Development of Solar Cooker With Temperature Controlled Thermal Energy Storage Units

Abstract

Solar cookers (SCs) have been a research focus worldwide because of their numerous advantages, such as no running costs, non-polluting nature, and ample availability of solar energy. Different sensible and latent heat storage materials are used to extend the usability of SCs in the late evening hours. Recently, the latent heat of phase change materials (PCMs) used as thermal energy storage (TES) medium has become remarkable because of its high energy density and constant temperature operating characteristics. In light of this, the main objective of the present study is to design, optimize, fabricate, and perform an experimental investigation of solar box cooker (SBC) with constant temperature heat storage unit incorporating PCM. Prediction models are also developed to forecast the component temperatures of SBC. As a preliminary work, three geometries of SBCs are developed and tested to familiarize the test procedures and performance assessment. A rectangular-shaped SBC (RSBC) is fabricated by incorporating an optimum mixture of sensible heat storage materials below the absorber plate. The optimum cooker surface area is estimated with analytical heat loss and design equations solved through an iterative procedure implemented in MATLAB. Next, a cylindrical-shaped SBC (CSBC) is designed and fabricated using the minimum entropy generation (MEG) method and iterative design procedure. The experimental investigation is carried out to check the effectiveness of the frustum of a decahedron-shaped reflector on the performance of CSBC. Through experiments, it is observed that the absorber plate attains peak temperature of about 138oC-150oC. Finally, the performance of trapezoidal shaped SBC (TSBC) fitted with four outer reflectors is assessed using water and glycerol as cooking load. The TSBC shows maximum absorber plate temperature of 171°C, making this an A-grade SBC. Comparing standardized cooking power and energy efficiency, TSBC fitted with four outer reflectors performs more than CSBC equipped with decahedron-shaped reflectors. The analysis of annualized life cycle cost and pay-back period show that TSBC is more economically feasible than CSBC. viii The next stage study focuses on developing prediction model for SBCs through computational and machine learning (ML) approaches. The objective is to forecast the component temperatures of SBC through ML techniques such as random forest (RF), k-Nearest Neighbor (k-NN), linear regression, and decision tree. A numerical model based on thermal balance is used to generate the data set for the ML algorithm. Heat transfer model is developed by considering all the components of SBC such as absorber plate, glazing cover, cooking pot, lid, air cavity, and cooking load. The total heat loss from the SBC to the surroundings is estimated by considering heat loss from the absorber plate to the ambient through all faces of the cooker. The absorber plate receives solar irradiance through the double-glazing covers. Among this, some heat energy is absorbed, and the rest is rejected via convection heat transfer to inner air cavity, radiation heat transfer to second glass, and conductive heat loss through bottom of absorber plate via insulation and casing. Experiments on the TSBC are conducted to validate the numerical model. The temperatures of different components obtained through numerical modeling agree with experimental values with less than 7% maximum error. The RF model outperformed the other models and has great accuracy in predicting the thermal parameters of SBC. The third stage study focuses on the design optimization of PCM based TES unit for SBC. A computational procedure is developed to estimate the optimum mass of PCM and dimensions of the TES unit. MATLAB code is written to implement the iterative procedure, simplifying exhaustive calculations required for optimizing and designing the TES unit. The computational procedure is validated by the present experimental study and also compared with previous works. A modified TES unit containing PCM as heat storage medium surrounding cooking vessel is designed and fabricated with the iterative procedure. The TES units developed in this study have the provisions for filling the PCM on all sides, including the lid, enhancing the heat storage. The present work also aims to design, fabricate and test different geometries of TES units using paraffin wax as the PCM. After six hours of the test, the cooking load temperature in all geometries of TES units reached the melting point of PCM. TES units with cylindrical shapes perform best among hexagon and square designs. ix The selection of optimum PCM is also important for effective heat storage in SCs. Therefore, the present work aims to select the optimum PCM among the alternatives for the TES unit incorporated in SBC. Based on the melting temperature, the PCMs are pre-screened among the alternatives used in earlier works. The optimum PCM is selected with multi-criteria decision-making (MCDM) techniques like TOPSIS, EDAS, and MOORA. The criteria weights required for the optimization algorithm are found by using AHP, ENTROPY, and CRITIC methods. All MCDM techniques show that erythritol is the best alternative for the TES medium incorporated in the SBC. The iterative solution procedure also selects erythritol as the best alternative since it requires less quantity than other PCMs. In the last stage, the performance of optimized TES units is experimentally assessed using SBC. The performance parameters of the RSBC having iron grits, sand, brick powder, and charcoal powder in the optimum ratio (mass) of 1:2:2:3 respectively as heat storage material is investigated. It is found that water temperature in the developed TES incorporated RSBC is maintained above 70oC till 6 PM in a day. The performance test on CSBC is carried out with the optimized cooking vessel surrounded by the TES unit filled with paraffin wax as the PCM. The results show that the TES maintains water temperature between 55oC-60oC during evening hours. Finally, the optimized TES unit containing erythritol as PCM is tested with TSBC using glycerol and water as cooking load. Glycerol and water show more than 115oC and 90oC, respectively, during night hours by absorbing latent heat energy from the TES unit. As a summary, this study focused on the design, fabrication, and experimental assessment of the performance of a novel solar box cooker with thermal energy storage unit using phase change material. It includes optimum design of solar box cooker and thermal energy storage unit, computational, and machine learning approaches for the prediction model development. The optimum phase change material is selected based on the multi criteria decision making techniques and computational procedure. As per literature study no other work incorporates x computational, experimental, and machine learning aspects of solar cooker assessment. Researchers can predict solar cooker performance through the study without requiring elaborate experiments, which saves both time and money. This study will inspire researchers to explore the possibilities of optimization, numerical and machine learning approaches for solar thermal conversion applications.