A Geothermal Primer
Heat pump systems using the ground as a source of energy are gaining in popularity with the recent climb in energy costs. Homeowners looking at their monthly budgets for their residences are finding the cost of energy to be a bigger slice of the “pie chart.” Doubling the initial cost of the heating and air-conditioning system is starting to make economic sense if the system is able to operate at 350 percent efficiency, compared to a traditional system that operates at 80-85 percent.
When one takes these “costs” over five years of energy bills (and no one really knows what the cost of fossil fuel will be in the next five years), then a safe bet is to add a few more dollars to your mortgage payment and the energy efficiency can achieve savings of an amount in excess of the increased mortgage payment.
Reduction in energy consumption does not have to result in reduction of comfort. Of course, it is also still very important to pay utmost attention to the building envelope with good insulation and window material selection. The heat delivery method is also important. Low temperature heat delivery methods - like radiant floor heating - work best with geothermal. Geothermal can also be used in combination with solar, electric, condensing or noncondensing boilers, or combinations of each to meet the needs of the system.
Ground-source heat pump systems use the consistent earth temperature that is typically at 42 to 78 degrees F year round in the United States, depending upon location. This heat is extracted through heat exchange via buried loops filled with refrigerant or a water/antifreeze mixture. The energy exchange is continued in a water-based system by using a water-to-refrigerant heat exchanger, or directly to the compressor in a direct geoexchange system. The compressor then sends the refrigerant to a “heat exchanger” that is either an air coil for refrigerant-to-air exchange or a brazed-plate heat exchanger for refrigerant-to-water exchange.
The great advantage of the heat pump using the ground as the “condenser” is that you have an air-conditioning source for cooling by reversing the cycle, and also a source for domestic hot water through de-superheating during the summer or full-time on-demand heating. Desuperheating can best be described as taking the excess heat produced during the compression cycle (recovers heat from air-conditioning cycle) and diverting it to the water heater tank rather than dissipation via the earth loop. This not only provides “free” hot water, but also increases the efficiency of the unit. Note that when the outside air is 95 degrees F, the ground is always cooler, which provides a much greater temperature difference, thus increasing efficiency.
Geothermal In Application: Now that a little background has been laid, let’s focus on the application of heat pumps and radiant floor heating. Radiant floor heating works best in a low-temperature, high-mass system. Heat pumps are, for the most part, limited to a high temperature output of 115 to 125 degrees F. Ground-source heat-pumps, however, are not as limited in their geographical location as air-source heat pumps. Air-source heat pumps do not perform well with freezing air temperatures.
For radiant floor heating, the best way to lower temperature requirements is to decrease pipe spacing and to use a thermal mass system, such as concrete or gypcrete, with a minimum thickness of 1 1/2 inches. A 6-inch pipe spacing in the perimeter zone or area 3 to 4 ft. off the outside walls of a space and 8-inch spacing in the occupied area or center of the space is recommended. Using this pipe spacing in an area with floor coverings with an R-value of less than 1, and walls that have an R-value of at least 20, typical water temperatures necessary to maintain comfort will be 105 to 115 degrees F when the outside temperature is 25 degrees F.
For air-conditioning, radiant heat contractors will find how simple geothermal heat pump systems are for controlling the water temperature. Set-point controls that measure the supply water temperature and outdoor temperature work well with heat pumps. Using a constantly operating circulator for the radiant heating water and a properly sized buffer tank will avoid equipment short-cycling.
The heat pump will simply cycle on and off as necessary to meet the temperature set point. That temperature is calculated based on a heating curve and the outdoor temperature that is measured at the north-facing wall of the building. In cases where the building size requires more than one heat pump, the control can lead/lag the heat pumps in rotation. This rotation will not only equally exercise the equipment, but also equally extract heat from the geothermal field, thus not excessively reducing the source temperature. It is important that each heat pump removes heat from each geothermal field as nearly equally as possible.
Geothermal systems can be used to heat domestic water in combination with other types of water heating equipment that can boost the temperature of the water from 120 degrees F to any desired higher level. Preheating with a geothermal system is proving cost recovery in less than three years for commercial projects that use large quantities of domestic hot water, such as apartments, nursing homes and motels.
Another advantage to geothermal heat pumps is their electrical energy source. The cost of fossil fuels has historically been lower than electricity. That has changed with OPEC, deregulation of natural gas prices, increasing global competition from developing countries and limited oil-refining capacity.
Energy delivery of electricity is less expensive and almost universally available in the United States. Electricity provides relatively stable pricing due to coal and atomic power plants and the emerging use of renewable energy generation. Heat pumps and solar supplemental heat work well together. Geothermal systems have the additional advantage of the buried heat exchanger, which is quiet and not exposed to salt air in seaside environments.
The greatest challenge to wider acceptance of ground-source heat pump systems has been the cost of the installation of the geothermal exchange field. Recent energy price increases and the development of small platform hydraulic drilling equipment have, however, made the payback much more attractive.
Hydronic heating contractors are also successfully subcontracting the refrigerant work to other contractors that specialize in heat pumps and have the necessary licenses to work with refrigerants. Companies that specialize in drilling and installing the tubing for geothermal exchange and HVAC contractors that understand both refrigerants and hydronics will lead the way in this technology. Innovative contractors that specialize in each system component cooperate to install hydronic systems using the geothermal heat supply to respond to customer demand for energy cost-savings and to allow North America to become less dependent on foreign sources of energy.
Europeans, who have experienced high energy prices for many years, were showing condensing boilers, solar and geothermal heat pumps at the 2006 EXPOCOMFORT show in Milan. We can learn from their experience while our energy prices are still relatively lower than theirs.