelectric device. What could possibly go wrong?  ::)
 

Offline Escorpiuser

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Can heat be converted into electricity?
« Reply #10 on: 07/12/2011 12:20:08 »
(...)
Of course, you could always use a thermoelectric device to drive a heatpump to increase the thermal gradient to increase the efficiency of the thermoelectric device. What could possibly go wrong?  ::)

Due to inefficiencies in the processes and imperfect adiabatic isolation, eventually one of the sides will reverse cold or hot, ending the thermal gradient and the processes based on it. When you are sending the energy forth and back continuously, what you get is accelerate the end.

Perpetual movement is not viable... so far  ;).
« Last Edit: 07/12/2011 12:38:25 by Escorpiuser »
 

Offline Escorpiuser

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Can heat be converted into electricity?
« Reply #11 on: 07/12/2011 12:33:11 »
Going to the point, Stirling motors seems to be the most efficient mechanical devices to convert heat into movement. After that, you have to attach an efficient electric generator to convert the movement into electricity. Unfortunately, Stirling motors don't produce many HP.
 

Offline peppercorn

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Can heat be converted into electricity?
« Reply #12 on: 07/12/2011 13:26:25 »
Stirling motors seem to be the most efficient mechanical devices to convert heat into movement. ... Unfortunately, Stirling motors don't produce many HP.
Isn't that an oxymoron?


More correctly, Stirling engines have a relatively low power density when compared with more practical (and therefore common) heat engines.

Further, multi- or combined- cycle systems will outperform a Stirling for almost every application in terms of efficiency and kW/Kg.


And, as far as a self-powered vehicle is concerned, power density (both kW/Kg and kW/L) starts to majorly trump outright efficiency in terms of realising a real-world full-cycle efficiency.     ie. you've got to carry the engine around!
« Last Edit: 07/12/2011 13:31:26 by peppercorn »
 

Offline Escorpiuser

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« Reply #13 on: 08/12/2011 23:26:52 »
(...)
More correctly, Stirling engines have a relatively low power density when compared with more practical (and therefore common) heat engines.
(...)

Thank you for the correction. My explanation wasn't accurate enough.

I just wanted to contribute to the thread because I was surprised that nobody mentioned the Stirling motor.

On the other hand, a Stirling motor could be practical if you want to produce some watts out of a wood stove as in, let's say, a mountain refuge.

Also, I have read (don't remember where) that a big company is developing a Stirling motor for, I believe, more ecological vehicles. But sorry, I can't show the link to the news right now.
« Last Edit: 08/12/2011 23:31:00 by Escorpiuser »
 

Offline peppercorn

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« Reply #14 on: 12/12/2011 15:04:53 »
Escorpiuser

If memory serves me well, one of the U.S. 'big-three' played with a Stirling-powered vehicle following the '70s Oil embargo.
It was of some success in terms of efficiency gains for the time I have read, but remained woefully underpowered and was dropped.

After I replied I did suspect that you were well aware of Stirling's limitations, so I apologise if the clarification was unnecessary (hopefully useful to other readers perhaps).   IMO Stirling's will never overcome their low kW/Kg in any configuration (hybrid or whatever) to work on a road vehicle - even a large truck.

.... anyone care to prove me wrong? :)
 

Offline Geezer

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Can heat be converted into electricity?
« Reply #15 on: 12/12/2011 20:00:28 »
.... anyone care to prove me wrong? :)

If you get stuck into that tool I told you about you prove youreself right [^]
 

Offline Escorpiuser

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Can heat be converted into electricity?
« Reply #16 on: 13/12/2011 01:04:20 »
(...)IMO Stirling's will never overcome their low kW/Kg in any configuration (hybrid or whatever) to work on a road vehicle - even a large truck.

.... anyone care to prove me wrong? :)

Probably you're right BUT...
...materials are getting better everyday. I'm thinking in materials with good strength/weigh ratio like carbon fiber, nanoparticles and so on. No one can predict what the future will bring us, except if there is some theoretical demonstration that something can not be overrun, as light speed in vacuum, etc.

