Geothermal heat pump systems, also known as ground source heat pumps, are growing in popularity as owners seek cost-effective ways to reduce energy use.
By using the ground as a heat sink, energy can be pumped out of the building in the summer and stored in the ground for later use. When the building needs heat or hot water, the refrigeration cycle is reversed. The earth will provide 75 percent to 82 percent of heating energy for free, and the moderate temperature underground makes cooling more efficient.
Residential-style units, mounted in a mechanical room, closet or above the ceiling, are the most common type of heat pump used for geothermal systems. Water-to-water heat pumps can be connected to most types of water-based HVAC systems and are often used for domestic hot water systems. Geothermal variable refrigerant flow is another popular option; it combines the best attributes of two high efficiency systems at a relatively affordable cost.
Geothermal systems don’t have to be expensive. A well designed system often costs less to install than traditional HVAC systems with boilers and chillers. Even when compared against cheaper systems, they often still have a lower life cycle cost. In addition to energy savings, the ASHRAE database indicates that geothermal systems are typically cheaper to maintain. They also last longer. The well field piping in the ground typically has a 50-year warranty, and the indoor heat pumps are protected from the elements and extreme temperatures.
Designing cost-effective geothermal heat pump systems requires a special skill set. Gas lines and cooling towers are sized for the worst day, and act as unlimited heat sources and sinks. A geothermal heat exchanger has finite heat storage ability. It functions like a thermal battery with some ability to transfer the annual heat imbalance to or from the surrounding ground. The rate of heat transfer is a function of the ground heat exchanger design and the ground conductivity. Engineers who are proficient at designing cost-effective geothermal systems are skilled in energy modeling, aggressive at cost cutting and knowledgeable in geology, drilling and heat exchanger design. Owners seeking experienced geothermal designers should consider engineers accredited through the Certified GeoExchange Designer program.
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