Low thermal efficiency is the remarkable drawback of solar air heaters. They have remarkable importance for low temperature applications. In a solar air heater due to formation of laminar sublayer between absorber plate and the flowing air convection coefficient of heat transfer is small. The laminar sublayer form on the absorber plate can be braked by applying the rib roughness of distinct shape and sizes due to which turbulence in the flowing air increases tremendously due to which friction factor increases between absorber plate and flowing air, resulting in increment in the convection coefficient of heat transfer and pressure drop. Design considerations of solar air heater, rib roughness geometries, fluid flow conditions and their effect on the turbulence, absorber plate temperature, convection coefficient of heat transfer and other thermo-hydraulic augmentation parameters have been minutely discussed. In this review, ongoing research and development on thermo-physical properties of solar air heater and effect of distinct roughness’s over absorber plate on heat transfer augmentation is summarized and to provide viewpoint for future research and development.
The nature of solar energy is intermittent due its variable magnitude in daytime and zero throughout the night or during overcast conditions, which limiting its use in hospitals and in various industrial and domestic applications particularly in water heating. Because of inconsistent nature of solar energy some type of thermal energy storage (TES) system is essential for its effectual utilization and to meet the energy demand at all the times. Phase change material (PCM) is a very good option for latent heat TES because of its high thermal storage density and isothermal behaviour at the time of charging and discharging. The PCM absorb 5 to 14 times more heat as comparison to sensible heat storge material such as water, stone, bricks or rocks etc. The PCM with nano-sized-particle can enhanced the performance of LHTS systems. In this review, ongoing research and development on the performance of solar water heater (SWH) incorporating with PCM’s and nano-embedded PCM is summarized and to provide viewpoint for future research and development.
Solar air heater is considered to be the most popular and widely used solar thermal system. Solar air heater (SAH) can be used in many applications, ranging from domestic to industrial purposes. However, it seems that the viability of SAH is not feasible due to the following two reasons: (i) the low convective heat transfer coefficient at the absorber plate is the reason that causes a low heat transfer rate to the flowing air, and (ii) the high temperature of the absorber plate insists on high heat losses, thus, reducing the thermal efficiency. The convective coefficient can be augmented by placing turbulators/roughness on the absorber plate, which induces turbulence in the flow passage near the absorber plate by disrupting and destabilizing the laminar sublayer. This comprehensive review has been presented to summarize the studies on artificial roughness/turbulators geometries to enhance the heat transfer rate. Various rib configurations (such as grits, grooves, blockages, baffles, winglets, protrusions, twisted taps, dimples, and mesh wires) and distinct arrangements of rib roughness (such as inclined, transverse, V shape, with gap) have been reviewed to present heat transfer and friction characteristics. Additionally, thermal efficiency and thermohydraulic efficiency (in terms of net effective efficiency) of various artificial roughnesses and rib configurations are presented under distinct operating conditions for comparing purposes. This comparative study has been presented to assess the most desirable ribs and their configurations. On the basis of net effective efficiency, a multiarc rib with gaps is found to be the best configuration among all and have the highest thermal and net effective efficiency of around 79%.
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