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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.
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.
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? 
Stirling motors seem to be the most efficient mechanical devices to convert heat into movement. ... Unfortunately, Stirling motors don't produce many HP.
(...)More correctly, Stirling engines have a relatively low power density when compared with more practical (and therefore common) heat engines.(...)
.... anyone care to prove me wrong? 
(...)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? 
If you get stuck into that tool I told you about you prove youreself right [^]
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.
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.
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.
These guys seem to think it can http://wattsupwiththat.com/2011/06/22/new-multiferroic-alloy-creates-electricity-from-waste-heat/
Quote from: CliffordK on 15/12/2011 19:22:27Even 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.
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%.
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.
Anyway, do you agree with my explanation of the difference between a resistor and a heat pump?
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.
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°CThe 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.
Very old news clippingCannot find anything else on the web
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..