Geothermal Energy

Can I Save Money With Geothermal Heat Here In Vermont?

We all know the frost only goes so deep arround here, and the gound underneath stays at a fairly constant temperature above freezing. It is possible to transfer some of the heat from the ground to your home. There are systems on the market that are designed to do this. In practice however, the amount of energy you can save from geothermal isn't going to pay for the equipment you would need. The reason APS does not endorse geothermal as a viable alternative energy source is detailed below.

Too Much to Pump

If you were to design a geothermal heating system, the first thing to consider is how much heat you need to pump. For arguments sake, lets use a standard whole home heating load of 70,000 BTU/hr (see boiler sizing). In other words, we need to extract 70,000 BTU per hour from the ground to heat an average size, well insulated home.

To transfer the heat, we'll use water pumped through pipes burried deep in the ground. If it were a perfect system, we might be able to get 48°F from the earth, get 10°F out of it, and send it back at 38°F through the loop where it would gain the 10°F back from the gound.

In order to figure out how much water we'll need to pump, we will need to use some well known scientific facts about water and the amount of heat it can carry. You can get these facts from a good dictionary or from your local library. The calculation goes something like this:

1 gram ≡ 1 cm³ water (at 4°C) 1 calorie ≡ Quantity of heat required to raise 1 gram of water 1°C (at 15°C) 1 BTU ≈ 252 calories 1 gallon = 3784.3 cm³ 10°F ≈ 5.6°C

Converting from our home heating load (BTU/hr) to gallons of water per minute:

70,000 BTU/hr × 252 calories/BTU = 17,640,000 calories/hr 17,640,000 calories/hr × (1 grams water • 1°C)/1 calorie = 17,640,000 grams water • 1°C per hr 17,640,000 grams water • 1°C per hr ÷ 5.6°C = 3,150,000 grams water per hour 3,150,000 grams water per hr ÷ 3784.3 cm³/gallon = 832 gallons per hour 832 gallons per hour ÷ 60 minutes per hour = 13.9 gpm

Already, we see that the volume of water we would need to pump is very large. It is possible to pump this ammount of water using the largest commonly available pumps. However, after adding for real world factors (no system is perfect), you would need to use at least two pumps and two burried loops. The system may well work, but the energy to heat your home is really comming from the electric meter, not the ground.

There are two cases which would change the feasibility of using geothermal energy: 1) if you have a high temperature source of water from which you can extract significantly more than 10 degrees worth of heat (a hot spring) or 2) you live in a milder climate where your home heating load is significantly less.

Unfortunately, this isn't the case for the majority of Vermonters. On the bright side, there are practicle alternative energy systems for us here in the north.