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Author Topic: Can heat be converted into electricity?  (Read 19113 times)

Geezer

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Re: Can heat be converted into electricity?
« Reply #25 on: 19/12/2011 08:17:56 »
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.

I'm not sure about the idea that conversion of heat to electricity has to be less than 50%. That is not dictated by Thermodynamics.

Isothermal conversion of heat to work is reversible, and adiabatic energy conservation is common. If it was not, the dampers on your car's suspension would not work very well.

Anyway, do you agree with my explanation of the difference between a resistor and a heat pump?

CliffordK

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Re: Can heat be converted into electricity?
« Reply #26 on: 19/12/2011 09:13:06 »
Anyway, do you agree with my explanation of the difference between a resistor and a heat pump?

We agree on how a heatpump operates.

Theoretically, one could have super-efficient energy conversions, solar, heat, or chemical to electricity.  There are some practical problems in the implementation which leads to significant differences between the calculated energy input and the actual energy output of the various systems.

Geezer

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Re: Can heat be converted into electricity?
« Reply #27 on: 19/12/2011 18:42:36 »
Theoretically, one could have super-efficient energy conversions, solar, heat, or chemical to electricity.  There are some practical problems in the implementation which leads to significant differences between the calculated energy input and the actual energy output of the various systems.

Theoretically you can't.   :D  These are not practical problems. Physics dictates the limits.

A heat engine (and a heat pump is just a heat engine) operates on the basis of moving thermal energy from one place to another place to do work or, in the case of the pump, doing work to move thermal energy from one place to another place.

Unfortunately, heat only travels "downhill" (from a hot thing to a cooler thing) and every time that happens, the Universe applies an inescapable tax in the form of entropy. So, even if a heat pump was perfect (no losses at all), it could never get back all the work put in when operating in reverse.

The only way to avoid the "tax" is to keep everything at the same temperature, but heat engines only operate because of temperature difference. It's just not fair!

Geezer

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Re: Can heat be converted into electricity?
« Reply #28 on: 20/12/2011 02:01:11 »
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.

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

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.

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.

I did take another look. I think you should try that tool I mentioned. It will let you model any of these configurations and let you know how much work each will do.

However, a couple of points.

There is no particular thermal benefit in using a phase change fluid in a heat engine to do work. The phase change takes in a large amount of heat without any great benefit because you have to give the heat up again to get the fluid to condense. You can skip the liquid/gas phase changes and heat the gas instead.

However, you also have to repressurise the fluid to get it to do work, but compressing a gas consumes a large amount of work. If you condense the fluid back into a liquid it takes hardly any work to compress it to the desired working pressure of the expansion device.

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Re: Can heat be converted into electricity?
« Reply #29 on: 21/12/2011 12:56:30 »
Heat from hot springs can definitely be turned to electricity.

ns8t

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Re: Can heat be converted into electricity?
« Reply #30 on: 18/01/2012 23:44:53 »
this week i purchased a mercury free thermostat for forced air furnace.  Inside is a coiled very thin ribbon of metal, i think two seperate metals fused together, so that the tension created from different thermal expansion and contraction creates movement that triggers a microswitch.  I would think that thousands of these ribbons of metal could turn a dynamo when temperature increased or decreased even a few degrees.

CliffordK

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Re: Can heat be converted into electricity?
« Reply #31 on: 19/01/2012 00:10:20 »
A mercury thermostat works in essentially the same fashion, except that it uses Mercury for the switch.

Thermal Expansion/Contraction could be very powerful.  It doesn't require a bimetallic structure, and could be used with either liquids or metals.  However, it does require the material to absorb the Energy related to the temperature change to cause the work to be performed, with the thicker the component, the more heat to be absorbed/dissipated.

One could imagine a generator designed to cycle on a diurnal cycle.

Piezoelectric may be a similar concept.

syhprum

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Re: Can heat be converted into electricity?
« Reply #32 on: 20/01/2012 22:09:58 »
When the London Festival Hall was built we were told that it would have a heat pump system using redundent Merlin aircraft engines.
Was this bizarre system ever built and how long did it last ?.
« Last Edit: 23/01/2012 06:57:14 by syhprum »

imatfaal

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Re: Can heat be converted into electricity?
« Reply #33 on: 23/01/2012 09:39:51 »
Very old news clipping

Cannot find anything else on the web

CliffordK

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Re: Can heat be converted into electricity?
« Reply #34 on: 23/01/2012 11:03:24 »
Very old news clipping
Cannot find anything else on the web
Interesting.
So, they used Natural Gas to run engines to run the heat pump.
And used the exhaust from the NG engines for heating.

So, say one wished to use Natural Gas to heat their home.  Rather than just burning the gas, one can burn the gas to power an engine to do work...  AND use the heat generated from burning the Natural Gas.

I know that many furnaces in the Midwest are already connected to an AC system.  It probably wouldn't take much to merge the two technologies, and perhaps double the efficiency of the system.

I suppose the biggest problem is finding a good source of "exchange heat" when it is 5°F (-13°C) outside.

And, of course, NOISE as well as additional system complexity.  Ok, so perhaps this is best kept to larger commercial systems.

Ahhh...
I thought I'd look to see what our local university is doing with their steam plant.
Apparently they are upgrading to add turbines to generate electricity with the energy before heating water.

Of course, I guess London was doing that a half century ago.

syhprum

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Re: Can heat be converted into electricity?
« Reply #35 on: 23/01/2012 16:41:06 »
Many thanks for finding that I wondered if my memory was playing tricks, I don't think natural gas was "invented" in 1951 they would have used the rather lethal stuff made from coal.
I don't think the system lasted long the Merlins made much to much noise to be used in a concert hall installation and even much detuned they would have needed a great deal off maintenance.

CliffordK

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Re: Can heat be converted into electricity?
« Reply #36 on: 23/01/2012 20:24:03 »
Many thanks for finding that I wondered if my memory was playing tricks, I don't think natural gas was "invented" in 1951 they would have used the rather lethal stuff made from coal..
Interesting.  There is a good article on Wikipedia about Coal Gas.

The use of Natural Gas in the USA began in the 40's and 50's, but came later in the UK, with the changeover occurring in the UK from 1959 to 1987.

Apparently the composition of the coal gas varied somewhat, but was primarily:
hydrogen 50%
methane 35%
carbon monoxide 10%
ethylene 5%

With the "lethal" part being carbon monoxide, which would be burnt to a large extent in the stoves, and should be burnt, or go up the chimney in properly vented furnaces.

I've read about carbon monoxide poisoning in the past in the UK.  Apparently a disturbing large portion are actually caused by electric appliances causing slow smoldering fires.
« Last Edit: 23/01/2012 20:26:40 by CliffordK »

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Re: Can heat be converted into electricity?
« Reply #36 on: 23/01/2012 20:24:03 »