1、 附录 SOLAR WATER HEATING BACKGROUND Using the suns energy to heat water is not a new idea. More than one hundred years ago, black painted water tanks were used as simple solar water heaters in a number of countries. Solar water heating (SWH) technology has greatly improved during the past century. To
2、day there are more than 30 million m2 of solar collectors installed around the globe. Hundreds of thousands of modern solar water heaters, such as the one shown in Figure 1, are in use in countries such as China, India, Germany, Japan, Australia and Greece. In fact, in some countries the law actuall
3、y requires that solar water heaters be installed with any new residential construction project (Israel for example). Figure1: Evacuated Tube Solar Collector in Tibet, China In addition to the energy cost savings on water heating, there are several other benefits derived from using the suns energy to
4、 heat water. Most solar water heaters come with an additional water tank, which feeds the conventional hot water tank. Users benefit from the larger hot water storage capacity and the reduced likelihood of running out of hot water. Some solar water heaters do not require electricity to operate. For
5、these systems, hot water supply is secure from power outages, as long as there is sufficient sunlight to operate the system. Solar water heating systems can also be used to directly heat swimming pool water, with the added benefit of extending the swimming season for outdoor pool applications. Solar
6、 Water Heating Application Markets Solar water heating markets can be classified based upon the end-use application of the technology. The most common solar water heating application markets are service hot water and swimming pools. Service hot water There are a number of service hot water applicati
7、ons. The most common application is the use of domestic hot water systems (DHWS), generally sold as “off-the-shelf” or standard kits as depicted in Figure 2. Figure2: Solar Domestic Hot Water System in Australia Swimming pools The water temperature in swimming pools can also be regulated using solar
8、 water heating systems, extending the swimming pool season and saving on the conventional energy costs. The basic principle of these systems is the same as with solar service hot water systems, with the difference that the pool itself acts as the thermal storage. For outdoor pools, a properly sized
9、solar water heater can replace a conventional heater； the pool water is directly pumped through the solar collectors by the existing filtration system. Swimming pool applications can range in size from small summer only outdoor pools, such as the one shown at a home in Figure 3, to large Olympic siz
10、e indoor swimming pools that operate 12 months a year. Figure 3: Unglazed Solar Collector Pool Heating System in Canada Description of Solar Water Heating Systems Solar water heating systems use solar collectors and a liquid handling unit to transfer heat to the load, generally via a storage tank. T
11、he liquid handling unit includes the pump (used to circulate the working fluid from the collectors to the storage tank) and control and safety equipment. When properly designed, solar water heaters can work when the outside temperature is well below freezing and they are also protected from overheat
12、ing on hot, sunny days. Many systems also have a back-up heater to ensure that all of a consumers hot water needs are met even when there is insufficient sunshine. Solar water heaters perform three basic operations as shown in Figure 4: Collection: Solar radiation is “captured” by a solar collector
13、Transfer: Circulating liquid transfer this energy to a storage tank； circulation can be natural (thermo siphon systems) or forced, using a circulator (low-head pump)； and Storage: Hot water is stored until it is needed at a later time in a mechanical room, or on the roof in the case of a thermo siph
14、on system. Figure 4: System Schematic for Typical Solar Domestic Water Heater Solar collectors Solar energy (solar radiation) is collected by the solar collectors absorber plates. Selective coatings are often applied to the absorber plates to improve the overall collection efficiency. A thermal fluid absorbs the energy collected. There are several types of solar collectors to heat liquids. Selection of a solar collector type will depend on the temperature of the application being considered and the intended season of use (or climate). The most common solar collector types are:
