Does a Geothermal Heat Pump Count as a Renewable Energy Source?
Here's another rant that goes in my Drives Me Crazy category of articles. I'm in good company, too. Green Building Advisor ran an article this week about making the choice between an air-source heat pump and a ground-source (aka geothermal) heat pump. At the end of the article, Peter Yost mentioned that ground-source heat pumps, "have been given quite the green 'pass' or 'seal of approval' because they are portrayed as using a 'renewable' energy source, and that makes me crazy."
So there's your answer to the question I asked in the title: No! Ground-source heat pumps are not a renewable energy source. Let's all go out and have a productive day now.
Oh, OK, I'll say a bit more. Remember last year when I wrote about how your air conditioner works? Before that I'd already written about how air-source heat pumps get heat out of cold air to heat your home in winter. A ground-source heat pump does exactly the same thing, with one difference. In an air-source heat pump or air conditioner, you're pulling heat from the outside air and putting it into the home (heating mode) or putting heat from the home into the outside air (cooling mode).
The diagram below shows what's going on. The heat exchange with the outside is going on in the part labeled as the condenser coil. In an air-source heat pump, that coil surrounds the noisy metal box that sits outside your home. The fan inside the condensing unit pulls outdoor air across that coil, and the refrigerant running through the coil either gives up heat to the outside (cooling mode) or picks up heat from the outside (heating mode).
The only substantial difference with a ground-source heat pump is that you're using the ground (or a body of water) instead of the outside air. The heat exchanger in a ground-source heat pump isn't a coil but a loop of pipe carrying the working fluid. That loop of pipe can be horizontal or vertical (see photo of well above), but its job is simply to exchange heat with the ground. It's doing exactly the same thing as the condenser coil above.
I think the source of the confusion about ground-source heat pumps and renewable energy is the unfortunate use of the term 'geothermal' in connection with these devices. When you hear the word 'geothermal,' you think of lava or geysers, of volcanoes blowing their tops. You think of beautiful Icelandic maidens in steaming pools of hot water surrounded by snow. (Or is that just me?) You think of heat engines being driven and doing useful work by harnessing the heat from within the Earth.
But that's not what ground-source heat pumps are or do. They're just like your regular air conditioner or heat pump except they use the ground instead of the air as the source or sink for heat. They still use electricity to power the pump that moves the working fluid through the loops. They still use electricity for the blower that moves the air through the duct system. They still use even more electricity to run the compressor, which is the pump for the refrigeration cycle. That electricity generally comes from outside the home, often from a power plant that burns coal or natural gas. Last I heard, most folks don't consider those fuels renewable.
I don't use the term 'geothermal heat pump.' It's too confusing. Even Thomas Friedman, the columnist for the New York Times who's written some well-received books about the environment, is confused about this. He wrote in a column a few years ago that he's using renewable energy in his home because he installed a geothermal heat pump.
Really now. If a ground source heat pump is a renewable energy source, then so is a refrigerator. Can you see why this drives me crazy?
How the Heck Does a Heat Pump Get Heat from Cold?!
The Magic of Cold, Part 1 - How Your Air Conditioner Works
What IS Heat Anyway? - Building Science Fundamentals
Photo of woman in hot spring by Andrea Pokrzywinski from flickr.com, used under a Creative Commons license.