Thermal loss factor Uc (constant)

What it is

The part of the module's heat-loss factor that does not depend on wind, in watts per square metre per kelvin. A larger value means the module runs cooler. The Prilliman and Faiman models use it; the NOCT model does not.

Typical values and how to get it

PVsyst's mounting classes: 29 W/(m²·K) for free-standing racks with air on both sides, 20 W/(m²·K) for modules mounted close to a roof, 15 W/(m²·K) for modules with an insulated back. If not given: 29 W/(m²·K).

How it is applied

$$T = T_{amb} + \frac{a\,G\,(1 - \eta)}{U_c + U_v\,v}$$

where

Symbol	Meaning	Unit
$T$	the cell temperature	°C
$T_{amb}$	the ambient temperature	°C
$a$	the absorptivity	—
$G$	the effective irradiance on the module plane	W/m²
$\eta$	the module efficiency: its operating value with Prilliman, its rated value with Faiman	—
$U_c$	this input	W/(m²·K)
$U_v$	the wind heat-loss factor	W/(m²·K) per m/s
$v$	the wind speed of the weather file	m/s

References

1. Faiman 2008, Assessing the outdoor operating temperature of photovoltaic modules, Progress in Photovoltaics 16(4) 307–315
2. pvlib-python 0.16.1 source code (github.com/pvlib/pvlib-python), reference implementation: pvlib/temperature.py, pvsyst_cell()

API

thermal.u_c

Symbols follow the symbol registry.