Design and Construction of a Sun tracker for Use in Low Cost Small Scale Multipurpose Thermal Solar Energy Concentrating Systems

摘要

Recently very promising advances in the field of thermal solar energy concentrating systems have been made. It is now proven that more than 90% of the energy in the direct solar flux can be converted to steam at 700°C, and that large concentrating solar energy systems (in the range 50 kW to 100 MW) feeding heat to a conventional thermal power station may in the near future be able to produce electricity at the same price level as coal fired power stations. These systems may also be scaled down to provide power in rural areas of developing countries. From the three different types of concentrating systems, namely parabolic through, dish and power tower systems, for the range of temperatures envisaged in villages applications, which is of about 400°C, a parabolic dish concentrating system can be more effective. This means systems of sizes of 4 to 50 kW, which are suitable to satisfy many needs in rural areas of developing countries. The heat produced by such systems can be used for many purposes, such as (i) food preparation; (ii) absorption cooling, refrigeration and freezing; (iii) sterilisation of instruments in health centres; (iv) food and chemical industry; (v) water desalination and pasteurisation; (vi) hot water preparation or space heating (vii) electricity production by using steam or sterling engines. Such systems may reduce the need for fuelwood, which is becoming a scarcity in many regions. In order to follow the movement of the sun, concentrating systems need tracking mechanisms. In dish systems it is common that the reflector and the receiver are moved as one unit, the dish receiver unit (DRU), to track the sun. For smaller systems, as it is in this case, the polar mount is more appropriate, which requires a rotation of 1o every 4 minutes around a polar axis (north-south axis). In addition a periodic adjustment for seasonal variation of the declination is considered, which can be done manually twice a week, as the maximum daily change for declination is 0.4 degree.