Tuesday, April 17, 2007

Heat Pumps and GeoThermal

I've mentioned previously that low grade geothermal systems are an effective way to substantially decrease our fuel usage. I've also mentioned that electricity is not an efficient way to create heat. However, electricity can be an efficient way to pump heat, and that is the key to low grade geothermal practicality.

Heat pumps are, of course, quite common. They effectively pump heat from lower temperatures to higher temperatures. In so doing they can make electricity quite effective at heating or cooling applications. While common usage generally applies the term heat pump to a device that warms your house, your air conditioner is also a heat pump, pumping heat from inside your house to outside. When the temperature difference across which the heat is pumped is not too large, heat pumps are quite efficient. Unfortunately, as the temperature difference increases, the efficiency declines. When heating your house with an outside temperature of 50 degrees, the system is only pumping against a temperature difference of 20-30 degrees. However, if the outside temperature is 20 degrees, you are pumping against a temperature difference of 50-60 degrees and will have substantially lower efficiency. This means that your peak demand coincides with the lowest efficiency of the system. The same applies to your air conditioner. And this fact makes a marriage between your heat pump and geothermal energy a sweet deal.

You see, just a few feet below the surface of your yard, the temperature is close to the average for your area year around. That means substantially less difference in temperatures between the heat source and the house at the extremes, when demand is highest. In fact, for most of the south, the ground temperature is close to 70 degrees, resulting in temperature differences of less than 10 degrees for both heating and cooling. Better yet, the variation in earth's temperature just a few feet down does vary a few degrees, but it lags the actual average air temperatures by 2-3 months, which means that the coolest temperatures don't occur until March/April, when the heating season is nearly over. The warmest temperatures occur in Sept/Oct, when the cooling season is nearly over.

Beyond this, in normal applications today, the benefit is collected for only one side of the application. When your air conditioner is running, it pumps heat to outside, where it is wasted. In winter, when your heat pump is heating inside, meanwhile cooling outside rather than using the cooling for applications inside your home. Low energy prices over the last several decades have resulted in chosing simplicity over efficiency, but higher prices will lead to using both the cooling and the heat that result from the heat pump. And, using the earth to make up the difference in heat and cooling demands and to lower the effective temperature difference will result in a much higher efficiency.

All just another reason why I believe energy prices will stay about where they are for the long term. The heating and cooling needs of your house use the majority of the energy used in your home. These can be made much more efficient. And, at today's prices, it is becoming economical to do so.

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