Experimental and computational analyses of solar pond based thermo-electric systems

Abstract

The solar pond is a radiation collecting and storing device used for low to medium-scale thermal applications. It traps the radiation either because of the halocline or any other method suppressing the natural convection to its upper layers. Heat loss occurs from the upper convective zone by surface radiation, evaporation, and natural convection. The heat transfer between the upper and non-convective zones and the non-convective and lower convective zones is governed by heat conduction and convection. Further, the heat transfer within the non-convective zone is primarily determined by heat conduction due to the halocline effect. Solar ponds are widely used in a plethora of essential applications, starting from the desalination of water, thermoelectric power generation, dairy plants, space heating, and crop drying, to name a few. In this research work, the focus is on the experimental and computational analysis of solar pond-based thermal systems with a particular focus on thermoelectric power generation applications. Various studies conducted in this direction are discussed below. In the first study, a genetic algorithm-based inverse methodology was developed to predict and optimize the suitable dimensions of various regions of a salt-gradient solar pond to ensure a temperature potential across the year for thermoelectric power generation. The system considered in this study was a combined thermosiphon and thermoelectric generators (TEGs) based power generation unit installed in a solar pond. The installed thermosiphon transports the heat stored in the lower zone of the pond to the hot end of the TEG attached to the thermosiphon at the upper zone. The desired parameter to be optimized thus becomes the difference in temperature between the lower and the upper zone of the solar pond. The results revealed that improved pond dimensions obtained using this methodology achieved a better temperature profile at a lower total height (18.11 % less) than that of the ponds available in the literature. The model demonstrated that for the diverse meteorological conditions of India, multiple combinations of convective and non-convective regions of the solar pond could ensure the required minimum temperature difference across the solar pond. This work was further extended to 5 parameters prediction model instead of three parameters. The area and volume were also optimized in addition to the thicknesses of the three zones of a solar pond. Next, a methodology based on the Eigen functions expansion was proposed to predict the temperature profiles within a two-zoned solar pond with heat extraction from the non-convective zone. This extracted was assumed to power a TEG module. The net energy and power output from the system for a specified period was obtained using the temperature field obtained from this model. The variation in power output and net energy for various cases of design and operating parameters during the maturation phase of the solar pond and after achieving the steady state was also analysed. It was observed that various combinations of pond operational parameters could result in the same output from the system. Further, it was observed from the study that either a maximum of 42.3% reduction in volume or 11.9% reduction in the solar pond’s area could be achieved to produce the same power from TEG as that of the reference model. Furthermore, the computational time of the developed model was found to be significantly lesser as compared to that of the numerical model. In the third study, a detailed investigation on the effect of heat dissipation from sidewalls of a salt gradient solar pond on its various thermal aspects has been carried out. The selection of an optimum side insulation thickness is crucial in the power enhancement of thermoelectric power units combined with the solar pond. Hence in this work, the effect of sidewall insulation on the influence of the variable size of different zones and area of cross-sections were studied using validated analytical and finite difference-based numerical approaches. It was highlighted that there is always a unique value of non-convective zone for a given insulation thickness where the pond’s performance under a given operating condition becomes maximum. But, such optimal size does not exist for upper and lower convective zones, and beyond a specific value, further change in these zones is futile. This study provided a mathematical background for the selection of circular cross-sections for designing vertical-walled solar ponds for given insulation and other operating parameters. The impact of insulation was also found to depend on the radiative intensity and temperature profiles of a given location. This work proposed cardinal guidelines regarding the suitability and the unsuitability of insulations along with the appropriate selection of the solar pond’s design. In the fourth study, a response surface analysis is carried out on a laboratory-scale experimental setup. A solar pond-powered combined thermosiphon and TEG system were simulated in the lab. A three-level Box-Behnken response surface method was adopted for the design of the experiment, and analysis of variance was carried out to gauge the contribution of each operating parameter on various performance parameters. Effects of operating parameters such as working pressure, filling ratio, evaporator length, and evaporator temperature are studied on the performance of the integrated thermosiphon and TEG system. Various correlations were developed that will be useful in computing the voltage and current output from the thermoelectric generators attached to the thermosiphon. Experiments revealed that the performance of the thermosyphon-assisted TEG system was mainly governed by pressure and evaporator temperature, whereas filling ratio and evaporator length had relatively less influence. Various thermoelectric power generation systems based on solar ponds requires either the heat extraction or the heat addition to lower zone of the solar pond based on their working. The heat extraction case has been analysed for stability by various researchers. However, it was observed that the stability analysis of the solar ponds under heat addition from external sources was not investigated. Therefore, in the fifth study, the stability and thermal performance of a salt gradient solar pond were analysed experimentally using a lab-scale setup. A lab-scale setup consisting of a mini cylindrical solar pond was fabricated, and experiments were conducted under simulated radiation using a halogen lamp. The external heat addition was simulated by circulating electrically heated water from an insulated storage tank to the solar pond using a copper heat exchanger installed in the lower zone of the solar pond. A numerical model based on the solution of differential heat balance equations in the gradient zone was also developed to predict the temperature and salinity variation in the pond with time. The stability criterion concerning upper and lower zones was tested for various cases of inlet temperatures and different upper zone thicknesses. Experiments revealed that the pond was stable for various cases of inlet temperature and upper zone thicknesses considered. The stability coefficients were found to be higher for the observation period compared to the heating period. The stability coefficients were found to be relatively lower at higher inlet temperatures than low inlet temperature in the heat exchanger. Furthermore, a study was conducted on a combined SGSP and biomass gasifier system particularly for the off-sunshine period. The effect of various design and operation parameters was studied on the performance of this system.