Geothermal Heating and Cooling System

A geothermal heat pump is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground (compared to variable open air temperatures) to boost efficiency and reduce the operational costs of heating and cooling systems.

Environmental Considerations

The U.S. Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources.

How Does It Work?

Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia.

Closed Loop Vertical System

The geothermal system being installed in the Southern Living Showcase Home at The Bluffs of Weiss is a closed look vertical system.

Most installed systems have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet within the house where it exchanges heat with a secondary water loop that is buried underground. The secondary loop is typically made of High-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning. The secondary loop is placed below the frost line where the temperature is more stable.

Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells. At the Southern Living Showcase Home we are using the vertical system. A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically 75 to 500 feet (23–150 m) deep. Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer. Thermally enhanced grouts are available to improve this heat transfer. Grout also protects the ground water from contamination, and prevents artesian wells from flooding the property. Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced at least 5–6 m apart and the depth depends on ground and building characteristics. For the Southern Living Showcase Home, our system is engineered for a house needing 10 kW (3 ton) of heating capacity. We are using xxx boreholes approximately xxx feet deep.

Economics

Ground source heat pumps are characterized by high capital costs and low operational costs compared to other HVAC systems. The initial cost can be two to five times that of a conventional heating system in most residential applications, new construction or existing. Their overall economic benefit depends primarily on the relative costs of electricity and fuels. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system. At the Southern Living Showcase Home we are estimating annual utility savings of approximately xx%.

Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally.