Solar thermal energy systems are future sustainable solutions for both domestic as well as industrial use. Solar thermal systems operating in medium temperature range (373-673 K) require concentrated solar-thermal heating (CSH). In this work, a comprehensive numerical tool is developed to design and study multipurpose on-sun CSH system. The model uses a combined Monte-Carlo ray tracing, finite difference method and all heat transfer modes. The model is validated with in-house experiment, which demonstrates its predictive capability. Next, the tool is used to optimise the cavity receiver geometry and predict the performance of the optimised CSH system under different direct normal irradiance (DNI) conditions. A CSH system using Therminol D12 as HTF is presented. Therminol D12 HTF based system is predicted to take longer time than the system using water as HTF, for heating water to a specified temperature because of the heat exchanger effectiveness. However, the designed CSH system using Therminol D12 can attain higher temperatures than water without pressurization and through the heat exchanger can be used as multipurpose system suitable for cooking, laundry, sterilization, process industry etc.

Steady-state and transient behaviors of single-phase natural circulation loop (SPNCL) are investigated using four thermal oils (Therminol VP1, Paratherm CR, Dowtherm A, and Dowtherm Q) and water-based ternary hybrid (various combinations of different nature and shaped nanoparticles: Al 2 O 3 , Cu, carbon nanotube (CNT) and graphene) nanofluids as loop fluid. The influences of nanoparticle volume concentration and loop height-to-width ratio on the mass flow rate and total entropy generation rate of SPNCL are investigated. Results disclose that ternary hybrid nanofluids enhance flow initiation, reduce fluctuation and are expected to attain a steady-state faster than water. Steady-state mass flow rate increases/decreases for ternary hybrid nanofluid depending on the shape of the nanoparticle, and the total entropy generation rate decreases as compared to water. Thermal oil shows a higher mass flow rate and total entropy generation rate as compared to water. Al 2 O 3 –Cu–CNT–water and Paratherm CR show the best result among all ternary hybrid nanofluids and thermal oils, respectively. The nanoparticle shape decides the optimum nanoparticle volume fraction. Increasing the height-to-width ratio decreases the total entropy generation and upsurges the mass flow rate at specified input power. The optimum height-to-width ratio depends on the loop fluid.

The open volumetric air receiver (OVAR)-based central solar thermal systems provide air at a temperature > 1000 K. Such a receiver is comprised of porous absorbers, which are exposed to a high heat-flux > 800 Suns (1 Sun = 1 kW/m 2 ). A reliable assessment of heat transfer in an OVAR is necessary to operate such a receiver under transient conditions. Based on a literature review, the need for developing a comprehensive, unsteady, heat transfer model is realized. In this paper, a seven-equations based, one-dimensional, zonal model is deduced. This includes heat transfer in porous absorber, primary-air, return-air, receiver casing, and their detailed interaction. The zonal model is validated with an inhouse experiment showing its predictive capability, for unsteady and steady conditions, within the reported uncertainty of ±7%. The validated model is used for investigating the effect of operating conditions and absorber geometry on the thermal performance of an absorber. Some of the salient observations are (a) the maximum absorber porosity of 70–90% may be preferred for non-volumetric and volumetric-heating conditions, (b) the minimum air-return ratio should be 0.7, and (c) the smallest gap to absorber-length ratio of 0.2 should suffice. Finally, suggestions are provided for extending the model.