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I believe high efficiency conversion of small temperature differences to energy would be a breakthrough technology. Geothermal power is the first that springs t

I believe high efficiency conversion of small temperature differences to energy would be a breakthrough technology. Geothermal power is the first that springs to mind.

adastra22 4y ago
There are fundamental limits there which can’t be avoided, alas.
doctor_eval 4y ago
Am I correct in understanding that if you could use a heat pump to convert electricity to heat at 300% efficiency, and then convert that heat back to electricity at 50% efficiency, you’d basically get “free” energy by cooling the air? Is this even feasible?
- coderenegade 4y ago
  Don't you need an external source of heat for a heat pump? They move heat from one place to another, rather than generate heat directly, so all you would be doing is moving existing heat, and then converting that into electricity, but we can already do that without the heat pump.
  - ben-schaaf 4y ago
    They're suggesting using the heat from the air as the power source.
    - coderenegade 4y ago
      I get that, but you need fairly high grade heat to generate power. The idea would be to move enough heat from lower grade sources to a reservoir that is then hot enough to generate more power than is required to move the heat in the first place. It's effectively harvesting the ambient air temperature. I don't know that you could pull in that much heat, and I suspect that an analysis of the system might show that it can't yield useful work.
- andrewxdiamond 4y ago
  Heat pumps move heat from one place to another. They pump the heat. That is how they achieve 300% returns, they move 300% of the heat energy that they consume to operate. But that heat isn’t free. It has to be produced somewhere. You must “consume” the heat to produce electricity. If you tried to generate power in the way you postulated, you would ignore the 300% number and instead look at the conversion rate between the energy it takes to produce that heat and the energy output.
  - XorNot 4y ago
    If you're pulling heat from the air though, you're just absorbing solar power efficiently. And this wouldn't be a bad way to do it if you could make it work: at any given time, there's always sun shining somewhere on Earth, just not where we need it. A heat pump which could extract enough of a differential to power itself using a turbine, isn't impossible. The real issue is such a thing at scale would functionally be a storm generator: you'd be pulling warmer air in the upper troposphere down to ground level, chilling it, and then releasing it.
  - chrismorgan 4y ago
    To frame this another way since I missed part of the point on first read: the heat pump has the same maximum theoretical efficiency (Carnot efficiency) as the heat-to-electricity generator does (just in the opposite direction), and cannot exceed it. If you model the two parts independently, then in the ideal scenario, your 300% efficient component is balanced by something that physically cannot exceed 33⅓% × some-other-variable efficiency. You therefore model the system as just a heat-to-electricity generator that consumes the atmospheric heat directly at some-other-variable efficiency. (I believe this to be accurate, but I am speaking far beyond my expertise and invite correction if I have erred.)
- dtgriscom 4y ago
  Rule of thumb: if your idea creates energy from nothing, it won't work.
  - doctor_eval 4y ago
    No it doesn’t. It takes the energy out of the environment. The result would be more energy, and cold air.
- ncmncm 4y ago
