Source: Geo-Heat Center.
This chapter provides information on geothermal heat pump shipments, based on the Energy Information Administration's (EIA) Form EIA-902, "Annual Geothermal Heat Pump Manufacturers Survey." It begins with a general discussion of heat pumps, followed by a discussion of the survey results and some salient characteristics of the data, and ends with a technical discussion of heat pump operation.
"Heat pump" is a term applied to a machine that can transfer heat both to and from a source. Therefore, a heat pump can be used both for space heating in the winter and for cooling (air conditioning) in the summer. The most common type of heat pump for domestic use is the air-to-air (air source) system in which heat is taken from air (heat source) at one location and transferred to air (heat sink) at another location.
Another type of heat pump transfers heat from air to water and has been designed for domestic hot-water heating. Geothermal heat pumps are a special type of air-to-water heat pump that use the earth as a heat source or sink, depending on the season. Since the earth's temperature a few feet below the surface stays relatively constant (about 55 degrees Fahrenheit) year round, one can extract heat from the earth when the air temperature is below the earth's temperature (winter), and transfer heat to the earth when the air temperature is above the in-ground temperature (summer). Geothermal heat pumps are generally more expensive to install but more efficient (costing less to operate and maintain) than an air-to-air heat pump. The technical section at the end of this chapter further explains the operation and rating of heat pumps.
The Energy Information Administration's (EIA) new survey, Form EIA-902, "Annual Geothermal Heat Pump Manufacturers Survey," shows that manufacturers shipped almost 58,000 geothermal heat pumps in 1997 and over 213,000 during the period 1994 through 1997. The survey was completed by approximately 18 known domestic manufacturers of geothermal heat pumps. Data from the survey are collected under a confidentiality provision. As a result, only aggregated statistics are released. The Form EIA-902 tracks shipments of the three main types of geothermal heat pumps, as certified by the Air Conditioning and Refrigeration Institute (ARI), and a small volume of non-ARI rated heat pumps (Tables 37-40). The three ARI-rated classifications for geothermal heat pumps are as follows:
Manufacturers shipped nearly 58,000 heat pumps of all types in 1997 (Table 37) compared with about 52,000 in 1996--an increase of almost 13 percent. Of those shipped in 1997, over 28,000 were ARI-320 rated, 9,700 were ARI-325 rated, 18,600 were ARI-330 rated, and approximately 1,300 were non-ARI-rated units.
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Note: Data for ARI-320 units are subject to substantial revision. See Appendix B.
Source: Energy Information Administration, Form EIA-902 "Annual Geothermal Heat Pump Manufacturers Survey." |
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Note: A capacity of one ton equals 12,000 Btu's. Data for ARI-320 units are subject to substantial revision. See Appendix B.
Source: Energy Information Administration, Form EIA-902 "Annual Geothermal Heat Pump Manufacturers Survey." |
The total rated capacity of heat pumps shipped in 1997 was almost 209,000 tons (Table 38). The average rated capacity of heat pumps shipped in 1997 was 3.6 tons compared to 3.4 tons in 1996. Average capacity increased for all ARI-rated geothermal heat pumps. ARI-320 units tend on average to be smaller at 3.3 tons per unit than ARI-325 or ARI-330 units at almost 4 tons per unit and non-ARI-rated units at 5 tons per unit. By comparison, a typical home central air conditioner has a rating of 3.0 tons. These results may indicate a niche market for larger geothermal heat pumps, where the savings in operating costs are greater than the higher initial investment for geothermal heat pumps.
The proportion of geothermal heat pumps shipped to each Census region in 1997 was as follows: the South (46 percent), the Midwest (23 percent), the Northeast (16 percent), the West 10 percent, and exports 4 percent (Table 39). The Northeast accounted for the greatest increase in units--up nearly 2,900 units in 1997 compared to 1996. In terms of the number of units and percentage of units, export shipments showed the greatest decline, falling 41 percent to just over 2,400 units in 1997. About 50 percent of total geothermal heat pumps were shipped to wholesalers (Table 40) of which nearly 40 percent were ARI-320 rated. Forty-four percent of geothermal heat pump shipments went to installers of which 58 percent were ARI-320 rated. Retail distributors represented less than 1 percent of heat pump shipments.
Collaborative alliances among government organizations, the Geothermal Heat Pump Consortium, the International Ground Source Heat Pump Association, and the geothermal heat pump industry have expanded consumer awareness and acceptance of geothermal heat pumps. Such efforts have resulted in increased installation of geothermal heat pumps where electric utilities and electric service companies provide attractive financing, rebates, guaranteed utility rates, shared savings contracts, and/or equipment leasing arrangements.
A heat pump is a device that operates on the concept of heat transfer from areas of lower temperature to areas of higher temperature--the reverse of normal heat flow. It employs a refrigeration cycle during which a refrigerant (known as the "working fluid") is compressed (as a liquid) then expanded (as a vapor) to absorb and release heat, respectively. Specifically, the heat pump absorbs from an outside medium (air, ground, groundwater) and then transfers ("pumps") the heat to a space to be heated during the winter season. By reversing the process, the heat pump extracts (absorbs) heat from the same space during the summer season, and then transfers it to the outside air. It also acts as a dehumidifier, removing moisture from the air and making it more comfortable.
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Note: The Midwest Census region consists of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin. The Northeast Census region consists of Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont. The South Census region consists of Alabama, Arkansas, Delaware, District of Columbia, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, and West Virginia. The West Census region consists of Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. Data for ARI-320 units are subject to substantial revision. See Appendix B.
