ASK THE EXPERTS: GSHP vs. SHW

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