Design and development of high performance concentrated solar collectors using passive and optical enhancement methods and nanofluids carrying absorber

With the rising concerns of fossil fuel usage and related environmental problems, the focus of global research is directed towards the development of methods utilizing renewable sources of energy and producing alternate fuels. Solar irradiation is a widely available source of free energy that can be harnessed to design a low-cost power production system. Concentrating solar collectors are promising devices to collect and transport solar energy for industrial processes and power generation. The concentrating solar collectors transform available solar energy into high-grade energy that can be transported at high temperature via heat transfer fluids such as thermal oils, molten metals, and salts. The solar collectors can be categorized as direct and indirect collectors. The direct absorption-based solar collectors have exhibited enhanced performance over the indirect collectors that consist of surface-based receivers. The last few decades have seen significant development in concentrating solar collector technologies and both Solar Tower Collector (STC) and the Parabolic Trough Collector (PTC) have been widely used in solar thermal applications. The non-uniformity of heat flux at the absorber has posed a great challenge to the safety of the system apart from exhibiting poor convection in the absorber tube. Further, the losses associated with the receiver has reduced the absorber efficiency significantly. The temperature uniformity of the absorber surface is inevitable to reduce the radiation and convection losses to the environment. The design modifications in the primary and secondary collectors can enhance the optical efficiency and create a more uniform heat flux at the tube surface. Further, the use of nanofluid as heat transfer fluid in the absorber can be used for enhanced energy transport, reduced temperature peaks and consequently reduced convective and radiation losses. These modifications need to be investigated in view of their applicability, safety, and techno-economic trade-off before proposing for commercial application. In view of the above-posed research issues, the proposed research is intended to design and develop an enhanced efficiency concentrated solar collector based on the synergetic effects of high optical efficiency of the reflectors, uniform flux, and temperature at the tube surface, minimizing, convection and radiation losses, and maximizing heat transfer through the absorber tube.

With the rising concerns of fossil fuel usage and related environmental problems, the focus of global research is directed towards the development of methods utilizing renewable sources of energy and producing alternate fuels. Solar irradiation is a widely available source of free energy that can be harnessed to design a low-cost power production system. Concentrating solar collectors are promising devices to collect and transport solar energy for industrial processes and power generation. The concentrating solar collectors transform available solar energy into high-grade energy that can be transported at high temperature via heat transfer fluids such as thermal oils, molten metals, and salts. The solar collectors can be categorized as direct and indirect collectors. The direct absorption-based solar collectors have exhibited enhanced performance over the indirect collectors that consist of surface-based receivers. The last few decades have seen significant development in concentrating solar collector technologies and both Solar Tower Collector (STC) and the Parabolic Trough Collector (PTC) have been widely used in solar thermal applications. The non-uniformity of heat flux at the absorber has posed a great challenge to the safety of the system apart from exhibiting poor convection in the absorber tube. Further, the losses associated with the receiver has reduced the absorber efficiency significantly. The temperature uniformity of the absorber surface is inevitable to reduce the radiation and convection losses to the environment. The design modifications in the primary and secondary collectors can enhance the optical efficiency and create a more uniform heat flux at the tube surface. Further, the use of nanofluid as heat transfer fluid in the absorber can be used for enhanced energy transport, reduced temperature peaks and consequently reduced convective and radiation losses. These modifications need to be investigated in view of their applicability, safety, and techno-economic trade-off before proposing for commercial application. In view of the above-posed research issues, the proposed research is intended to design and develop an enhanced efficiency concentrated solar collector based on the synergetic effects of high optical efficiency of the reflectors, uniform flux, and temperature at the tube surface, minimizing, convection and radiation losses, and maximizing heat transfer through the absorber tube.

