Energy recovery at thermodynamic expansion and thermal boosting through convection in flat plate solar thermal systems.

Abstract

Fossil fuels have served mankind to meet energy needs in both domestic and commercial applications for a considerable length of time. However, fossil fuels have environmental implications such as emission of harmful gases, depletion of ozone layer and climate change. Moreover, the ever rising prices and limited resources of fossil fuels have obstructed the uninterrupted supply of energy. Therefore, there is a dire need to develop renewable energy technologies that can sustain energy supply with increasing demands. Due to inexhaustible amount of clean solar energy, engineers and researchers are engaged in developing technologies to minimise dependency on fossil fuels. Solar collectors are utilised to use solar heat to increase the heat energy of thermo-fluids or heat transfer fluid (HTF), which can operate Organic Rankine Cycle (ORC) to generate electricity. However, the extensive gain from solar energy is restricted due to unreliability of solar energy during changing weathers and lack of availability at nights. Therefore, thermal energy storage (TES) system can provide a viable solution to respond to varying levels of solar energy. Literature review indicated that phase change materials (PCM) based latent heat storage (LHS) systems are promising TES technique due to their high thermal storage density, operation at isothermal conditions and wide range availability of PCMs. However, large-scale practical utilisations of LHS systems are limited due to restrained charging and discharging rates caused by low thermal conductivity of PCM. Hence, this research is focused on numerical and experimental analyses conducted in developing an efficient and effective TES technology with novel heat transfer mechanism and novel thermal storage materials to sustain continuous generation of heat and power for low temperature practical applications. In this research, numerical investigations were conducted to propose an optimum and novel design solution for shell and tube heat exchanger with multiple tube passes and longitudinal fins for improved thermal performance. Parametric investigations were conducted to examine the influence of number and orientations of tube passes in the shell container, geometrical dimensions of longitudinal fins, construction material for shell, tube passes and longitudinal fins, and operating temperature of HTF on phase transition rate and overall enthalpy of LHS system. Further, the proposed design was developed and commissioned with a connection to flat plate solar collector to examine thermal performance at varied operating conditions. Paraffin (RT44HC) was employed as PCM in shell container and water was utilised to circulate in tube passes to transfer thermal energy gained at solar collector to paraffin in shell container. Thermal performance was evaluated by conducting series of charging and discharging cycles at varied operating conditions to examine the charging/discharging rate, accumulative thermal energy gain/release and mean charging/discharging power. Furthermore, numerical and experimental analyses were conducted to evaluate nano-additives enhanced paraffin samples, which were developed by incorporating aluminium oxides (Al2O3), aluminium nitride (AlN) and graphene nano-platelets (GNP) nano-additives in base paraffin. Based on numerical and experimental results and recommendations, numerical simulations were conducted on coupled thermal performance enhancement techniques with longitudinal fins and graphene nano-additives enhanced paraffin samples. It was noticed that phase transition rate for coupled thermal performance enhancement techniques was significantly enhanced by 75.46% as compared to no longitudinal fins orientation with pure paraffin. Likewise, the proposed LHS system can efficiently charge and discharge 14.36 MJ and 12 MJ of thermal energy in as less as 3 h and 1.5 h, which ensures the large-scale practical utilisation in both domestic and commercial applications.