Ԫ Build By Design :: Geothermal (part 2)

What is Geothermal?

What is Heating and Cooling?

This may sound like a silly question, but it's really the place we have to start. What really is heating and cooling your home? Why do we do it?

The answer seems obvious, but let's break it down for a minute. After all, people living in subtropical rain forests don't heat or cool their homes. We heat our home in the winter because it gets cold outside, and cool our homes in the summer because it gets hot. But this is only true at the surface of the earth. Not that far below the surface in any given area the earth remains a remarkably constant temperature year-round. In fact, it remains so constant there may be no deviation in temperature from one season to the next. If we lived in mines 50 feet below the surface we would need to heat or cool our homes only a very small amount, if at all.

We don't live in mines, but if we could use the constant temperature below the surface somehow to stabilize the temperature in our homes...

We would have geothermal heating and cooling!

What is Geothermal?

We have to start by understanding that the heat source for geothermal heating is the sun. Solar radiation heats the surface of the earth in winter and summer. That energy radiates down into the ground and is stored in bedrock, dirt and water tables. The temperature below ground doesn't get too hot in the summer because most of the solar radiation is reflected by the earth's surface back into the atmosphere. Below-ground temperature doesn't get too cold in the winter, because the ground absorbs enough radiation to provide some heat, and because the sheer mass of the earth acts as a giant heat sink, and is thus very slow to change temperature. The temperature above the ground may vary a great deal from one season to the next, but the temperature far enough below the ground (not very far, really) remains very constant throughout the year1.

A geothermal heating and cooling system heats by transferring the solar heat stored in the ground below your home to your home's interior; it cools by working in reverse, transferring heat from your home to the ground beneath it. That heat transfer could be done in a number of ways, but it turns out the most efficient is to use the same principle that runs your refrigerator. Specifically, sealed pipes or tubes containing a refrigerant carry the coolant down to the ground and back up again. That heat is then transferred to air which circulates throughout your home. (To see how a refrigerator works, go to How Does a Refrigerator Work?)

A geothermal heating and cooling system is nothing more, then, than a simple sealed system which uses energy stored in the ground by mother nature to raise the temperature of your home in the winter and cool it in the summer to the same temperature, all without any boost by burning fossil fuels!

If you think about it, if you own the land, you already own the energy stored in it. You're just not using it today...

How Does it Work?

A geothermal system works using the same principal that makes your refrigerator work. Loops or coils of pipe are buried in the ground deep enough to be in earth of relatively constant temperature. The loop is a sealed system filled with an EPA-approved, environmentally-friendly gas similar to that in your refrigerator. You notice how your refrigerator is cold on the inside, but hot near the heat-exchange coils on the back or bottom of the unit.

A conventional heater draws cold air from the outside and has to heat it up considerably to warm your home. A conventional A/C unit (running on the same principal as your refrigerator) has to use very hot outside air to try to cool the gas used to refrigerate your home. (The air outside your home when your A/C is running could be 100 degrees or more.)

The direct-exchange geothermal heating and cooling unit is in essence a refrigeration unit, with the heat-exchange coils buried in the ground. By running these coils through a medium of constant temperature the refrigerant enters the system at a stable, moderate temperature, and has far less work to do to exchange energy (in the form of heat.) Your heat pump therefore has less of a temperature change to achieve, and therefore uses less energy.

Once heated or cooled by the earth, the refrigerant is run through a heat-exchanger to heat or cool the air which is then blown through your home.

It is kind of cool to observe that there is no new technology needed for this system. In fact, the technology involved in heating or cooling your home through geothermal is over 100 years old! And the first geothermal systems using "modern" technology were installed in the 1940's2! The only difference between a geothermal system and conventional furnaces or air conditioners is that they use the sun's energy stored in the earth to give the compressor a huge "head start" on what it needs to heat or cool the gas, so that your system has far less work to do.

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1Jennifer Anderson, "At what depth in the earth can you reach a constant, sustained temperature?", 3/27/2002,
http://www.madsci.org/posts/archives/2002-03/1017275290.Es.r.html, (11 July 2008)
2Russell Boniface, "Geothermal Heat Pumps Provide Sustainable Alternative for Architects", n.d.,
http://www.aia.org/aiarchitect/thisweek08/0222/0222p_heatpump.cfm, (9 July, 2008)


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