Solar Mechanical Refrigeration

Solar Mechanical Refrigeration

Solar mechanical refrigeration uses a conventional vapour compression system driven by mechanical power that is produced with a solar-driven heat power cycle. The heat power cycle usually considered for this application is a Rankine cycle in which a fluid is vaporised at an elevated pressure by heat exchange with a fluid heated by solar

collectors. A storage tank can be included to provide some high temperature thermal storage.

The vapour flows through a turbine or piston expander to produce mechanical power, as shown in Figure 4. The fluid exiting the expander is condensed and pumped back to the boiler pressure where it is again vaporised.

The efficiency of the Rankine cycle increases with increasing temperature of the vaporised fluid entering the expander, as shown in Figure 5 (bold line). The Rankine cycle efficiency in Figure 5 was estimated for a high-temperature organic fluid assuming that saturated vapour is provided to a 70% efficient expander and condensation occurs at 35°C (95°F). The efficiency of a solar collector, however, decreases with increasing temperature of the delivered energy. High temperatures can be obtained from concentrating solar collectors that track the sun’s position in one or two dimensions. Tracking systems add cost, weight and complexity to the system. If tracking is to be avoided, evacuated tubular, compound parabolic or advanced multi-cover flat plate collectors can be used to produce fl uid temperatures ranging between 100°C – 200°C (212°F – 392°F).