# Harness energy from temperature difference between Earth and space

1. Mar 8, 2015

### Stanley514

The second law of thermodynamics say that we could harness thermal energy only if there is temperature difference between a two bodies. Will it work if we have a forcible heat rejection from one body to another? Difference of temperatures between the oceans and the outer space is around 300 K. Let say we take a liter of water (300 K) and cool it down quite a much by forcing all the heat to be emitted as infrared radiation through athmosphere transparency window to the outer space. We could spend one kilowatt of energy to reject a few kilowatts of heat. And then we will have temp. difference between cooled and non-cooled water. Could we gain some energy in total using a process of that kind?

2. Mar 8, 2015

### Svein

How do you define the "temperature of outer space"?

3. Mar 8, 2015

### Stanley514

I do not know exact definitions, but thermodynamically it should be closer to absolute zero (zero Kelvins). Though ideal vacuum contains no matter, blackbody radiation tends to expand in empty space (in vacuum) and it effects in possibility to harness energy. One of the proves is solar cells. Vacuum contains small amount of photons (of any kind) per squire meter, much less than room temperature matter and it may result in some kind of effective temperature.

4. Mar 8, 2015

### Svein

Agree, but on the other hand vacuum is a perfect thermionic isolator (that is why we use it in thermos bottles). I have not calculated the radiation balance for a black body in earth-near space lately, but I know that it is nowhere near absolute zero.

5. Mar 8, 2015

### Staff: Mentor

Hi Stanley,
What you suggest could be done - and other than the equipment would be completely free - but it wouldn't produce much energy. You can calculate using the stefan-boltzmann law the theoretical maximum capacity of such a device, but keeping it isolated from other enviromental thermal energy sources would be difficult.

6. Mar 8, 2015

### Staff: Mentor

Thermos bottles use reflective coatings to limit thermal radiation.

The black body temperature of empty space is about 3K. http://en.wikipedia.org/wiki/Cosmic_microwave_background

7. Mar 8, 2015

### Stanley514

Do you mean low power or low efficiency? And which isolation do you mean?

8. Mar 8, 2015

### Staff: Mentor

Efficiency doesn't matter when the energy is free and different sources aren't comparable with each other anyway: it would be low power.
In order to make a chamber or object colder than the surrounding environment, it needs to be insulated.

9. Mar 8, 2015

### Drakkith

Staff Emeritus
How do you propose to do this?

10. Mar 8, 2015

### Staff: Mentor

A solar oven (at night) would work. This issue is why cars (not to mention telescopes) get dew/frost on them so easily.

11. Mar 8, 2015

### Drakkith

Staff Emeritus
Ah. I should have known from all the time I spent unfogging my telescope mirrors with a hairdryer.

12. Mar 9, 2015

### Stanley514

Could you explain how?

13. Mar 10, 2015

### Staff: Mentor

Take the solar oven, but make it large and flat. The four sides would be insulated, the glass would be double-paned, also for insulation (but of a type that transmits IR well). The back of the chamber would be made of aluminum, painted flat black for maximum emissivity (would need to confirm the paint's IR emissivity). Evacuate most of the air out of the chamber to limit the convection loss.

Heat will flow from the ambient air, through the aluminum panel, where it would be radiated into space. Like a water-wheel interrupting a stream, a heat engine interrupts a flow of heat energy and harnesses it. So you connect the back panel of the thermal radiator to a Sterling engine, since Sterling engines are capable of functioning with a low delta-T.

As I said, you can use the Stefan Boltzmann equation to calculate how much radiation you should emit from such a setup. Then with that energy input, heat loss calculations based on the insulation of the chamber and the efficiency equation for a Sterling Engine, you could calculate how much energy you can generate this way. It would be very small, but it would be an interesting project to see if it could be made to work at all.

14. Mar 11, 2015

### Stanley514

What if we try to intensify this process with help of a heat pump?

15. Mar 11, 2015

### Staff: Mentor

Heat pumps improve energy quality (delta-T) at the expense of quantity (or vice versa). So adding a heat pump would only further reduce the efficiency.

16. Mar 18, 2015

### Stanley514

From what I know heat pumps are pretty efficient in theory and even in practice, for example for each 1 KW of energy spend in frig or conditioner you could obtain 5 KW of "cold". It seems like you could move lot of quantity sacrificing little quality. Stefan Boltzmann equation it seems shows quantity, but I would be glad to know efficiency (quality) for a start. If temperature of oceans are close to 300K and outer space to 4K, it should be pretty large difference?

17. Mar 18, 2015

### Staff: Mentor

The radiator transfers heat to space: the heat pump would only be transferring it from the radiator to some intermediate storage between the radiator and the environment (or did you have something else in mind? Please be specific ). I'd be surprised if you could get any better than 5-10C difference between the radiator and the environment and the heat flux is set by the radiator. There really isn't anything for a heat pump to do, that I can see.

Last edited: Mar 18, 2015
18. Mar 18, 2015

### CWatters

I wondered if the OP meant to send the water into space on a rocket, have it cool down and then some time later wrap it in insulation and return it to earth. Unfortunately I calculate it would take 33MJ to raise 1Kg of water up to say the International space station. Even if it could return at say 3K it would take less than 1MJ to return it to room temperature.

19. Mar 18, 2015

### CWatters

The problem is how to radiate the energy to space. I calculated above that it takes too much energy to carry the water into space so the radiator would have to be on earth. Radaitors can either conduct or radiate heat away. Clearly they can't conduct it to space because the surrounding air is a lot warmer than 4K. The problem with radiation is that the power emitted is proportional to T4. Consider how little heat a human radiates at 309K. It's not very much but infra red cameras can detect it. Divide that by 3054 and you can see that the radiator would have to be truly enormous making it very hard to insulate.

Edit: Oh and there is the major problem of the radiator absorbing radiated heat from the sky (the atmosphere) which is hotter than space.

20. Mar 19, 2015

### Stanley514

If we take a litre of water with temp. 300K and will insulate it from the environment. Surround it with deep vacuum and polorized glass which allow IR to go out but not to go in. How much this water will cool down with time?