Learn What is the Right Geothermal Ground Loop Design for your Needs
Geothermal heat pump systems are a great alternative to conventional HVAC systems.
A Geothermal heat pump uses the heat stored in the ground as both a hot and cold reservoir in a heat pump to provide both heating and cooling for installations of almost any size. By avoiding the use of cooling towers and boilers, energy savings are very significant, the decreased running cost completely offsetting the higher installation cost of a geothermal system. Additionally, but lowering the power requirements for HVAC (one of the highest consumers of power) there is an additional benefit in having a lowered carbon footprint, making Geothermal heat pump systems not only saves money, but also helps to preserve the environment.
Finally, depending on the ground loop design, Geothermal adds some design flexibility helping to make more attractive building design by eliminating the need for ventilation of boilers and significant amounts of real estate to house cooling towers.
Geothermal heat pump systems are theoretically simple; use pumps to circulate a working fluid and heat pumps to move energy into and out of the fluid while dumping or absorbing energy from the earth. However, the design process is complex, not because of the technology, which is well understood and which uses components that have already been substantially optimized, but because of the number of options available to the engineer. Depending on the location and size of the project, the configuration of a particular system will change because of types of ground loops available. Also, there are many engineering tradeoffs such as variable or fixed pumping speed which must also be weighed in the cost/benefit equation. In this article, the main focus will be on geothermal ground loop design, as it is the single biggest determining factor for geothermal heat pump designs.
Types of Geothermal Ground Loop Design
There are two main types of geothermal ground loop designs: Open and Closed Loop. Closed loop systems use a closed loop of working fluid in contact with the ground to transfer energy between the heat pump and the ground such that the working fluid is continually recirculated. The main mode of heat transfer with the Earth is through conduction of the pipe wall. Open loop design, on the other hand, uses ground water directly which is not recirculated. The main advantage of an open loop design is the higher overall heat transfer given that the fluid is in direct contact with the ground. However, open loop designs have significant drawbacks.
Open loop designs are not available in all areas because of the fact that ground water is not always readily available. The water reservoir can be either natural, such as an underground river, or manmade, by using a well. However, because the water contains impurities in the form of minerals, there will be increased wear on the water pumps and significant filtration is an absolute necessity. At the same time, open loop geothermal ground loop designs are often the lowest initial cost, especially when the load on the system is large.
Closed loop geothermal ground loop design yields many different options as far as configuration; however, the concept remains the same in all cases. Obtain either a heat source or sink by circulating a working fluid through tubes in contact with a heat reservoir at a constant temperature. Closed loop designs at one point were single loop designs called direct exchange geothermal heat pump systems.
Direct exchange systems pump the working fluid in the heat pump directly to the heat source or sink in the earth, rather than using a secondary loop, usually water, to transport the energy. Given the ability of the refrigerant to hold energy in the form of the latent heat of vaporization, and the efficiency losses due to the use of heat exchangers in an indirect closed loop ground loop design, direct exchange systems held, at one time, significant efficiency advantages. However, using a refrigerant means the use of more expensive copper tubing with brazed joints with larger concern for leaks and higher cost in needing far more refrigerant than a double loop system. Given advances in efficiencies of heat exchangers and pumping systems, most geothermal ground loop designs today are nearly the same efficiency regardless of direct or indirect exchange.
Closed loop systems have several different configurations depending on geographical conditions. Characteristics such as ground temperature and composition can make designs of different types more attractive. Below is a summary of some considerations of each major type of closed loop geothermal ground loop design configuration.
Depending on the depth into the Earth, the variation of temperature over the course of a year varies less and less farther down into the crust, meaning that in Ohio, for example, the ground temperature remains close to 55 degrees Fahrenheit all year round, and below 30ft, this variation falls to less than 1 degree between summer and winter extremes. Vertical loop geothermal ground loop designs take advantage of this seasonal consistency to attain higher thermal efficiency in the entire system, meaning lower running costs, but with an increased initial installation cost due to the nature of drilling.
Design for vertical ground loops requires careful consideration of the terrain composition and properties. Rocky environments make deep drilling more costly while areas with significant variation in soil composition will have different heat conductivities making each hole more or less effective. The best option when putting in a large installation of 50 tons or more would be to run a ground test where a hole is bored into the ground and then hot water supplied from a calibrated source is used to measure how much heat absorption is done by the ground over a 48 hour period. As with all engineering testing, these values come at a price, however, this additional cost (around 2000$ per test hole) is offset by closer approximation of the necessary number of vertical shafts of the optimum depth severely decreasing the cost of the overall system.
Additional design considerations include the cost of real estate when setting up the ground loops. Given that holes cannot intersect and that the closer the holes are spaced to each other, the more interference between ground loops, an optimum condition for hole spacing must be found for each project. Ideally, a 25 foot center for each shaft would provide the minimum necessary clearance; however, in practice, closer spacing between 15 to 20 feet is more practical.
Horizontal loop geothermal ground loop designs leverage the ease of installation by not requiring deep holes, but rather a shallow trench, against the decreased efficiency from having seasonal variations of temperature. Typically, a horizontal loop setup will require far more area per ton than any other configuration. Because of this, again, the location and ground temperature will greatly influence the cost of the design. Also, because the loop operates at a lower efficiency, more pipes are necessary to attain the same heating and cooling power as a vertical loop system, also contributing to the cost equation. Finally, trapped air in a vertical configuration is very difficult to flush out without the use of auxiliary pumps, a consideration that needs to be accounted for when deciding between configurations that do not exhibit an obvious fault or strength in a situation.
What are slinky coils
Another design worth considering is the geothermal slinky coils. This is a coil of plastic tubing that is installed horizontally at the hole typically measuring 3-foot wide. This concentrates the transfer of the heat into small volume using less space and short trenching. The most common measurement for the slinky is the 10-inch pitch and this is equivalent to 12 feet of pipe per foot of trench. Its use will reduce the length by 2/3 if compared to 2 pipes at 4 and 6 foot depth. An extended slinky is available, and this comes at 56-inch and equal to 4 feet of pipe for every foot of trench. There are also other design and measurement configurations for the slinky coils depending on the contractors and installation needs.
For the direct exchange system, water sourced from pond (lake) is used instead of anti-freeze. In the typical operations and exchange, the water is pumped into the heat pump and heat is extracted. Once done, the water is returned back to the source such as the pond. Under this set-up, around 1.5 to 3 gallons of water is needed for every ton of cooling capacity. Say for example your home measures 3000 square feet and well-insulated. This means that the water requirement is 8 to 15 gallons for every minute of operations. Just a little reminder if this design is selected- make sure that the water is of good quality. Impurities will surely damage the geothermal system.