Published on July 25th, 2008 | by Low Impact Living11
Low Impact Living: Are Geothermal Heat Pumps For You?
Over the past few months we’ve noticed quite a bit of interest in geothermal heating and cooling amongst our site visitors, and in particular in geothermal heat pumps. We’ve also had many questions from people about exactly what they are and how/if they should consider them as an eco-friendly heating/cooling option. If this describes you, then read on – these systems ARE incredibly promising technologies to heat and cool your home, but they’re also more complicated than your typical AC or furnace unit. We’ll try to help clear the air!
We get into quite a bit of detail below, but before you get into that here’s a very quick summary of geothermal heat pumps:
- Geothermal (or ground source) heat pumps can be incredibly efficient, delivering 3-6x as much energy for heating and cooling as you use to power the equipment;
- They are in some ways a renewable energy system, since they use the heat contained in the earth to provide heating / cooling;
- They do require extensive installation work, including excavation or drilling to install subsurface pipes; and
- They are more expensive than traditional heating/cooling equipment, but the payback period is less than five years almost everywhere in the country due to their greater efficiency.
What is a Heat Pump?
First things first, though: what exactly is a heat pump? Well, just like it sounds, a heat pump moves heat from one place to another rather than creating heat or cooling by burning a fuel (like a furnace or boiler). They do this by taking advantage of the fact that liquid refrigerants absorb huge amounts of heat when they turn into gas via evaporation, and release that same heat when they are condensed back into liquids.
The most common kind of heat pump, called an air-source heat pump, uses the energy in outdoor air to heat and cool. To cool a warm space, a heat pump evaporates the liquid refrigerant in copper coils indoors and condenses it (via a compressor) in similar coils outdoors. To heat a cold room, a valve is activated that reverses the process: the gaseous refrigerant is condensed indoors where it gives off heat, and it evaporates in the outdoor coils, picking up heat from outdoors in the process. Air conditioners and refrigerators use the same exact process to deliver their cooling performance.
Why are they such great heating and cooling options? For one, heat pumps can be incredibly efficient: because they move rather than create heat, they can often deliver 3-4x more energy into your home than you use to power the heat pump (high efficiency air conditioners have the same benefit). Unlike air conditioners, heat pumps also provide both heating and cooling, meaning you don’t need two separate systems that only get used for half the year.
With all of these benefits, you might expect to see air source heat pumps everywhere, so what’s the catch? Well, because they are more complicated than typical air conditioners and furnaces, they’re a bit more expensive up front. And, they work best in relatively moderate and humid climates: the greater the difference between the indoor and outdoor temperatures, the harder they have to work. Once outdoor temperatures drop below 40 degrees or so, heat pumps are no longer efficient as heaters and you need some kind of auxiliary heating. In very hot climates with low heating needs, air source heat pumps are no more efficient than air conditioners but are more expensive.
The Joys of Geothermal
Fortunately, there’s a great way around these limitations on traditional air source heat pumps. In even the most extreme climate regions, the temperature several feet underground is between 45 and 75 degrees Fahrenheit. Enter the geothermal heat pump (also called GeoExchange heat pumps and ground source heat pumps). These heat pumps circulate a fluid through piping buried in the ground, discharging heat to the ground in summer and pulling it from the ground in winter. The heat pump coils are in contact with this fluid rather than the outside air as in a standard heat pump, thereby avoiding the huge temperature swings of our atmosphere. Geothermal heat pumps can be incredibly efficient, delivering from 3-6x the amount of energy used to power the pump’s compressor and fans/pumps.
There are several options for installing a ground source heat pump. The choice of which one makes sense in your area involves many factors, such as how much land you have available, what the underground conditions are like, and the skill / experience of installers in your area. As you scan these options you might think “wow, these must be expensive!”, but due to the incredible energy efficiency of these systems payback periods can often be less than five years.
The most cost-effective option for residential applications is called closed loop horizontal installation. In this type of installation, plastic pipe is laid in horizontal trenches at least four feet deep. The pipes can be installed straight or in loops resembling a big Slinkly – this requires deeper but shorter trenches, which can help in smaller yards. For a typical home, you might have to install 1,000 – 2,000 feet of tubing/piping, so this isn’t a small project!
A second option is called the closed loop vertical system, in which u-shaped sections of pipe are installed in borings drilled 150+ feet deep. These systems are more expensive because of the drilling costs, but they can be used in tighter places or where the soil is very rocky or difficult to dig in. Vertical systems have been employed in areas as densely populated as New York City.
A final common option is referred to as an open loop system. Here the heat exchange is done via groundwater withdrawn from a well rather than through a closed loop of piping. Heat is transferred from the water to the building via the heat pump, and then the water is reinjected into the groundwater aquifer via a second well some distance away. This can be the easiest approach from a technical standpoint (and can work in dense urban areas as well), but it can introduce some permitting hassles in areas where groundwater is used for drinking or is tightly regulated.
There are some other options (such as using a lake or pond as your heat source), but the three above are the most common options in most residential situations. There are also some interesting innovations in the technology of the heat pump equipment itself (such as using the waste heat for hot water, hooking the heat pump up to in-floor radiant heating systems, and other higher-tech approaches), but we’ll cover those in a separate posting.
Geothermal Heat Pumps Near You
Clearly, finding a contractor skilled in this type of system is critical! We have a directory of geothermal heat pump installers around the country here. If you don’t find any in your area, then check out the installer lists provided by some of the top heat pump manufacturers:
Oh, one more thing to note. A geothermal heat pump is NOT the same as geothermal heating, where you heat your house directly using hot water pulled from deep underground. There aren’t that many parts of the country with the necessary underground geothermal energy to do this (primarily in the West), and even in these places almost all systems are commercial-scale operations. So if you’re thinking of harnessing the earth’s energy for your heating / cooling needs, most likely a geothermal heat pump is the way to go!