When was asked about mankind ability to fly sometime, "Heavier than air flying machines are impossible" answered Lord Kelvin. Well, he was a great scientist, but it seems clear that there he put his leg on it. Also:
"Aerial flight is one of that class of problems with which man will never be able to cope". Simon Newcomb, American astronomer (1903).

(There are many of this kind...).
 

Offline Geezer

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Can heat be converted into electricity?
« Reply #17 on: 13/12/2011 03:55:41 »
One of the apparent advantages of the Stirling cycle is its very high thermal efficiency. However, that is based on the assumption that the expansion and compressions processes are isothermal. I was looking at the Wiki page on the subject and it seems the expansion and contraction are a lot more adiabatic (isentropic) than the typical model portrays.

If that's true, the thermal efficiency won't be much different from any other heat engine, so where's the advantage?
 

Offline peppercorn

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« Reply #18 on: 15/12/2011 15:47:14 »
If you get stuck into that tool I told you about you prove youreself right [^]
I assume you're talking about that thermodynamics s/w tool - think I let the registration lapse on that sadly.

No one can predict what the future will bring us, except if there is some theoretical demonstration that something can not be overrun, as light speed in vacuum, etc.
... funny you should mention that - the speed of light thing - considering the oddities at CERN at the moment.
Seriously though, although I take onboard what you are saying, I think there is somewhat of a difference between the many and varied examples of the type of famous (or infamous) quotes of great minds that you are alluding to and the limits here - for a device like a Stirling engine - That is there are actual unavoidable upper limits to a Stirling's compactness w.r.t. HP available.  So that's not to say that it can't be improved on, but, for instance, the fact that it is (for one thing) an External combustion engine almost certainly makes it by nature, well, Big!


One of the apparent advantages of the Stirling cycle is its very high thermal efficiency. However, that is based on the assumption that the expansion and compressions processes are isothermal. I was looking at the Wiki page on the subject and it seems the expansion and contraction are a lot more adiabatic (isentropic) than the typical model portrays.
Surely there will always be a realistic limit simply as we are forced to accept a finite heat transfer index from the heat reservoirs (between hot and cold and the regenerator).
Or is there a more fundamental oversight that is also in play here?
« Last Edit: 15/12/2011 15:58:14 by peppercorn »
 

Offline CliffordK

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Can heat be converted into electricity?
« Reply #19 on: 15/12/2011 19:22:27 »
As far as performance maximums.

There was an earlier discussion that a resistance heater is essentially 100% efficient.

According to Wikipedia,, they introduce a coefficient of performance indicating that if a resistance heater has a COP of 1, a Heat Pump has a COP of 3 to 5, or about a 300% to 500% efficiency rating which surely isn't too bad.  Hopefully nobody has said that one can't extract more than 100% thermal energy from electricity.  Obviously it is gaining thermal energy from ambient temperatures...  but....

Even with the relatively low efficiency ratings of many generators, I'm surprised that nobody is combining an air/air heat pump with a very efficient thermal-electric generator.  Or, perhaps using air as a heat source in the summer, and water/earth as a heat source in the winter.

Perhaps the future will bring a thermal-electric generator coupled with a heat-pump as the perfect method to generate power while combating global warming, perhaps even combating the urban heat islands.  Overall efficiency may be low, but if input is ZERO, it couldn't be bad.
 

Offline Geezer

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Can heat be converted into electricity?
« Reply #20 on: 15/12/2011 20:29:36 »
Even with the relatively low efficiency ratings of many generators, I'm surprised that nobody is combining an air/air heat pump with a very efficient thermal-electric generator.  Or, perhaps using air as a heat source in the summer, and water/earth as a heat source in the winter.

You will notice I "proposed" something like that earlier in this thread.

It won't work. The thermal efficiency is limited (per Carnot) by the temperature difference between the hot and cold sinks. If you use air/water or something similar, the difference (in absolute temperature terms) is very small, so there is almost no "headroom" to cover the losses in the mechanism.

If you try to use a heatpump to boost the temperature difference, the heatpump will consume more power than the heat engine can produce, so that's not going to help.

