Coolest kid on the block
August 21, 2007
Appeared in Consulting-Specifying Engineer
Geothermal energy, or literally, heat derived from the earth, is the third most employed renewable resource in the United States. Used to warm and cool buildings without polluting the environment or straining operating budgets, the number of geothermal energy applications has skyrocketed in the last two decades.
As much as 2,200 MW of geothermal electric power generation is drawn to support motels, strip malls, nursing homes and modest residential and office buildings across the country, a number equal to the amount of annual power generated by four large nuclear power plants, says the Geothermal Resources Council.
But, it didn’t use to exist this way. The first domestic ground source heating pump (GSHP) system installed in Boise, Idaho, in 1892, left much to be desired, with metal piping that deteriorated when it came in contact with ground minerals and water. The inefficient makeup of the first systems, coupled with high initial costs, left geothermal energy less than affordable for almost a century.
However, high-density polyethylene (HDPE) piping, efficient heat pump equipment and first-cost incentives offset by utility companies have since come to the rescue. GSHP systems now rival stan-dard electric centrifugal chiller and boiler machines in energy efficiency and occupancy comfort, mak-ing them today’s “coolest” power generation solution.
Testing the waters
Geothermal resources are site specific—as precise geological conditions must be present to exploit this potential energy source. Most commonly found internationally in earthquake zones such as the Pacific Ring of Fire, and domestically in the western region of United States, Alaska and Hawaii, geothermal energy can be a significant design component to any sustainable facility.
The supply piping typically used to acquire this renewable resource is a GSHP vertical loop. Before this can be designed and installed, however, the first step is to conduct a thermal conductivity (TC) test by drilling a sample borehole, or narrow shaft (typically 5 in. in diameter), into the ground, to measure the heat transfer between the earth and the water in the pipe.
An experienced geothermal TC test operator will use a generator to power the testing equipment in order to avoid power fluctuations and obtain accurate TC values. TC test values should range from 0.80 to 1.00 BTU/hr, otherwise a GSHP system is not advisable on the site. Thermal diffusivity also plays an important role in the TC test. Investigating this aspect of the site will uncover the range of materials present in the underground soils and rock formations, revealing how they diffuse heat at different rates.
Drafting the system
Once the viability of geothermal energy has been confirmed, it is time to design the GSHP system. Accuracy should be the No. 1 design consideration. Precise calculations are crucial in creating a system that has the right-sized loop field (an oversized one will add undue costs to the initial system), balances heating and cooling loads and reduces energy expenditure.
Additionally, it is important to note that heat pump systems do not handle outdoor air loads well. Therefore, it is best to temper the outdoor air separately from the terminal heat pump units. In ven-tilation systems requiring more than 15% outdoor air, a dedicated outdoor air system is recommended, clad with a total energy heat recovery wheel. Further energy savings can be realized at the circulating pump with the use of variable speed drives on pump motors greater than 5 hp.
The geothermal energy source is tapped into by drilling several boreholes 15 ft. to 20 ft. apart and as deep as 250 ft. to 350 ft. down into the earth. The loop field size and the amount of boreholes is dependent on the heating and cooling loads of the building, thermal diffusivity and conductivity, while the depth of the loop is determined by the rock and soil materials encountered, thermal diffusivity, conductivity and the space between boreholes.
HDPE piping is then inserted into the boreholes and filled with water, while the area around the boreholes is filled with grout to provide efficient thermal energy transfer between the earth and the wa-ter. Subsequently, the piping is brought into the building where it is connected to the main header. The main header, in turn, is attached to a circulating pump for distribution to heat pump units throughout the building. Standard schedule 40 steel piping and type-K copper HDPE piping is used inside the building. While freeze protection is not required, rust inhibitors should be used where non-HDPE piping is speci-fied.
Advantages of geothermal energy use GSHP systems provide many benefits over traditional central boiler/chiller plant equipment:
- Lower operating and maintenance costs. The average cost for a ground source heat pump system in the United States is $0.098/sq. ft. versus $0.22/sq. ft. for a pour-pipe central boiler and chiller plant. This provides a potential annual savings of 44% on maintenance costs, an attractive advantage to this energy-efficient system. Closed-loop geothermal systems are low maintenance because no fresh minerals are added to the water after initial treatment, limited fresh oxygen means less corrosion and depending on the region's climate, partial or no supplemental heat is required.
- Environmentally friendly. The Environmental Protection Agency (EPA) report "Space Conditioning: The Next Frontier" (EPA 430-R-93-004, April 1993), credits GSHP systems for having the lowest CO2 emissions of all heating /cooling technologies and the lowest overall environmental impact.
- High-comfort level and air quality. In the heating mode, GSHP systems provide high efficiency without raising the evaporating temperature. This allows heat pumps to maintain lower indoor humidity levels, thus providing greater comfort and reducing the risk of health problems associated with higher moisture levels. This is especially important in institutional and commercial buildings.
- Save on space. Standard air conditioning systems require air-cooled HVAC equipment to be located out-doors. In a geothermal system, the loop piping is underground. The mechanical room for a loop field can be substantially smaller than one needed to support boilers, chillers and air-handling units. Heat pumps can be located beneath window sills, vertically in closets or horizontally above hung ceilings.
- Water heating. In warmer regions, where a pump system primarily provides cooling, the GSHP system adds heat to the loop field. A water-to-water heat pump unit used to generate hot water will remove the added heat from the loop field and provide the building with low-cost domestic hot water.
- Low-demand characteristics. In commercial buildings, geothermal systems produce a significant de-mand reduction when compared to conventional systems— an advantage of 0.5 kW/ton versus a rooftop unit and 0.3 kW/ton versus a chiller.
- Simple controls. GSHP systems inherently provide very low operating costs without sophisticated, pro-prietary or expensive controls and equipment. Instead, they require self-contained controls, including an on/off/auto/fan switch, a cooling/heating mode switch and a low/high fan speed switch, all located on each individual heat pump unit, serving its own zone.
- Efficiency ratios. Pumps can be sized to meet the load at 7.5 hp/100 ton maximum. Water-to-air heat pump energy efficiency ratios (EERs) can be better than 13 at ARI 330 conditions compared to chilled water VAV systems whose average EER is about 8.2.
- Return on investment. Despite their higher installation costs, GSHP systems have three qualities that are important to keep in mind when performing a life-cycle cost analysis: low energy consumption, ex-tended equipment life and low maintenance costs. Furthermore, most utility companies and state gov-ernments including California, Illinois, New York and Oklahoma as well as national agencies, including the Department of Energy Conservation, provide financial incentives. For more information visit: www.geoexchange.org.