fixed temperature controller

the fixed temperature controller (ftc) method was adopted by the solar industry in india for a long period. even now some manufacturers are supplying with this design. the ftc method involves the arrangement of collectors in series-parallel or parallel-series configeration. cold water is let into the collector and it remains there without circulation until the desired temperature, say 80?c, is reached. at the end of the bank of collectors on the delivery-load side, a thermastat and an electro-hyduraulic valve are fixed. as soon as the water in the collectors reaches the set temperature, the electro-hydraulic valve opens and releases the hot water either to the process or to a storage tank. this type of system is not efficient because for efficient heat transfer, there should be a good temperature difference and an optimum flow in the collectors. the flow should be neither too slow nor too fast. in the former case the absorber loses heat because of its high temperatures and in the latter case the time is insufficient to transfer the heat from the absorber to the liquid in this method the thermal losses are high and hence it is not a desirable practice.