PERFORMANCE OF THE PV MODULE COOLED BY THE SYSTEM

The performance is evaluated from the rainwater pumped by the device with a 0.16-m3 secondary water tank connecting to the a 1-m3 gas cham­ber. The secondary water tank is designed to protect gas infiltration and increase air tightness so that the whole system can work with the high­er efficiency. The variation of the gas temperature in the chamber dur­ing a day was illustrated in Figure 7. The initial gas temperature is 293 K, which is quickly heated to 342 K at 7 am, because low heat capacity of gas, small mass of gas in the chamber and intensive solar radiation

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8 9 10

Time (h)

 

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FIGURE 8: Accumulated rainwater volume in each hour.

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on the design day cause that gas can be heated to a high temperature. With the increase of solar radiation, the gas temperature gradually increas­es to a maximum value 387 K at 1 pm. After that, gas temperature reduces due to the reduction of solar radiation and the rise of heat loss from a gas chamber to the outside environment.

TABLE 1: Climate data of the day of calculation.

Time

Temperature

(0C)

Wind speed (km/h)

Humidity

(%)

Beam solar radiation (w/m2)

Diffuse solar radiation (w/m2)

7

16.3

6

80

517

122

8

18.2

4.6

72

528

207

9

20.3

4.3

67

634

193

10

22.3

4.3

62

744

166

11

24

4.3

59

712

232

12

25.2

6.5

56

682

249

13

26.2

6.5

54

798

149

14

27

5.4

52

768

153

15

27

4.3

51

558

216

16

26.7

7

53

646

128

17

25.6

10.3

57

381

134

18

24.2

6.5

61

463

59

TABLE 2. Hourly rainwater supply with temperature sensitive control for cooling.

Time Water volume (l)

10

28.37

11

30.13

12

30.62

13

31.57

14

31.31

Under this condition, the device is able to push 152 l of water to the PV panel (Figure 8). As discussed earlier, without control, the majority amount of the water is pumped at the early time of the day when the demanding for cooling is low. A control to the flow may be needed, for example, by a temperature sensitive valve to delay the water pumping to address this issue. The operating temperature of PV is primarily determined by the solar radiation. On the day of 29th July, between 10 am and 14 pm, solar radiation was.850 W/m2 and its temperature reached 50oC. The detail of the climate data for the day is shown in Table 1. In order to maximize the cooling benefit to the PV panel, a temperature sensitive valve can be used to adjust the flow rate of water according to the roof temperature. Table 2 shows that a total of 152 l of water can be pushed at different hourly rates with respect to the roof temperature from 10 am to 2 pm on the day. It can be seen that with the temperature sensitive valve, more water is pumped when the roof temperature is higher at late hours, which allows more cool­ing to the PV penal when it receives high solar radiation.

During the working time, the cooling to the PV panel is very effective when the PV panel temperature is high as shown in Figure 9. It can be seen that at 1 pm, a maximum temperature reduction of 19oC is achieved and at other time temperature reduction ranges from 12.5 to 18.5oC. Figures 10 and 11 present the efficiency and the power output of the PV panel with and without cooling, respectively. The cooling maintains the efficiency of the cells above 14.5% each hour in a design day, particularly, between 12 pm and 2 pm during which the PV panel has very low efficiency without cooling. The cooling also increases the power output by 16W on average. In summary, solar-driven cooling system is able to reduce the operating temperature of the cells by 16.5oC on average, and it has a better cooling effect when the temperature of the cells becomes higher. In addition, daily electrical yields of the PV module will grow 80Wh, achieving an incre­ment of 8.3%. However, variable environmental conditioning has impacts on gas chamber expansion, so does on water pumping and the cooling effect. Therefore, it is meaningful to evaluate annual performance of the solar-driven water cooling system under a stable environmental condition.

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