Subject Literature: Concentrating Solar Collectors: Concentrating solar collectors (CSC) have primary and/or secondary reflectors that reflect incoming solar radiation into a specific point or line where the receiver is located. There are several variants of reflector configurations and designs. The receivers which absorb reflected solar radiation is available in a variety of configurations and carry a heating fluid often called Heat Transfer Fluid (HTF) which too available with a spectrum of thermophysical properties. The heated fluid can be of many usages in the industry such as process steam and electrical power generation. Over the last few decades, several studies were carried out to realize the potential of solar thermal technology successfully. As early as in 1976, Ramsey et al. (Ramsey et al. 1976) reported results on the concentrating solar collector using parabolic. The authors carried out experimentation and obtained a peak efficiency of 63% in the absence of losses. Dudley et al. (Dudley et al. 1994) carried out an experiment to test the efficiency and heat losses from a parabolic trough solar collector (surface absorption method) using cremet and black chrome as selective receiver coating for three receivers configurations, i.e., glass tube envelope surrounding steel pipe, with vacuum, with air, and the one without envelope. Vacuum in tube annulus performed best, then air in tube annulus. The cremet performed better than black chrome as receiver coating due to its better radiative properties. The linear Fresnel collectors require smaller land area as compared to PTC, however, its efficiency enhancement is yet to be proved. Bernhard et al. (Bernhard et al. 2008) conducted a project for designing, fabricating and testing a Linear Fresnel solar collector for the purpose of comparing it with a parabolic trough solar collector in terms of cost and efficiency and discussed the challenges to assure the durability and performance. Kalogirou (Kalogirou 2004) carried out analysis on the optical, thermal and thermodynamic properties of the collectors and provides a description of the methods used to evaluate their performance. One of the major challenges that the CSP has the non-uniformity of the heat flux which is detrimental to the safe and efficient operation of CSP (He et al. 2019). The solar energy concentrated on the absorber is mainly focused at the bottom of the receiver that results in non-uniform heat flux and temperature distribution as well as the non-uniform temperature of the carrier fluid. These nonuniformities give rise to temperature gradient which poses serious challenges to the safety of the system. Several design modifications have been proposed to reduce these uniformities such as variable focus PTC, secondary reflector, and annular space. Ries and Spirkl (Ries and Spirkl 1996) proposed a secondary reflector which is applicable to any rim angle of the parabolic reflector. The secondary concentrating reflector proposed by the authors increased the concentration ration by a factor of more than 2. Further, Tsai and Lin (Tsai and Lin 2012) proposed a variable focus PTC with a heat pipe receiver to overcome these non-uniformities. They proposed a method for optimizing the geometry of the concentrator to distribute irradiation and observed up to 84% increase in uniformity. Montesa et al. (Montesa et al. 2014) carried out experimental investigation for PTC consisting of aluminum mirror film reflectors. The authors observed an average efficiency of 60%. Cau and Cocco (Cau and Cocco 2014) Carried out theoretical modeling and analysis for comparing the performance of medium-sized concentrating solar power plants using parabolic trough and linear fresnel solar collectors and observed that the parabolic trough solar collectors collect higher energy per unit area of solar collector due to their higher optical efficiency. Bellos and Tzivanidis (Bellos and Tzivanidis 2019) reviewed several studies on PTCs and summarised the measures taken to overcome the heat flux non-uniformities and to enhance the heat transfer. The use of secondary reflectors and optical modifications are highly effective for efficeincy enhancement. Further, the use of turbulators and nanofluid are promising options for efficeincy enhancement. The suspensions of nanoparticles in a liquid medium called nanofluids have been extensively used in the last few for conventional and non-conventional heat transport applications. Several studies (Murshed et al. 2008; Leong et al. 2010; Madhesh et al. 2014; Ray et al. 2014; Ghozatloo et al. 2014) have shown that the nanofluids flow exhibit improved thermal properties such as thermal conductivity and heat transfer coefficient. However, recently some studies (Otanicar et al. 2010; Saidur et al. 2012a; Lee and Jang 2013; Sajid et al. 2014; He et al. 2019) have indicated an improvement of optical properties as well. Otanicar et al. (Otanicar et al. 2010) carried out experimental investigations of Direct Absorption Solar Collector (DASC) for a variety of nanofluids such as carbon nanotube, graphite and silver as working fluid. Authors demonstrated the unique features and advantages of