Performance enhancement of parabolic dish solar cookers using thermal energy storage

Abstract

The performance enhancement of a parabolic dish solar cooker using thermal energy storage (TES) is evaluated. A detailed literature review of solar parabolic cookers with thermal energy storage for off-sunshine cooking is presented. Both parabolic trough and parabolic dish solar cookers are included in the review. The review concludes that most studies are on the experimental evaluation of parabolic dish solar cookers with limited numerical studies done. The review also suggests that other crucial factors, such as the choice of sensible or latent heat storage materials, environmental impact, different foods to be cooked, and social acceptance should also be investigated. Cooking experiments are conducted during solar and storage cooking periods with two identical solar cooking storage pots. Sunflower oil is used as the sensible heat storage material in one storage pot, while erythritol is used as a phase change material in the other pot. The two pots are enclosed in insulated wonderbag slow cookers to assess their thermal performance during non-sunny periods. The sunflower oil storage cooking pot shows faster cooking times (1.8-5.0 h) and higher maximum storage temperatures (124-145 oC) compared to the erythritol storage cooking pot (3.8-6.6 h; 118-140 oC) during solar cooking periods. The heat utilisation efficiencies of the erythritol pot (4.8-14.3 %) are greater than those of the sunflower pot (3.7-6.0 %) during storage off-sunshine cooking periods. A comparative experimental study on the effect of different cooking loads for two solar cooking pots incorporated with different thermal energy storage (TES) materials is presented. Five different loads are utilized in the cooking experiments (0.5, 1.0, 1.5, 2.0, 2.5 kg) using water and sunflower oil as the heating fluids. The effect of the load on the storage and heat utilisation efficiencies is investigated. When utilizing water as the test load, the erythritol pot shows slightly greater heat utilization efficiencies (13-49 %) compared the sunflower oil pot (17-46 %). When sunflower oil is used as the cooking fluid, the sunflower oil pot shows greater heat utilisation efficiencies (9.4-28 %) compared to the erythritol pot (9.2-19 %). A thermal model of a solar cooking pot is developed using computational fluid dynamics (CFD) for sunflower oil as the storage medium. The model shows good agreement with experimental results, and the variances between the model and experimental results are attributed to solar flux fluctuations in the experiments. Natural convection is the primary heat transfer mechanism in the TES system at first, creating a rather flat temperature profile. After 4 hours, the cooking pot attains a constant state temperature of 405.8 K, which is adequate to cook a variety of foods and vegetables. A combined solar cooker with a sunflower oil storage tank (dual-purpose) is preliminary experimentally investigated during charging and discharging processes using 1.0 L of water. During the charging cycle, water is heated up to around 75 oC in a cooking pot with storage tank temperatures attaining temperatures above 100 oC. During discharging, the heat transfer was poor with the heated water only achieving temperatures just above 40 oC. Experimental charging energy and exergy thermal performance parameters of the dual-purpose solar cooking and TES system are also evaluated. Four different charging flowrates (2, 3, 4 and 5 ml/s) are used in the experimental charging tests. The average charging energy rate and the charging efficiency generally increases with an increase in the flowrate except for the highest flowrate (5 ml/s), due to lower average direct solar radiation conditions during the experimental test with the highest flowrate. Detailed energy and exergy thermal performance evaluations of the dual-purpose solar cooking and TES system during charging and discharging cycles are presented. The effects of the flowrate (2, 3, 4 and 5 ml/s) and load (0.5, 1.0, 1.5 and 2.0 kg) are investigated using sunflower oil and water as the heating loads. Charging results show that increasing the charging flowrate enhanced the average energy and exergy rates, as well as their related efficiencies. Water outperformed sunflower oil in terms of average charging energy and exergy efficiency. The average charging energy and exergy rates for water, as well as their related efficiencies, declined slightly as the mass load increased. The average charging energy, exergy rates, and efficiencies for sunflower oil decreased significantly as the mass increased, and sunflower oil showed lower values when compared to water. For the discharging results, the correlations between the energy and exergy thermal performance parameters with respect to flowrate and heating load are not well defined for both heating loads, possibly due to different initial storage tank temperatures at the start of discharging, and an inefficient discharging process that needs to be optimized in the near future.