ANNUAL ENERGY SAVING AND PAYBACK PERIOD

The certain amount of rain water can cool more PV modules if heat is removed by evaporation, under the premise that rainwater uniformly cov­ers the PV modules. Based on available rainwater in each month, it is estimated that this solar-driven rainwater cooling system can increase 33.4 kWh of electrical yields for a domestic house when six PV modules are applied. To comprehensively analyse the benefits of a new system, the economic analysis of the solar-driven water cooling system is conducted by the extra cost of equipment required to construct this cooling system, against additional energy benefits obtained from the modified PV panels. The total cost of this passive cooling system is estimated as £197., i. e. £80

image088

FIGURE 12: Volume of daily pushed rainwater by a gas chamber in different months.

to the cost of the rainwater harvest system and £117 to the cost of the gas expansion chamber, a secondary water tank, pipes and valves. The saving in electrical yields per year equals to £20 when feed-in tariff equals 0.45£/ kWh and electricity rate equals 0.145£/kWh [22].

TABLE 4: Payback period analysis.

Year

Power generated at 1% discount

rate (kWh)

Electricity supply rate (£) at 2.8% inflation rate

Feed-in-tariff (£) at 2.5% inflation rate

Annual

operating

saving (£)

Total saving (£)

Initial capital growth (£) at 3% annual interest rate

1

33.40

0.15

0.45

19.87

197.00

2

32.73

0.15

0.46

19.98

202.91

3

32.40

0.15

0.47

20.28

209.00

4

32.06

0.16

0.48

20.59

215.27

5

31.73

0.16

0.50

20.90

221.73

6

31.40

0.17

0.51

21.21

228.38

7

31.06

0.17

0.52

21.53

235.23

8

30.73

0.18

0.53

21.84

242.29

9

30.39

0.18

0.55

22.16

249.55

10

30.06

0.19

0.56

22.48

257.04

11

29.73

0.19

0.58

22.80

264.75

12

29.39

0.20

0.59

23.13

272.69

13

29.06

0.20

0.61

23.45

280.23

280.87

14

28.72

0.21

0.62

23.78

304.01

289.30

15

28.39

0.21

0.64

24.11

297.98

16

28.06

0.22

0.65

24.44

306.92

17

27.72

0.23

0.67

24.77

316.13

18

27.39

0.23

0.68

25.10

325.61

19

27.05

0.24

0.70

25.44

335.38

20

26.72

0.25

0.72

25.77

345.44

21

26.39

0.25

0.74

26.10

355.80

22

26.05

0.26

0.76

26.44

366.48

23

25.72

0.27

0.77

26.77

377.47

24

25.38

0.27

0.79

27.10

388.80

25

25.05

0.28

0.81

27.44

587.50

400.46

A simple payback formula was used to calculate the payback period as follows with an inflation rate of 2.8% being taken into account:

Payback period = (initial cost) / (annual operating saving)

The annual saving in the equation is calculated from:

Annual operating saving = kWh x (electricity rate + feed in tariff)

Assume all the costs of this solar-driven rainwater system are paid up front; the power output of PV discount rate at 1% a year; the electricity inflation rate at 2.8%; the feed-in-tariff inflation rate at 2.5% and annual saving rate at 3%. Based on the assumptions mentioned earlier, the calcu­lation results are shown in Table 4. It can be seen from Table 4 that under this conservative assumption, the payback period is 14 years. Considering that the cost (including water tanks, gas chambers and other equipment) could be reduced with mass production and the additional rainwater col­lection can be recycled for domestic use in non-operating period, the eco­nomic analysis results make this cooling approach quite attractive.

6.2 CONCLUSION

This paper reports a passive cooling system, which can be used for cool­ing the PV modules on the roof of a domestic house in order to increase electrical efficiency. The simulation results for this cooling system show:

• The influences of the absorbing surface area on the water supply volume are not obvious, whereas a gas chamber with larger volume significantly in­creases the water supply. However, the actual chamber size should be com­prehensively considered with roof area and available rainwater capacity.

• On the design day, the solar-driven rainwater cooling system is able to pump 152 l of water to PV modules. The maximum reduction in the temperature of the cells reaches 19oC and average electrical yield is increased by 8.3%.

• For the solar-driven rainwater cooling system operating between April and September, this cooling system can increase the electricity generation by 33.4 kWh annually.

• The simple payback period of the solar-driven rainwater cooling system was found to be equal to 14 years under a conservative assumption. It still has potential and the initial cost will be reduced if it incorporates with the guttering system.

The most significant point of this approach is that it utilizes rainwater and solar energy to cool the PV panels—improving PV system efficiency with no requirement for additional energy input. The authors believe that it has the potential for further exploration.