Ground-source heat pump loops.
Ground-source heat pumps extract heat from the earth using a well, several bore holes, or loops laid in shallow trenches.
Ground-source heat pump loops.

I don’t see ground source heat pump articles much in Home Power, but I thought I’d ask this question: Theoretically, what is the more cost and environmentally effective choice—a ground source heat pump (GSHP) or a solar hot water (SHW) system for heating a home in a cold climate?

You would have to hold some things constant (size of home, kind of backup heat, location, etc.) in the comparison, but I would guess a GSHP would use more electricity than a SHW system (increasing pollution in my coal heavy state) A GSHP may cost more, but it will also be more effective in winter when you need it. Any back-of-the-envelope calculations you can offer?

Mike • via E-mail

Good question. There seems to be more than a little confusion about the different technologies. Ground (water) source heat pump performance is measured by coefficient of performance (COP), the ratio of energy out to energy in. The higher the COP, the higher efficiency and the more money saved and emissions avoided. Grid-connected electrical water heaters (and furnaces) have a COP of 1, the baseline. Ground source heat pumps have COPs of 3 to 4 in home heating applications. Because domestic hot water requires a higher temperature than needed for home heating, the COP of heat pump water heaters is closer to 2. A COP of 2 means the heat pump water heater uses half the electricity needed to heat the same amount of water using an electrical resistance water heater.

Passive solar water heaters and active water heaters powered by PV have an infinite COP—they do the same job with zero grid electrical usage and cost, and zero emissions. Active solar water heaters using a grid powered pump and control have COPs of about 8 to 20, depending on system type and the location of the installation.

A quick and rough calculation for a single pump active solar water heater using a Taco 006 pump and Goldline GL-30 control: The pump draws 60 watts and the control 3 watts when the system is on, an average of 6 hours, so about 380 watt-hours per day. In addition, the control uses about 1 watt on standby for an additional 18 watt-hours, so we can round up to 400 watt-hours a day. This system is located in a sunny climate and is capable of producing 40 gallons of hot water per day (60°F temperature rise). It takes about 20,000 BTUs to heat 40 gallons of water 60°F. A KWH of electricity will produce about 3,400 BTUs, so it takes 5.6 KWH to heat the 40 gallons per day. The COP of this solar water heating system would be about 14 (5.6 ÷ 0.4 = 14).

The initial cost of the system, the total life cycle cost, and the amount of the total energy load displaced are also important factors that you may wish to consider to decide what will be best for you.

Chuck Marken

Comments (1)

Fred Golden's picture

This is a very good question, and timely as I am considering options while building a home soon. If I could store some heat from solar panels in a large pool, say 26,000 gallon indoor pool, and then use that heat to run a water source heat pump, that heat pump can have a higher COP than if air source on a cold winter day.

I will not consider geothermal heating because of the huge cost in wells and less than exceptional return on that additional investment. I can install my own 16 SEER heat pump with a COP of about 9 in the heating mode. That well water system might add $15,000 above the normal $3,500 parts cost for a 4 ton unit I can install myself.

I like your cost comparison for a single panel, collecting 20,000 Btu's of heat daily for about 400 watts = COP of about 14. I guess if you installed 3 panels, and tank size around 150 gallons, the pumping cost would rise a nominal amount, while heat output can rise to around 60,000 btu's increasing the COP to over 20.

My plan is evacuated tube SHW heating a 800 gallon tank to a target of about 160F. Then use that to warm water going into a HPWH, that will be set at about 115F. Out of the HPWH I will have a tempering valve to reduce the water to a usable 125F if the water in is above 125F.

Warm air around that water tank can be used to warm the home, or vented outside during summer. Also it can feed my clothes dryer.

Now I can calculate the SHW system COP. If it consumes 1,000 watts per day, and collects 102,000 Btu's of heat, or 30 KW worth of heat, I can see it having a COP of about 30. I plan on having more than 4 KW in PV power too.

On a recent article that I was reading about a Net Zero home in Colorado, I was thinking they are spending to much on insulation. While this lowers their total cost - due to not having a electric heating bill, it does increase their mortgage costs by spending $5,000 more on the home. Insulation will not heat a home, just lessen the time the heater is running. Would it also be cost effective to have a $5,000 solar heater to make up for the heat lost by using a "Reasonable" amount of insulation?

So instead of spending a extra $5,000 on an extra 8" of wall thickness and 8" of sprayed on foam insulation, spend that money on a SHW heating system that will provide say 50,000 Btu's daily to warm the home or it's hot water needs. The home might lose a additional 10,000 Btu's per day due to R-30 insulation instead of R-60 insulation, but how cares, you are making up for it with the solar water heater. Or install a 10 foot high X 20 foot wide X 1/2 foot deep solar heat collector, with the ability to send warmed air into the home, or blow it outside on a hot day.

Great information, keep it coming!

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