Can conversion from heat to electricity be made more practical/efficient?

[sweeteviljesus]

Thermocouples and Peltier-effect devices are grossly inefficient and aren’t good for much besides sensors, etc. Is there a technology that could make transferring energy directly from heat to electricity viable for power applications?

Thanks,
Rob

[Iggy]

Not sure it’s what you’re thinking of but Ocean Thermal Energy Conversion or Geothermal electricity production both tap into pre-existing thermal differences to generate electricity.

[Michael63129]

Most electricity is in fact made from heat, just not directly (with a thermoelectric generator); heat engine theory (which also applies to TEGs) states that the maximum efficiency is limited by the temperature differential, so if you want to increase efficiency, increase the temperature:

Like all heat engines, the maximum power-generation efficiency of a thermoelectric generator is thermodynamically limited by the Carnot efficiency (DeltaT/Th). If temperature is assumed to be independent and n-type and p-type thermoelectric properties are matched (alpha, sigma and kappa), (an unrealistic approximation in many cases) the maximum device efficiency is given by the above equation with Z = z.

One problem with TEGs is that they are semiconductor-based and semiconductors can’t handle high temperatures, which requires metal-based thermocouples, but these don’t appar to produce much voltage (41 microvolts per Kelvin for copper-constantan), which requires many more junctions in series, leading to a high output resistance (e.g if you want 12 volts and have a 1,000K difference, you need about 300 junctions in series, which will have an output resistance in the order of 10,000 ohms, or about 1.2 mA of short-circuit current, or 0.6 mA at 12 volts for 600 junctions (and twice the resistance), with an efficiency of 50%, not including the actual conversion efficiency).

[engineer_comp_geek]

You may find this interesting (MIT article on thermal diodes, from 2009):

Theory says that such energy conversion can never exceed a specific value called the Carnot Limit, based on a 19th-century formula for determining the maximum efficiency that any device can achieve in converting heat into work. But current commercial thermoelectric devices only achieve about one-tenth of that limit, Hagelstein says. In experiments involving a different new technology, thermal diodes, Hagelstein worked with Yan Kucherov, now a consultant for the Naval Research Laboratory, and coworkers to demonstrate efficiency as high as 40 percent of the Carnot Limit. Moreover, the calculations show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.

Rest of article here:

Turning heat to electricity

[Michael63129]

@Iggy

Not sure it’s what you’re thinking of but Ocean Thermal Energy Conversion or Geothermal electricity production both tap into pre-existing thermal differences to generate electricity.

Note that neither of these actually convert directly from heat, which is what the OP was asking, for the reasons I stated in my last post (efficiency and especially practicality of a large-scale TEG based generating plant); much easier to use a turbine or similar to make electricity indirectly. Although TEGs, aka radioisotope thermoelectric generators, have been used in applications like space probes, and probably other applications where a (usually) small amount of power is needed and a heat source is available.

[supery00n]

@Michael63129

Note that neither of these actually convert directly from heat, which is what the OP was asking, for the reasons I stated in my last post (efficiency and especially practicality of a large-scale TEG based generating plant); much easier to use a turbine or similar to make electricity indirectly. Although TEGs, aka radioisotope thermoelectric generators, have been used in applications like space probes, and probably other applications where a (usually) small amount of power is needed and a heat source is available.

I’m not very informed on what “direct” conversion of heat into electricity means. Why wouldn’t geothermal electricity be a direct conversion?

[beowulff]

@supery00n

I’m not very informed on what “direct” conversion of heat into electricity means. Why wouldn’t geothermal electricity be a direct conversion?

Direct conversion is just that: Heat->Electricity (usually using metallic junctions).
All other conversions are indirect: Heat->Working fluid->Mechanical motion->Generator.