You can model any ideas you have using tools you will find here  http://www.thermofluids.net/
 

Offline yor_on

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Re: Can heat be converted into electricity?
« Reply #21 on: 18/12/2011 17:18:53 »
These guys seem to think it can http://wattsupwiththat.com/2011/06/22/new-multiferroic-alloy-creates-electricity-from-waste-heat/

That will be very cool with laptops.
=

Or maybe not, 'strongly magnetic' may foil its benefits?
« Last Edit: 18/12/2011 17:30:02 by yor_on »
 

Offline CliffordK

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Re: Can heat be converted into electricity?
« Reply #22 on: 19/12/2011 02:21:37 »
Even with the relatively low efficiency ratings of many generators, I'm surprised that nobody is combining an air/air heat pump with a very efficient thermal-electric generator.  Or, perhaps using air as a heat source in the summer, and water/earth as a heat source in the winter.

You will notice I "proposed" something like that earlier in this thread.

It won't work. The thermal efficiency is limited (per Carnot) by the temperature difference between the hot and cold sinks. If you use air/water or something similar, the difference (in absolute temperature terms) is very small, so there is almost no "headroom" to cover the losses in the mechanism.

I'm still trying to wrap my head around this one. 

Earlier, I think you said that an electric resistance heater is 100% efficient.
But, a heatpump is "better"...  so in effect it should be better than 100%.

The classic Heat Pump isn't amplifying a temperature difference like the Stirling Engine.  Rather, it's heating and cooling coils are independent.  In fact, one wants the heating or cooling coils to be as close to the desired temperature as possible. 

Anyway, if one was to use an ideal gas to heat another ideal gas...
PV=nRT.

Then one ends up with a dog chasing it's tail.  Transferring energy from one state to another.

Perhaps the trick is that by using liquid/gas transitions, one can improve over the energy efficiency that would otherwise be predicted by the ideal gas law.

So...  say you have two gases.

Gas A & Gas B.
Applying 60 PSI (2 ATM)
If you increase the energy density of Gas A by 10 fold (vapor pressure/liquifying), and Gas B by 3 fold (ideal gas law)

Then you have suddenly created an energy difference between the two states.  In fact, we use this gas/liquid phase change a lot.  Every steam engine (including those in modern Nuclear Plants) utilizes it.

So, choosing the right gas/liquid phase change, one should be able to amplify this potential energy change, using a minimum energy input.

Hmmm.
Ok, so this is why at moderate temperatures/pressures, Freon is much more efficient than Nitrogen (or Helium) for refrigeration.  Both would technically work, but the Freon is much more compressible by pushing it through the phase change.

One would need to efficiently further amplify the energy, perhaps by using multiple stages.  Or, find a better way to generate electricity from the captured energy.

-------------------------------------------------------------------------------------------------------

Ok...  how about this hypothetical engine.

Take Gaseous Freon, say at 20°C.
Pressurize it to liquify it.  Say raise the temperature to above the BP of Methanol (65°C).  Say up to the temperature of 80°C.

At this point, your methanol should boil, and could be used to generate power.  Until your freon drops down to close to the BP of Methanol. (65°C).

Now, you can use the ambient temperature (20°C) to cool the Methanol back to the liquid phase (FREE ENERGY)

Your Freon, now at 65°C should still boil.  And, could be used to generate pressure/power too.  It will be a slightly lower pressure than when it was at 80°C. 

When the Freon boils, it will drop down to... say 0°C.  Depending on your gas, it could be a lot lower, but it really all depends on your pressures & potentially the starting temperature (which we said was 65°C).

You use the ambient temperature to raise the temperature of the Freon gas back up to the ambient temperature (20°C), (FREE ENERGY) and repeat the process.

So...  In this system, you have:
Energy Gain (Boiling Methanol)
Energy Loss (loss of Vapor Pressure of Freon with drop in temperature from 80°C to 65°C).

So, the question is whether this hypothetical energy loss and gain would be equal, or if it would be different.

One is both heating and cooling with ambient temperatures...  but, presumably one could have a net change, or cooling of the ambient temperature which would reflect the energy captured. 

