Geothermal Heating and Cooling Systems
A geothermal system is a system used for heating and/or cooling that utilizes the earth as a heat source (to obtain heat in winter) or as a heat sink (to discharge heat for cooling in summer). Geothermal systems simply take advantage of the relatively constant temperature within the earth. In Minnesota, the earth’s temperature ranges at 40-50 degrees Fahrenheit at a depth of 50 feet throughout the year.
Geothermal systems have the potential for significant savings in energy costs and for reducing reliance on fossil fuels like oil and natural gas. However, some of these systems also have the potential for adverse environmental effects if installed or operated improperly or if they use inappropriate materials.
Types of Geothermal Systems
Geothermal systems include a wide variety of designs and operations. These systems may be described by various terms – geothermal heat pumps, earth-coupled heat exchangers, horizontal or vertical heat exchangers, groundwater-source heat pumps, or groundwater thermal exchange devices. But all systems include the same basic operation – circulating a fluid that is in contact with the earth through a heat exchanger in order to obtain heat for heating a structure or remove heat when cooling a structure. However, different regulations and requirements may apply, depending on the specific type of geothermal system being installed.
A closed-loop system consists of piping installed in the earth and which circulates a heat-transfer fluid through the piping to a heat exchanger and, then, returning the fluid to the earth in a completely closed-loop system. The heat transfer fluid does not come in direct contact with the earth. A closed-loop system may be installed in horizontal trenches or borings, in vertical borings, or even in a surface water body (see Figure 1). Closed-loop systems can be designed to meet a wide range of heating and cooling needs, from those of an individual house to very large commercial buildings.
An open-loop system is a system that typically pumps groundwater from a water-supply well through a heat exchanger and, then, discharges the water to the ground surface, infiltration gallery, a surface water (lake/river), or an injection well. The discharge from an open-loop geothermal system must never be discharged to a Subsurface Sewage Treatment System (SSTS), sometimes referred to as a septic system. The hydraulic loading may damage the SSTS, causing it to not perform properly. Open loop systems, especially when the water is discharged to the ground surface or surface waters, have the potential for adverse environmental impacts, including general warming of surface waters, reduction of lake oxygen levels, and damage to lake ice (a safety concern).
Minnesota Department of Health Permits
The Minnesota Department of Health (MDH) regulates the construction of vertical heat exchangers (vertical heat loops) and groundwater thermal exchange devices (supply well – heat exchanger – injection well). The requirements for each of these systems are as follows:
Vertical Heat Exchanger: A vertical heat exchanger is a type of closed-loop geothermal system where high-density polyethylene piping is installed in vertical bore holes and which circulates food-grade or USP-grade propylene glycol as the heat transfer fluid. This fluid is required in order to minimize groundwater contamination and related drinking water concerns in the event of a system leak. Bore hole depths can vary depending on geology, but are commonly 150-200 feet deep. Bore holes must be grouted in order to promote efficient heat transfer between the earth and the piping, support the piping in the bore hole, and prevent contaminants migrating down the bore hole from the surface and contaminating groundwater.
A vertical heat exchanger must be installed by a well contractor or vertical heat exchanger contractor licensed by the MDH. Prior to installing this system, an applicant must submit a completed application form with fee to the MDH for approval. Minnesota Rules, Chapter 4725, establishes the requirements for the location, design, construction, testing, repair, and sealing of vertical heat exchangers, including standards for piping materials, heat transfer fluids, and grout mixes.
Groundwater Thermal Exchange Device: This system, a type of open-loop geothermal system, consists of a water-supply well, which pumps water through a heat exchanger, and uses an injection well for disposing of the water leaving the heat exchanger (see
Figure 2). The supply and injection wells must be completed in the same aquifer, to avoid mixing waters that may have different chemistry. The requirements for the location, design, construction, operation, repair, and sealing of the wells for the groundwater thermal exchange device system are found in Minnesota Rules, Chapter 4725, and Minnesota Statutes, section 103I.621.
A groundwater thermal exchange device must be installed by a MDH-licensed well contractor. Prior to installing this system, an applicant must submit a completed application form, with fee, to MDH for approval.
An open-loop system that withdraws groundwater, directs that water through a heat exchanger, and then disposes of that water to the land surface or surface water (sometimes referred to as “Pump and Dump”) requires a water appropriation permit from the Minnesota Department of Natural Resources (DNR) if more than 10,000 gallons per day or a million gallons per year is withdrawn. Minnesota Statutes, section 103G.271, prohibits once-through cooling/heating systems that withdraw greater than 5 million gallons per year. If the water is discharged to a surface water body, a National Pollutant Discharge Elimination System (NPDES) permit may also be required by the Minnesota Pollution Control Agency.
A closed-loop system installed in the bed of a public water requires a public waters work permit from the DNR. A permit will not be issued for installations in designated trout streams or designated wild and scenic rivers, where the system poses a navigational hazard, or where the system may damage the aquatic ecology of the water body.Questions?
Contact the MDH Well Management Section
651-201-4600 or 800-383-9808
Minnesota Department of Health