# Can coldness be converted to heat energy?

We know that the heat can be converted into heat energy with the help of thermoelectric generators, but why can't we generate energy from coldness?

Like the temperature of the universe in 1 K, can this be used in the near future to be used as an energy resource for probes or satellites?

Here is the link to the article that made me think about this. Somewhere in the middle it is written that scientists can harness the cold energy using some active input method.

• I think you need to clarify something. When you said "generating energy from coldness" it seemed to me you were asking if it were possible to use the coldness of space as a SOURCE of thermal energy. However, the answer you accepted is based on using the coldness of space as a heat sink and not a source of thermal energy. Is that what you meant? – Bob D Nov 18 '20 at 19:29
• @BobD - yeah, I think there's an underlying issue here - cold is a lack of energy, heat is the presence of energy. So it would be equivalent to ask, "Can a lack of energy be converted to energy?" – Don Branson Nov 18 '20 at 22:11
• answers by the community members made me realized my mistake. – Anonymous Nov 19 '20 at 2:42
• You can extract useful work from a temperature gradient-- e.g. a different in temperature. Many thermodynamic processes will only care about the difference in temperature rather than absolute values (e.g. Heat Equation is invariant, still satisfied, under the addition of a constant term to temperature). Energy is by its very definition a numerical quantity which is conserved due to the time-invariance of laws of physics. It so happens that something which is 'hot' has a high amount of this quantity, and something which is 'cold' has a small amount. – Myridium Nov 19 '20 at 5:02
• @JimmyJames - I think your observation that people (somewhat naturally) think of cold and heat as opposites is an important one. It usually isn't obvious to experts what the confusion is of a layman. To leverage your understanding of the source of the OP's question, you may want to provide an answer that appeals to the fact that the feeling of 'coldness' really is indicative of useful work being available. Intuition is correct here. But what we feel is not the absolute temperature of something, but the flow of heat from our hand into it. A.k.a. temperature difference is the source of work. – Myridium Nov 22 '20 at 4:15

Strictly speaking, heat is not converted into energy - instead heat is energy. A thermoelectric generator is sometimes loosely described as turning heat into energy, but what actually happens is that a temperature difference between a heat source and a cold sink (usually the surrounding environment) causes heat/energy to flow between them and some part of this heat/energy is used to do work e.g. to generate a current which drives an electric motor etc.

If everything was at the same temperature, no matter whether this was $$1$$ K or $$1$$ million K, you could not use the heat/energy to do work because there would be no temperature difference and so no flow of heat/energy.

• Comments are not for extended discussion; this conversation has been moved to chat. – ACuriousMind Nov 20 '20 at 18:10

You need a difference in temperature between two places to generate useful energy. It is possible to use the coldness of space to generate energy, but only if you also have something warm nearby. Some satellites and space probes carry radioisotopes to be the warm thing. https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator .

• It seemed to me the OP was speaking of using the temperature of space as a source of energy from the statement "generating energy from coldness". But I guess not since your answer is using space as the cold temperature reservoir, which it can obviously be used as the heat sink. – Bob D Nov 18 '20 at 15:11
• @BobD the misconception is that heat does the work, when it's actually the heat difference. – kutschkem Nov 18 '20 at 20:03
• @kutschkem What do you mean by "heat difference". That makes no sense because there is no "difference" in heat. Its a temperature difference that causes heat to flow and that flow to result in work. – Bob D Nov 18 '20 at 20:07
• @BobD my bad, temperature difference then – kutschkem Nov 18 '20 at 20:08
• @AccidentalBismuthTransform if the other thing is colder then you're using the warmness of space, not the coldness. – user253751 Nov 19 '20 at 14:57

There's a more recent scientific study about this topic, which was then covered in articles like this and this. I also asked a similar question a few months ago.

Basically, this is called "thermoradiative photovoltaics" and involves generating energy by emitting heat (as infrared rays) to a heat sink. The proposed technology would use the Earth as a heat source and the night sky as the heat sink. In a sense it's the opposite of traditional solar panels, which is why some have referred to it as "anti-solar panels".

