A heat pump is a thermodynamic heating/refrigerating system used to transfer heat. Cooling and heating heat pumps are designed to utilize the heat extracted at a low temperature and the heat rejected at a higher temperature for cooling and heating functions, respectively.
The household refrigerator can provide a simple analogy. A refrigerator is actually a one-way heat pump that transfers heat from the food storage compartment to the room outside. In so doing, the inside of the refrigerator becomes progressively cooler as heat is taken out. In a closed room, the heat coming out of the refrigerator would make the room warmer. The larger the refrigerator is, the greater the potential amount of heat there is to transfer out. If the refrigerator door were open to the outdoors, there would be an almost unlimited amount of heat that could be transferred to the inside of a dwelling. Thus it is possible to design a refrigeration system to transfer heat from the cold outdoors (or any other cold reservoir, such as water or the ground) to the insides of a building (or any medium that it is desired to heat).
Heat pumps can move heat energy between any form of matter, but they are typically designed to utilize the common heating and cooling media, i.e., air or water. Heat pumps are identified and termed by transfer media. Thus terms air-to-air, water-to-air, or air-to-water are commonly used. Most heat pumps are designed to transfer heat between outside air and inside air, or, in the case of so-called geothermal or ground-source heat pumps, between the ground or well water and air. The principle application of heat pumps is ambient heating and cooling in buildings.
There are three types of heat pump applications: heating, cooling or heating and cooling. "Heating only" heat pumps are designed to transfer heat in one direction, from a cold source, such as the outdoors, to the inside of a building, or to a domestic hot water plumbing system, or to some industrial process. There is no term "cooling only heat pump," as the term would simply be describing what is commonly referred to as an air conditioner or cooling system.
"Heating and cooling" heat pumps have refrigeration systems that are reversible, permitting them to operate as heating or cooling systems. An analogy would be a window air conditioner that was turned around in winter, so that it was cooling the outside and blowing hot air inside. Most heat pumps in use are of the heating and cooling type, and are used to heat residences, or commercial and industrial buildings. Residences consume the bulk of those sold: in 1999 about a quarter of new single-family homes were equipped with a heating and cooling heat pump.
Most heat pumps utilize a vapor-compression refrigeration system to transfer the heat. Such systems employ a cycle in which a volatile liquid, the refrigerant, is vaporized, compressed, liquefied, and expanded continuously in an enclosed system. A compressor serves as a pump, pressurizing the refrigerant and circulating it through the system. Pressurized refrigerant is liquefied in a condenser, liberating heat. Liquid refrigerant passes through an expansion device into an evaporator where it boils and expands into a vapor, absorbing heat in the process. Two heat exchangers are used, one as a condenser, and one as an evaporator. In the case of an air-to-air heat pump, one heat exchanger is placed outside, and one inside. In a ground-source heat pump, the outdoor heat exchanger is placed in contact with well water, a pond, or the ground itself. In both cases, the refrigerant flow is made reversible so that each heat exchanger can be used as an evaporator or as a condenser, depending on whether heating or cooling is needed. The entire system is electrically controlled. (See Figures 1 and 2.)
Some heat pumps, called thermoelectric heat pumps, employ the Peltier effect, using thermocouples. The Peltier effect refers to the evolution or absorption of heat produced by an electric current passing across junctions of two suitable, dissimilar metals, alloys, or semiconductors. Presently, thermoelectric heat pumps are used only in some specialized applications. They have not been developed to a point to make them practical for general heating and cooling of buildings.
The energy efficiency of heat pumps is measured by calculating their coefficient of performance (COP), the ratio of the heat energy obtained to the energy input. The capacity of modern heat pumps in the United States is rated in British thermal units per hour (Btu/h). The COP at a given operating point can be calculated by dividing the Btu/h output of the system by the energy input in Btu/h. Since system input for most heat pumps is electricity measured in watt-hours, the watt figure is multiplied by a conversion factor of 3.412 to obtain the energy input in Btu/h.
There are two major factors that impact the COP: temperature difference and system component efficiency. A heat pump requires energy to move heat from a lower temperature to a higher one. As the difference in the two temperatures increases, more energy is required. The COP of a heat pump is higher when the temperature difference is less, and less energy is consumed to transfer a given amount of heat. Thus the COP of a heat pump varies during its operation, and is relative to the system operating point, the temperature of the medium heat is transferred from, and the temperature of the medium it is transferred to. Comparison of the COP of two different systems is meaningless unless the operating points are the same. For this reason, although heat pump efficiency continues to improve, specific historical comparisons of heat pump efficiencies are difficult because system operation data was recorded at different operating points.
