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Article Excerpt
INTRODUCTION
Encouraged by the successful worldwide effort to protect the ozone layer, scientists and engineers have been committed to minimize and reverse the harming environmental effects of global warming. Global warming occurs when carbon dioxide, released mostly from the burning of fossil fuels (oil, natural gas, and coal) and other gases, such as methane, nitrous oxide, ozone, CFCs, HCFCs and water vapor, accumulate in the lower atmosphere. As results of the rapid growth in world population and the economy--especially in developing countries--total world energy consumption has increased and is projected to increase by 71% from 2003 to 2030 (US DOE 2006). Fossil fuels continue to supply much of the energy used worldwide, and oil remains the primary energy source. Therefore, fossil fuels are the major contributor to global warming. The awareness of global warming has been intensified in recent times and has reinvigorated the search for energy sources that are independent of fossil fuels and contribute less to global warming. Among the energy sources alternative to fossil fuels, renewable energy sources such as solar and wind garner the public's attention, as they are available and have fewer adverse effects on the environment than do fossil fuels. Four energy sources represent renewable energy: hydro-energy, geothermal energy, solar energy, and wind energy. The prime source of renewable energy is solar energy.
Many countries and organizations promote renewable energy through federal research, taxes, and subsidies. As an example, the United States started the Solar America Initiative in 2005 to enhance widespread commercialization of solar energy technologies by 2015. The Solar America Initiative will accelerate the development of advanced photovoltaic materials that convert sunlight directly to electricity, with the goal of making solar photovoltaic cost-competitive with other forms of renewable electricity by 2015 (National Economic Council 2006). Air-conditioning systems are the dominating energy consumers in buildings in many countries, and their operation causes high electricity peak loads during the summer. The solar cooling technology can reduce the environmental impact and the energy consumption issues raised by conventional air-conditioning systems. Therefore, the purpose of the current paper is to provide an overview of the cooling technologies that utilize solar thermal energy and review their prospects as alternative cooling technologies as compared to conventional ones.
SOLAR COOLING TECHNOLOGY
Solar cooling technology can be classified into three categories: solar electrical cooling, solar thermal cooling, and solar combined power and cooling, as illustrated in Figure 1. Detailed discussion of each solar cooling technology follows.
[FIGURE 1 OMITTED]
Solar Electrical Cooling
Photovoltaic Devices. Solar energy can be transformed to electricity by using photovoltaic (PV) devices. PV devices convert light energy into electrical energy through photoelectric effect, as French physicist Edmond Becquerel discovered as early as 1839. Individual PV cells (or so-called solar cells) are electricity-producing devices made of semiconductor materials. Among many types of solar cell material, silicon wafers, polycrystalline thin films, and single-crystalline thin films represent the typical solar panel material. Bulk silicon was used to make some of the earliest PV devices. Single crystal silicon-wafer-based PV cells plated on the glass are the more widely used compared to other materials, but their efficiency is relatively low and their costs are relatively high. With the advent of the transistor and accompanying semiconductor technology, the various thin-film technologies have been developed. These thin film technologies have reduced the manufacturing cost from that of bulk materials, and their efficiency exceeds those of bulk silicon wafers by having a multi-layer thin film structure. Polycrystalline thin film has an efficiency between 10% and 17% (Wu 2004). Single-crystalline thin film using multi-junction solar cell structure is the most efficient one between 15% and 20%. The latest development for the single-crystalline film using a multi-junction solar cell structure resulted in a world-record conversion efficiency of 41% in an optical concentrator solar cell produced by Boeing-Spectrolab (Rainbow Power Company 2006; Fthenakis and Alsema 2006).
Solar Electric Cooling. In solar electric cooling, dc power produced by the solar PV devices is supplied either to the Peltier cooling systems or to the vapor-compression systems with a dc compressor motor. Since the solar electrical cooling systems rely on the electricity supplied by the PV devices, the overall system efficiency is predominantly determined by the efficiency of the PV devices. The overall energy conversion efficiency of the solar-powered vapor-compression cooling systems is obtained from the energy conversion efficiency of the PV cell and the COP of the vapor-compression system. Similarly, the overall energy conversion efficiency of the grid-powered vapor-compression system is obtained from the energy conversion efficiency of the grid power plant and the COP of the vapor-compression system. Therefore, the energy conversion efficiency of the solar-powered vapor-compression cooling system is higher than that of the grid-powered one only if the energy conversion efficiency of the PV cell is higher than the grid efficiency. This means that the current PV cell technologies are not as efficient as the grid-based systems, but new PV cell technology can reverse this situation. However, solar power from PV cells is renewable energy with minimum impact on the environment. From this point of view, the efficiency of the solar cells is a secondary factor. If on the other hand, the conventional power plant is fired with biofuels, then the conversion efficiency, including the factor for the conversion from solar radiation into the biofuel, will be more informative. Klein and Reindl (2005) investigated the issues of matching the electrical characteristics of the compressor motor with the power produced by the PV cells. They pointed out that it is important for the voltage imposed on the PV cell to be close to the voltage that provides maximum power in order for the PV-powered refrigeration system to operate at high efficiency. They suggested several ways of doing this, including using a maximum power tracker, using a battery, and selecting an electric motor having current-voltage characteristics closely matched to the maximum power output of the module.
Solar Thermal Cooling
Thermal energy produced from the solar energy can be transformed to useful cooling and heating through the thermochemical or thermophysical processes by using thermally activated energy conversion systems. Thermally activated energy conversion systems are further classified into three categories: open sorption cycles, closed sorption cycles, and thermo-mechanical systems. Further details of each technology are as follows.
Open Sorption Cycle Solar Cooling. Open cycle refers to solid or liquid desiccant systems that are used for either dehumidification or humidification. Basically, desiccant systems transfer moisture from one airstream to another by using two processes. In the first process, called the sorption process, the desiccant system transfers moisture from the air into a desiccant material by using the difference in the water vapor pressure of the humid air and the desiccant. If the desiccant material is dry and cold, then its surface vapor pressure is lower than that of the moist air, and moisture in the air is attracted and absorbed to the desiccant material. During the sorption process, the latent heat of the water vapor is released so the temperature increases. After the desiccant material becomes wet, the second process starts. In the second process, called the desorption process or regeneration process, the captured moisture is released to the airstream by increasing the temperature of the desiccant. After regeneration, the desiccant material is cooled down by the cold airstream. Then it is ready to absorb the moisture again. When these processes are cycled, the desiccant system can transfer the moisture continuously by changing the desiccant surface vapor pressures, as illustrated in Figure 2. To drive this cycle, thermal energy is needed during the desorption process. The desiccant has to be heated to release the moisture from point 2 to point 3 in Figure 2, and then cooled to start the process again. This is an opportunity to use the waste heat instead of an electrical-driven refrigeration cycle to transfer the moisture. Although desiccants can either be solid or liquid, both behave in the same principle so that their water vapor pressure is a function of temperature and moisture content. The difference between solid and liquid desiccants is their reaction to moisture. Solid desiccants, such as silica gel, mostly adsorb the moisture, which means there is no chemical reaction. However, liquid desiccant materials usually absorb the moisture by...
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