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The setup in this study was tested in various ways to ascertain its cooking potential, as well as its heat-generation abilities. The behaviour of the heat-transfer fluid, the charging and discharging process as well as the general analysis of the system when it was not performing any cooking, were determined using a no-load heat-transfer experiment in which the HTF was just heated, with no cooking load added to the system.

6.1.1 Goals

The no-load solar cooking experiment was used to understand the manner in which and the time it takes the heat-transfer fluid in the system to be fully charged and discharged. This was to be used to predict the time it takes the system to cook various food items. This test prevented wastage of food items during the initial testing, as the temperatures that the HTF would rise to could be used to determine whether the overall system should be used for solar cooking or not. The no-load test also served as a means to determine the behaviour of the solar cooker during cooking, and the heating ability of the system under various climatic conditions.

6.1.2 Experimental procedure

The start-up procedure given in Appendix G was performed; the storage tank was filled with 50 litres of the heat-transfer fluid, leaving an expansion and pressure gap. The tank was not also filled because of the anticipated low DNI for the day. Two experiments were performed – one on 22 May 2015 and the other on 10 June 2015. The average DNI during the experiment on 22 May was 513.9 W/m2 _{during peak hours, the data recording was done} between 08:00 and 22:00, the whole body of the storage tank was well insulated, but the top cover of the tank was left uninsulated (open) during the day to check the effect of direct irradiation on the tank top. This would determine if the secondary DNI falling on the pot would add heat to the system or otherwise in order to recommend whether to wrap up the

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cooking during day cooking or to expose it. In the second experiment, carried out on 10 June 2015, the whole tank was insulated, with no part exposed. The two experiments were performed outdoors and the results are discussed below.

6.1.3 Evaluation and summary of results

Figure 6-1: Storage oil temperature as a function of day time: no cover insulation Figure 6-1 shows the available DNI during the first experiment, the average oil temperature, the ambient air temperature and the temperature of the exposed cover of the tank as a function of the time of the day from 08:00 to 23:00. The temperature of the storage tank cover, which was not insulated, rose up to 55 ℃. This temperature was really high compared to the ambient air temperature outside the tank, creating a high temperature difference, thus the average temperature of the heat-transfer oil inside the tank could not reach 200 ℃. The oil temperature attained its maximum temperature at around 13:30, which was four and a half hours after charging began. The system, however, retained a temperature of above 150 ℃ for four hours and above 100 °C for more than nine hours. It was also noticed that the temperature of the tank top cover did not drop, even when the sun set. This means that more heat was lost through the top cover when it was exposed. It was then inferred that the effect of secondary irradiation (the DNI straight onto the tank cover) falling on the tank could be neglected in operation, as it was stationary and did not track the sun. The only time when the sun fell directly on the top was between 11:00 and 13:00, and this only caused its temperature to rise to a maximum of 57 ℃; at sunset, the temperature difference between the ambient air and the uninsulated cover stood at 35 ℃ average and it thus acted as a huge convective heat-loss medium. It was then suggested that another experiment be performed to compare the results when the cover was insulated. These results are shown in Figure 6-2.

0 100 200 300 400 500 600 700 800 900 0 20 40 60 80 100 120 140 160 180 8 10 12 14 16 18 20 22 24 T em per at ur e (℃ ) Day time (h) T_oil-avg T_cov T_ambAvg DNICalc_Avg DN I Is (W /m 2 )

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Figure 6-2: Storage oil temperature as a function of day time – insulated cover (8 June 2015)

In Figure 6.2, the same experimental setup as for Figure 6-1 was repeated, but the cover of the storage tank was completely insulated. The experiment was performed on 8 June 2015 and the average DNI during the experiment was 645.884 W/m2_{. The heat-transfer oil during} this experiment attained a maximum recorded temperature of 225 ℃ at around 13:30. The peak average temperature of around 210 ℃ was maintained for four hours. The system reached 150 ℃ a few minutes before 12:00. The average temperature of the ambient air was around 35 ℃ throughout the day. The insulated cover reached 50 ℃ and the temperature was noticed to drop as the sun set. The average temperature difference between the insulated cover temperature and the ambient temperature was 14 ℃ at sunset, compared to the 35 ℃ noticed in first experiment in Figure 6-1.

From these two experiments, it can be concluded that the average charging time for the heat-transfer oil is five hours on days with average insolation of 600 W/m2_{. After the} working conditions of the system had been established, there was a need to test its cooking ability, heat dissipation rate at load conditions and the heat loss coefficient, which are discussed in the next sections.