With reference to Figure 4, the expression of hydraulic head in the secondary water tank varying with the gas expansion can be derived as follows.
At initial state, the secondary water tank is filled with water and the gas in the chamber does not expand. The state of the gas can be expressed as follows:
PJV = m RT0
atm gas gas 0
where T0 denotes initial temperature of the chamber (oC); Patm indicates atmosphere pressure (Pa).
On receiving heat from the solar radiation, the gas in the chamber starts to expand. If the gas volume is expanded by 5 V (volume expansion) (m3), the same volume of water will be pushed out of the tank. The change of the state follows the following equation:
Ф + pg(0.9 + H)](V + 5V) = m RT
atm gas V gas gas
where mgas represents mass of gas (kg); Vgas denotes volume of gas (m3) and 5V = H x AW; where AW = 0.25: From Equations (5) and (6), a relationship between the volume of the water pushed and gas temperature can be obtained:
pgAwH2 + (Patm^w + PgVgas + 0.9 pgAw) H + Patm Уд
p V T
, Л Г» . _.ТЛ atm 3asgas л
+$.9pgVgas————- 7——– = 0
FIGURE 5: Schematic diagram of cross section of the PV panel.