Evacuated Tube solar Collectors (ETC)

Evacuated tube solar collectors are extensively and widely used because it’s good thermal insulation characteristics and insensitivity to the direction of sunlight. There are three common types of evacuated tube solar collectors, which are (a) Water-in glass evacuated tube solar collector, (b) U-type evacuated tube solar collector and (c) evacuated tube heat pipe solar collector. The function of an evacuated tube heat pipes is to absorb the incident solar radiation over the evacuated tube and to transfer as much of that absorbed energy in the form of heat to the inside heat pipe, which in turns heats up a secondary medium, e.g. water or oil, that is in contact with the tip of the heat pipe.

The evacuated tube consists of two concentric tubes, i.e. an inner and an outer tube, such that the air in between the tubes is evacuated, and then the tubes are sealed together at the end. The vacuum between the pipes plays an important role in the thermal performance of the evacuated tubes, such that it eliminates the convection loss and improves the thermal insulation. The inner tube is coated with a selective coating material, which has a high absorbing capacity and a low emissivity. The inner tube absorbs the incident solar irradiance and transfers that heat energy to the medium inside the inner tube. Inside the inner tube are a circular aluminium fin and a heat pipe. The heat pipe contains a heat transfer fluid, which has a low boiling temperature, such that the fluid can absorb the latent the heat of evaporation and evaporates at normal operating conditions of solar irradiance and temperature. The vapour rises upwards to the tip of the heat pipe carrying a large amount of energy. Heat is transferred from the vapour inside the heat pipe to the fluid that is in contact with the tip of the heat pipe, such that the vapour condenses and goes back to the bottom of the heat pipe and the cycle of heating is repeated. Mostly two types of working fluids that are used in the heat pipe, i.e. water and methanol, and they found that the heating performance was a bit better in the case of using water than methanol. There is an aluminium fin inside the evacuated tube, which is in contact with both the inner tube and the heat pipe, such that heat is transferred from the inner tube to the heat pipe by conduction via the fin surface. The efficiency of heat transfer from the evacuated tube to the heat pipe is dependent on the contact between the finned surface and the evacuated tube as well as the heat pipe. However, irregular expansion of the fin occurs due to uneven exposure to Solar radiation, i.e. the bottom part of the tube is exposed to a different radiation than the top of the tube due to the orientation of the tube and the inclination of the Sunlight, which is a function of daytime. The irregular expansion of the fin causes the fin to lose contact with the evacuated tube and the heat pipe, which results in deteriorating the rate of heat transfer. Also, the fin surface can lose contact with the evacuated tube due to overheating due to excessive solar irradiance, the influence of the air gap between the evacuated tube and the circular fin on the performance of U-type evacuated tube solar collectors, and it was found that the air gap can intensely influence the rate of heat transfer and the efficiency of the collector. The U-type evacuated tube with compressed graphite in order to prevent the adverse effect of the air gap and to improve the rate of heat transfer by conduction from the evacuated tube to the U tube. The influence of filling a U-type evacuated tube with compressed graphite, and it was found that the efficiency of the filled-type evacuated tube with U-tube is 12% higher than that of the U-tube evacuated tube with a copper fin.

However, the filling material, i.e. compressed graphite, is heavier and more expensive than copper or Aluminum fins, which can affect the lifetime and feasibility of using such a material for improving the rate of heat transfer.

                                                                                                                              By Supreme Products


Visit Us : Supreme Solar Projects Private Limited.