Although geothermal system technologies have been used for many years, interest in these systems for HVAC applications has increased over the past ten years. There are several reasons for this growing interest, including the building owner’s desire to obtain LEED® and ENERGY STAR certification, concerns over global warming, depletion of domestic oil supplies, and the practical interest in reducing operational utility costs.
Traditional air conditioning systems use outdoor air to absorb or reject heat for building heating and cooling purposes. Outdoor air temperatures can fluctuate greatly throughout the day. A geothermal system is more efficient because it uses the nearly constant temperature of the ground to absorb or reject this same heat.
Both the air cooled and geothermal air conditioning systems require a heat pump to heat or cool the building. A heat pump is an electrically powered device that utilizes a mechanical refrigeration system to transfer heat from one place to another. In the cooling mode the pump removes heat from the building and in the heating mode it adds heat to the building.
A Matrix of Vertical Wells
The main component of a geothermal system is the well field. A well field is a matrix of vertical wells drilled into the ground at depths between 200 to 500 feet. The wells are spaced at 15-20 feet apart and are connected together by polyethylene tubing. The tubing, usually ¾” to 1″ in diameter, extends to the bottom of each well in a “U” shape with all wells connected in parallel (not series) fashion. A rule of thumb is that each well will provide approximately one ton (12,000 Btu/hr) of cooling. Geothermal well fields can be quite large for commercial buildings with the well field area equaling the total usable area of the building it serves. Well fields can be located under sports fields and parking lots but should not be located under buildings.
The geothermal well field can be very expensive with each individual well costing approximately $5,000. For a 40,000-square-foot commercial building, the total well field cost would be approximately $500,000 not including piping, pumps and controls. Although expensive, once installed, the well field should last the life of the building.
Return on Investment: Energy Savings
The return on investment for a geothermal system can vary greatly depending on the size and complexity of the building it serves. Typically these systems yield a payback in 10 to 15 years as compared to a traditional air-cooled variable-air-volume (VAV) system. This is not an extremely low pay-back period but it does fall within the useful life of the indoor heat pump equipment. After approximately 15 years, the original heat pumps should be replaced, but the piping, pumps and well field serving the building can be used to deliver continued energy savings.