  That would be a perpetual motion machine, so no. Separating room-temperature air into hot and cold fractions removes entropy from that air, and that needs more energy input than can be extracted by a heat engine working from that temperature difference.
  - deleted 4y ago
    [deleted]
- colechristensen 4y ago
  > Is this even feasible? No. A heat pump moves heat from one pool to another and adds whatever input energy required to move the heat to the whole system. When you have two pools at different temperatures you can harvest energy from that difference but only as much as is there because harvesting the energy involves bringing the two pools closer in temperature. There is nothing you can do to cheat this, whatever you do will bring the two pools closer together. If you bring in outside energy you will always be able to harvest less than you brought in.
- deleted 4y ago
  [deleted]
- _carbyau_ 4y ago
  Lots of comments here jumping at impossible/perpetual motion machine etc Think of it as mining the air/water for heat energy and turning it to electricity. No one said energy is being made from nothing. This is not being touted as a solution to climate change. To rephrase the idea, is the following equation possible: heat pump running costs <= heat extracted x electricity generation efficiency At a single human scale there is near infinite air/water heat energy to run through the device. But could you do it efficiently enough? This is the closest thing I could find on short notice: https://www.pv-magazine.com/2021/08/19/the-worlds-hottest-heat-pump/ https://www.pv-magazine.com/2021/08/19/the-worlds-hottest-he...
- Schroedingersat 4y ago
  This is the problem with characterising heat pumps as '300%' efficient. They're not, they're actually around 30-60% efficient like most other heat engines. It's just the thing they do is move heat so people characterise the 'output'/ work as efficiency when in reality the relevant factor is something like output / output_of_reverse_carnot_cycle. A 100% efficient heat pump would produce exactly as much electricity putting the heat back where it started as it used in the first place.
jbay808 4y ago
The limit is the Carnot efficiency 1 - (Tc/Th). As the temperature difference gets smaller, the maximum efficiency approaches 0. Although, the closer that you can get to that ideal, the better!
aaaaaaaaaaab 4y ago
Thermodynamical impossibility.
sandworm101 4y ago
Geothermal doesn't really convert small deltas into power. It takes a small delta, then amplifies it using compression/expansion to alter the temperatures into a usable delta. So it is subject to all sorts of inefficiencies as it compresses/expands/moves the medium around. The real breakthrough would be a machine that can harvest power without a temperature gradient/delta, something that could take the vibration of molecules and convert it directly to electricity. Something like this... https://www.youtube.com/watch?v=KiLTEjm8zLw https://www.youtube.com/watch?v=KiLTEjm8zLw https://phys.org/news/2020-10-physicists-circuit-limitless-power-graphene.html https://phys.org/news/2020-10-physicists-circuit-limitless-p... "The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman's well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Thibado's team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible."
- Schroedingersat 4y ago
  Without overwhelming evidence there's no reason to think this isn't just another mirage like emdrive. The second law has stood up to everyone everywhere who ever made a machine being on the lookout for a single counterexample starting from well before we had science.
- yccs27 4y ago
  > "The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman's well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Thibado's team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible." The crucial question here is: Can this alternating current do work?

