Solar cooling systems

Solar cooling systems are attractive because cooling is most needed when solar energy is most available. If solar cooling can be combined with solar heating, the solar system can be more fully utilized and the economic benefits should increase. Solar cooling systems by themselves, however, are usually not economical at present fuel costs. Combining solar heating and cooling systems is not easy because of the different system requirements.

ABSORPTION COOLING. 

Absorption cooling is the most commonly used method of solar cooling. An absorption refrigeration machine is basically a vapor-compression machine that accomplishes cooling by expansion of a liquid refrigerant under reduced pressure and temperature, similar in principle to an ordinary electrically operated vapor-compression air conditioner. Two refrigerant combinations have been used: lithium bromide and water, and ammonia and water. There have been a number of proposed solid material absorption systems also.

In the absorption cooler, heat is supplied to the generator in which a refrigerant is driven from a strong solution. The refrigerant is cooled in the condenser and allowed to expand through the throttling valve. The cooled, expanded refrigerant receives heat in the evaporator to provide the desired cooling, after which the refrigerant is reabsorbed into the cool, weak solution in the absorber. The pressure of the resulting strong solution is increased by pumping and the solution is available to repeat the process.

The performance of the system is governed largely by the temperature difference between the generator and the condenser and absorber units. Since the generator temperatures in solar driven systems are only moderate, it is important to keep the condenser and absorber temperatures as low as possible. The LiBr system is preferred over ammonia systems for solar energy applications because of the lower generator temperatures required. Permissible generator temperatures for a water-cooled LiBr system range from 170 deg. F to 210 deg. F (76 deg. C-99 deg. C) compared to the 205 deg. F to 248 deg. F (95 deg. C-120 deg. C) temperatures required for a water-cooled ammonia absorption system. Most, if not all, of the commercially available absorption units use LiBr and water as the absorbent-refrigerant fluid pair. Because the LiBr will crystallize at the higher absorber temperatures associated with air cooling, these units must be water cooled. A prototype ammonia-water unit, amenable to direct air cooling, has been built by Lawrence Berkeley Laboratories.

Parabolic trough systems concentrate solar radiation, specifically direct normal insola(DNI), onto a receiver tube located along the focal line of a single-axis tracking parabolically curved, trough-like reflector. Heat transfer fluid flowing through the receiver tube absorbs the thermal energy. The heat is collected and used to generate steam which is produces electricity by a Rankine cycle turbine-generator. Troughcan be hybridized (natural gas can be burned to produce steam when the sun isn’t shining) or can use thermal storage to dispatch power to meet utility peak load requirements.

The operating temperature of trough plants is limited by the thermal property of the heat transfer fluid (HTF) that is suitable for pumping through miles of piping in the solar fIn typical applications, oil flowing through the receiver tube is heated to about 390°C and used to boil water to produce steam. The resulting steam is used in a Rankine power cycle and expanded through a turbine connected to an electric generator. As with any steam cycle, the exhaust steam is cooled and condensed back to liquid water to be recirculated in the cycle. The condensers can be either water-cooled or air-cooled, or a hybrid combination.