IN THIS MONTH'S column we will begin looking at ground source heat pumps as a means of providing efficient heating and cooling to buildings of all shapes and sizes.
Ground source heat pumps (GSHP) have been in use for many years. Although not new to the heating market, their use has been boosted by colleges such as Oklahoma State University, home of the International Ground Source Heat Pump Association, a nonprofit organization dedicated to the research, use and marketing of this excellent energy resource.
Simply stated, GSHP systems utilize a highly efficient means of transporting low-grade energy, compounding it and compressing it for later use. The means of transportation is done through the use of a refrigeration cycle. Essentially, during the heating season, low-grade heat (30°F to 50°F earth temperature) is collected, compressed and release in the form of 120°F water. Any large surface radiator, such as a radiant floor, wall or ceiling system, can use this hot water or it can be distributed via forced air using fan coils.
The coefficients of performance are typically about 2.5 to 1. This means that for each watt of electricity consumed, the system delivers 2.5 watts of thermal energy. This can result in a very low "per therm delivered" cost of operation, with delivered energy costs less than those of natural gas, historically the least expensive means of heating a building.
In addition to the heating side of the equation, the GSHP system can also provide air conditioning at an extremely high efficiency on a year-round basis. This is because the typical air conditioning condenser is exposed to the worst possible working condition when it is needed the most. It is trying to reject heat from the refrigerant into hot air in July. If it had a nice cool heat sink, like deep down in Mother Earth, its efficiency would be much higher and its electrical consumption much lower.
This is exactly what a GSHP system does. GSHP systems can typically be twice as efficient in providing cooling as their air-cooled cousins.
In order to use the low-grade thermal energy provided by GSHP systems in a radiant panel design, it typically requires the tube to be installed at a higher density than would be used with a higher temperature of fluid. In most cases, it is also not entirely possible to carry 100% of the building's thermal load with a GSHP system. It is technically possible, just not financially recommended. GSHP systems are typically sized to carry 80% to 90% of the building's heating loads and 100% of the building's cooling load.
This requires the installation of some way to augment the system to carry the minor balance of the heating load capacity. This is typically done using direct resistance heaters. While the cost of operation of these thermal boosters is obviously quite high, they are not on for very long and the overall fuel savings still warrant the installation costs. Augmentation can also be performed with other fuel sources, such as gas, oil or solid fuels.
Mother Earth is the most common heat source for the GSHP system. These systems can be as simple as a "pump and dump" system that draws water from an underground aquifer, runs it through the heat pump and then returns the water to the aquifer that it drew the water from in the first place.
Another means of extracting heat from the earth is to drill a series of wells, install a parallel reverse return grid of fusion-welded polyethylene tubing and circulate a mixture of alcohol and water. This method typically requires one well per ton of heating capacity with the well being up to 300 ft. deep, depending on the thermal conductivity of the soil in which the system is installed. This requires some pretty substantial soils knowledge that is usually provided by geological engineers and experienced well drillers.
In addition to the two abovementioned methods, another body of water such as a large lake or nearby pond can serve as the heat source for the closed-loop system if the body of water has some substantial depth (at least 10 ft. deep) to it.
The last method of providing heat is the horizontal method, which requires trenches dug at depths from 4 ft. to 8 ft. below grade, and the tubing installed in these trenches. The tubing can be laid straight, or spread out like a "Slinky," depending upon soil structure, thermal conductivity, and groundwater availability and movement. The length of the trench is a function of the aforementioned conditions, and typically one trench is requiredper ton of heating capacity. Spacing-between the trenches is usually 10 ft. or greater, again depending upon soil conditions.
Tune in next month as we continue to look at this exciting means of providing extremely highly efficient heating and cooling. Until then, Happy I.G.S.H.P.A. Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via email at [email protected] or by phone at 303/778-7772.