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Geothermal Specifics
 Closed-Loop – Vertical-Bore Systems
Closed-Loop systems are made up of a series of pipes buried under ground that are connected to one another and to the building, creating a continuous loop from the building to the ground. The sealed piping system contains a fluid made up of water, antifreeze, and various additives.
The piping is made from high density, high grade polyethylene, similar to that used for cross country natural gas piping, and often comes with a fifty year warranty. In a Vertical-Bore application, the ground loops are placed in deep vertical holes drilled in the earth. The holes in the bore field are about 4 to 6 inches in diameter and are typically 100 to 300 feet in depth and are spaced 8 to 20 feet apart. Each boring contains a U-shaped pipe which allows the fluid to make one complete loop per hole. The remaining space inside the boring, not occupied by the piping, is filled with a grout to seal the hole from potential ground water penetration.
 The grout also provides the means for thermal contact between the pipe and surrounding earth. New grout mixtures are available that improve the ability of the grout to exchange heat with the surrounding ground.
Thermally enhanced grout, developed for use in geoexchange systems, can double or nearly triple the ability to exchange heat with the ground by controlling the sand particle size used in the grout formation. The depth and number of bores are directly related to the size of the heating and cooling loads and the ground thermal properties. The loads are defined by the size of the building, type of construction, use of the building, duration of the heating and cooling seasons, and climate.
Making the Connection
Every hole containing a U-Tube is connected together via a manifold with shut-off valves allowing each circuit to be isolated. The header combines the flow through all circuits before going to the building portion of the loop.
The header can be connected outdoors in a valve pit or all of the circuits can be brought into the building before being combined. The manifold is connected to a flow center where the fluid is pumped through the heat exchanger.
Heat Exchangers
The pumping or exchanging of energy is done so by the use of heat pumps or heat exchangers. A heat pump is a refrigeration device that works by the same concept as a household refrigerator. Refrigerators, air conditioners, and heat pumps all operate by pumping refrigerant through a closed loop in a way that creates two distinct temperature zones – a cold zone and a hot zone. Heat pumps use the principle that heat always flows from a hot area to a cold area, and use a refrigeration cycle to move the heat from a colder to a hotter temperature, concentrating it. They create a cold zone in the area where heat is to be extracted and a hot zone in an area where heat is to be dumped.
 When a geothermal heat pump heats, fluid from the ground loop flows across the heat exchanger tubes containing refrigerant which is colder that the loop fluid. Since the primary principle of heat transfer is that heat always flows from a higher to a lower temperature, the colder refrigerant absorbs heat from the hotter fluid flowing through the ground loop and evaporates within the tubing. The cool refrigerant gas is then compressed and pumped to the high temperature section which is often configured as a refrigerant coil with air blowing across it. Because the refrigerant becomes hotter that the air when it is compressed, it gives up the heat to the relatively cooler air from the space. As the refrigerant gives up heat to the airstream, it condenses back to the liquid. 
The liquid passes through a restriction that maintains the pressure difference between the hot and cold zones. As the pressure of the liquid drops, it vaporizes and its temperature drops to the cold zone temperature where it begins the refrigerant process again.
In order to provide cooling, a heat pump has a reversing capability so the hot zone and the cold zone can be swapped. With the zones reversed, heat is extracted from the indoor air and transferred to the ground loop.
Inside the Building
Geothermal systems are a distributed system rather that a central system. The distributed nature of the geoexchange system contributes to its overall efficiency. Thermal energy is moved around the building efficiently with water rather than air via the water loop. A heat pump in each space (zone) rejects or extracts heat from the loop to maintain the desired temperature. Other systems circulate large volumes of air to provide space conditioning. A central system may supply cooled air to all spaces with individual spaces reheating the air to maintain the desired temperature. Geothermal systems often save on fan energy as they use many smaller fans to blow air through short ducts at low pressure. Other systems use extensive duct systems that transport air greater distances at a higher pressure.
Fresh air is often introduced through a dedicated outdoor air system. This system preconditions the outdoor air by recovering energy from the exhaust air stream through a heat exchanger. A heat pump tempers the ventilation air to neutral conditions before it is distributed to the heat pumps serving each room. Providing ventilation air, via a separate system, ensures that the proper amount of fresh air is delivered to each space. There is no mixing of fresh air with re-circulated air until it reaches the room heat pump.
The air distribution system is smaller than the air system in a conventional system because it contains no re-circulated air. Only the required outdoor air is delivered to each space as opposed to a central system that often over-ventilates many zones. The fan energy is minimized because the air can be delivered at lower pressure and there is no damper or coil to pass through at each room.
For the most part, the space conditioning of each room is independent of other rooms. The only common reliance is on the ground loop. Any problem with a heat pump only affects the room it serves and cannot impact the performance or energy use of the entire system.
Large Spaces
Large open spaces often require more heating and cooling capacity than a single heat pump can provide. In moderately sized spaces, multiple heat pumps can meet the space needs. In larger spaces, systems often employ standard two-speed air handler units with heating and cooling supplied by water-to-water heat pumps. These heat pumps condition water rather than air like those used within a space. The water-to-water heat pumps come in larger sizes and can be ganged together to achieve larger capacities.
Water Heating
A geothermal system moves heat from the ground to heat the building. It is also possible for the geothermal system to move heat from the ground to heat hot water to 125 degrees Fahrenheit. Integrating water heating with a geoexchange system is particularly effective when the cooling load dominates the sizing of the ground loop. The need to reject heat to the ground can be reduced by using some of that energy to heat hot water. These systems require additional engineering and design planning.
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