Air-to-air heat pumps are particularly prone to varying COPs, due to fluctuating outdoor weather conditions. In the winter, the COP decreases as the outdoor temperature decreases, due to the need for the system to transfer heat energy at greater temperature differences. The heat output decreases as the outdoor temperature drops at the same time that the heating needs of the building are increasing. In fact, the COP may drop so low that the heat pump cannot meet the thermal needs of a building. For this reason an auxiliary heat source such as electric resistance heaters or a fossil-fueled furnace is needed in geographic areas that have cold winters. The COP of air-to-air heat pumps is further reduced by the use of defrost cycles needed to clear the outdoor heat exchanger of frost buildup when the air temperature drops below about 40°F. During defrost, the refrigeration system is reversed into comfort cooling mode, heating the outdoor heat exchanger to melt the frost. Since the defrost mode is cooling the indoor air, supplemental heat is usually necessary to maintain comfort levels, reducing the overall COP of the system. The reduction varies with the duration of the defrost.
In the summer, the COP of an air-to-air heat pump decreases as the outdoor temperature rises, reducing the cooling capacity. Normally the thermal needs of the building are met since it is common practice to size a heat pump so that it will deliver adequate cooling capacity in all but the most extreme summer conditions. The winter heating capacity of the system is then determined by this tradeoff, and if the heating capacity is inadequate, supplemental electric or fossil fuel heat is required.
Because deep ground temperature has little change, geothermal heat pumps using well water function at a fairly stable COP. Geothermal types using the ground for thermal mass will see some variance in COP depending on the dryness of the soil. Wet ground conducts more heat than dry and theoretical soil conductivity may vary up to 1,000 percent. For this reason the ground loop heat exchanger should be buried deep enough to minimize soil moisture fluctuations.
System components can affect the COP because their design and performance can vary. In vapor-compression systems, the transfer efficiency of the heat exchangers and the energy efficiency of the compressor, and how these components are matched, help determine the operating COP. Compressors are performance-optimized for narrow operating ranges. If the optimization is done for heating, cooling operation may suffer. The opposite is also true.
The operating condition of the components also affects the COP. For example, if the heat exchangers become clogged or corroded, or if a homeowner fails to change a furnace filter, the operating COP of a heat pump will decrease. Any heating and cooling system will suffer some performance degradation once it has been put into use. The amount of degradation depends on the conditions under which the system has to operate, as well as how carefully system components are maintained.
Most heat pumps for residences are unitary systems; that is, they consist of one or more factory-built modules. Larger buildings may require built-up heat pumps, made of various system components that require engineering design tailored for the specific building. Sometimes, multiple unitary units are used in large buildings for ease of zone control; the building is divided up into smaller areas that have their own thermostats.
Oliver Evans proposed the closed vapor-refrigeration cycle in 1805 in The Young Steam Engineer's Guide. Evans noted: "Thus it appears possible to extract the latent heat from cold water and to apply it to boil other water." By 1852 William Thompson (Lord Kelvin) had proposed that a refrigeration system be used to either cool or heat the air in buildings, and outlined the design of such a machine. In Austria after 1855 Peter Ritter Von Rittenger constructed working heat pumps that were said to be 80 percent efficient. These devices were used to evaporate salt brine, and similar devices were constructed in Switzerland after 1870.
A theoretical discussion of the heat pump appeared in the Journal of the Franklin Institute in 1886. T. G. N. Haldane of Scotland comprehensively pursued the application of heat pumps to the heating of buildings after the mid-1920s. Haldane tested air-to-water heat pump systems in his home and concluded that the vapor-compression refrigeration cycle could, under certain conditions, provide a more economical means to heat buildings and swimming pools than fossil fuels. Haldane further proposed using a reversed cycle so the heat pump could be used to cool buildings or make ice. Haldane's heat pump had a coefficient of performance (COP) between 2 and 3, depending on operating conditions.
During the 1930s and 1940s a number of residential and commercial heat pump installations were made in the United States. They were of all types, and the heating COPs of these systems, where results were known, seem to have ranged from 2 to 5. Hundreds of articles and papers were published discussing the theory, application, and installed examples of heat pumps. Despite this activity, most heating systems installed were conventional fossil-fuel furnaces and boilers due to their lower first cost, broad acceptance, and well-established manufacturing, sales, and service infrastructure.
Comfort cooling was one benefit of a heat pump that was typically cited in the literature of the 1930s and 1940s. However there was not a great consumer demand for general comfort cooling at that time, particularly for residences. True, there were isolated pockets of interest, as in movie theaters and some commercial buildings, but genuine mass consumer demand for comfort cooling did not develop until the mid-twentieth century. Thus, there was no financial incentive for mass production of unitary comfort cooling air conditioners in the 1930s and 1940s, and therefore no incentive to produce unitary heat pumps. There were a few isolated attempts, such as a package heat pump marketed by DeLaVergne in 1933, but the efforts were short-lived.
