MONTHLY WATER CONSUMPTION BY THE SYSTEM

The supply of the rainwater depends on the gas expansion in a chamber, which varies with solar radiation and ambient air temperature. As shown in Figure 12, in January and December, little water can be pumped by this device; however, in June, intensive solar radiations and high air tempera­tures make the device to pump 110.8 l of water to PV panel for cooling in each day. According to solar radiation and rainwater supply, the system was designed to work between April and September.

For a well-constructed roof, the runoff coefficient is usually assumed as 0.8 [21]. Therefore, monthly rainwater collection can be estimated from the following equation:

Rainwater volume = monthly rainfall x catchment area x runoff coefficient

TABLE 3: Comparison between collected rainwater and required rainwater.

Month

Daylight hours

Equivalent sunny day

Operating day ratio

Rainwater

collected (l/day)

Required rainwater (l/day)

April

146.32

14.60

0.95

123

89

May

200.9

18.26

1.56

110

104

June

205.1

18.65

1.64

239

111

July

174.96

14.58

0.95

177

107

August

164.24

14.93

1

158

103

September

141.42

14.14

0.89

104

81

It is not efficient and cost-effective to design this solar-driven rainwa­ter cooling device to work every day, especially for the rainy and cloudy days. Thus, equivalent sunny days in each month can be predicted based on an assumption of 10-12 sun hours in a sunny day in different months. In an ideal scenario, sunny days and rainy days occur intermittently and an operating day ratio (number of sunny days/number of rainy days) is calculated to evaluate the relationship between collected rainwater and re­quired rainwater. Table 3 shows that, except in May, the amount of collected rainwater can meet the requirement of the cooling system in each month. A 1000-1 water tank allows it to meet the water consumption up to 10 days under the worst-case scenario like continuous sunny days.