Here’s another rant that goes in my “drives me crazy” bin of articles. I’m in good company, too. Another article that ran at Green Building Advisor recently discussed making the choice between an air-source heat pump and a ground-source (a.k.a. “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.
Can a heat pump be classified as an energy source?
OK, I’ll say a bit more. Remember that article I wrote about how your air conditioner works? And the one before that 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 or air conditioner, 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 the photo of a well, above), but its job is simply to exchange heat with the ground. It’s doing the job of the condenser coil above. (Actually, there are two closed loops and three heat exchanges going on, but I’ve taken the liberty of simplifying the process here, which doesn’t change anything regarding our original question.)
I’ve focused on air conditioners and heat pumps here, but the same analysis applies to refrigerators, which are just another kind of heat pump that uses the refrigeration cycle. All of these need an input source of energy to run the refrigerant through the system. In an air-source heat pump, you also need a blower to move air across the evaporator coil and through the duct system and another fan in the condensing unit. In a ground-source heat pump, the fan in the condensing unit is replaced with a pump for the working fluid in the ground loop.
So, is a refrigerator a renewable source of energy?
A poor choice of terminology
I think the main 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’re still using even more electricity to run the compressor, which is the pump for the refrigeration cycle. All that electricity generally comes from outside the home, often from a power plant that burns coal or natural gas. The last I heard, most folks don’t consider those fuels renewable.
Ah, but now that you mention Iceland, Allison, I hear you thinking, isn’t a ground-source heat pump doing the same thing they do when they pump heat from their numerous hot springs and use it in buildings? They’re using pumps to move the heat, too, just like a ground-source heat pump does? Well, yes and no. It appears at first to be the same thing, and we do indeed call the heat they’re using a “renewable” source of heat. The difference, though, is that they’re pumping high-grade heat that comes in at a temperature high enough to be used directly. They don’t have to “concentrate” it through the use of a refrigeration cycle as a ground-source heat pump does.
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?
Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.