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HEAT PUMPS

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Heat pumps are devices that operate in a cycle similar to the vapor-compression refrigerator cycle illustrated in Figure 1.

In its most basic form, a vapor-compression refrigeration system [see Van Wylen (1985)] consists of an evaporator, a compressor, a condenser, a throttling device which is usually an expansion valve or capillary tube and the connecting tubing. The working fluid is the refrigerant, such as freon or ammonia, which goes through a thermodynamic cycle (see also Refrigeration).

The thermodynamic cycle is shown schematically in Figure 2 [Althouse et. al. (1982)]. Four important processes take place during the cycle. First, heat (Q1) is transferred to the refrigerant in the evaporator from stations 4 to 1 in Figure 1 and 2, where both its pressure and temperature are lower than a thermal source such as air, water, or the ground. Evaporation of the refrigerant occurs from a liquid to a saturated vapor, theoretically at a constant pressure. In practice, however, a pressure drop is associated with fluid flow and heat transfer through the evaporator. Secondly, work (W) is done on the refrigerant as saturated vapor at low pressure and temperature enters the compressor and undergoes adiabatic compression (from 1 to 2). The result is a compressed refrigerant vapor at high pressure and temperature at the compressor outlet (point 2 in Figures 1 and 2). Third, heat (Qy) is transferred from the hot vapor in the condenser (from 2 to 3), where its pressure and temperature are higher than a thermal sink that is at a higher temperature than the source. Condensation from a vapor to a saturated liquid occurs in the condenser, theoretically at constant pressure, but again in practice a pressure drop occurs for the same reasons as in the evaporator. The refrigerant leaves the condenser as saturated liquid. The last process before the refrigerant reenters the evaporator is the throttling of the refrigerant through the expansion valve or capillary tube from 3 to 4. During this process the pressure drop is adiabatic, resulting in a decreased refrigerant pressure and temperature. Usually the refrigerant enters as a liquid and leaves as a mixture of liquid and vapor.

A simple vapor-compression refrigeration cycle.

Figure 1. A simple vapor-compression refrigeration cycle.

Thermodynamic cycle of the vapor-refrigeration cycle.

Figure 2. Thermodynamic cycle of the vapor-refrigeration cycle.

The basic heat pump cycle is identical to the vapor-compression refrigeration cycle shown in Figures 1 and 2, the only difference between a heat pump and a refrigerator being their basic functions. A refrigeration system cools the external fluid flowing through the evaporator, whereas a heat pump heats the external fluid flowing through the condenser. The main difference between a refrigerator and a heat pump is in the manner of operation regarding cooling or heating. If the application is cooling then you would be interested in the cooling aspect, Q1 in Figure 1, occuring over the evaporator, and the cooling device will, for example, be called refrigerator, air conditioner, chiller, crycooler, etc. On the other hand, if the application is heating, then you would be interested in the heating aspect, Qy in Figure 1, occuring over the condenser, and the heating device will be called a heat pump.

Heat pumps are mostly used for heating water and air. Water can be heated for swimming pools and household purposes by using ambient air (a so called air-to-water heat pump) and air is usually heated during winter for space heating inside houses, buildings, factories, etc. also by using ambient air as the source (air-to-air heat pump). The ground can, however, also be used as a source for space heating. The heat pump will then be called a ground-coupled, ground source or ground-to-air heat pump. Another source of heat is water. Here, the evaporator is placed in a borehole, pond or lake for space heating. This type of heat pump is called a water-to-air heat pump. In the industry water-to-water heat pumps are used, for example, to produce hot and cold water simultaneously. These types of systems are also called dual-purpose systems.

Dual-purpose systems have been designed to be applied in both heating and cooling capacities at the same time. In a reverse cycle system, the functions of evaporator and condenser can be reversed. Both "hot" and "cold" can thus be delivered to the same thermal reservoir at different times. Dual systems are also called heat pumps, although the focus is not solely on their heating capacities.

