Midcoast Green

Midcoast Green Collaborative > Home > Articles > Geothermal Comfort

Geothermal Comfort

by: Paul Kando

When solar radiation hits the earth it is converted to heat. This heat is absorbed by the ground where it is stored. A pound of soil absorbs around 1/4 Btu for every degree it is warmed. Four to six feet below the surface the earth maintains a steady temperature of 45 to 50�F. Geothermal systems draw on this stored energy to heat homes and also use this storage capacity of the soil to absorb heat removed from the house during summer cooling.

Ground source heat pump wells
photo credit: Climate Master

Geothermal systems have three main components: (1) a heat exchange loop of refrigerant buried underground, (2) a heat pump to boost the temperature of the refrigerant to make heat flow in the desired direction, and (3) a liquid to air heat exchange system inside the house to either distribute or absorb heat, depending on whether heating or cooling is desired. In heating mode the underground refrigerant loop absorbs moderate temperature heat from the ground and the heat pump boosts this temperature by compression, to be delivered to the indoor heat exchangers. When cooling, heat is collected for the hot interior, boosted by the heat pump and returned, via the buried refrigerant loop, to the ground.

The heat pump and the indoor heat exchangers together work like an air conditioner that can move heat either into or out of a house. The buried refrigerant loop consists of a network of tubes. The quantity of heat transferred across any plane is the product of the thermal transmissivity of the plane, the surface area of the plane and the temperature gradient across the plane. In this case the plane is the thin wall of the refrigerant tubing, and the temperature difference between the ground and the refrigerant circulating in the tubing is small. Therefore, for sufficient heat exchange to take place, the surface area of the tubing must be large. For this reason the tubes must be long.

The refrigerant tubes can be buried horizontally in a 5-6 foot deep trench (e.g. under the lawn); immersed on the bottom of a lake or pond (if such is available); or coiled in one or more relatively deep vertical bores similar to ordinary water wells (except they can be dry). The length of tubing and the rest of the system must be sized to the heating and cooling loads of the house. It is, therefore, important that the house itself be energy efficient. Btu for Btu, energy efficiency usually costs less than mechanical systems and geothermal systems are no exception.

What about costs? The most expensive component is usually the buried refrigerant loop, because of the labor involved in drilling a well or trenching, except when there is access to a lake or pond near the house. A horizontal loop - if sufficient room is available and we don't mind tearing up the lawn or back yard temporarily by trenching - is usually cheaper than drilling a deep well. For most of us wells are still the answer because they take up the least room and involve the least disturbance to the landscape.

According to supplier data, a typical single family residential system may cost $25,000, subject to a 30% tax rebate or $7,500. This leaves a cash balance of $17,500 to be recovered through operating the system. There will be no oil or other fuel to buy, however there will be electric power to be paid to run circulating pumps, fans and the heat pump. Taking all that into account, supplier calculations show that a home that now burns 823 gallons of oil (currently costing around $2,950) can expect $1,832 in first year savings, followed by increasing annual cash savings as oil prices rise, totaling over $24,450 in ten years and over $51,000 by the end of 20 years. (The $17,500 cash investment will be fully recovered in 7 years).

If, on the other hand, the $17,000 is financed with a 20 year loan at 5% interest, cumulative savings at the end of 10 years will still total more than $7,850 and in 20 years more than $43,600. (The $17,500 initial investment, in this case will take a little over 14 years to recover). In both cases cumulative fuel oil costs would total $32,268 in ten years and $98,640 in 20 years, assuming a very conservative 3% annual oil price inflation.

To minimize costs, upgrade your home's energy efficiency first; energy not used is always the least expensive. Leave your existing heating system in place; why pay to remove it? Besides it is backup security, even if you never have to fill that oil or propane tank again.

All things considered, geothermal comfort looks like a good deal to me, especially for those of us now heating with oil or propane. Combined with solar electricity on a south-facing roof (also subject to a tax incentive), it even offers the prospect of a zero energy house at a fairly reasonable cost. Worth investigating - especially for seniors "aging in place" with lifetime savings to invest. $51,000 returned on an investment of $17,000 is not a bad deal - especially when it accrues annually as cash savings, with care-free comfort in the old homestead thrown in as a bonus.