Monday, 4 April 2011

Geothermal Heat Pumps - A Technology That We Should Seriously Consider For Our Homes

Geothermal Heat Pumps - A Technology That We Should Seriously Consider For Our Homes

In yesterday's post I focused on one of my pet technologies - geothermal energy. However, despite the long term sustainability of this technology, the initial capital cost to tap into warm water sources of direct geothermal energy can often be too expensive with too few suitable sites to make a big impact. Now, consider geothermal heat pumps, a technology that we can use at our homes without the need for a "hot springs" nearby. The basics.

If we dig down about 10 feet, we will find temperatures in the soils typically ranging from 50 - 54 degrees F - and very stable through all seasons. Heat can be extracted from about any source no matter how cold, even in Minnesota and Michigan. A ground source heat pump uses the shallow ground or ground water as a source of heat, thus taking advantage of its seasonally mild temperatures. For example, let's say that it's 40 degrees F outside. To heat our home, we need to raise the temperature to say 70 degrees to be comfortable. And we typically rely on either electricity or natural gas to make up this difference - heating from 40 up to 70. However, if we have an in ground source of energy that is already at 54 degrees with a heat exchanger, then we need only rely on electricity or natural gas to get us from 55 to 70 - a considerable savings in energy expended to heat our home.

Similarly, in the summer, if the outdoor ambient temperature is, say, 80 degrees F but we want our home to be air conditioned and maintain in home temperatures in the 70 to 75 degree range, then we can draw on the lower temperatures in the ground, with a heat exchanger, to do much of the work of reducing in home temperatures. And this is what geothermal heat pumps do - reduce both our home heating and air conditioning expense. Following is a graphic of "closed loop" geothermal heat pump systems typically used in the Pacific Northwest:

Note both the supply and return lines. Geothermal pipe is installed in the gound in a closed loop system. In the winter when heating is needed, a carrier fluid (typically a water/antifreeze mix) is circulated through pipes located in the ground. As the fluid circulates underground, it absorbs heat from the ground and on its return the now warmer fluid passes through the heat pump - requiring less natural gas or electricity use by the heat pump to raise temperatures to the desired level. Spent fluid is recycled back into the ground to be heated up again. The same is true in the summer, except in reverse to provide cooling.

The above graphic shows two alternative closed loop systems - (1) a shallow system spread out across reasonable large spaces and (2) a deep system designed to minimize the land footprint area required, but requiring much deeper depths. Both work very well depending on your homesite footprint area available.

I have learned a lot about the realities of geothermal from a contractor in Seattle who specializes in "green building" - David Delfiner aka Lisa's Parson's husband. For those not aware, Lisa is executive director of the Middle Green River Coalition and she has contributed enormously to the open space areas we now have available to us in the Green River Gorge and watershed. David reports very satisfied customers, because it's so simple. Makes sense to me.

Looking at the economics, I'm not sure that we can make this pencil for us at this time. We have more than enough land to make it work but the total installed cost of about $14,000 seems hard to get a reasonble payback on. According to one supplier, we would save about $950 per year from our investment. The only problem for us is that this includes savings in air conditioning costs during the summer - and we just don't air condition today. As it turns out, we have another geothermal resource called a basement. If it gets too hot upstairs, just go sleep in the basement and all is well.

I will say this. If we were building a new home - there is no question that geothermal heat pumps would be part of our design.

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