Thermal Aspects of Solar Air Collector

The amount of solar radiation striking the earth's surface not only depends on the season, but also depends on local weather conditions, location and orientation of the surface. The average value of this radiation is about 1000 2 w m when the absorbing surface is perpendicular to the sun's rays and the sky is clear. There are several methods exist to absorb and use this free, clean, renewable and very long lasting source of energy [1]. The solar collectors are one of the devices which can absorb and transfer energy of the sun to a usable and/or storable form in many applications such as drying the agricultural, textile and marine products as well as the heating of building [2]. There is variety of designs for the solar thermal collectors depending on their applications. For example, parabolic trough solar air collector is widely used in solar power plants where solar heat energy is used to generate electricity [3]. Flat plate solar collectors are the most common types of solar collectors used in many applications such as solar hot water panels to provide hot water or as solar air heater for pre-heating the air in building heating or industrial HVAC systems. In the solar hot water panel, a sealed insulated box containing a black metal sheet, called absorber surface, with built-in pipes is located faced to the sun. Solar radiation heats up water inside pipes, causing it to circulate through the system by means of natural convection. Then, water is delivered to a storage tank located above the collector. Such passive solar water heating devices used widely in hotels and home especially in southern Europe. Several models of thermal solar flat plate collector are available and generally all of them consist of four major parts: 1. A flat-plate absorber, which absorbs the solar energy 2. A transparent cover(s) that allows solar energy to pass through and reduces heat loss from the absorber 3. A heat-transport fluid (air or water) flowing through the collector (water flows through tubes) to remove heat from the absorber 4. And finally, a heat insulating backing. The exergy of a system defined as the maximum possible useful work during a process that brings the system into equilibrium with a heat reservoir [3]. Exergy can be destroyed by irreversibility of a process. One of the powerful methods of optimizing complex thermo dynamical systems is to do an exergy analysis, which is called the second law analysis as