Water Well Journal

The heat pump is connected to a network of piping in- stalled outdoors through which fluid travels and exchanges heat with the earth or a body of water. The fluid is transferred through the pipe by circulator pumps. The majority of systems are closed loop, which means the same fluid stays within the piping network; groundwater is not extracted from the ground. Ground source heat pump (GSHP) systems do the job of air-to-air heat pumps and fossil-fuel heating appliances with much higher efficiency and lower operating costs. For example, most GSHP systems deliver a coefficient of performance (COP) of 4-to-1 or better, which means that for every unit of electricity purchased to operate the heat pump, four units of heat energy are delivered to the building. These systems are 400% efficient or better. Also, heat pumps usually contain a heat transfer coil for heating domestic hot water. As the price of fossil-fuel energy and electricity are in- creasing unpredictably, GSHP systems can deliver significant cost savings to their owners. Geothermal systems can dramat- ically reduce carbon emissions and help to flatten the energy demand spike for electricity. A critical aspect of GSHP systems is the underground net- work of plastic pipes and fittings that are buried in the ground or submerged in water. The network of pipe and fittings— sometimes referred to as the ground heat exchanger or simply the ground loop—is the thermal energy source during heating cycles and the thermal sink during cooling cycles. With all these advantages, the popularity of GSHP systems is increasing fast as homeowners, governments, businesses, and universities are turning to geothermal technology. The Plastics Pipe Institute, the Texas-based nonprofit trade associ- ation, estimates growth in demand for GSHP systems of 20% per year over the next five years. Geothermal Ground Loop Piping Materials The piping material in the outdoor ground loop is critical to the overall success of any geothermal system. Other than direct- exchange systems, where refrigerant is piped through the earth in copper pipes, practically 100% of all GSHP systems have relied upon plastic piping materials for performance and longevity. With pipes and fittings buried or encased directly in the earth, piping materials must provide corrosion resistance, chemical resistance, temperature resistance, flexibility, impact resistance, resistance to slow crack growth, and long-term hydrostatic strength (pressure capability). In addition, the ground loop heat exchanger materials must provide suitable heat transfer capabilities, since conducting heat with the earth is the primary function of the ground loop. Moreover, all ground loop pipes must meet the require- ments of standard NSF/ANSI/CAN 61 for drinking water safety to ensure that any aquifer or water reservoir into which the piping system is installed is not contaminated by the pipe itself. Considering all these challenges, the three types of plastic piping materials which are approved for geothermal ground loops are HDPE, PEX, and PE-RT. Each of these piping mate- rials delivers long-term reliability proven over decades of use around the world. Geothermal specifiers and installers need to be aware that only certain sub-types of each of these piping materials are recommended and approved for geothermal ground loops. High-Density Polyethylene (HDPE) HDPE is the predominant piping material used for ground heat exchangers and is approved in all modern codes for ground loops. It is joined primarily by heat fusion to provide leak-free connections. HDPE pipes (Figure 1) have improved significantly over the past 20 years, thanks to new bimodal resins that deliver higher strength with greater toughness and resistance to slow crack growth. The highest performing grade, known as PE 4710, is flexible, tough, and strong. For horizontal piping systems, HDPE is typically supplied in long coils as needed for the specific project. Some coils can be more than 1000 feet long, depending on the pipe diameter. Larger diameter pipes (e.g., 3-inch), which are often used for headers, are typically supplied as 40-foot straight lengths. For vertical boreholes, pipes can be provided with a molded U-bend fused to two parallel pipes. For example, for a 400-foot-deep borehole, the piping manufacturer can provide two 405-foot pipes which are fused in the factory to a molded U-bend fitting. Figure 1. Coil of HDPE piping with molded U-bend already fused to pipe ends. Image courtesy Versaprofiles. Figure 2. PEX tubing with factory-formed double U-bend. Image courtesy REHAU. 22 n July 2022 WWJ waterwelljournal.com PIPE FOR GEO from page 20

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