CLOSED LOOP SYSTEM
A closed loop system, the most common, circulates the fluid through the loop fields' pipes and does not pull in water from a water source. In a closed loop system there is no direct interaction between the fluid and the earth; only heat transfer across the pipe. The length of vertical or horizontal loop required is a function of the ground formation thermal conductivity, ground temperature, and heating and cooling power needed, and also depends on the balance between the amount of heat rejected to and absorbed from the ground during the course of the year. A rough approximation of the initial soil temperature is the average daily temperature for the region. Although copper and other metals can be used, polyethylene seems to be the most common tubing material used currently by installers; often 3/4 inch (19mm) inside diameter tubing.
There are four common types of closed loop systems; vertical, horizontal, slinky, and pond. (Slinky and pond loops depicted below.)
VERTICAL CLOSED LOOP FIELD
A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically, 150 to 250 feet deep (45-75 m). Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock to maximize the heat transfer. Vertical loop fields are typically used when there is a limited square footage of land available. Bore holes are spaced 5-6 m apart and are generally 15 m (50 ft) deep per kW of cooling. During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.
HORIZONTAL CLOSED LOOP FIELD
A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped coils are placed horizontally inside the same trench. A trench for a horizontal loop field will be similar to one seen under the slinky loop field; however, the width strictly depends on how many loops are installed. Horizontal loop fields are very common and economical if there is adequate land available.
SLINKY CLOSED LOOP FIELD
A slinky closed loop field is also installed in the horizontal orientation; however, the pipes overlay each other. The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then move your hands in opposite directions. A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation.
CLOSED POND LOOP
A closed pond loop is not as common, but is becoming increasingly popular. A pond loop is achieved by placing coils of pipe at the bottom of an appropriately sized pond or water source. This system has been promoted by the DNR (Department of Natural Resources), who support geothermal systems and the use of ponds for geothermal systems. A pond loop is extremely similar to a slinky loop, except that it is attached to a frame and located in a body of water versus soil.
OPEN LOOP SYSTEMS
In contrast to the closed loop systems, an open loop system pulls water directly from a well, lake, or pond. Water is pumped from one of these sources into the heat pump, where heat is either extracted or added. The water is then pumped back into a second well or source body of water. There are three general types of systems: First water can be pumped from a vertical water well and returned to a nearby pond. Second, water can be pumped from a body of water and returned to the same body of water. Third, water can be pumped from a vertical well and then returned to the same well. While thermal contamination (where the ground temperature is affected by the operation of the system) is possible with any geothermal system, with proper design, planning, and installation any loop configuration can work very well for a very long time. Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually much more efficient than closed systems because they will be heat exchanging with water always at ground temperature. Closed loop systems, in comparison, have to make do with the inefficient heat-transfer between the water flowing through the tubing and the ground temperature.
One of the benefits of an open loop system is that for most configurations and depending on the local environment you are dealing with ground water at a constant temperature of about 50°F/10°C. In closed loop systems the temperature of the water coming in from the loop is often within 10°F/6°C of the temperature of the water entering the loop showing how little heat was exchanged. The constant ground water temperatures significantly improve heat pump efficiency.
STANDING COLUMN WELL
A standing column well system is less expensive and more efficient than a comparably sized closed loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where traveling downwards it exchanges heat with the surrounding bedrock. The choice of a standing column well system is often dictated where there is near-surface bedrock and limited surface area is available. A standing column is typically not suitable in locations where the geology is comprised of mostly clay, silt, or sand. If bedrock is deeper than 200 feet from the surface, the cost of casing to seal off the overburden may become prohibitive.
A multiple standing column well system can support a large structure in an urban or rural application. The standing column well method is also popular in residential and small commercial applications. There are many successful applications of varying sizes and well quantities in the many boroughs of New York City, and is also the most common application in the New England states. This type of Earth-Coupling system has some heat storage benefits, where heat is rejected from the building and the temperature of the well is raised, within reason, during the Summer cooling months which can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. As with closed loop systems, sizing of the standing column system is critical in reference to the heat loss and gain of the existing building. As the heat exchange is actually with the bedrock, using water as the transfer medium, a large amount of production capacity (water flow from the well) is not required for a standing column system to work. However, if there is adequate water production, then the thermal capacity of the well system can be enhanced by periodic discharge during the peak Summer and Winter months.
Since this is essentially a water pumping system, standing column well design requires critical considerations to obtain peak operating efficiency. Should a standing column well design be misapplied, leaving out critical shut-off valves for example, the result could be an extreme loss in efficiency and thereby cause operational cost to be higher than anticipated. The development and promotion of Standing Column Well technology is generally credited to Carl Orio CGD from Atkinson, New Hampshire.