Geothermal exchange heat pump
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A geothermal exchange heat pump is a heat pump that uses the Earth as either a heat source or a heatsink depending on the mode of operation: heating or cooling. Geothermal heat pumps are either open or closed systems and typically consist of a loop of pipe, a compressor, and a pump. Closed systems circulate a fluid which never leaves the heat pump system. Open systems pump a fluid from one place to another, remove or add heat, but do not return that fluid.
The pipe in a closed loop system is installed under the surface of the Earth below the frost line in either a horizontal layout or a vertical layout for space restricted areas. The length of the pipe varies with the heating or cooling requirements, the soil type, pipe installation layout, and the location of the installation. The pipe can be installed in several different configurations including both a spiral pattern and a straight pattern.
A typical 2,000 sq. ft. (185.8 m²) residence will require four tons of heating capacity. Each ton of heating capacity requires approximately 91 m (300 ft) of vertical loop or 122 m (400 ft) of horizontal loop. Thus the typical residence will require 366 m (1200 ft) of vertical loop or 488 m (1600 ft) of horizontal loop.
A fluid is circulated through the pipe and absorbs heat from the Earth in heating mode or sinks heat into the Earth in cooling mode. The fluid is typically water. In the case of closed systems an antifreeze such as propylene glycol, denatured ethanol or methanol is often added.
The Earth below the frost line remains at a relatively constant temperature year round, usually between 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on geographical location. This property allows geothermal heat pumps to perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces.
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Heating mode
In the heating mode, the external fluid returns from the earth (heat field) at 8-16 degrees Celsius and passes through the heat exchange unit. Within the heat exchanger the internal fluid is allowed to expand and change state into a gas (vaporization) drawing the heat of vaporization from the external fluid. This gaseous fluid is then pumped to the compressor which compresses and liquefies the fluid releasing the heat of vaporization into the airflow within the household heating system. The cooled external fluid is then pumped back into the loop (closed loop system) running outside the house, where its temperature is lower than the temperature of the surrounding soil. It once again absorbs the heat from the earth and the cycle repeats.
Cooling mode
In the cooling mode, household air is drawn through a heat exchange unit where the internal fluid is allowed to expand and evaporate absorbing the heat of vaporization from the air. The gaseous fluid is then pumped to the compressor where it is compressed back into a liquid releasing the heat of vaporization into the external loop via a second heat exchanger. The fluid in the external loop is then pumped out into the heat field where its temperature is higher than the temperature of the surrounding soil. The soil absorbs the heat and the cooled fluid then returns to the house to repeat the cycle.
External links
- Canadian Geoexchange Coalition (http://www.geo-exchange.ca/EN/home.html)
- Geothermal Heat Pump Consortium (http://geoexchange.org/about/how.htm)
- US Dept. of Energy (http://www.eere.energy.gov/consumerinfo/heatcool/hc_space_geothermal.html)