DASC with nanofluid as energy harvesting systems such as volume heating instead of surface heating as in case of surface collectors, higher spectral absorption due to nanoparticles, enhanced thermal conductivity and heat transfer surface. The authors observed an increase in efficiency by 5%. Taylor et al. (Taylor et al. 2011a) carried out a two-dimensional heat transfer analysis of DASC using nanofluid and compared the performance with surface absorbers. Further, the authors carried out experimental investigations on a laboratory-scale model of a parabolic dish collector. The efficiency enhancement of 5-10% was observed by using nanofluid as a working fluid (Otanicar et al. 2010; Taylor et al. 2011a). Yousefi et al. (Yousefi et al. 2012) Carried out an experiment investigating for Al2O3 in water nanofluid for a flat-plate solar collector and observed the maximum improvement in efficiency by 28.3% with respect to water as working fluid. The efficiency is further increased by 15% using a surfactant in nanofluids. Khullar et al. (Khullar et al. 2012) carried out a numerical analysis of nanofluid-based concentrating parabolic solar collectors using Therminol VP-1 as base fluid with aluminum nanoparticles. The authors investigated the collector performance and observed that nanofluid-based solar offers 5-10% higher efficiency due to improved conduction, convection and radiation properties. Later on, Saidur et al. (Saidur et al. 2012b) investigated direct absorption collectors (DAC) performance after incorporating nanofluids as the working fluid instead of conventional fluids (aluminum water-based nanofluid was tested) with varying nanoparticle sizes and volume fraction and observed that the aluminum nanofluid, in particular, is very good for DASC applications. Said et al. (Said et al. 2014) conducted experiments on flat plate solar collectors using water-based nanofluids with TiO2 particles. The best efficiency was found for the minimum volume fraction and minimum flow rate due to the increased pumping power with the increase of TiO2 volume fraction as the flow rate is increased. Nasrin et al. (Nasrin et al. 2014) carried out a flat plate solar collector analysis using a two-dimensional finite element method. Authors used a nanofluid correlation for all the combinations of nanofluids investigated in the study such as water-based Cu, CuO, Al2O3 and Ag nanofluid and observed water-Ag as better performer compared to other nanofluid combinations. Luo et al. (Luo et al. 2014) developed a simulation model of nanofluid based direct absorption solar collector and carried out performance analysis. The authors observed an improvement in outlet temperature and efficiency by the amount of 30-100 K and 2-25%, respectively, as compared to the base fluid. Lee and Jang (Lee and Jang 2015) analytically solved a direct solar receiver with multiwall carbon nanotube-based heat transfer fluid and observed that the efficiency of the receiver is directly proportional to Peclet number and nanofluid concentration and inversely proportional to aspect ratio. Khan (Khan 2015) carried out a heat transfer analysis of a flat plate solar collector using a nanofluid model. The authors used a single- and two-phase model to analyze nanoparticles in base solution. The CuO based nanofluid exhibited higher efficiency than that of Al2O3 based nanofluid. Further, the wall shear increased with an increase in nanoparticle volume fraction in the nanofluid. Keeffe (Keeffe 2018) mathematically analyzed several variants of direct absorption solar collectors with nanofluid and as working fluid using a continuum mechanics approach to resolve heat transfer and flow field. Authors noted an increase in performance with the reflecting base panel as compared to nonreflecting one and an increase in nanoparticle volume concentration increases the collector performance. A very low volume fraction of nanoparticles will not be able to absorb solar radiation while at a very high volume fraction, a thin layer of nanoparticles is formed resulting in thermal energy losses (Taylor et al. 2011b). The optical properties of nanofluids can be substantially enhanced and tailored by varying the volume fraction, size and shape of nanoparticles, therefore a clear understanding of nanoparticle behavior is necessary to design a high-efficiency collector. An increase of about 5-10% in efficiency was observed with the use of nanofluids as absorber fluid for concentrating solar collectors (Kessentini et al. 2011; Khullar et al. 2012). In view of the above literature review, it can be concluded that the solar collectors have great potential for efficiency enhancement and optimization. The synergetic application of enhanced heat transfer fluid such as nanofluids, inserting turbulators in the absorber, direct absorption and enhanced characteristics of nanofluid are unexplored areas. This research proposal is aimed to get insight into the implementation of these technologies to get enhanced performance and functionality of concentrating solar collectors. The project is targeted to conceptualize and realize this technology by designing, fabricating and carrying out performance analysis.