Briefly describe the major losses ocuurs in flat plate collector
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INTRODUCTION
Solar water heating is a technology that is simple to adopt for both urban and rural applications. It basically consists of the flat-plate collector, the flow pipe network and the water storage tank. The storage tank is placed at a height 30-60 cm relative to the top of the collector to prevent the reverse circulation during off-sunshine hoursA natural circulation solar water heater is a passive energy system which operates on the principles of conduction, convection and radiation without assistance of a mechanical device (Hunt, 1982). This makes them generally more reliable, less expensive, easier to maintain and possibly longer lasting than active systems. Hence, the option of utilizing natural circulation solar water heater for rural application is particularly attractive given the level of deforestation in these areas and the absence of national grip for alternative power supply.
Flat-plate collector is one of the most important components of a solar water heater. Flat-plate collectors are the simplest and most commonly used collectors for converting the sun’s radiation into useful heat. They are designed for applications requiring energy delivery at moderate temperature (less than 100°C) as in water and space heating (Duffie and Beckman, 1974). They have the advantages of using both beam and diffuse solar radiation, not requiring orientation towards the sun and requiring little maintenance. A conventional flat-plate collector is made up of a flat absorbing plate, normally metallic upon which solar radiation falls and is absorbed, changing to heat energy. The absorb plate is usually black in colour for optimum collection and its area is the same as the area intercepting the radiation. Attached to the collector absorb plate are tubes, channels or passages which circulate the heat transfer fluid. A transparent or translucent cover of glass or plastics is attached on the absorber plate mainly to reduce the upward convection and radiation heat losses from the collector. Translucent low iron glass is a common glazing material for flat-plate collectors because low iron glass transmits a high percentage of the total available solar energy . Insulation is provided at the back and sides of the absorber plate to minimize heat losses by conduction. Flat-plate collectors are usually mounted in a stationary position with an orientation optimized for a particular location (Garg, 1987). As the sun heats the collector, the hot water inside rises by natural convection and the colder storage tank water leaving from its bottom flow into the collector by gravity. Thus the circulation loop is automatically established whenever there is sufficient insolation and circulation automatically stops during insufficient insolation when the upward buoyancy force is unable to overcome the fluid friction losses inside the pipes.