August 13th, 2020
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 . 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.
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.