|Fig. 1: Pictorial representation of a GSHP with vertical underground loops. The condenser, valve and compressor would generally be located in the basement while the evaporator could be run throughout the house to absorb the heat from the air.|
An estimated 31.2% of the energy used in US households goes toward heating and cooling (HVAC), with an additional 9.1% going to water heating.  Ground source heat pumps (GSHP) offer the ability to cut that usage by up to 1/3. Ground source heat pumps transfer heat "uphill" from a cold source to a hot one. During the winter, the heat from the ground would be transferred into a building. However, during the summer the heat from the building will be transferred to the ground.
There are many ways to design a GSHP. The design most appropriate for an urban setting would have three main sections: an underground closed liquid loop, a "heat absorption" subsection (H.A.S.) and a "heat rejection" subsection (H.R.S). The details of the three main sections will be described in their summer mode. In the winter, the same system can be used to heat the building with the use of a bypass valve that reroutes the liquid medium.
The best way to describe GSHP in summer mode is to start with the H.A.S., located in the building in contact with the indoor air. This section absorbs the heat of the building in order for the H.R.S. to ultimately transfer it to the ground. The H.A.S. consists of an expansion valve and an evaporator. A pressurized refrigerant is expanded over the valve and into the evaporator. During the process, the refrigerant goes from a liquid to a vapor by absorbing heat from its environment. The vapor then goes into the H.R.S.
The goal of the H.R.S. is to dump heat from the H.A.S. to the underground closed liquid loop. This portion of the GSHP consists of a compressor and a condenser. The compressor compresses the refrigerant to a higher pressure, and doing so increases the temperature of the fluid above the temperature of the ground water loop. The refrigerant then enters the condenser where vapor condenses into a liquid in a process that rejects heat to its environment, which in this case is the underground water loop. 
The underground liquid loop is a closed loop of a water and glycol solution that dumps heat to the ground. There are two types of designs for soil-based underground loops. One type uses vertical loops submerged 50 to 100m below ground level (see Fig. 1). This design is most suitable for cities, as it uses less land. It is, however, more expensive than the second design that uses shallow horizontal loops submerged 1 to 2m below ground.  The hollow loops require less drilling, but involve more land. In either case, the water or glycol is pumped through the underground loops, and ultimately transfers heat from the heat rejection subsection to the ground.
The Carnot cycle is the gold standard by which heat engines, refrigerators and heat pumps are measured. From analysis of the Carnot cycle the figure of merit, meaning the best that a heat pump can do, is quantified with the equation
The coefficient of performance 1/η gives the highest efficiency that one can expect for a given environment. For example, η = 1 means that 1 KWh of energy was used to transfer 1K Wh of heat into a desired space (winter mode) or out of a desired space (summer mode).  A heat pump 1/η can vary from 2.8 - 3.6 depending on the environmental temperature and on the design of the model.  It is worth noting that in the USA, the coefficent of performance 1/η for the cooling mode of a heat pump is called the EER (Energy Efficiency Ratio). The EER has units of BTU/W-h; thus it is a ratio of heat output in BTU to the energy input in Watt-Hours.
A back-of-the-envelope calculation can be done to determine the energy savings if a home were to switch to a GSHP for summer mode operations. Let's assume that a typical home has a heating load of about 10.5kW, which is utilized for 1000 hours per year.  This gives a total energy usage of 36000 kBTU per year of heat removed from a home. Let's also assume that a typical air conditioning unit has an EER of 10 BTU/W-h, and the GSHP has an EER of 14. [4,5] Thus, this typical home would need 3600 kW-h of electricity for a standard air-condition while a home with a heat pump would only need 2800 KW-h of electricity. With a cost of 12 cents a kW-h, a home with a GSHP would spend $342 for cooling a year, while a home with a conventional air conditioner would spend $504 . Given that the cost of installation for GSHP can be in the thousands, it can be easy to see why the average home may be reluctant to make the investment to a GSHP.
© Etosha Cave. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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