Source: Energy Information Administration, Form EIA-902 "Annual Geothermal Heat Pump Manufacturers Survey." | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
An air-source heat pump--the most common type of heat pump--absorbs heat from the outside air and transfers the heat through the working fluid to the space to be heated. In the heating mode, however, air-source heat pumps lose efficiency and generally require a back-up heating system when the outside air approaches 32o F or less. In the cooling mode, the heat pump absorbs heat through the working fluid from the space to be cooled and rejects the heat to the outside air.
In a geothermal heat pump, the refrigerant exchanges heat with a fluid circulating through piping in contact with the earth. The fluid circulates in a variety of loop (pipe) configurations, depending on the temperature of the ground. Loops may be installed horizontally or vertically in the ground or submersed in a body of water (Figure 12). Although the fluid in most types of loop configurations circulates in a closed system, open loops (normally vertical systems) are sometimes used when a sufficient supply of water is available from wells.
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W = Data withheld to avoid disclosure of proprietary company data. Data for ARI-320 units are subject to substantial revision. See Appendix B.
Source: Energy Information Administration, Form EIA-902 "Annual Geothermal Heat Pump Manufacturers Survey." | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Figure 12. Ground Coupled Heat Pumps-Loop Configurations |
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| Note: For more in-depth information on geothermal heat pumps, visit the Geo-Heat Center's consumer information website at http://www.oit.edu. Source: Geo-Heat Center. |
Heat pumps are rated according to their heating or cooling capacity and their operating efficiency. The capacity of a heat pump for either heating or cooling is commonly expressed in terms of British thermal units (Btu) per hour of heat delivery or removal. The heating or cooling capacity is sometimes stated in tons, where 1 ton is equivalent to a heat delivery or removal rate of 12,000 Btu per hour. Although an average home requires a unit of approximately 36,000 Btu (3 tons), the size of a house and the quality of insulation determine the necessary heat pump capacity. An under-sized unit will not keep the house comfortable during peak heating and cooling demand times and may run continuously, resulting in high operating costs. On the other hand, an over-sized unit is more costly to purchase and operate and during cooling seasons may not run long enough, causing conditions of high humidity.
The efficiency of a heat pump or performance rating (distinct from its capacity) is useful for comparing units of the same type. The ratings used for different types of equipment are not generally comparable. Heating performance is rated by the coefficient of performance (COP)--the rate of heat delivery relative to the total rate of energy input required to operate the heat pump. The average COP of a heat pump in heating mode may be in the range 2 to 3, meaning that heat delivered to a building is 2 to 3 times as great as the energy input required to operate the heat pump. The excess energy delivered (as heat) over the input (as electricity) is derived from the heat source--outdoor air for a conventional air source heat pump and ground or ground water for a geothermal unit).(15)
The COP of a heat pump is not the same in the heating and cooling modes but varies with conditions, in particular with the temperatures of the heat source and heat sink. This is important for heating, because the efficiency decreases as the temperature of the heat
source decreases, i.e., the outdoor air in an air-to-air heat pump. When the outdoor air temperature falls to below about 32oF, the efficiency of an air-to-air heat pump is so low that auxiliary heating is required. The auxiliary heating system is usually resistance heat that supplements the heat pump, though an independent gas- or oil-fired furnace may be used. Electric resistance heat is very expensive and is best used as auxiliary heat or in climates where little heat is required. Hence, air-to-air heat pumps are not recommended where low winter temperatures are common and large amounts of auxiliary resistance heat are required. By comparison, geothermal heat pumps operate more efficiently by taking advantage of the fairly constant and warm temperature of the earth. Thus, they do not require auxiliary heating equipment.
The energy efficiency rating (EER) is commonly used to rate the efficiency of cooling performance. The EER in Btu per watthour is 3.412 (the Btu equivalent of a watthour) times the COP. A COP of 3 (i.e., 3.412 Btu per watthour x 3) equals an EER of 10.2 Btu per watthour. The EER can also be defined as the cooling capacity (size in Btu/hour) divided by the wattage of the unit (i.e., 36,000 Btu/hour/3,600 watts equals 10.0 Btu per watthour).
The newest term used by the air conditioning and heating industry to rate the efficiency of air conditioners and heat pumps is "Seasonal Energy Efficiency Rating" (SEER). While both the COP and EER values are valid only at specific test conditions used in the rating, the SEER is based on average or typical operating conditions.
Because of advances in technology and design, newer units have a SEER rating as high as 15 or 16. The Federal Government now requires new air conditioners and heat pumps to have a SEER of at least 10. Units that are 10 or more years old probably have a SEER of 10 or less. A new unit with a SEER of 12 will be one-third more efficient than an older unit with a SEER of 8.(16) In air-to-air heat pumps, when the outside air is below average temperatures (heat source) during the heating season or above average temperatures (heat sink) during the cooling season, the unit will not operate at the design SEER. Geothermal heat pumps that take advantage of the relatively constant temperature of the earth will be able to operate at the design SEER over wider temper-ature variations in the air.
15. The heat in the outdoor air is derived from the sun, so that an air-to-air heat pump makes use of solar energy.
16. To pump a specified amount of energy, in this example 24, two heat pumps with a SEER rating of 8 and 12 will require 3 and 2 units of input energy respectively, resulting in the heat pump with the SEER rating of 12 saving one-third the energy used by the less efficient unit.