
The previous page explained what influences how much energy we need to heat or cool the home along with some of the traditional fuel choices. A modern approach however can also use a combination of electricity and geothermal (renewable energy) to heat and cool the home.
This is perhaps one of the very best methods of both heating and cooling your home or office (and you also get cheap hot water in the summer). It works because unlike the air temperature which can vary greatly, the temperature of the earth is relatively constant (once you get deep enough). Here I am not talking about going very deep, only a few meters; once you start getting deeper, then the temperature of the earth increases as you get closer to the hot core. But at a few meters down, the temperature will be a constant value. It is called geothermal energy because it is energy from the ground, but it is actually mostly stored solar energy.
Not only can this stored solar energy be used to heat your home, but it can also cool your home and provide hot water in the summer.
Watch this introduction to geothermal energy from the Department of Energy (2:31 minutes)
We all want to save money heating or cooling our house or office, right?
The answer may be under your feet, literally. Much of the heating and cooling can come from the ground, below the surface, with something called a geothermal heat pump. You see, below the frost line about 10 feet down, the Earth maintains a nearly constant temperature of 54 degrees. We can tap into this energy to provide heating in the winter and cooling in the summer.
OK, now, here’s how it works. Bury a loop of pipes called a heat exchanger just below the surface, and fill them with water or a water and antifreeze solution. During the winter months, the air is usually cooler than the temperature below ground. The solution circulates in a loop underground and absorbs the Earth’s heat. This heat is brought to the surface and transferred to a heat pump. The heat pump warms the air, and then your regular heating system warms the air some more to a comfortable temperature. Finally,
ducts circulate the air to the various rooms. Now, a huge benefit is that the geothermal system doesn’t have to work as hard to make people inside comfortably warm, and you save lots of money on your heating bill. In the summertime, the system works in reverse. When it’s hot outside the temperature below the surface is cooler than the summer heat. So the fluid in the loop absorbs heat in the building and sends it underground. The ground’s lower temperature cools it, and it’s circulated again and again. Now you’re saving money on air conditioning.
Now, this church uses a large geothermal heat pump to heat and cool the building. It has a very big parking lot, which lets it spread out is loop horizontally. But if you don’t have all that space, you can go straight down and use a vertical loop system instead. Geothermal heat pumps can be used just about anywhere in the U.S. because all areas have nearly constant shallow-ground temperatures, although systems in different locations will have varying degrees of efficiency and cost savings.
The constant temperature of the Earth just below our feet is a sustainable resource literally in our own backyard. It’s a clean energy source ready for us to use to heat and cool our homes and buildings while
Heating

So, it is a cold winter day, the outside air temperature is 30 °F, but the temperature of the ground 10 feet down is a balmy 50 °F. By putting pipes in the ground, we can exchange the heat from the ground to the house. A fluid is pumped through a closed loop of piping into the earth where it warms up. See more detailed information on the Geothermal heat pump page of the Dept. of Energy website.
In the image to the right, pipes enter and exit the vertical hole in the ground. Most systems will be closed-loop systems like this, although you could take the water out of the ground in an open-loop system as the water temperature will remain constant.
Cooling
So, it is a balmy 90 °F outside, but the ground is a cool 50 °F. We can now move heat from the house into the ground. All we need to pay for is the electricity to circulate the cooling fluid. You can also produce hot water via this method, more cheaply than using electricity, to heat cold water to hot water for your showers or clothes washer.
Geothermal heat pumps are sold by the weight of the cooling fluid. Some of the facilities require lots of pipes to provide enough heating and cooling for large buildings. This is the barrier to using a heat pump - the high initial cost (capital cost). After that, the cost of electricity is low and no fuel costs, thus producing cheap heating and cooling without air pollution (apart from the electricity needed to run the pumps).
How does it Work?
Okay, the above is a tad simplistic. We could, if we wanted to, flow the heating/cooling fluid around the house, but we tend not to. A cooling system works by turning a liquid into a gas. This liquid to gas process requires energy, and so it cools its surroundings (we actually lower the pressure surrounding the liquid). We use a compressor to compress the gas and turn it into a hot gas. We also need energy (electricity) to pump the fluid. This is how we would cool the house by expanding the liquid to a gas (absorbing heat) which cools the house. The gas is then compressed to produce a higher temperature gas (heat exchange here to get the hot water for the house) and then allow the hot gas to heat exchange with the earth, cooling the gas so it turns back into a liquid, so we can do the expansion again and cool the house.
To heat the house, we pump liquid into the pipes (which are in the ground). There, the liquid warms up and forms a gas. Unfortunately, the gas is not hot enough to directly warm the house, but if we increase the pressure, we can turn the gas into hotter gas (concentrate the heat). This process does require electrical energy. But, for a little energy, we are getting a great deal of free energy from the geothermal source —the earth. Now that the gas is much hotter than the air temperature, we have a heating cycle.
Cost
The geothermal heat pump in this house provides all the heating, cooling, and hot water needs for the entire house. For a home of 1,500 square feet with a good building envelope (well-sealed so a low air-infiltration) and a geothermal heat pump, energy costs are about \$3 a day. This is much cheaper than the average energy cost but they are not cheap systems to install at about $7,500 in a new house, but they only use a small amount of energy (electricity), and they both cool and heat the house (and provide hot water). Payback time for this investment is about 6 years, so it is worth doing. However, the cost is more expensive if the house does not already have the ductwork in place for air handling. If you look back at the insulation page, you will see that the Department of Energy thinks that geothermal heat pumps can be used in PA. I only know of a few houses, however, that have an in-ground heat pump.
We will see that, in comparison to the other methods of heating and cooling the house, this will have a much lower environmental impact.
Note:
In lesson 2, we will also discover that the energy from the much deeper ground can also be used to generate electricity. Don't confuse the two types as it is a very common error:
- Home heating and cooling use stored solar energy (warmth) in relatively shallow sites. Heat is moved around using a heat pump. Also, the system can cool when the air temperature is hotter than the ground.
- Geothermal for electricity generation typically uses deep geothermal energy for electricity generation from very hot steam.