Vapour absorption refrigeration systems (VARSs), which utilise eco-friendly refrigerants (water), can work based on low-grade thermal energy, such as the solar energy. Using solar energy can relief the high electrical load on many national grids around the world as the peak load almost coincides with the high solar intensities time during summer. However, high initial cost, big specific size and low coefficient of performance are the main challenges that face the VARSs. Therefore, improving the efficiencies of the components of a solar refrigeration system, such as the solar collector, generator and absorber, is crucial to improve the overall efficiency of that system and to reduce its size and the cost.
To improve the efficiency of a solar VARS, nanofluids are proposed in this work through direct and indirect ways. The direct way is seeding functional nanoparticles in the aqueous solution of a VARS. The expectations are that refrigerant (steam) can be generated efficiently via direct volumetric absorption of the solar energy at the generator and can be absorbed effectively via the Brownian motion of the nanoparticles at the absorber. While, the indirect way is; using aqueous nanofluids in direct solar collectors can harvest the solar energy in an efficient way, saving it in a storage tank. This stored energy is used later to generate the steam in the generator of the VARS.
This work aims to investigate fundamentally the applicability of utilising nanofluids for solar absorption refrigeration systems through performing three main studies:
Firstly, a comparative study of gold, copper oxide, gold and copper oxide hybrids, and carbon black nanofluids has been conducted to investigate the photo-thermal conversion efficiency. The results have shown that gold nanofluids are not feasible for solar application due to the high cost and low performance comparing to the carbon black nanofluids. Moreover, this study has demonstrated that blending different nanofluids of different narrow spectral absorption peaks can really broaden the effective spectral absorption peak but reduces its value due to the accompanied dilution of the blended nanofluids as the overall volume unavoidably increases.
Secondly, a comparative study of solar steam generation among gold nanofluids, carbon black nanofluids and a thin carbon-based porous medium has been conducted. The results have also shown the infeasibility of using gold nanofluids comparing with the carbon black nanofluids due to the high cost and low absorptivity. Furthermore, this study has shown the superiority of using a thin, carbon-based porous medium in producing steam due to its capability to absorb most the solar energy in micro-sized thickness. While, very high nanoparticles concentration is required to trap and absorb the solar energy in such a thin layer, which consequently leads to an instability, high viscosity and high-cost issues.
Finally, a study of the steam absorption by and generation from aqueous lithium bromide solutions seeded with carbon black and carbon nanotubes has been conducted. The results have demonstrated that a very low concentration of carbon black nanoparticles can reduce the transparency of the solutions to zero. However, seeding nanoparticles in the solutions has shown the negligible effect on the steam absorption rate, which demonstrated that the Brownian motion of the nanoparticles has a negligible effect on the steam absorption.
Although the experiments conducted in this project showed negligible enhancement in the steam absorption, obvious enhancements in the photo-thermal conversion efficiency and steam generation were achieved by using nanofluids. Recommendations are suggested for future work to study other affecting aspects of seeding nanoparticles in aqueous solutions.