• yes. That is interesting – Anonymous Nov 19 '20 at 11:34
• But they are not very efficient, are they? – Peter Mortensen Nov 20 '20 at 22:26

Heat is not “converted to heat energy”. Heat is the transfer of energy due solely to a temperature difference. Without a temperature difference there can be no heat. The consequences of that transfer can, but doesn’t necessarily, result in work.

In the case of the thermoelectric effect, heat can generate a voltage which in turn can produce electrical work. So in order for the thermoelectric effect to generate a voltage using a 1 K temperature source, you would need a sink with a temperature between 0 K and 1 K.

I hope this helps.

• I somewhat disagree with this answer. It is possible to generate heat through a material without having any temperature difference. For example the Joule effect is the generation of heat through the volume of the material (at any point/position in it). One does not need any temperature difference for it to occur. All is required is a material with finite resistance and an electric current. The system can be isothermal (no temperature difference anywhere), it would still work. – AccidentalBismuthTransform Nov 19 '20 at 7:52
• @AccidentalBismuthTransform You are mixing up heat with temperature. The term "Joule heat" used by electrical engineers (of which I am one) is sloppy from a thermodynamics standpoint. Current in a resistor causes an increase in the temperature of the resistor. Meaning it causes an increase in the internal microscopic kinetic energy of the atoms and molecules of the resistor. The increase in the temperature of the resistor results in heat transfer from the resistor to its cooler surroundings. THAT is heat. Anyway, don't feel bad. it's a common mistake to confuse heat with temperature. – Bob D Nov 19 '20 at 8:10
• I don't think I am confusing heat with temperature (they have different units, and Joule heat is not a temperature, for sure). I do not see the relevance of the heat transfer with the surroundings (whose temperature can be adjusted anyway, if needed), if you consider the surroundings at a fixed temperature then the heat equation of the material will have a Fourier conduction term, a Joule term and a radiation term (and convection term if you really enjoy over-complicating things for no purpose regarding this question). I.e. you can completely supress the heat transfer from the wire with its su – AccidentalBismuthTransform Nov 19 '20 at 10:36
• rroundings if you wish so. There is no need at all to have parts of the system at different temperature. Also, I do not understand why you say that Joule heat is sloppy from a thermodynamics standpoint. It is just part of irreversible thermodynamics, there is nothing mystical about it, we can compute the increase of entropy with time due to it, and other things if we want. It's nothing we don't know how to deal with. – AccidentalBismuthTransform Nov 19 '20 at 10:38
• What I’m trying to tell you, if you would listen, is that the temperature of a resistor increases not due to heat but due to electrical work With that I am ending this discussion – Bob D Nov 19 '20 at 11:01

If you want to transfer heat from a cold environment to a warm environment, you need a heat pump but then part of the provided heat will come from the work done by the heat pump, so it comes from the fuel that drives the heat pump. To get all the heat from only the cold environment, you need to use a heat pump that exploits temperature differences in the cold environment. This means that a pure transfer of heat from only a cold environment to the warm environment requires at least two different cold environments at two different temperatures.

Suppose then that we have 3 heath baths at temperatures of $$T_1. If due to some process the heat added to heath bath $$i$$ by the other heath baths is $$q_i$$, then by the First Law of thermodynamics (conservation of energy), we have:

$$\sum_{i=1}^3 q_i = 0$$

If this is a reversible process, the total entropy does not change, we then have:

$$\sum_{i=1}^3 \frac{q_i}{T_i} = 0$$

We can then compute what fraction of the heat extracted from heath bath 2 ends up in heath bath 3. This is then ratio $$-\frac{q_3}{q_2}$$, solving for this using the above two equations yields:

$$-\frac{q_3}{q_2} = \frac{T_3}{T_2}\frac{T_2 - T_1}{T_3 - T_1}$$

So, we see that heat can flow from cold to warm, without having to rely on external work or having to rely on some other system at a temperature that is higher than the system where the heat is flowing to.

Edit- here is the link to article that made me think about this. somewhere in middle it is written that scientists can harness the cold energy using some active input method.

