Lesson 1 EGEE 101 Header

Energy —Geothermal Heat Pumps

Image of the piping gowing into the ground as required for geothermal heating and cooling.
Source: DOE
Pipes need only go down about 8 feet before the earth is at a relatively constant temperature. Pumping a fluid through the pipes allows for heat exchange and hence heating and cooling .

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. We make use of this feature in PA since we have some caves in the mountains where the thermal mass of the mountain is so large that the temperatures in the caves are constant— great for Yuengling - to cool their beer, or for the growing of mushrooms. 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, it can also cool your home and in the summer provide hot water, too.

Heating


Picture of a whole in the ground with the geothermal piping going into it.
Source: NREL
Pipes enter and exit this 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 be the same constant temperature.

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 the GeothermalGenius animation (3 min.)

Note: the system isn't 400% efficient as claimed, that is not possible. It does use electricity to take advantage of the heating and cooling available through "free" geothermal energy.

Cooling


Picture of piles of geothermal tubing.
Source: NREL
Some large establishments will require lots of heat exchange like this one. Geothermal heaters are sized by the mass of the coolant fluid.

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 geothermal 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. How a cooling system works is by turning a liquid into a gas. This liquid to gas process requires energy, and so it cools its surrounds (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 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 (we can 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


Picture of a house which uses a geothermal heat pump.
Source: NREL
This nice looking house in Aurora, Colorado has a geothermal heat pump system that provides all the heating, cooling, and hot water needs. For a home of 1,500 square feet with a good building envelope and a geothermal heat pump, energy costs are about $1 a day. Much cheaper than the average energy cost.

These are not cheap systems at about $7,500 for installation 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. We will see that, in comparison to the other methods of heating and cooling the house, this will have a much lower environmental impact.

The cost is more expensive if the house does not already have the duct work 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.

 

In lesson 02, we will also discover that the energy from the ground can also be used to generate electricity. Don't confuse the two types as it is a very common error:

Geothermal for home heating and cooling uses solar energy in relatively shallow sites.

Geothermal for electricity generation typically uses deep geothermal energy for electricity generation. (See this Department of Energy simple geothermal power plant animation.)