Sandia Model

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Summary

The Sandia model is an empirical temperature model developed at Sandia National Laboratories (King et al., 2004) based on outdoor measurements. Unlike the Heat Balance model, it calculates back-of-module surface temperature first using an empirical exponential relationship for thermal losses with conductive and convective contributions, then derives cell temperature from it.

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Inputs

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Outputs

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Detailed Description

The Sandia model calculates back-of-module surface temperature using an empirical exponential relationship for thermal losses with conductive and convective contributions: $$T_m = G_{POA,front,eff} \cdot e^{a + b \cdot v_w} + T_a$$ where:

• $T_a$ is the ambient air temperature in °C (from weather data)
• $G_{POA,front,eff}$ is the front-side after optical corrections, in W/m² (from irradiance calculation)
• $a$ is the empirical conductive coefficient (from DC field definition)
• $b$ is the empirical convective coefficient in s/m (from DC field definition)
• $v_w$ is the wind speed in m/s (from weather data)

The exponential term $e^{a + b \cdot v_w}$ represents the thermal resistance between the module and the environment. Both $a$ and $b$ are negative, so higher wind speeds reduce the exponential and thus lower module temperature. These coefficients depend on mounting configuration and module construction and are empirically determined.

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Reference Coefficients

The following reference coefficients were empirically derived by Sandia National Laboratories for open-rack mounting (King et al., 2004). These are guidelines only—users can set $a$, $b$, and $\Delta T_{c-m}$ freely in the DC field definition.

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Cell Temperature

Cell temperature is derived from surface temperature by adding an irradiance-proportional offset: $$T_c = T_m + \frac{G_{POA,front,eff}}{G_{ref}} \cdot \Delta T_{c-m}$$ where:

• $G_{ref}$ is the reference irradiance in W/m² (typically 1000 W/m²)
• $\Delta T_{c-m}$ is the cell-to-module temperature difference at the reference irradiance, in °C (from DC field definition)

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References

• King, D. L., Boyson, W. E., & Kratochvil, J. A. (2004). Photovoltaic array performance model. SAND2004-3535, Sandia National Laboratories. DOI: 10.2172/919131