From a sustainability perspective, directly using solar as a primary energy source is attractive because of its universal availability, low environmental impact, and low or no ongoing fuel cost. Research has demonstrated that solar energy is an ideal source for low temperature heating applications such as space and domestic hot water heating…
Refrigeration systems that use environment-friendly refrigerants provide a sustainability advantage when compared to other refrigerant selections. However, the energy use associated with refrigeration system operation and the environmental impacts associated with its generation and distribution often outweigh the choice of refrigerant. To minimize environmental impacts associated with refrigeration system operation, it is reasonable to evaluate the prospects of a clean source of energy. From a sustainability perspective, directly using solar as a primary energy source is attractive because of its universal availability, low environmental impact, and low or no ongoing fuel cost. Research has demonstrated that solar energy is an ideal source for low temperature heating applications such as space and domestic hot water heating.
Photovoltaic-based vapor compression, is presently a viable solar refrigeration technology.
Vapor Compression Refrigeration
Prior to discussing how solar energy could potentially provide refrigeration, it is appropriate to review the basic principles of operation for vapor compression refrigeration cycles that form the foundation for nearly all conventional refrigeration. A schematic of the vapor compression cycle is shown in Figure 1a and a corresponding enthalpy-pressure diagram for the refrigerant is shown in Figure 1b.
In the vapor compression cycle, cooling is provided in the evaporator as low temperature refrigerant entering the evaporator as a mixture of liquid and vapor at State 4 is vaporized by thermal input from the load. The remaining equipment in the system reclaims the refrigerant and restores it to a condition in which it can be used again to provide cooling. The vapor exiting the evaporator at State 1 in a saturated (1a) or slightly superheated (1b) condition enters a compressor that raises the pressure and, consequently, the temperature of the refrigerant. The high pressure hot refrigerant at State 2 enters a condenser heat exchanger that uses ambient air or water to cool the refrigerant to its saturation temperature prior to fully condensing to a liquid at State 3. The high-pressure liquid is then throttled to a lower pressure, which causes some of the refrigerant to vaporize as its temperature is reduced. The low temperature liquid that remains is available to produce useful refrigeration.
Figure 1(a): Schematic of a vapour compression refrigeration system. Figure 1b: Pressure-enthalpy diagram for the cycle.
Photovoltaic Operated Refrigeration Cycle
Photovoltaics (PV) involve the direct conversion of solar radiation to direct current (dc) electricity using semiconducting materials. In concept, the operation of a PV-powered solar refrigeration cycle is simple. Solar photovoltaic panels produce dc electrical power that can be used to operate a dc motor, which is coupled to the compressor of a vapor compression refrigeration system. The major considerations in designing a PV-refrigeration cycle involve appropriately matching the electrical characteristics of the motor driving the compressor with the available current and voltage being produced by the PV array.
The rate of electrical power capable of being generated by a PV system is typically provided by manufacturers of PV modules for standard rating conditions, i.e., incident solar radiation of 1,000 W/m2 (10 800 W/ft2) and a module temperature of 25°C (77°F). Unfortunately, PV modules will operate over a wide range of conditions that are rarely as favorable as the rating condition. In addition, the power produced by a PV array is as variable as the solar resource from which it is derived. The performance of a PV module, expressed in terms of its current voltage and power-voltage characteristics, principally depends on the solar radiation and module temperature. Figure 2 shows current (solid lines) and power (dotted lines) vs. voltage for a 1.32 m2 (14 ft2) single crystalline PV module at the reference condition and four operating conditions. At any level of solar radiation and module temperature, a single operating voltage will result in maximum electrical power production from the module. The module represented in Figure 2 shows the voltage that yields maximum power ranges between 30 and 35 volts for this PV array.
The efficiency of the solar panels, defined as the ratio of the electrical power produced to the incident radiation is between 8% to 10% at maximum power conditions for the PV array represented in Figure 2. If the PV refrigeration system is to operate at high efficiency, it is essential that the voltage imposed on the PV array be close to the voltage that provides maximum power.
Figure 2(left): Current (solid lines) and power (dotted lines) vs. voltage for a single crystalline PV module at different operating conditions.
Figure 3(right): current-voltage characteristics for a PV module and two dc motor types.
This requirement can be met in several ways. First, a maximum power tracker can be used which, in effect, continuously transforms the voltage required by the load to the maximum power voltage. If the system includes a battery, the battery voltage will control the operating voltage of the PV module. PV panels can then be chosen so that their maximum power voltage is close to the voltage for the battery system.
The battery also provides electrical storage so that the system can operate at times when solar radiation is unavailable. However, the addition of a battery increases the weight of the system and reduces its steady-state efficiency. Electrical storage may not be needed in a solar refrigeration system as thermal storage, e.g., ice or other low temperature phase storage medium, may be more efficient and less expensive.
A final option for systems that do not use a maximum power tracker or a battery is to select an electric motor having current-voltage characteristics closely matched to the maximum power output of the module.
Figure 3 superimposes the current-voltage characteristics of a series dc motor and separately excited motor on the photovoltaic module. In this case, the separately excited motor would provide more efficient operation because it more closely matches the maximum power curve for the photovoltaic module. However, neither motor type represented in Figure 3 is well-matched to the characteristics of the PV module over the entire range of incident solar radiation. Studies of solar-powered motors have shown that permanent magnet or separately excited dc motors are always a better choice than series excited dc motors in direct-coupled systems that are not equipped with a maximum power tracker.
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