Heat transfer analysis and optical characterization of nanoparticle dispersion-based solar thermal systems

Abstract

Majority of the present solar thermal systems are constructed using surface absorption-based absorbers, wherein solar energy is first absorbed by a selectively coated solid surface and then transferred to a working fluid through conduction and convection heat transfer. Selective coatings have high absorptivity across the incident solar energy spectrum (0.3 µm - 2.5 µm) while simultaneously having low emissivity in the midinfrared region (2.5 µm - 10 µm). These characteristics make selective coatings quite suitable for harnessing solar thermal energy. However, during such a process an inherent thermal barrier is present in the form of conduction and convection resistances between the absorbing surface and the working fluid. As a result, conversion of solar energy into thermal energy of the working fluid is not a very efficient process in surface absorptionbased solar thermal systems. This problem becomes more pronounced in the case of concentrating solar collectors as the flux that must be transferred across this barrier increases. In this case, excessively high surface temperatures can develop relative to fluid temperature. Since radiative losses are proportional to the fourth power of the surface temperature, high surface temperatures lead to excessively high radiative heat losses and stringent material requirements. This problem can be reduced by allowing the incident solar radiation to interact directly with the working fluid without heating any other structures within the receiver. Although typical heat transfer fluids are transparent in significant portion of solar spectrum, dispersing trace amounts of nanoparticles can significantly enhance the solar irradiance absorption capability of these basefluids. Excellent stability (relative to micron-particle suspensions) coupled with high solar energy absorption capability make nanoparticle dispersions (popularly known as 'nanofluids') suitable for solar thermal applications. Moreover, the fact that the optical properties of metallic and semiconductor nanoparticles are dependent on their shape, size, and the surrounding dielectric media makes them easily usable for engineering highly solar selective nanoparticle dispersions at very low volume fractions. Furthermore, critical analysis of the previous studies relevant to nanofluid-based solar thermal systems reveals that these novel solar thermal systems hold huge potential and warrant further exploration. The present study is an attempt to understand the optical behavior of nanoparticle dispersions and investigate the key heat transfer mechanisms involved in the volumetric absorption solar thermal systems employing these nanoparticle dispersions. Firstly, an attempt has been made to model optical properties of nanoparticle dispersions. It has been found that optical properties of metallic nanoparticles can be calculated from that of the corresponding bulk values after incorporating intrinsic size effects in the classical Drude model. Opposed to nanospheres, nanospheroids have optical signatures which are characteristic of particle shape and size. Thus anisotropic geometries are far more optically tunable as compared to their isotropic counterparts. Next, a detailed heat transfer model was developed for cylindrical volumetric receiver (simulating a linear parabolic trough) employing nanoparticle dispersion as the working fluid. Furthermore, it was demonstrated that it is possible to make these volumetric receivers ‘solar selective’ through usage of heat mirrors. This exercise reveals that there is a definite improvement in terms of thermal efficiency in the case of the nanofluid-based volumetric receivers relative to their surface absorption-based counterparts. The modeling results being encouraging paved the way to carry out experimental demonstrations of laboratory scale solar thermal systems. As a first step in this direction, an attempt was made to identify the optimal nanoparticle-basefluid pair for solar thermal systems. For this optical characterization of various basefluids and nanoparticle dispersions was carried out using UV-VIS and FT-IR spectrophotometers. This exercise confirmed that most of the common basefluids are nearly transparent in most part of the short wavelength region (0.2 µm - 1.5 µm) and highly absorbing in long wavelength region (2.5 µm - 25 µm). Hence, as such these basefluids alone are not good enough for harnessing solar energy and definitely some additives need to be added to make them good solar absorbers. Furthermore, it was found that addition of nanoparticles to the basefluid significantly enhanced the solar irradiance absorption capability. Rate of increase of solar weighted absorptivity with increase in mass fraction of nanoparticles was found to be highest in the case of amorphous carbon. Now that optimum nanoparticle material was identified, next step was to test these nanoparticle dispersions for their heat transfer behavior. For this purpose a proof-of-concept experiment was carried out. In this experiment, nanoparticle dispersion (amorphous carbon nanoparticles in ethylene glycol) was employed as the working fluid in the volumetric absorption system. Furthermore, in the same configuration, pure liquid (ethylene glycol) was placed in contact with a selectively coated (TiNOX) copper substrate to test the baseline surface absorption system. It was found that under similar operating conditions, higher average stagnation temperatures are achievable if the electromagnetic radiation is allowed to directly interact with the bulk of the fluid (which is the case in a volumetric absorption system), compared to the conventional surface absorption system. Moreover, the performance of the volumetric absorption system was found to be sensitive to the amount (mass fraction) of the nanoparticles dispersed - which, in the limit, approached surface absorption at very high mass fractions. Furthermore, the spatial temperature distribution was found to be consistent with the underlying theory of volumetric absorption. Based on the optical characterization and heat transfer analysis of nanofluid-based volumetric absorption solar thermal systems it can be concluded that these systems promise enormous potential in harnessing solar energy. Anisotropic metal nanoparticles and broad absorption carbon nanoparticles are most suitable for harnessing solar thermal energy. For high temperature operating conditions, heat mirrors should be employed to curb radiative losses. As a whole there is a definite relevance of these nanofluid-based solar thermal systems and further research into these systems is warranted.