Although the devices have different names, two features are common in both. First, the same basic refrigeration cycle takes place and second both are heat-pumping systems. Heat is "pumped" from a thermal source at low temperature to a thermal sink at a higher temperature. These heat-pumping systems or heat pumps have two major advantages over conventional technology. First, depending on the application, more than one unit of heating per unit of energy input required can often be delivered, i.e., the coefficient of performance (COP) value is greater than one. Usually, for every one kilowatt of power required by the compressor more than one kilowatt of heating capacity is available at the condenser. In most practical applications the coefficient of performance is between two and six for a heat pump. To heat a swimming pool, the COP may be as high as six; whereas in a hot water system, where water is heated to a temperature of 55°C, the COP may be as low as three. The coefficient of performance (COP) of a heat pump is defined as

A COP value of four would mean an energy saving of 75%. Heat pumps therefore offer the advantage of energy conservation and lowered costs compared to other methods of heating. Another advantage of heat pumps is that heat-pumping devices may be either heat-actuated or work-actuated. Heat-actuated heat pumps allow for use of lower-grade thermal energy, which in other cases might often remain unused. Heat pumps therefore may also help mitigate thermal pollution and environmental concerns.

The advantages of heat pumps have long been recognized. Research into improving performance, reliability, energy-efficiency, and environmental impact has been an ongoing concern for industrial, governmental, and academic organizations. Studies have centered on advanced cycle design for both heat- and work-actuated systems, improved components (including choice of refrigerant), and use in a wider range of applications. Specific areas of activity have included: a search for refrigerant replacements, advanced mobile air conditioning for transportation-applications, advanced vapor-compression technology, absorption heat pumps, ground-coupled (or geothermal) heat pumps and air cycle heat pumps. A few of these activities will now be discussed.

Replacement Refrigerants

In response to increasing concerns that certain chlorine-containing compounds, such as the fully halogenated fluoroalkanes, may be catalyzing a decrease in stratospheric ozone levels has led to an active search for replacement refrigerants. The ozone layer is vital for mankind because it protects us from the dangerous ultraviolet rays emitted by the sun. Chlorofluorocarbon (CFC) refrigerants have been identified as major contributors in the ozone layer depletion problem. Ultraviolet radiation breaks CFC molecules apart, releasing its free chlorine as the CFC molecule reaches the upper level of the stratosphere. The free chlorine disrupts the delicate equilibrium of the ozone layer. Concern for the effect of CFCs on the ozone layer has previously led to an international meeting in Montreal, in September 1987. The Montreal Protocol on Substances that Deplete the Ozone layer was born out of this meeting. The Protocol was originally signed by forty-five nations. The Protocol was later amended after agreement was reached that CFCs should be completely phased out by the year 2000 and an intended phase-out of hydrochlorofluorocarbon (HCFC) refrigerants by 2020, with 2040 as the absolute deadline. This Protocol can, however, be expected to be amended again.

The search for replacement refrigerants has led to HCFCs and hydrofluorocarbons (HCFs), An HFC molecule contains no chlorine and poses no threat to the ozone layer. HCFC-22 is perhaps the most widely used refrigerant in heat pumps. It has a relatively low ozone-depletion potential, but since it contains chlorine, a replacement fluid is being sought. The important factor is to find replacement fluids with thermodynamic properties similar to the refrigerants being replaced. It is also desirable to match the enthalpy of vaporization. If these parameters can be matched closely, the need for system redesign would be minimal. (See also Refrigeration and Refrigerants.)

Ground-Coupled or Geothermal Heat Pumps

In the past ten years, home-owners all over the world have discovered that geothermal (ground source) systems are ideal for heating and cooling. In winter, water or other fluids circulating through a "loop" of underground pipe absorbs heat from the earth and carries it to the geothermal unit, which extracts the heat at a higher temperature, and distributes it throughout the home. In summer, the unit extracts heat from your home and transfers it back to the circulating water in the underground loop system, where it is dissipated into the cooler earth. Loops are installed in two basic types: closed and open. Closed loops are buried in the earth or submerged in lakes or ponds. Open loops use ground water pumped from a well. The loop configuration will depend on the following factors: the subsoil geology of the land; the local cost of trenching and drilling; availability of quality ground water and availability of land area. If land area is limited, the closed loops can be inserted into vertical boreholes. U-shaped loops of pipe are inserted into the holes. The holes are then back filled with a sealing solution.

