What’s the COP of a Solar Thermal System?
COP is a term that is used to refer to the performance of heat pumps. Coefficient Of Performance is the ratio of the energy to drive the compressor versus the energy transferred.
For any solar thermal system there is a “COP” which refers to the energy gained vs the energy to run the system. More simply, it is the energy produced by the solar thermal system divided by the energy to run the pump and controls (which is trivial).
Catching Some Rays
For our system recently commissioned at the military hospital in Bethesda, MD, we’ve gathered the following performance data…
Energy produced = 49ºF rise for 2500 gallons of water = 1,016,750 Btu (1 MBtu/4hrs = 250kBtu/hr)
The pump consumes 1.3 hp (55.13 Btu/min) to run the system. Total pump energy = 55.13 x 4 x 60 = 13,231 Btu
So, the WTU system COP (on a clear day) is 1,000,000/13,231 = 75.5:1.
Actually, it is a little higher, because the energy gained is slightly less than 4 hours, but we won’t quibble over that. COP = 76:1 would be OK.
Note that the building is 6 stories high, which means it has a relatively high pump hp. If the building were 2-3 stories, the COP could have been 150:1 or higher.
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Dr. Ben
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Chuck
So, for a Thermo-siphon solar hot water system with a natural convection COP must be infinity?!
Theoretically, yes. But there are several real-world limitations that would make such a system commercially undesirable. Primarily the fact that the solar thermal system energy output would be constrained by the availability of the natural convection. It has been shown that installing a real pump increases the energy collected by much more than the energy to run the pump.