Your "Energy" is that, say, 20°C is equal to 293°K, which is an energetic state.

Obviously you would have to take a lot of care to make the entire system as efficient as possible.

If you could have 90+% energy conversion, my guess is that you could design such a system that would actually give you a net energy gain.  However, with the typical less than 50% energy conversion, it would be a lot more difficult to do.

I suppose I will have to get some real numbers with actual temperatures and vapor pressures to prove that. 

If one looks at this vapor pressure chart from Wikipedia.


This is on a logarithmic scale.
But, each substance also has a slightly different curve.

Maybe I'm thinking about this all backwards.

Looking at this chart...
Say from 10°C to 40°C
The Vapor Pressure of Methyl Chloride increases from 3.5 ATM to 8.5 ATM (5 ATM difference).
The Vapor Pressure of Diethyl Ether increases from 0.4 ATM to 1.5 ATM (about 1ATM difference).

I think you could utilize that difference, but you would probably use low pressure and substances with higher boiling points for the refrigerant.   
 

Offline Geezer

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Re: Can heat be converted into electricity?
« Reply #23 on: 19/12/2011 03:48:44 »
Earlier, I think you said that an electric resistance heater is 100% efficient.

Yes - in terms of the heat it produces, it is 100% efficient in converting electrical energy into thermal energy.

Quote
But, a heatpump is "better"...  so in effect it should be better than 100%.

Yes - it's better because the electrical energy is converted into work by a motor and the work done by the motor removes thermal energy from one source of heat and delivers it to sink at a higher temperature. The important bit here is that work was done. The motor is not 100% efficient, but it can still transfer a lot of heat by doing work. Essentially the heat pump is "squeezing" the heat out of some ambient heat source. (The heat generated because of the inefficiency of the motor should end up in the heat supplied, so none of the electrical energy goes to waste in the heating case.)

The heat pump only works because there is a source of thermal energy. If the house was surrounded by space at 0K (zero kelvin) the heat pump could not supply heat, but the resistor could.

"Efficiencies" are very trixy things. Unless the conditions are very carefully defined it's possible to make some remarkable claims.

In this case, as resistive heating is 100% efficient (and it really is) heat pump heating systems can be much more than 100% efficient, which is obviously ridiculous!

The reality is that a resistor is 100% efficient at producing heat from electricity, but it is 0% efficient at pumping heat using electricity.

(I'll try to respond to the rest of your post as time allows.)
 

Offline CliffordK

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Re: Can heat be converted into electricity?
« Reply #24 on: 19/12/2011 06:08:45 »
You can calculate the efficiency of an electric motor and electric generator quite easily.

Connect the motor to the generator.
Measure the power in, and the power out, and calculate the efficiency.

One can do something similar with a solar-electric panel.
Use electricity to produce light to shine on a solar panel, and calculate the output.
However, the efficiency of the solar panels is generally quite low.

Unfortunately, one doesn't have a direct generator from a heat source, although that was what started this topic.

However...
One has basic unit conversions...
And can calculate...  1 calorie of energy can heat up 1cc of water 1°C.  Or, whatever your favorite units are.  Anyway, 4.18 Watt Seconds = 1 calorie, heats 1cc of water 1°C.

So, you feed into your resistance heater, 4.18 Watt Seconds, and it should raise 1cc of water 1°C.

If you could recover the energy back from that resistance heater, you should be able to recover 4.18 Watt Seconds for that 1cc of water, 1°C.

Now, if you feed it into the heat pump, and that same 4.18 Watt Seconds raises 1cc of water 5°C, or 5cc of water 1°C.  Then you have a problem.

And, you should be able to put that 4.18 Watt Seconds of power in, and recover 20.9 Watt Seconds back out.

Obviously, you are recovering energy from a secondary source, the sun through its heating of your heat exchange medium, the air, water, whatever.

Unfortunately we are still stuck if our conversion efficiency from heat to electricity is still less than 50%.  But, even at 50%, we might be able to get somewhat of a net gain.
 

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Re: Can heat be converted into electricity?
« Reply #24 on: 19/12/2011 06:08:45 »

 

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