This past week I received an email from a WEL instrumentation ( http://www.welserver.com/ ) user asking for my comment on how to compute COP, how to do it with a WEL, and how accurate can the COP value be.

The portion of my response that isn't WEL specific is at the top here, for the benefit of anyone here.  I'd certainly value comment from anyone to help point out where there may be a mistake in this writing.

DEFINITION:

COP = BTU Output / BTU Input

where BTU Output and Input are in BTU/hr, and thus COP is unitless.

COP is only applicable when a GSHP is in heating mode.

ASSUMPTIONS:

1. DSH, if present, is not in operation.
2. Loop fluid is water – i.e., no antifreeze is present.
3. Temperature units are °F

DETAILS:

COP = (HE + (Power * 3.42)) / (Power * 3.42)

where:
• HE is Heat of Extraction from the ground loop, in KBTU/h.
• Power is electricity used to run the GSHP (but not including water pumps)
• 1 KW = 3.42 KBTU/h

COP = ((LWT - EWT) * GPM * 0.5) + (Power * 3.42)) / (Power * 3.42)

where:
• LWT and EWT are in °F.
• GPM is gallons/min.
• 0.5 accounts for the specific gravity of water and usage of °F


EXAMPLE:

EWT = 60.0 °F
LWT = 54.5
GPM = 14.0 gpm
Power = 2.89 KW

HE = 38.3 KBTU/h

COP = (38.3 + 2.89*3.42) / (2.89*3.42) = (38.3 + 9.88) / 9.88

COP = 4.88

THOUGHTS REGARDING USING A WEL TO DISPLAY/RECORD COP REAL-TIME:
 (this portion is WEL specific)

A highly accurate, real-time computed COP measurement is well-achievable using a WEL, a WEL-compatible power meter, and a WEL-compatible flow meter.

In advance you’ll need to measure typical loop water pump power, and subtract this factor from real-time GSHP power. You’ll need to use a few WEL logic Expressions to do this, and to do it in a manner that avoids negative COP values during time periods when the GSHP isn’t running (sample & hold function).

All other values are easily obtainable via the WEL’s one-wire network with connected temp sensors, and via the WEL’s pulse counting inputs for the flow meter and power meter.

But, most people aren’t going to have an installed flow meter, and retro-installation of a flow meter for COP purposes may be overly expensive.

Still, reasonably accurate, and real-time COP display is well within the WEL’s capabilities, by making an assumption that water flow is constant regardless of water temp. I.e., the pump’s speed is constant.

(In fact it’s not perfectly constant, but, the variation with respect to water temp is small.)

A constant value GPM can be determined by measuring the water temp static pressure difference across the GSHP’s internal water heat exchanger, and then looking up a corresponding GPM via tables provided by the manufacturer.

Then, the computed GPM is inserted into the COP formula, again using WEL logic expressions.

Best regards,

Bill