Wednesday, September 8, 2010

Full Scale Test of Evapro/Radiation Cooling System

These are the results for the first full scale test of the Evapro/Radiation cooling system that I experimented a bit with last year.

This cooling scheme uses trickles of water down a surface that is pointed at the cold night sky.  The water trickling down the surface is cooled by a combination of radiation to the sky and evaporation of some of the water.  The cooled water is saved in a "coolth" storage tank for use on the next hot day.  For climates like ours which normally have cool nights even when the days are hot this works well.  Surfaces that are free to radiate to the night sky will cool significantly below ambient air temp, so the water can be cooled below the ambient air temperature.  

This full scale version uses the North roof surface of my Solar Shed as the cooling radiator to cool a 430 gallon tank of water during the night time.   When cooling is needed in the house during the following day, the cooled water in the tank is pumped over to the house and through the radiant floor heating loops in the house for cooling.
The solar heating side of the Solar Shed

 The same water storage tank and radiant floor loops that are used for heating in the winter can be used by this system for cooling in the summer with very little new work to be done.  Even the controls are unchanged except to switch from heat mode to cool mode.  So, you can basically add the cooling function for only a few dollars and a few hours of labor.

The cooling side of the Solar Shed in operation.
Normally this would be done  at night.
The picture above shows the Solar Shed in cooling mode.  Water from the "coolth" tank is distributed along the ridge line by a pipe with small holes.  It trickles down the roof surface, cooling it down in the process.  The cooled water is then collected by the gutter at the bottom, and routed back to the cooth tank.  Really simple, but the combination of cooling due to both radiation and evaporation is quite effective -- several times more efficient than conventional AC. 

We had such a cool summer here this year that I really did not have much motivation to get the full scale system set up, and only ended up with one hot day of testing.  But, I've received quite a few questions on the system, so I thought I would pass along the results from the one day test.  I plan to do more for next summer.

The link just below gives quite a bit of detail on how the test went, but the quick summary is that we were able to collect quite a bit of coolth in the tank efficiently, and that it kept our house comfortable through our one day (96F) heat wave.   The estimated COP of the system is about 22 (roughly SEER 75) -- several times more efficient than conventional air conditions. 

All the details on the full scale test of evpro/radiation cooling system... 



  1. Hi Gary,

    I'd seriously look into straight well water for cooling. Well water temps in MT should be less than 50F. You might need to watch for sweaty floors if the air humidity every got high. However, your climate is typically non-humid, right? My brother in Iowa has cooled his house for years using cold (45-55F) well water. He pumps it directly to a finned coil he has mounted in his air handler. The "spent" warm water he discharges to his lawn. You'd have to do a calculation to see how much energy you'd need to pump well water vs the energy consumed by your evaporative cooling system to see which comes out ahead on lower operating costs.

    Monte Peterson
    Pearland, TX

  2. Hi Monte,
    I have been considering about doing that.
    Our well water is consistently 50F, so there is enough coolth there to be helpful.

    If I figure 43,000 BTU of cooling for a hot day, this would be about (43K BTU)/(62F-50F)(1 BTU/lb-F) = 3580 lbs (430 gallons) of water on days that require cooling.

    This water would be used for watering plants, but given that we are trying to cut down on our water use, it seems like a lot of water?
    Water is becoming more of an issue in our valley now, and I'd just as soon not do anything to aggravate the problem.


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  3. Clever idea using the North side of the shed to cool water. Would be nice to see the relationship between humidity and temp drop..

  4. Hi John,
    Yes -- agree that it would be nice to know how well this method would work in more humid climates and climates that don't get as low a night temperature as we get. Surely not as well, but maybe still good enough to be worthwhile.

    The FSEC (Florida Solar Energy Center) has a really interesting project that uses night radiation cooling via a metal roof to reduce AC costs. This is one of their reports:
    There is a lot more on their site about it.
    The FSEC has done some great work on cooling methods for hot humid climates.


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  6. Hi Gary,
    Was not able to get that link to work -- maybe you could check? I'd like to see the details.



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