Adaptation and Evaluation of a PV Model for Urban Climate Modeling Systems

By 2040, solar photovoltaics (PV) are projected to make up the largest share of renewable energy
production worldwide (International Energy Agency, 2017). This transformation could lead to a
considerable increase of rooftop solar PV. The potential direct effects of PV systems on urban climate
have been seldomly studied so far and generally with simplified approaches such as an effective
albedo, which takes the conversion efficiency of PV modules into account (Taha, 2013). What these
studies do not account for is the fact that PV modules installed on roof tops might generate an
additional sensible heat flux compared to a roof that has the effective albedo of a PV system. Also,
there seems to be a discrepancy between numerical modeling studies who found that PV can reduce
urban heat islands (UHI)(Masson et al., 2014; Salamanca et al., 2016) and experimental studies who
found a PV heat island (Barron-Gafford et al., 2016; Broadbent et al., 2019). To elucidate this
problem, a more detailed energy balance model is needed which takes into account the two sides of
PV modules that are exposed to the surrounding air layer. This approach should also reflect that
these PV systems are typically mounted on metal frames with a small air gap between the roof and
the module. Therefore, the wind speed below the PV panel must be considerably lower than above
the PV panel.
In this study a PV energy balance model (Heusinger et al., 2019) is adapted and evaluated with
measured surface temperatures of a rooftop PV system installed in Braunschweig, Germany together
with local meteorological data. The results indicate that the thermal behavior of the investigated PV
panel is dependent on wind direction. With north-western winds, the panel is at the windward edge
of the PV array and therefore more exposed to the wind than when wind direction is south-east and
the panel is at the downwind edge of the PV array. This can result in module temperature differences
of up to 15 K on clear sky days. However, the PV model can be adapted to both situations by
adapting the forced and natural convective heat transfer calculation of both sides of the panel. After
thorough evaluation the PV model is planned to be integrated in the BEP module of the Weather
Research and Forecasting (WRF) model to examine the potential impact of rooftop PV systems on the
urban climate and urban heat islands in detail.
References
Barron-Gafford, G.A., Minor, R.L., Allen, N.A., Cronin, A.D., Brooks, A.E., Pavao-Zuckerman, M.A.,
2016. The Photovoltaic Heat Island Effect: Larger solar power plants increase local
temperatures. Sci. Rep. 6, 35070.
Broadbent, A.M., Krayenhoff, E.S., Georgescu, M., Sailor, D.J., 2019. The observed effects of utilityscale photovoltaics on near-surface air temperature and energy balance. J. Appl. Meteorol.
Climatol. 58, 989–1006.
Heusinger, J., Broadbent, A.M., Sailor, D.J., Georgescu, M., 2019. Introduction, evaluation and
application of an energy balance model for photovoltaic modules. Sol. Energy. Under Review.
International Energy Agency, 2017. World energy outlook 2017 – Executive Summary. International
Energy Agency, Paris. 8 p.
Masson, V., Bonhomme, M., Salagnac, J.-L., Briottet, X., Lemonsu, A., 2014. Solar panels reduce both
global warming and urban heat island. Front. Environ. Sci. 2, 14.
Salamanca, F., Georgescu, M., Mahalov, A., Moustaoui, M., Martilli, A., 2016. Citywide Impacts of
Cool Roof and Rooftop Solar Photovoltaic Deployment on Near-Surface Air Temperature and
Cooling Energy Demand. Boundary-Layer Meteorol. 161, 203–221.
https://doi.org/10.1007/s10546-016-0160-y
Taha, H., 2013. The potential for air-temperature impact from large