Air-Cycle Heat Pumps

Air is the ultimate refrigerant. It is nontoxic, noncorrosive and does not harm the ozone layer or contribute to global warming. Leaks have no impact on the environment. No particular service precautions or refrigerant recovery procedures are needed. The price of air is obviously right–it is free. Air is readily available and delivery to the site is immediate. No recovery equipment is needed. From a design viewpoint, air is ideal, especially if the problem calls for very high or very low temperatures. Air does not change phase in the normal operating regimes and can be used over an exceptionally wide range of temperatures, Many thousands of air cycle systems are in use today. Virtually every jet propelled aircraft in production today, whether commercial or military, uses air cycle refrigeration. In addition, air cycle systems can operate over the broad range of temperatures to which they will be subject in the course of normal flight.

An electrically driven air cycle heat pump is a very simple device. The rotating group typically consists of a compressor and a turbine mounted on the same shaft as a high-speed motor. This assembly is the only moving part. The single stage, centrifugal compressor has a relatively low pressure ratio that is typically in the 1.4:1 range. The single-stage, radial inflow turbine wheel has a slightly lower pressure ratio. The motor is powered by the output of a variable frequency, variable voltage inverter so that the speed of the motor, compressor and turbine can change. Finally, one, two or three heat exchangers are needed, depending on the application and the concept.

Energy is required for compressing air. Mechanical energy is converted into thermal energy when air is compressed and the air becomes hotter. This is the source of heating. Correspondingly, when air is expanded and does work, thermal energy is converted to mechanical energy and the air becomes colder. This is the source of refrigeration. The compression process therefore consumes mechanical energy and the expansion process produces mechanical energy. By mounting the compressor and the turbine on the same shaft, the turbine helps drive the compressor. Unfortunately, the laws of physics are such that the turbine does not produce enough mechanical energy to drive the compressor by itself and therefore an electric motor is used to supplement the mechanical energy produced by the turbine.

Consider a system in which both heating and cooling are needed simultaneously. Perhaps a restaurant wants to heat 20°C incoming city water up to 82 °C for dishwashing. Simultaneously, they would like to air-condition the kitchen. Assume that the outside air temperature is 25°C. If outside air at atmospheric pressure is compared with a 1.8:1 pressure ratio, the discharge temperature will be about 89°C. This is sufficient to heat the water to the required temperature using an appropriate heat exchanger. In heating the water, the compressed air is cooled to a lower temperature. If the compressed air is now expanded in the turbine back to atmospheric pressure, it will discharge at a temperature substantially below freezing–assuming that the air was dry and no condensation occurred. This is well below the temperature required for air conditioning and so the air will be mixed with room air before being discharged. The air remains breathable as there is no lubricating oil in the system to contaminate it. It is difficult to present a comparison with conventional equipment because the air cycle heat pump is not yet in production and it must be matched against a wide range of combinations of heating and cooling equipment. COP heating values, however, of between 1.9 and 3.6 are expected.

REFERENCES

Althouse, A. D., Turnquist, C. H., and Bracciano, A. (1982) Modern Refrigeration and Air Conditioning, The Goodheart-Willcox Co., Inc.

Van Wylen, G. J. and Sonntag, R. E. (1985) Fundamental of Classical Thermodynamics, John Wiley and Sons, Toronto, 3rd edn.

References

  1. Althouse, A. D., Turnquist, C. H., and Bracciano, A. (1982) Modern Refrigeration and Air Conditioning, The Goodheart-Willcox Co., Inc.
  2. Van Wylen, G. J. and Sonntag, R. E. (1985) Fundamental of Classical Thermodynamics, John Wiley and Sons, Toronto, 3rd edn.

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