Enhancing the optical and thermal efficiency of a parabolic trough collector - A review

Among the various available renewable energy sources such as wind, geothermal, tidal, bio mass, etc., harnessing of solar energy has become quite popular in most of the countries. Different collectors have been modeled, designed, fabricated and tested to operate in different range of temperatures such as low temperature collectors (flat plate 30-80 degrees C, evacuated tube 50-200 degrees C & compound parabolic collector 60-240 degrees C), medium temperature collectors (linear Fresnel reflector 60-250 degrees C, Cylindrical trough 60-300 degrees C, parabolic trough 60-400 degrees C) and high temperature collectors (parabolic dish reflector 100-1500 degrees C, heliostat field collector 150-2000 degrees C). In applications such as delivery of process heat and steam generation, the parabolic trough collector is found to be most popular one among the other collectors. Numerous research investigations both theoretical and experimental have been carried out for nearly more than three decades to enhance the optical and thermal efficiency of the system. The optical efficiency depends on the property of the materials such as reflectance of mirror, transmittance of glass cover, absorptance-emittance of receiver, intercept factor, geometry factor and angle of incidence. The properties of the reflecting, absorbing and transmitting surfaces are greater than 95% and the emissivity has also reached values as low as 0.02. A few research works on end losses have been carried out too. The thermal efficiency depends on overall loss coefficient which includes conduction, convection and radiation losses. Convection and radiation losses have been minimized by the use of metal glass evacuated tubes and selective surface coatings on the receiver. Conduction losses persist and depend on the material of structure. Investigations reveal that failure of vacuum, hydrogen accumulation in receiver and breakage of tubes at end of the receiver are quite a few problems that result in heat loss. A narrow gap alone exists in the enhancement of the property of materials except for a huge prospect lies in minimizing the degradation of coatings and its properties at high temperature. Thus a huge opportunity for further investigation lies in the heat transfer enhancement of receiver tube, development of a low cost and highly rigid structure, less expensive and more accurate tracking mechanism. Numerous numerical and/or experimental investigations of the performance of a novel cavity absorber to determine thermal or optical efficiency and/or pumping loss while replacing the conventional tubular receiver in a parabolic trough, linear Fresnel and parabolic dish collector have been performed. Investigations using passive methods to enhance the rate of heat transfer in heat exchanger domain have been carried out and hence a similar practice has been tried out in the receiver of PTC where some of the inserts used in heat exchanger have been tried out in the receiver tube of the PTC too. A review of such works have been presented in this paper along with the review of the other works carried out in the enhancement of optical and thermal efficiency of the solar PTC. Lastly the delivery of process heat and steam generation has been discussed along with the economic analysis of the PTC followed by the future research prospects in a parabolic trough collector and economic assessment models.