Geothermal Heat Pumps
Geothermal heat pumps use the constant temperature of the earth as an exchange medium for heat. Although many parts of the country experience seasonal temperature extremes—from scorching heat in the summer to sub-zero cold in the winter—the ground a few feet below the earth's surface remains at a relatively constant temperature.
Depending on the latitude, ground temperatures range from 45°F (7°C) to 75°F (21°C). So, like a cave's, the ground's temperature is warmer than the air above it during winter and cooler than the air above it in summer. Geothermal heat pumps take advantage of this by exchanging heat with the earth through a ground heat exchanger.
Geothermal heat pumps are able to heat, cool, and, if so equipped, supply homes and buildings with hot water. A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork), and a heat exchanger—a system of pipes buried in shallow ground. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water.
There are four types of geothermal heat pump systems. Three of these—horizontal, vertical, and pond/lake—are closed-loop systems. The fourth type is open-loop. Which is best depends on the climate, soil conditions, available land, and local installation costs at a particular site. All of these approaches can be used for residential and commercial building applications.
Closed-Loop Systems
Horizontal
This type of geothermal heat pump is generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. It requires trenches at least 4 feet deep. The most common layouts use two pipes, one buried at 6 feet and the other at 4 feet, or two pipes placed side-by-side at 5 feet in the ground in a 2-foot-wide trench. The Slinky™ method of looping pipe allows more pipe in a shorter trench, which cuts installation costs and makes horizontal installation possible in areas that would not be viable for conventional horizontal applications.
Vertical
Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops is prohibitive. Vertical loops are also used where the soil is too shallow for trenching, and they minimize the disturbance to existing landscaping. For a vertical system, holes (approximately 4 inches in diameter) are drilled about 20 feet apart and 100–400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building.
Ponds or Lakes
If a site has an adequate body of water, a water-source heat pump may be the lowest-cost option. A supply line pipe is run underground from the building to the water and coiled into circles at least 8 feet under the surface to prevent freezing. The coils should be placed only in a water source that meets minimum volume, depth, and quality criteria.
Open-Loop System
This type of system uses well or surface body water as the heat exchange fluid that circulates directly through the heat pump system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or surface discharge. This option is practical only where there is an adequate supply of relatively clean water and all local codes and regulations regarding groundwater discharge are met.