The following statement from the article is poorly worded:

“Essentially, a sky-facing surface passes its heat to the atmosphere as thermal radiation, losing some of its heat to space and reaching a cooler temperature than the surrounding air”.

You don’t “lose” heat to space. Heat is not something you lose or store. Heat is defined as energy transfer due solely to temperature difference.

What appears to be happening is that there is a decrease in the internal microscopic kinetic energy of the atoms/molecules of the material of the device due to heat transfer by thermal radiation to the night sky. As a result the temperature of the device drops below the temperature of the surrounding air. Then there is heat transfer from the surrounding air to the device and that is what generates electricity.

Bottom line: It is the thermal energy of the surrounding air that is harnessed to generate electricity. The reduction in the temperature of the device below the temperature of the surrounding air is what enables that harnessing of energy. It is not the thermal energy of space that is harnessed.

Hope this helps.

• I upvoted this answer Bob D, I judge it clear and to the point (and not wrong). The other answer though, I cannot do the same now, at least as it is. – AccidentalBismuthTransform Nov 19 '20 at 20:02

I don't have a definite answer for you. But I don't believe the existing answers have given this an appropriate treatment and I would like to leave an extended comment.

You can extract useful work from a temperature gradient-- e.g. a different in temperature. Many thermodynamic processes will only care about the difference in temperature rather than absolute values (e.g. Heat Equation is invariant, still satisfied, under the addition of a constant term to temperature). Energy is by its very definition a numerical quantity which is conserved due to the time-invariance of laws of physics. It so happens that something which is 'hot' has a high amount of this quantity, and something which is 'cold' has a small amount.

...a sky-facing surface passes its heat to the atmosphere as thermal radiation, losing some of its heat to space and reaching a cooler temperature than the surrounding air.

What this means is that the device creates a temperature difference between itself and the ambient air. That temperature difference can then be harnessed to do 'useful work' (e.g. charge an electrical battery). What's happening is that the device is radiating some of Earth's heat energy to space, and heat from the ambient air must flow in to replace it. That heat flow comes from a difference in temperature between the device and the ambient air. That is where the 'useful work' is coming from. What happens is that some energy is taken from the higher-temperature heat source (ambient air) and dumped into the lower-temperature one (the device), and some of it goes toward a practical purpose like charging a battery.

We know that the heat can be converted into heat energy with the help of thermoelectric generators but why can't we generate energy from coldness. Like the temperature of the universe in 1K, can this be used in near future to be used as an energy resource for probes or satellites?

At first I was going to say a tentative no. However, thinknig more about it I suppose it is possible in principle to radiate the ambient 'heat' of the probe away faster than it is absorbed from space. This could be done if the probe has a surface highly reflective to infrared light, but still acts effectively as a heat radiator. So this may be an engineering problem rather than a physical impossibility. With clever optics perhaps it is even possible to focus and trap heat in an optical well, so that a higher temperature (I don't know, maybe 10K) is achieved from which a heat difference (10K versus 1K of ambient space) can be used to extract useful work. I do not know if this is possible but I don't see any obvious physical principle preventing this.

I hope an expert can elaborate on this.

• You seem right and i hope that this should be used with the probes. – Anonymous Nov 20 '20 at 12:06

If you have a barrel with nothing but air in it, you can’t put it to much use. But if someone helpfully liquefies the air for you, suddenly you can use it to power a mechanism, despite the fact that all the helpful person did was take away energy.

One way of thinking about it is that all machines run on entropy, not energy. If you have access to a very cold object, it allows you to build a low-entropy system which can do useful work as it increases the entropy. Or from a different viewpoint, the colder your cold object, the more eagerly energy will flow to it from hotter objects, the easier it is to cause that energy to do something as it flows.

I like physics. And I just wanted to comment that coldness is observed in temperature. While heat is a type of energy. There is no relation I know of with heat and temperature except the one with "Specific heat" which is, Energy = mass * Specific Heat * Temperature $$H = mS\theta$$ where $$\theta$$ is change in temperature.

So energy sorts of flow from hotter to colder objects. So to extract some energy from a cold object you need a colder object.