I believe high efficiency conversion of small temperature differences to energy would be a breakthrough technology. Geothermal power is the first that springs to mind.

adastra22 4y ago

There are fundamental limits there which can’t be avoided, alas.

doctor_eval 4y ago

Am I correct in understanding that if you could use a heat pump to convert electricity to heat at 300% efficiency, and then convert that heat back to electricity at 50% efficiency, you’d basically get “free” energy by cooling the air? Is this even feasible?

coderenegade 4y ago
Don't you need an external source of heat for a heat pump? They move heat from one place to another, rather than generate heat directly, so all you would be doing is moving existing heat, and then converting that into electricity, but we can already do that without the heat pump.
- ben-schaaf 4y ago
  They're suggesting using the heat from the air as the power source.
  - coderenegade 4y ago
    I get that, but you need fairly high grade heat to generate power. The idea would be to move enough heat from lower grade sources to a reservoir that is then hot enough to generate more power than is required to move the heat in the first place. It's effectively harvesting the ambient air temperature. I don't know that you could pull in that much heat, and I suspect that an analysis of the system might show that it can't yield useful work.
andrewxdiamond 4y ago
Heat pumps move heat from one place to another. They pump the heat. That is how they achieve 300% returns, they move 300% of the heat energy that they consume to operate. But that heat isn’t free. It has to be produced somewhere. You must “consume” the heat to produce electricity. If you tried to generate power in the way you postulated, you would ignore the 300% number and instead look at the conversion rate between the energy it takes to produce that heat and the energy output.
- XorNot 4y ago
  If you're pulling heat from the air though, you're just absorbing solar power efficiently. And this wouldn't be a bad way to do it if you could make it work: at any given time, there's always sun shining somewhere on Earth, just not where we need it. A heat pump which could extract enough of a differential to power itself using a turbine, isn't impossible. The real issue is such a thing at scale would functionally be a storm generator: you'd be pulling warmer air in the upper troposphere down to ground level, chilling it, and then releasing it.
- chrismorgan 4y ago
  To frame this another way since I missed part of the point on first read: the heat pump has the same maximum theoretical efficiency (Carnot efficiency) as the heat-to-electricity generator does (just in the opposite direction), and cannot exceed it. If you model the two parts independently, then in the ideal scenario, your 300% efficient component is balanced by something that physically cannot exceed 33⅓% × some-other-variable efficiency. You therefore model the system as just a heat-to-electricity generator that consumes the atmospheric heat directly at some-other-variable efficiency. (I believe this to be accurate, but I am speaking far beyond my expertise and invite correction if I have erred.)
dtgriscom 4y ago
Rule of thumb: if your idea creates energy from nothing, it won't work.
- doctor_eval 4y ago
  No it doesn’t. It takes the energy out of the environment. The result would be more energy, and cold air.
ncmncm 4y ago
That would be a perpetual motion machine, so no. Separating room-temperature air into hot and cold fractions removes entropy from that air, and that needs more energy input than can be extracted by a heat engine working from that temperature difference.
- deleted 4y ago
  [deleted]
colechristensen 4y ago
> Is this even feasible? No. A heat pump moves heat from one pool to another and adds whatever input energy required to move the heat to the whole system. When you have two pools at different temperatures you can harvest energy from that difference but only as much as is there because harvesting the energy involves bringing the two pools closer in temperature. There is nothing you can do to cheat this, whatever you do will bring the two pools closer together. If you bring in outside energy you will always be able to harvest less than you brought in.
deleted 4y ago
[deleted]
_carbyau_ 4y ago
Lots of comments here jumping at impossible/perpetual motion machine etc Think of it as mining the air/water for heat energy and turning it to electricity. No one said energy is being made from nothing. This is not being touted as a solution to climate change. To rephrase the idea, is the following equation possible: heat pump running costs <= heat extracted x electricity generation efficiency At a single human scale there is near infinite air/water heat energy to run through the device. But could you do it efficiently enough? This is the closest thing I could find on short notice: https://www.pv-magazine.com/2021/08/19/the-worlds-hottest-heat-pump/ https://www.pv-magazine.com/2021/08/19/the-worlds-hottest-he...
Schroedingersat 4y ago
This is the problem with characterising heat pumps as '300%' efficient. They're not, they're actually around 30-60% efficient like most other heat engines. It's just the thing they do is move heat so people characterise the 'output'/ work as efficiency when in reality the relevant factor is something like output / output_of_reverse_carnot_cycle. A 100% efficient heat pump would produce exactly as much electricity putting the heat back where it started as it used in the first place.

jbay808 4y ago

The limit is the Carnot efficiency 1 - (Tc/Th). As the temperature difference gets smaller, the maximum efficiency approaches 0. Although, the closer that you can get to that ideal, the better!

aaaaaaaaaaab 4y ago

Thermodynamical impossibility.

sandworm101 4y ago

Geothermal doesn't really convert small deltas into power. It takes a small delta, then amplifies it using compression/expansion to alter the temperatures into a usable delta. So it is subject to all sorts of inefficiencies as it compresses/expands/moves the medium around. The real breakthrough would be a machine that can harvest power without a temperature gradient/delta, something that could take the vibration of molecules and convert it directly to electricity. Something like this... https://www.youtube.com/watch?v=KiLTEjm8zLw https://www.youtube.com/watch?v=KiLTEjm8zLw https://phys.org/news/2020-10-physicists-circuit-limitless-power-graphene.html https://phys.org/news/2020-10-physicists-circuit-limitless-p... "The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman's well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Thibado's team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible."

Schroedingersat 4y ago
Without overwhelming evidence there's no reason to think this isn't just another mirage like emdrive. The second law has stood up to everyone everywhere who ever made a machine being on the lookout for a single counterexample starting from well before we had science.
yccs27 4y ago
> "The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman's well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Thibado's team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible." The crucial question here is: Can this alternating current do work?