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Would this require a temperature gradient to function? My understanding is that a device that converts heat into power would violate thermodynamics.
by cowthulhu 2y ago
Would this require a temperature gradient to function? My understanding is that a device that converts heat into power would violate thermodynamics.
- BuildTheRobots 2y ago> My understanding is that a device that converts heat into power would violate thermodynamics. I'm probably missing the point entirely, but aren't we doing this already with Thermocouples as used in RTGs?
- abdullahkhalids 2y agoThere are a number of different effects that turn heat into electric power. The review article I linked in the opening comment discusses all of them.
- abdullahkhalids 2y agoYes indeed. There is quite a lot of discussion on the wiki page about what constraints that laws of thermodynamics place on such a device.
- jjtheblunt 2y ago> My understanding is that a device that converts heat into power would violate thermodynamics. isn't a coal burning steam engine converting heat into power? perhaps the violation applies if requiring efficiency in the conversion?
- Qwertious 2y agoSteam engines operate off the temperature gradient - the water (cool stuff) is turned into steam (hot stuff), which expands 16x and creates a huge pressure spike that pushes the piston/turbine.
- urban_winter 2y ago> steam (hot stuff), which expands 16x 1600 times. Gas at room temp occupies 24 dm3/mole.
- semi-extrinsic 2y agoLet's be precise. When we say "convert heat to power", what we mean is actually "if we have some energy flowing from a heat source to a heat sink, we can extract a percentage of that energy as useful work, like turning a steam turbine to produce electricity". Carnot proved in 1824 that the maximum theoretical efficiency is purely a function of the hot and cold temperature (in Kelvin): Eff = (T_hot - T_cold)/T_hot So for instance a thermoelectric generator operating between 25 °C and 5 °C has a maximum efficiency of Eff = (298 - 278) / 298 = 6.7%
- someplaceguy 2y ago> So for instance a thermoelectric generator operating between 25 °C and 5 °C has a maximum efficiency of > Eff = (298 - 278) / 298 = 6.7% Does that mean a thermoelectric generator operating at a minimum of 0K has a maximum efficiency of Eff = (N - 0) / N = 100%?
- 11101010001100 2y agoOr just (2*N-N)/N.
- someplaceguy 2y agoAccording to the formula you'd have to divide by 2*N.
- 11101010001100 2y agomy mistake!
- tullianus 2y agoYes, although in practice you will have issues sinking heat at 0K without using additional energy to do so, since that's below the radiation sink temperature of space (~2.73K).
- jbay808 2y agoA photovoltaic cell also requires a temperature gradient. In the case of a solar panel, it's the high temperature of the sun that shifts its blackbody spectrum into a range where the cell can generate electricity. But if you were to heat the PV cell to that same temperature as the surface of the sun (somehow without melting it), it would glow and radiate away just as much light as it absorbs from the sun, rather than converting any sunlight into usable electricity. Likewise the phonovoltaic would need to operate on a phonon spectrum that is shifted away from that of the material's own temperature, in order to not violate the laws of thermodynamics.
- cowthulhu 2y agoThat is fascinating, thank you!
- almostnormal 2y ago> But if you were to heat the PV cell to that same temperature as the surface of the sun [...] Much easier to replace the sun by some other source of EM-waves of suitable wave length and put it into a freezer, below the temperature of the cell. How does the wave poking some electron to a higher state remember the temperature of its source?
- jbay808 2y agoGood question. Temperature is a funny thing. When a system is far from equilibrium, the notion of temperature becomes a little unfamiliar. The thermodynamic temperature of the radiation is connected to the entropy of its power spectrum. LEDs and especially lasers emit very low-entropy light, which can be focused and heat a surface up to very high temperatures. (Sunlight focused onto a surface cannot heat that surface above the temperature of the sun). Thermodynamically speaking, a low-entropy power source like a laser -- and whatever is driving it -- must have a very high exergy, which is equivalent to behaving like a high temperature heat source, even though it might feel cold to the touch. Some more details here: https://en.wikipedia.org/wiki/Exergy#Quality_of_energy_types https://en.wikipedia.org/wiki/Exergy#Quality_of_energy_types Stored electricity's equivalence to a high-temperature heat source is one of the things that makes it so useful. It's intrinsically connected to why it takes a lot of low-grade heat to produce a small amount of electricity in the first place, and also why electric furnaces can produce such high temperatures. So while a battery can drive an LED that shines on a PV panel that generates power with everything feeling equally warm to the touch, a temperature gradient is still necessary; it's just been moved outside the boundaries of the system, to the process that distilled the entropy out of the energy that became the battery's stored charge. We can reversibly recreate this temperature gradient by driving a Carnot engine with that battery, instead of a laser.