The Science of Sustainability

A Realistic Look at Geothermal Heat Pumps

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Here's some of the equipment you can us to create your own geothermal heat pump. And you'll need a shovel.

Henry Gifford is a man who designs mechanical systems for very energy efficient, comfortable, and affordable apartment buildings in New York City, along with his partner, architect Chris Benedict. In a recent article in Fine Homebuilding, Henry explained how geothermal heat pumps work in a way that I will always remember. I paraphrase:

Dig a hole in the ground. Put some buckets of water in the hole. If you are deep enough below ground, the temperature of the water in the buckets, after a while, will be about 550F. Take the bucket into your house and put it in your refrigerator. The fridge will cool the water down to say 500F, and the heat produced in the coils behind the refrigerator will add some heat to your house. Voila! You’ve created a geothermal heat pump.

Notice that the heat produced is not free. It takes electricity to run the refrigerator. And if you don’t want to spend your days hauling water in buckets from the hole in the ground to your refrigerator, you’ll want to install a water pump, which uses more electricity.

The very best residential geothermal heat pump system, according to Henry, has a coefficient of performance (COP) of about 3. This means that for every 2 watts of energy the system pulls from the ground, you have to provide only 1 watt of electricity. You get 3 watts out for 1 watt in. But a typical system has a COP of about 2.

Given that electricity is produced at power plants that use fossil fuels, and depending on the mix of fuels your utility uses to produce electricity, you will probably burn more fossil fuels using a geothermal heat pump with a COP of 2 than you would using an efficient gas- or oil-fired furnace. And geothermal heat pumps are much more expensive to install than traditional furnaces.

At Home Energy Magazine, where I work, we always tell people that if you have your house air sealed, insulated, and provided with the right amount of ventilation to keep you healthy, you can do just fine with a medium-efficiency furnace and burn much less fuel than you would with a high-end system—ike a high efficiency gas furnace—and a leaky house. For most of us, that’s the best choice of all, for heating and for cooling.

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Jim Gunshinan

About the Author ()

Jim Gunshinan is the editor of Home Energy, the magazine of sustainable home building and renovation.
  • durbrow

    Does anyone know why we can't use our unheated swimming pools are MORE EFFICIENT heat sinks? Is there a device that circulates air to and from our cool pools?

  • Jim Gunshinan

    Haven't heard of one. You'd have to figure out if the cost of moving the air from the pool to the house would be comparable to cooling the air in the house some other way.

    I have heard of research that tried to take the hot air coming from the back of a refrigerator in kitchen to an air-source heat pump water heater in a garage. Haven't heard how that turned out.

  • Ted Cushman

    I have an even better idea: Go down to the package store, pick up a case of warm beer (not in the cooler – out on the aisles – it has to be warm), and bring that home and put that in your refrigerator, then carry on. 2 birds with one stone, eh? That beer should be at 70 degrees, anyway. You've gained 15 degrees on the bucket.

    You could also pee in the bucket. But that's your call.

    My patent application is still pending.

  • Dave

    I see articles saying Geo costs more because it there are costs to deliver electricity. Where I am in the country, it costs money to get propane/fuel oil delivered too. I would think that the propane trucks driving all over creation are not terribly energy efficent either. I guess it is all

  • wave

    you said "The very best residential geothermal heat pump system, according to Henry, has a coefficient of performance (COP) of about 3", I doubt that. As I know COP of GSHP, especially in heating mode, is much higher. Even Air souce <a href="http://www.waveheatpump.com/domestic/domestic.htm">Domestic heat pumps</a> have COP value of 4 in the ambient air temp of 20C.

  • Mike

    Is Geothermal heat realistic for the northeast. Upstate NY to be exact

  • JPGunshinan

    Hi Mike

    "We always tell people that if you have your house air sealed, insulated, and provided with the right amount of ventilation to keep you healthy, you can do just fine with a medium-efficiency furnace and burn much less fuel than you would with a high-end system—like a high efficiency gas furnace—and a leaky house. For most of us, that’s the best choice of all, for heating and for cooling."

    You can hire a contractor with knowledge and skill in installing geothermal systems to come and do an assessment.