Geothermal Heat Pumps, the most common use of geothermal energy systems in green homes, use stable ground or water temperatures near the earth’s surface to provide heating, air conditioning, and in most cases, hot water. Because they use the earth's constant temperatures, they are among the most efficient and comfortable (not to mention sustainable) heating and cooling technologies currently available – and are becoming a popular alternative to oil, coal, and gas. With the ability to deliver comfortable heat even on the coldest days, cost effective equipment, and requiring only a small amount of electricity to operate, geothermal systems are a good consideration for a green home.
Three kinds of systems – Open Loop, Closed Loop, and Standing Column Well Systems – circulate water in pipes below the earth, where water is heated, then delivered into the home for use.
Open Loop Systems are the easiest to install, but because of local codes and the amount of ground water available, are not the most popular kind of geothermal system. In this system, ground water from an aquifer is piped directly to a heat pump inside the home. After it leaves the building, the water is pumped back to the same aquifer by a second well, called a discharge well, located a good distance from the first.There are three types of Closed Loop Systems available depending on your site characteristics: horizontal ground, vertical ground, and pond systems.
Horizontal ground closed loop systems: If your site permits and trenches are easy to dig, this may be the best system for you. Backhoes or trenchers are used to dig trenches three to six feet below the ground surface and a series of connected plastic pipes are laid in long loops or in a slinky pattern, depending on the amount of area available. A typical system will be 400 to 600 feet long per ton of heating and/or cooling capacity. This system is easy to install while the home is under construction, but can also be installed as a retrofit to an existing home with minimal site disturbance.
Vertical ground closed loop systems are favorable for sites where yard space is limited or rocky, or for retrofitting projects with the least amount of site disturbance. Vertical holes are bored into the ground between 150 and 450 feet deep. Each hole contains a single loop of pipe with a U at the bottom and a horizontal pipe under the ground near the home which carry fluid to and from the geoexchange system. While these types of systems can be more expensive to install, they require less piping than the horizontal loops.
Pond closed loop systems are very economical if your site is near a lake or pond. Polyethylene piping is run underground to the water source, and then long sections of pipe are submerged under the water. While pond closed loop systems do not have any adverse affects on the water source, experts do recommend using this system only if water levels never drop below six to eight feet at its lowest level for sufficient heat-transfer capability.
Standing Column Well Systems are common in the northeast United States. Standing wells are typically six inches in diameter and as deep as 1500 feet. Temperate water is drawn from the bottom of the well, circulated through the heat exchanger, and returned to the top of the water column. Most of the year, they re-circulate water between the well and the water pump, but during peak temperature months, they can bleed some of the water from the system. This causes the groundwater to make up the flow, cooling the column and the surrounding ground in summer and conversely heats the column and surrounding ground in winter, restoring the well water temperature.
Be sure to discuss your plans to implement a geothermal energy system with your local environmental board, check with local authorities regarding permits, and always use experienced contractors.
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