Anyone who has a refrigerator
or air conditioner is already familiar with the operation of a geothermal heat pump. Contrary to common belief, cold is not something that is produced, but is a condition that results when heat has been removed. If you remember your high school physics class, you know that heat is produced by a molecular motion. All substances are made up of tiny molecules that are in a state of rapid motion. As the temperature of a substance is increased, the molecular motion increases, and as the temperature decreases, the molecular motion decreases. Molecules move faster on a warm surface than on a cool surface. Heat will flow from a warm substance to a cool substance. Reminder: Second Law of Thermodynamics.
The household refrigerator removes heat from inside the cabinet by means of a refrigeration process that creates a cold surface on a heat exchanger (also called an evaporator) which transfers (absorbs) heat from the food stored inside the refrigerator. The heat extracted from the interior of the refrigerator is not destroyed, but is pumped from the inside to the outside of the cabinet by an electrically operated vapor compression device called a compressor, and released to the surrounding space by another heat transfer device known as a condenser. The refrigerator continues to perform its magic of removing heat until a preset temperature is reached.
A heat pump works just like a refrigerator, except that by the addition of a reversing valve, the refrigeration process of a heat pump can be reversed, so that we can actually put heat INTO the cabinet, as well as extracting it.
The output of these types of refrigeration equipment is measured in Btu’s/Hour. You remember, a Btu stands for British thermal unit and is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. Although the basic principles are the same, a whole-house air conditioner or heat pump is capable of producing huge amounts of Btu’s/hour when compared to your refrigerator. We commonly refer to the output in terms of tons of refrigeration, i.e. 12,000 Btu’s/hour is equal to one ton of cooling.
Now that we know how a heat pump works, by moving heat from a high temperature source to a low temperature source, we can compare the distinct differences and advantages of a geothermal heat pump to an air source heat pump.
Two 36-ton geothermal heat pumps
used at the College of Southern Idaho.
In the space heating mode, a typical air source heat pump extracts heat from the outdoor air and pumps it into a building. In summer, the air source heat pump works in reverse, removing heat from the inside of a building and pumping it to the outdoors. You’re already asking yourself “How can an air source heat pump heat my house in the winter when it’s 10° F outside?” Good question! As we discussed earlier, heat will move from a high temperature surface to a low temperature surface. If the outdoor evaporator temperature of an air source heat pump is kept low enough, say 0° F, then, even at 10° F outdoor temperature, enough heat is in the air to transfer to the cold surface of the evaporator.
At such cold temperatures, the air source heat pump would have to run for long periods of time to produce enough heat to satisfy indoor heating requirements. Thus, we can see, that the capacity of a typical air source heat pump is dramatically reduced as outdoor temperatures drop below 45° F. Conversely, if we wish to cool a structure in the summer, when outside temperatures reach 90° F and above, we are now using 90° F air to remove heat from the outdoor condenser. Are you still with us? Remember, heat flows to a cool substance. This means that to remove the heat that is pumped from inside your house to the outdoor condenser, the temperature of the refrigerant has to be a lot warmer than the 90° F air in order for it to condense in the outdoor condenser. Elevating the temperature of refrigerant to a higher level by using a mechanical pump (the compressor) in order to remove the heat makes the system work much harder. Voila! Your cooling bills go up as the compressor works harder and consumes more electricity.
Although the basic principles of operation are the same for an air source heat pump and a geothermal heat pump, the actual system design and components are different. The geothermal system is a self-contained unit. The air source heat pump is a split system, with an indoor air handler and an outdoor, compressor-bearing, condensing unit.
The air source heat pump requires a means to defrost its outdoor heat exchanger when temperatures fall below freezing. This defrost cycle wastes energy and consumes more energy dollars.
A geothermal heat pump has no defrost cycle and no unsightly, noisy outdoor unit. The geothermal heat pump has a much simpler design, and since it is a packaged indoor unit, it requires less maintenance than an air source heat pump.
By now I think you get the point. Air source heat pumps work fine in mild climates. We call them our fair weather friends. Geothermal heat pumps work efficiently in mild and harsher climates. Geothermal heat pumps use the thermal properties of good old mother earth for their heat transfer energy source. That’s how geothermal gets it’s name.
Why are geothermal heat pumps more efficient than air source heat heat pumps?
The geothermal system cycles water from a well, pond, lake or river, or water piped through underground plastic tubing, to warm or cool the heat pump’s refrigerant. Liquid such as water or an antifreeze solution is a much better heat transfer medium than air. If it were not a better means for heat transfer, we would all be driving those big SUV’s with air cooled engines! Overall, the liquid medium for heat transfer is about 30% more efficient than air.
In addition to the advantages of using liquid as a heat transfer medium, the geothermal heat pump also takes advantage of mother nature’s renewable energy. The earth acts like a big solar collector, providing stable temperatures beneath the surface, year round. The soil below frost level, 4-6 feet deep, stores the earth’s energy at a fairly constant level, with temperatures dependent upon latitude. In most northern parts of the U.S., soil temperatures will get as low as 40° F. In the far south, soil temperatures will usually not be lower than 70° F. The central states and the midwest have extreme temperature differences from one season to the next, but these regions also have many mild days. In the central states and midwest, ground temperatures average about 55° F, the mean average of the outdoor temperature for the entire year.
In the central states and midwest, a geothermal system needs to extract heat from the 55° F earth temperature and elevate it 15-20° F to maintain a comfortable indoor temperature. In the summer, the 55° F ground temperature is a perfect heat sink, which helps keep condensing temperatures of the geothermal heat pump as low as possible. Contrast the constant 55° F ground temperature used by a geothermal heat pump to an air source heat pump or conventional air conditioner, where outside air temperatures drastically affect the operation and economy of the system.
In the winter, a geothermal system thinks it is operating in Florida, and in the summer it thinks its been moved to Alaska. Using a liquid as the heat transfer medium, and taking advantage of the earth’s thermal properties, results in a 20-40% savings for geothermal over conventional heat pumps and air conditioners.
Of course, lower soil temperature will reduce heating efficiencies, and warmer soil will cut air conditioning savings. On average, however, geothermal systems deliver 3-4 times the energy they consume. How is this possible? Remember, geothermal systems transfer heat energy, unlike fossil fuel furnaces which rely on the consumption (burning) of natural gas, fuel oil or propane. With geothermal, the only energy consumed is that which operates the pump (compressor) and the fan that distributes the conditioned air.
With a high efficiency geothermal system, over two-thirds of the heat output comes from the earth, and it’s free. Imagine: millions of Btu’s of energy are already in your backyard, waiting to be delivered to your home or office. The geothermal energy in the ground will be there forever, free of charge. Energy independence is yours, now and for the future, with geothermal heating and cooling.
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