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Solar thermal collectors are emerging as a prime mode of harnessing the solar radiations for generation of alternate energy. Heat transfer fluids (HTFs) are employed for transferring and utilizing the solar heat collected via solar thermal energy collectors. Solar thermal collectors are commonly categorized into low temperature collectors, medium temperature collectors and high temperature collectors. Low temperature solar collectors use phase changing refrigerants and water as heat transfer fluids. Degrading water quality in certain geographic locations and high freezing point is hampering its suitability and hence use of water-glycol mixtures as well as water-based nano fluids are gaining momentum in low temperature solar collector applications. Hydrocarbons like propane, pentane and butane are also used as refrigerants in many cases. HTFs used in medium temperature solar collectors include water, waterglycol mixtures -the emerging "green glycol" i.e., trimethylene glycol and also a whole range of naturally occurring hydrocarbon oils in various compositions such as aromatic oils, naphthenic oils and paraffinic oils in their increasing order of operating temperatures. In some cases, semi-synthetic heat transfer oils have also been reported to be used. HTFs for high temperature solar collectors are a high priority area and extensive investigations and developments are occurring globally. In this category, wide range of molecules starting from water in direct steam generation, air, synthetic hydrocarbon oils, nanofluid compositions, molten salts, molten metals, dense suspension of solid silicon carbide particles etc., are being explored and employed. Among these, synthetic hydrocarbon oils are used as a fluid of choice in majority of high temperature solar collector applications while other HTFs are being used with varying degree of experimental maturity and commercial viability -for maximizing their benefits and minimizing their disadvantages. Present paper reviews the recent developments taking place in the area of heat transfer fluids for harnessing solar thermal energy. Refrigerants/phase changing materials: These are low boiling point but high heat capacity substances used in the solar collectors to transfer heat in applications like solar space cooling and heating, refrigerators, air conditioning, etc [7]. These materials absorb heat from the solar collectors, produce work either by expanding in a turbo-generator of a vapor compression cycle or by dissociating the refrigerant from its absorbent in a vapor absorption cycle [8]. In some of the relatively high temperature applications, higher boiling point refrigerants are used as indirect heat transfer fluids, wherein the heat collected from the solar collectors is transferred to another fluid like water from where the refrigerants pick-up heat to do the required work in the turbo-generators [9,10]. Recent Developments in Heat Transfer

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Bulk-heterojunction (BHJ) electrodes are expected to play a significant role in electrode designs aimed to increase the efficiency of photoelectrochemical applications owing to their combined, efficient charge extraction and charge transportation. The photoelectrochemical behavior of BHJ nanoparticulate photoelectrodes with CuO (donor, p-type) and TiO 2 (acceptor, n-type) semiconductors is examined with a focus on random distribution nature. Electrodes with different donor/ acceptor ratios are prepared and characterized using various spectroscopic and electrochemical tools to reveal interconnected, crystalline, randomly distributed CuO (∼150 nm) and TiO 2 (∼25 nm) nanoparticles. A photocathodic current of ∼1.5 mA/ cm 2 is measured at 0 V RHE for CuO photocathodes (FTO/npCuO) under simulated sunlight (AM 1.5G) in 0.5 M Na 2 SO 4 (pH 5.5). We identify the cathodic photocurrent to be due to internal reduction of Cu 2+ to Cu + ; photocathodic hydrogen evolution was not observed. BHJ electrodes (FTO/npCuO:npTiO 2 ) display dual (p-type and n-type) semiconducting behavior and correspondingly exhibit cathodic and anodic photocurrents, the magnitude of which is dependent on the donor/acceptor ratio present. The addition of TiO 2 increases the electrode resistance and also promotes charge recombination in the photoelectrode. Compared to non-homogeneously distributed donor/ acceptor BHJ photoelectrodes, homogeneous electrodes obtained via the co-precipitation method exhibit higher photocurrents but still undergo photodegradation, implying the need for a protective layer or electron shuttle. We demonstrate photocurrents of 0.2 mA/cm 2 from TiO 2 -and Pt-protected npCuO:npTiO 2 BHJ photoelectrodes with improved stability and hydrogen evolution albeit with a low Faradaic efficiency (<30%).

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The paper presents energy–exergy–economic–environment–ethics analysis of a concentrated solar thermal power plant. Design basis of a concentrated solar power for 24 h operation on parabolic trough collector technology in best suited direct normal irradiation location and least capital cost analysis has been presented. An unconventional approach of reducing the capital cost is analyzed by intentionally designing the power plant for sub-critical conditions using a low-cost mineral oil with permissible operating temperature of 320°C in place of the conventional synthetic solar grade oil of 400°C. Using low pressure and temperature steam in the plant, it has been shown that while there is a reduction of 0.1% in energetic efficiency, there is a gain of 0.28% in the exergetic efficiency of the solar power plant conditions, gross thermal efficiency decreases by 1.18% and the net thermal efficiency decreases by 2.91%. However, the energetic and exergetic utilization factor for heat transfer fluid is increased by 0.84 and 5.58%, respectively. By suitably adjusting the solar field configuration and inlet oil temperature, energy savings to the tune of 45% is possible apart from 2.5 times of cost saving. An attempt has been made to quantifiably assess the ethics of switching to renewable electricity through shared responsibility as a novelty in the study. The payback period for the investment has also been shown to reduce from 20 years to 5 years assuming that the carbon price increases, concentrated solar power cost comes down by 25%, and cost at which electricity can be sold increases to US $0.14 (Rs. 10) per unit.