Solar Cooling

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The quest for a safe and comfortable environment has always been one of the main pre-occupations of the human race. In ancient times, people used the experience gained over many years to utilize in the best possible way the available resources to achieve adequate living conditions. Central heating was pioneered by the Romans, using double floors and passing the fumes of a fire through the floor cavity. Also in Roman times, windows were covered for the first time with materials such as mica or glass. Thus, light was admitted in the house without letting in wind and rain (Kreider and Rabl, 1994). The Iraqis, on the other hand, utilized the prevailing wind to take advantage of the cool night air and provide a cooler environment during the day (Winwood et al., 1997). Additionally, running water was employed to provide some evaporative cooling.

As late as the 1960s, though, house comfort conditions were only for the few. From then onward, central air-conditioning systems became common in many countries, due to the development of mechanical refrigeration and the rise in the standard of living. The oil crisis of the 1970s stimulated intensive research aimed at reducing energy costs. Also, global warming and ozone depletion and the escalating costs of fossil fuels over the last few years have forced governments and engineering bodies to re-examine the whole approach to building design and control. Energy conservation in the sense of fuel saving is also of great importance.

During recent years, research aimed at the development of technologies that can offer reductions in energy consumption, peak electrical demand, and energy costs without lowering the desired level of comfort conditions has intensified. Alternative cooling technologies that can be applied to residential and commercial buildings, under a wide range of weather conditions, are being developed. These include night cooling with ventilation, evaporative cooling, desiccant cooling, and slab cooling. The design of buildings employing low-energy cooling technologies, however, presents difficulties and requires advanced modeling and control techniques to ensure efficient operation.

Another method that can be used to reduce energy consumption is ground cooling. This is based on the heat loss dissipation from a building to the ground, which during the summer has a lower temperature than the ambient. This dissipation can be achieved either by direct contact of an important part of the building envelope with the ground or by blowing into the building air that has first been passed through an earth-to-air heat exchanger (Argiriou, 1997).

The role of designers and architects is very important, especially with respect to solar energy control, the utilization of thermal mass, and the natural ventilation of buildings, as was seen in Section 6.2.6. In effective solar energy control, summer heat gains must be reduced, while winter solar heat gains must be maximized. This can be achieved by proper orientation and shape of the building, the use of shading devices, and the selection of proper construction materials. Thermal mass, especially in hot climates with diurnal variation of ambient temperatures exceeding 10°C, can be used to reduce the instantaneous high cooling loads, reduce energy consumption, and attenuate indoor temperature swings. Correct ventilation can enhance the roles of both solar energy control and thermal mass.

Reconsideration of the building structure; the readjustment of capital cost allocations, i.e., investing in energy conservation measures that may have a significant influence on thermal loads; and improvements in equipment and maintenance can minimize the energy expenditure and improve thermal comfort.

In intermediate seasons in hot, dry climates, processes such as evaporative cooling can offer energy conservation opportunities. However, in summertime, due to the high temperatures, low-energy cooling technologies alone cannot satisfy the total cooling demand of domestic dwellings. For this reason active cooling systems are required. Vapor compression cooling systems are usually used, powered by electricity, which is expensive and its production depends mainly on fossil fuel. In such climates, one source abundantly available is solar energy, which could be used to power an active solar cooling system based on the absorption cycle. The problem with solar absorption machines is that they are expensive compared to vapor compression machines, and until recently, they were not readily available in the small-capacity range applicable to domestic cooling applications. Reducing the use of conventional vapor compression air-conditioning systems will also reduce their effect on both global warming and ozone layer depletion.

The integration of the building envelope with an absorption system should offer better control of the internal environment. Two basic types of absorption units are available: ammonia-water and lithium bromide (LiBr) water units. The latter are more suitable for solar applications since their operating (generator) temperature is lower and thus more readily obtainable with low-cost solar collectors (Florides et al., 2001).

The solar cooling of buildings is an attractive idea because the cooling loads and availability of solar radiation are in phase. Additionally, the combination of solar cooling and heating greatly improves the use factors of collectors compared to heating alone. Solar air conditioning can be accomplished by three types of systems: absorption cycles, adsorption (desiccant) cycles, and solar mechanical processes. Some of these cycles are also used in solar refrigeration systems and are described in the following sections.

Solar cooling can be considered for two related processes: to provide refrigeration for food and medicine preservation and to provide comfort cooling. Solar refrigeration systems usually operate at intermittent cycles and produce much lower temperatures (ice) than in air conditioning. When the same systems are used for space cooling they operate on continuous cycles. The cycles employed for solar refrigeration are absorption and adsorption. During the cooling portion of the cycles, the refrigerant is evaporated and re-absorbed. In these systems, the absorber and generator are separate vessels. The generator can be an integral part of the collector, with refrigerant absorbent solution in the tubes of the collector circulated by a combination of a thermosiphon and a vapor lift pump.

Many options enable the integration of solar energy into the process of "cold" production. Solar refrigeration can be accomplished by using either a thermal energy source supplied from a solar collector or electricity supplied from photovoltaics. This can be achieved by using either thermal adsorption or absorption units or conventional vapor compression refrigeration equipment powered by photovoltaics. Solar refrigeration is employed mainly to cool vaccine stores in areas with no public electricity.

Photovoltaic refrigeration, although it uses standard refrigeration equipment, which is an advantage, has not achieved widespread use because of the low efficiency and high cost of the photovoltaic cells. As photovoltaics operated vapor compression systems do not differ in operation from the public utility systems, these are not covered in this book and details are given only on the solar adsorption and absorption units, with more emphasis on the latter.

Solar cooling is more attractive for the southern countries of the Northern Hemisphere and the northern countries of the Southern Hemisphere. Solar cooling systems are particularly applicable to large applications (e.g., commercial buildings) that have high cooling loads for large periods of the year. Such systems in combination with solar heating can make more efficient use of solar collectors, which would be idle during the cooling season. Generally, however, there is much less experience with solar cooling than solar heating systems.

Solar cooling systems can be classified into three categories: solar sorption cooling, solar-mechanical systems, and solar-related systems (Florides et al., 2002a).

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