Given rapid depletion of conventional energy sources and environmental degradation caused by their over exploitation, the renewable energy sources are believed to be the future. Technologies utilizing renewable energy sources differ significantly from one another, not only with regard to technical and economic aspects but also in relation to their reliability, maturity, and operational experience in utility scale conditions. Technologies used to harness solar energy have emerged as the most promising and mature since solar energy is abundant, freely available, and it has commercial potential too. This paper presents a review of advancements made in the field of solar thermal technology with a focus on techniques employed for its performance enhancement. It also covers the description of different types of solar collectors to facilitate the systematic understanding of solar thermal technology and the novel modifications realized in each category of solar collectors have been highlighted to promote the use of solar energy in routine activities. Performance enhancement techniques such as geometrical modifications on the absorber plate, use of solar selective coatings and nanofluids have been given a special attention.

Viscoplastic fluids are special kind of non-Newtonian materials which deform or flow only when applied stresses are more than a critical value known as yield stress. In this work, a numerical investigation of natural convection in a square enclosure filled with viscoplastic fluids has been reported. The enclosure has been partially heated from the bottom wall by a heating source and symmetrically cooled from both the side walls. The rheology of the viscoplastic fluids has been modeled with Bingham fluid model. A scaling analysis has been presented to establish the gross dependence of heat transfer on different values of operating parameters of the problem. The effects of yield stress of the fluid on heat and fluid transport inside the enclosure have been investigated for different values of temperature difference, across the hot and cold surfaces and also for different fluids. The effects of different lengths of heated zone on the flow phenomena and heat transfer characteristics have been investigated for three different values of the heated zones. All the important results have been expressed in terms of Bingham number (Bn), Rayleigh number (Ra), and Prandtl number (Pr). It has been observed that with the increase in Bingham number, the buoyancy induced fluid circulation and convection effect decreases inside the enclosure. For each Rayleigh number, there correspond a critical Bingham number for which the heat transfer inside the enclosure takes place solely by conduction mode. This critical value increases with the increase in Rayleigh number. For fixed value of Bingham number, i.e., fixed value of yield stress, the effects of Rayleigh number and heated length on heat transfer have been observed similar to the case of natural convection in Newtonian fluid. It has been also observed that at high Bingham number the effect of increase in Rayleigh number on average Nusselt number is lesser compared to the effect of increasing Rayleigh number at low Bingham number. Using the present numerical results, a correlation of average Nusselt number as a function of other nondimensional numbers has been established.

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