# PVsyst Cell Temperature Model

The PV performance modeling application, PVsyst, implements the following cell temperature model:

$$T_{c}= T_{a}+E_{POA}\frac{\alpha \left (1-eta_{m} \right)}{U_{0}+U_{1}\times WS}$$

where

• $$T_{c}$$ is cell temperature (°C)
• $$T_{a}$$ is ambient air temperature (°C)
• $$\alpha$$ is the absorption coefficient of the module (PVsyst default value is 0.9)
• $$E_{POA}$$ is the irradiance incident on the plane of the module or array ($$W/m^{2}$$)
• $$eta_{m}$$ is the efficiency of the PV module (PVsyst default is 0.1)
• $$U_{0}$$ is the constant heat transfer component ($$W/m^2{K}$$)
• $$U_{1}$$is the convective heat transfer component ($$W/m^{3}sK$$)
• $$WS$$ is wind speed (m/s)

PVsyst says little about what values to use for $$U_{0}$$ and $$U_{1}$$.  Note that the current default values assume no dependance on wind speed ($$U_{1}$$)

• For free-standing arrays the current default is : $$U_0$$ = 29 $$W/m^{2}K$$; $$U_1$$ = $$0$$ $$W/m^{3}sK$$
• For fully insulated arrays (close roof mount) the current default is: $$U_0$$ = 15 $$W/m^{2}K$$; $$U_1$$ = $$0$$ $$W/m^{3}sK$$

PVsyst users can also enter a NOCT (Nominal Operating Collector Temperature) in place of $$U$$ values.  The program then automatically calculates $$U$$ values based on $$\alpha=0.9$$ and $$eta_{m}$$.