Heating and Cooling from The Ground Up

As the earliest cave dwellers knew, a good way to stay warm in the winter and cool in the summer is to go underground. Now scientists and engineers are using the same premise—and using existing technology in a new, more efficient way—to heat and cool aboveground homes for a fraction of the cost of conventional systems.

"At any given occasion, the earth temperature is the seasonal average temperature," said Gunnar Walmet, of the New York State Energy Research and Development Authority (NYSERDA). "In New York state, that's typically about 50°F all year long."

Although the average specific heat capacity of earth has a smaller value than the specific heat capacity of air, the earth has a greater density. That means there are more kilograms of earth than there are of air near a house and that a 1°C change in temperature involves transferring more energy to or from the ground than to or from the air. Thus, in the wintertime, the ground will probably have a higher temperature than the air above it, while in the summer, the ground will likely have a lower temperature than the air. An earth-coupled heat pump enables homeowners to tap the earth's belowground temperature to heat their homes in the winter or cool them during the summer. The system includes a network of plastic pipes placed in trenches or inserted in holes drilled 2 to 3 m F to 10 ft) beneath the ground's surface. To heat a home, a fluid circulates through the pipe, absorbs energy from the surrounding earth, and transfers this energy to a heat pump inside the house. The heat pump uses a compressor, tubing, and refrigerant to transfer the energy from the liquid to the air inside the house. A blower- and-duct system distributes the warm air through the home. According to NYSERDA, the system can deliver up to four times as much energy into the house as the electrical energy needed to drive it. Like other heat pumps, the system is reversible.

In the summer, it can transfer energy from the air in the house to the system of pipes belowground. There are currently tens of thousands of earth-coupled heat pumps installed throughout the United States.

Although the system can function anywhere on Earth's surface, it is most appropriate in severe climates, where dramatic temperature swings may not be ideal for air- based systems.