![]() Perhaps the greatest advantage, which is a unique feature of the evacuated heat pipe collector, is the fact that because no evaporation or condensation above the phase-change temperature is possible, the heat pipe offers inherent protection to the collector from freezing and overheating. The circulated heated liquid is then directed either through a heat exchanger to supply heat to a process or is stored in a storage tank for later use. Water or water-glycol mixture usually flows through the manifold heated up from the condensation of the vapor. The cycle is repeated as long as there is sunshine and thus solar heat to evaporate the fluid. The condensed fluid then returns back to the solar collector due to gravity. The solar heat evaporates the liquid and converts it into a vapor which due to its lower density risers to the heat sink (metallic vessel) where it condenses by transferring its latent heat to the flowing fluid, usually water. The heat pipe contains a small amount of volatile fluid (usually methanol or ethanol) which, as long as there is sunshine, undergoes an evaporating-condensing cycle. Each tube is terminated to a metallic vessel which is attached to the sealed pipe and acts as a condenser. The heat pipe is a sealed copper pipe, which is attached to a black thin copper fin and forms the collector absorber plate as shown in Fig. ĮTCs use the liquid–vapor phase change as a heat pipe to transfer heat effectively from the collector to the heat transfer medium. ![]() Like FPCs they collect both direct and diffuse radiation, their efficiency however compared to FPCs is higher at low incidence angles, which gives an advantage to the ETC in day-long performance. ![]() Schematic diagram of an evacuated tube collector and cross-section detail.Ĭompared to FPCs, higher temperatures can be obtained from ETCs due to the combination of selective surface which is more feasible to be used in these collectors due to the small size of the fin and the effective convection suppressor achieved because of the vacuum insulation. ![]()
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