How Geothermal Works

Geothermal Heating and Cooling Systems

The Sun in fact heats much of the Earth’s top layer of surface, thus Geothermal is a derivative of Solar energy. Every part of the Earth’s atmosphere experiences changes in temperature, depending upon the season, and location relative to the Equator. No matter where you are, however, the temperature of the ground just a few feet below the earth’s surface remains relatively the same, regardless of the temperature of the earth’s atmosphere. This condition is able to be maintained principally because of the earth’s absorption of almost half of the sun’s energy as it shines on the earth every day, resulting in an almost uniform 45° F to 62 ° F degree temperature range of the ground 10 to 40 feet below the Earth’s surface. This temperature uniformity can be tapped for both winter heating and summer cooling, through the use of an underground loop system or loop field that acts a heat sink for cooling in the summer and a heat source for heating, as described below.

Geothermal Heating

During the cold season, the loop system circulates fluid that extracts heat from the ground. The heat extracted from the ground is passed through a radiant floor heating coil, or through a standard heat exchanger and air duct system creating warm air. Domestic hot water heating may also be assisted with the warm fluid circulated through the loop system.

Geothermal Cooling

In the warm season, the opposite effect is produced. Warm air is passed over air conditioning coils that have been assisted by the loop system’s cool temperature, extracted from the Earth, and the heating effect is reversed. The loop system acts as a heat sink and draws warmth out of the air.

Geothermal Loop System

Although geothermal loop field systems are initially more expensive to install, the lifetime maintenance and management expenses of these types of systems are much less than other renewable energy systems, since they have fewer moving parts, and much less exposure to the elements.

Loop Field System Design Factors

There are several methods to create a loop field for movement of heat and cooling effects in a geothermal system. These fields can be placed horizontally at a depth of from 10 to 40 feet underground and vertically from 100 to 400 feet deep. Loop fields are generally composed of polyethylene piping, and costs can range from $0.20 per foot of bore for 3/4 inch pipe to $1.00 per foot of bore for 1 1/2 inch pipe. The range of costs of vertical ground coils range from $2.50 to $7.00 per foot. The required bore lengths range from 125-150 feet per ton of heating capacity for cold climates in high initial load buildings and 300 feet or more per ton for warm climate systems; the larger the polyethylene piping, the shorter the bore lengths. The use of hybrid designs can reduce this ratio.

In the case of a vertical loop field, the drilling costs can range from $1.00 to $12.00 per foot and perhaps more, if there are low water table issues. The added costs of vertical ground coils typically range from $500 to $1,000 per ton for residential installations and more for complex commercial installations. Final installed cost can be measured in dollars per ton of BTU heating and cooling and will vary depending on the amount of ground, type of soil, latitude of the building, age of any existing HVAC equipment on site and other factors that need to be assessed on a case-by-case basis. Many factors need to be considered in order to arrive at a properly designed system for each application, and no two system profiles will ever be exactly the same.

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