Electric utilities did have a vested financial interest in heat pumps, since most designs used electricity for heating and cooling. But utilities were not manufacturers or consumers. Despite their attempts to promote heat pumps with articles and showcase system installations, they failed to create the demand for the product.
By the 1960s, central residential air conditioning was becoming increasingly popular. Equipment had developed to the point that a number of manufacturers were producing and marketing unitary air conditioning equipment. Some of these manufacturers did attempt to resurrect the idea of applying the heat pump to residential, store, and small office heating and cooling. Systems were designed and marketed, but suffered dismally in the market. System components used did not stand up to the demands of summer and winter operation over wide weather conditions. Compressors and reversing valves in particular saw enough failures that most manufacturers withdrew their products from the marketplace. The air-to-air heat pumps of the 1960s used timed defrost cycles, initiating defrosts even when they were not necessary. Thus the system efficiency was unnecessarily reduced.
Spurred on by the energy crisis of the 1970s interest in heat pumps renewed. Rising price of fossil fuels caused manufacturers to take another look at heat pumps. Owners of "all electric homes," stung by the high cost of electric heat, were looking for a way to reduce heating costs with minimal existing heating system redesign. A heat pump could save 30 to 60 percent of the cost of electric resistance heating.
Remembering the problems of the 1960s, manufacturers redesigned system components. The system efficiency was generally higher than in the 1960s because system components were more efficient. Most air-to-air heat pumps of the period used a demand-controlled defrost cycle, further increasing overall energy efficiency. By the mid-1970s most of the major manufacturers of unitary air conditioning equipment were offering heat pumps. They were particularly popular in areas of moderate winters where air-to-air heat pumps could operate at higher COPs.
Conventional heat pumps continued to be available through the 1970s to the 1990s. The trend has been toward progressively higher energy efficiencies. The average efficiency of new heat pumps increased 60 percent (based on cooling performance) between 1976 and 1998. Minimum efficiencies were mandated for residential heat pumps in 1987 and for commercial equipment in 1992, a significant factor contributing to the rise in efficiency over the past twenty years. For heat pumps used in commercial buildings, the efficiency standards were derived from the model standards published by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). The U.S. Department of Energy may review these standards in the next few years and setting more stringent standards (for both residential and commercial heat pumps) if technically and economically feasible.
CURRENT PRACTICE AND THE FUTURE
Heat pump systems are now rated with a Heating Seasonal Performance Factor (HSPF) and a seasonal energy efficiency ratio (SEER). The HSPF is calculated by the annual heating system output in BTU by the heating electricity usage in watt-hours. The SEER is an estimate of annual cooling output in Btu divided by the cooling electricity usage in watt-hours. The HSPF and SEER are calculated at specific rating points, standardized by the Air Conditioning and Refrigeration Institute (ARI), an industry trade organization that performs testing advocacy, and education. ARI conducts a certification program, participated in by almost all manufacturers of heat pumps. The program gives the consumer access to unbiased and uniform comparisons among various systems and manufacturers. Rating various systems at the same operating point allows accurate comparisons between systems. Heat pumps introduced in 1999 have an NSPF of 6.8 or greater and a SEER of 10.0 or greater, with high efficiency units having an HSPF of as much as 9 and a SEER of 13 or greater.
There is more interest than ever in the geothermal heat pumps. The cost of such systems has decreased with use of plastic piping for water or ground loops. Geothermal systems are proving particularly advantageous in colder winter areas, since ground temperature is much higher than outdoor temperatures. In addition, there is no efficiency-robbing defrost cycle that is necessary in air-to-air heat pumps.
New compressor technology employing rotary scroll type compressors is replacing previously-used reciprocating technology. Scroll compressors operate at higher efficiencies over wider operating conditions. Heat pumps employing scroll compressors use less supplemental heat at low outdoor temperatures. Electrical and electronic technology are making variable-capacity compressors cost-effective for the next century. Compressor performance can be optimized for a wider operation range, and matching compressor capacity to the actual demand for heating or cooling increases the system efficiency, saving energy. Application of microprocessors to control systems further permits fine-tuning of system operation. Energy efficiency is also being increased by development of higher efficiency electric motors for compressors and fans.
Increased environmental awareness will no doubt spur increasing interest in solar-assisted heat pumps in the future. Such systems can operate at higher heating COPs than more conventional heat pumps. A solar-assisted heat pump system uses a solar heating system in parallel with a conventional heat pump. The solar heating system reduces the operating time of the heat pump, and also reduces the need for supplemental electric or fossil-fuel heating during colder weather.
Bernard A. Nagengast
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