Step 8: Two days of real testing (May/5 and May/6)
Photo 1: First day of real testing (May/5). Half of the panel is powering some 12 V halogen lamps. Connecting 1 lamp this half panel was providing 9.3 V at 1.47 A (power = 14 W), connecting 2 lamps the voltage dropped to 7.9 V at 2.69 A (power = 21 W). Extrapolating to the whole panel this would be at least 42 W of electrical power (not too bad for doing this experiment at 4:30 pm, considering that the theoretical maximum for this panel is 60 W, assuming 1 kW/m^2 solar power and 12% efficiency of the PV system). The other half of the PV panel powers the water pum, which makes water circulate between the panel and the cooler. There was 3 kg of water in the system.
Photo 2 and Photo 3: Results from May/5. Given water's heat capacity [4.2 kJ/(C kg)], one can conclude that the thermal power transferred was about 200 W maximum (again not bad considering that the panel received at most 500 W of solar light!). Note that I am neglecting possible other possible heat transfers (cooler, etc). See the second day of experiments below. The max. temperature reached was 52 C (126 F). Quite hot to the touch!
Photo 4 and Photo 5: Results from May/6. Second day of experimentation. Again I used 3 kg of water in the cooler. Note that at the 94 minute I turned off the water pump and the water started to cool off (at an energy loss rate of ~ 56 W). See the next picture. I estimated that in the "more stable" region of temperature rise the thermal power was about 210 W. This day the solar flux was similar, notice that the electrical power for 2 lamps was almost the same as the previous day. Also notice that connecting 3 lamps in parallel reduces the net electrical power: this is an important factor when designing a complete PV system, you need to optimize the IV operational point! In the plot, the very high rate of temperature rise in the first few minutes is probably an artifact of having used cold water rather (BTW: the air temperature was about 24 C). At the 94 minute I turned off the water pump and the water started to cool off (at an energy loss rate of ~ 56 W). Notice that as happened before the maximum temperature was about 52 C (126 F).