When the water flows over a tilted PV panel, the heat transfer between the water and panels can be complicated by involving water evaporation in addition to the normal radiation and convection heat transfer. With refer­ence to Figure 5, the energy balance leads to:

G = Qe + Qc + Qr + AU where G denotes energy generated from solar radiation (J), U indicates internal energy (J), and Qe, Qc, and Qr denote heat loss by evaporation (J), heat loss by convection (J) and heat loss by radiation (J), respectively.

The heat transfer mechanism is quite complex due to temperature variation along the water-solid interface. A two-dimensional steady-state model is used, and some assumptions should be made to simplify the calculation.

• Solar radiation irradiates on the PV panel, 15.4% is converted to electricity energy, 5% is reflected and the rest part is converted to heat energy.

• Assume the water mass is uniformly distributed over the PV panel and water is ultimately heated to a temperature that is same as the cell temperature Tc.

• Water temperature increase caused by solar radiation is neglected. [18]

• Convective heat loss at the back of Pv panel and radiation heat transfer is not considered.

Air flowing at the air-water interface essentially accelerates water evapora­tion rate. Thus, the air convection could be measured accompanying with water evaporation. Smith et al. [19] predicted evaporation heat transfer flux by the following equation, which approximately estimates how much latent heat is removed from the PV panel by water evaporation (w/m2).

Подпись: (8)qe = (0.0638 + 0.0669V) (Pw – Pa)

where saturation pressure of water is as follows:

l0g(їмо) = 30-59051 – 8-2 ios(Ta + 273.16)

3142 31

+0.00248(Ta 4 27^ –

partial pressure of water vapour at surrounding air is as follows:

P = mP

a T s

partial pressure of water vapour at water is as follows:


5132 ‘

°-5( Tw, in + Tc).




Pw = exp





FIGURE 6: Accumulated rainwater volume pushed by gas chambers with different sizes.


where T denotes water inlet temperature (8C).

w, in

Therefore, total heat transfer via water evaporation can be expressed as follows:


Qe AcelA




where Acell denotes area of cells (m2).

The internal energy change of water can be expressed as follows:

_ ^waterCpiTw’Out TWlin ^water fp ^w, m

_ 3600 _ 3600


where Twout denotes water outlet temperature (oC).

Thus, after a water film flowing down to a PV panel, the cell tempera­ture can be approximately calculated as follows:





where Uw denotes wind speed (m/s).