What is Geothermal?
I'm sure you heard the word before, but just what does it mean. Well I'll try to explain it simple as possible.
First let's consider a Refrigerator. Ever wondered how a refrigerator works? A refrigerator extracts heat from inside the icebox and expels the heat outside the refrigerator via coils on the back or bottom of the unit, which in turn makes the air colder in the box. This is performed via a Heat pump, which basically just moves heat from one area to another. How does this work? All air above absolute zero has heat in it. You might say it's cold outside when it's 20 degrees outside, but that cold air contains heat, if it didn't the air would be frozen solid. Now granted 90 degree air has more heat than 20 degree heat, but the concept is the same. So a heat pump extracts heat from the air and moves it elsewhere.
Now an Air Source Heat Pump for a house is just a larger version of your refrigerator's heat pump. OK, so how's that help me heat my house? With a reversing value, the process can be reversed. So if we installed a reversing value on the refrigerator, we could make it heat items inside it instead of cooling, making is De-refrigerator, or an anti-refrigerator. With the reversing value, we would be extracting the heat from the air outside the refrigerator there by making it hotter inside the icebox.
In the winter an Air Source Heat pump extracts heat from the outside air and used that to heat your house and in the summer it extracts the heat from your house and expels it outside, there by making your house cooler.
So how is that better than Gas or Oil? Using a Heat pump the efficiency is something on the order of 2 to 2.5 more time efficient (COP) heating then oil of gas. If you were to have a 90% "High Efficiency" Gas furnace, for every 100 units of fuel you burned, you were get 90 units of heat for your house and 10 units would be lost in the process. With an Air Source Heat Pump, for every 100 units of fuel (electricity) burned (used) you would get 200 to 250 units of heat for your house. So even if electricity is more expensive then gas, the increased efficiency more and makes up for the difference, there by saving you $.
But there a problem. As it get colder outside, the Air Source Heat pump (ASHP) has to work harder and harder to extract enough heat out of the outside air which has less heat in it as it becomes colder and colder. Once the air get down to around 30 degrees, the ASHP can not extract enough heat out of the air to keep your house warm, also it loses it efficiency which made it such a bargain in the first place over gas.
But what if we could use something else besides air to extract heat from? Water from a well is the prefect alternative. For most of the northern United States, the ground temperature deeper than 30 feet down remains around 50 to 55 degrees all year round, thus any water you get from your well is 50 to 55 degrees. Now if we extract the heat from water, instead of the cold outside air, we could heat our house efficiently no matter how cold it was outside. Infact, with Water Based Heat Pumps, or Geothermal as it’s commonly called, can get as high as 5.0 COP, that is for every 100 units of fuel used, we could get 500 units back!
So what happens it you have a well that pumps water from the ground and to your geothermal heat pump. The heat pump extract heat from the water, there by making the water a little colder as it extracts heat from it. The water is then re-injected into the ground via a return well. It’s important to note that the well and the return well have to be far enough apart so that the colder water from the return well is not being sucked up by the main well. There has to be enough space between them so that the colder water has time to be heated by the surrounding water underground before being sucked up again.
No not all ground water can be used with geothermal heating systems, some ground water is unsuitable because it has too many impurities in it that adversely affect the geothermal equipment. In this case you would install a closed loop with consists of plastic pipes buried 8 to 10 feet underground. The water is then pumped in an endless loop under the ground to the geothermal heat pump and back to the underground loop again. As the water is pumped thru the closed loop it absorbs the heat from the surrounding ground before it returns to the heat pump again.
It’s important to understand that the earth is like a sponge. It absorbs heat from the sun in the summer and expels heat with it’s cold in the winter. This is why it stays around 50 degrees all year round for the northern United States. So if it’s hot as 90 degrees in the summer and cold as 20 degrees in the winter, the average would be somewhere around 50 degrees. Places where it’s colder year round, like Alaska, the ground temperature is lower and in places where it’s warmer like Hawaii the ground water is around 85 degrees because it’s not cold enough long enough for the earth to expel it’s heat.
So why it this important? When your heating your house with a closed loop geothermal system, your stealing heat from the earth. If your ground loop is too short, it’s possible to extract all of the heat from the ground over time, thus making it freeze underground. And once the temperature of the ground water drops low enough, the geothermal system is unable to more extract heat from it. A type of anti-freeze is in the close loop pipe to prevent it from forming ice so the pipes don’t crack, and the geothermal system has automatic lock out so it turns off when the water is too cold, but neither of them help you heat your house, they are protection measures.
So how big do your closed loops have to be? Well that would depend on the heating load of your house. The larger the house, the greater the heating load, in theory. It’s possible to have a small house with really lousy installation that has a greater heating load than a much larger house with really good installation. What heat/loss calculation is what really should be performed to give you a correct heating load, rather than run of thumb estimation based on square footage per ton. Once you know how big your load it, your system can be designed with the collect tonnage geothermal heat pump and how large of a ground loop you will need. What’s important to remember, bigger is better. It’s better to add extra footage of pipe to your ground loop, while it will cost you more to install it, it will never hurt your system. Nothing is worse than paying for a geothermal system and finding out half way thru winter your loops are too short and it stops working. The loops need to be large enough so as your extracting heat from the ground, it has time to recover the heat from the surrounding earth before you extract more heat from it.
Last edited by TechGromit; 01-29-2009 at 05:53 AM.
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