Word is getting out, and TVA’s Large Commercial Marketing Manager, Bill Stalker, is a busy man these days. “As soon as a system is installed at a school or business, folks start calling me,” he says. “They’ve heard how much money it’s saving, and they want to see how they can get the same thing.”
Stalker is describing the reaction in the Tennessee Valley to geothermal heat pumps, which have been installed at a number of schools and businesses and have earned a growing reputation for low-cost, low-maintenance cooling and heating.
Frequently asked questions:
How do geothermal heat pumps work?
Underneath the frost line, the ground stays at a constant temperature of about 57 degrees Fahrenheit. That happens to be an extremely efficient operating temperature for heat pumps. Geothermal heat pump systems circulate water between an underground water loop, called an earth heat exchanger, and water-to-air heat pumps located throughout a large building.
The earth heat exchanger is most often a network of high-density polyethylene piping buried in vertical boreholes 150 to 300 feet deep. Each of the heat pumps in the building is connected to the circulating water loop. The temperature is controlled independently at the individual heat pumps located in rooms throughout the building.
The constant temperature of the earth heats or cools the circulating water loop as needed to balance the building’s year-round heating-and-cooling requirements. If your room needs heat, you turn up the thermostat and heat is pumped from the water loop into the room. If your room needs air conditioning, you turn down the thermostat and excess heat is pumped out of the room into the water loop.
Have there been studies done that can aid in the design of a system?
TVA has done much work in developing software and design procedures to properly size geothermal units based on building loads and other factors. One of those factors is the thermal conductivity (TC) of the earth. TVA has completed a study that documents the TC testing done in the Tennessee Valley and overlays the sites on a geology map in order to assist the design engineer in the preliminary system design. The results of that study can be viewed here.
What advantages do geothermal heat pump systems offer?
- A 25 to 40 percent reduction in heating and cooling costs
- Standard, simple controls
- No need for a highly specialized chiller technician or boiler operator
- Highly durable piping (the life expectancy is between 30 and 50 years)
- No high-maintenance, freezing-prone cooling tower
- No boiler to clean or maintain
- No air conditioning equipment on roof to cause leaks
- No harmful chemicals
- No danger of fire, asphyxiation, or explosion from coal, gas, or oil
- Nothing outside to vandalize or steal
- No central system to fail or shut down the entire building.
What does it cost to install a geothermal heat pump system?
Costs vary from project to project. Installation of the system at Daniel Boone High School, a 160,000-square-foot facility in Washington County, Tennessee, was completed in April 1996 at a cost of $551,000. Although the installation costs for geothermal systems can be higher than for other less-efficient systems, the cost is rapidly offset by substantially lower utility bills. The Daniel Boone system should pay for itself in a little more than five years.
What if there is an underground leak in the system?
Leaks are rare, but the piping system is designed to make it easy to find and repair a leak. Pipes can be grouped in clusters of 20, with each cluster having its own valve. Even if a leak occurs, the system will continue to run while one of the clusters is closed off from the system for repairs.
