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Most efficient way to recover heat from 130 degree F water and transfer this heat to 140 degree F water? Answered

I am making a solar still and I need to recover as much heat as I can from the 130 degree F water (bringing the water temp down to near ambient, 72 degrees F) and then transfer this heat to 140 degree F seawater. I have a limited amount of solar collectors to heat brackish water, send it through an evaporator tower and then a condenser tower to make fresh water. As the fresh water goes through the condenser it gains a great deal of latent heat, thus the 130 degree F water. In order to increase production and regain otherwise lost energy I am trying to recapture a portion of this heat to further warm up the brackish water (currently at 140 degrees after exiting the evaporator tower) before being further heated with the solar collectors. I am operating off grid, so the solution must utilize solar heat or PV panels as the power source. 

Does anyone know of a heat pump that operates best in this temp range? What about using TEC (aka TEG, TEP or TEHP) to perform this task? Is there a better or more obvious solution that I am oblivious to? Any advice would be much appreciated. 

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Josehf Murchison (author)2013-05-21

Ammonia heat pump like the ones used in trailer refrigerators.
They run on heat.

Heat evaporates the ammonia and compresses the gas, the compressed gas in the condenser then cools by transferring the heat to the second medium. The second medium takes the heat and returns it back to the evaporator.

It is not perfect but it will create cold in one place and heat in another with no more heat than a candle.

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user

Could you forward me a site or two with such pumps?

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user

I was referring to combining the Ammonia heat pump on the first page with the same system as the geothermal heat pump on the second page.

http://www.polarpowerinc.com/products/refrigerator/ref-tech-overview.htm

http://www.examiner.com/article/the-efficiency-of-geothermal-heat-pumps

Joe

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user

Great advice. I will research this idea further. Thank you!

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user
steveastrouk (author)2013-05-22

Of the "competing" methods,
A standard mechanical refrigerator has a COP of 3 to 5.
An Absorption refrigerator has a COP of roughly 0.5
A TEG has a COP of around 0.4 to 0.7

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user

Thanks for those helpful numbers. Sounds like solid state is not the way to go, although I believe I would be getting more efficient results because I would be getting the benefit from cooling the 130 degree freshwater before sending it back through the condenser to assist with condensation, and I would also be getting the benefit of heating the brackish water as well?

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rickharris (author)2013-05-21

Unless I totally miss your point you would need to operate some kind of heat pump to gather the energy over a longer time period (and very good insulation) or you will just cool down the 140 deg water.

However in an reasonably sunny area you should be able to boil water easily with solar power - Insulation is the key,

Many years ago I could boil 45 gals of water over a sunny day with little more than a black painted radiator and a triple glazed and insulated cold frame

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user

TEP stands for ThermoElectric Pump (or TEHP for ThermoElectric Heat Pump). It is a solid state heat pump that uses a doped silicon chip to transfer heat with no compressor or moving parts.

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user

Where are you getting the electricity from to run a "TEP" - usually called a TEC - thermoelectric cooler. or TEG - thermoelectric generator - same thing, backwards. And they're not made of silicon.

Not very efficient. A mechanical pump, running with propane as the working fluid is vastly more efficient.

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user

The mechanical pump with propane as the working fluid is outside my field of knowledge, so I will need to research the pump idea further. Any links or further info would be much appreciated.

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user

The electricity for either the mechanical or thermoelectric heat pump would have to come from PV panels. I have not been able to dig up much information about the COP of TEC's, so I am not sure how they would compare performance-wise, to a standard heat pump. I am hoping advances in technology and the lack of moving parts would make TEC's a viable solution.

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user

Yes, I believe you got the point. I have a limited amount of solar collectors to heat brackish water, send it through an evaporator tower and then a condenser tower to make fresh water. As the freshwater goes through the condenser it gains a great deal of latent heat, thus the 130 degree F water. I am trying to recapture a portion of this heat to further warm up the brackish water (currently at 140 degrees after exiting the evaporator tower) before being further heated with the solar collectors.

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