Rooftop PV performance during heatwaves


From pv magazine Global

Scientists from research institutions across China have developed a model to evaluate the impact of rooftop PV on the energy use of residential buildings and their surrounding air temperatures during heatwaves. They revealed their findings in “Thermal and energy benefits of rooftop photovoltaic panels in a semi-arid city during an extreme heatwave event,” which was recently published in Energy and Buildings.

The study assesses the impact of rooftop PV arrays of varying energy efficiencies on mid-rise residential buildings. In particular, it investigates the impact on air conditioning use and surrounding urban temperatures. The researchers conducted the study in northeastern China, in a semi-arid area that was hit by an extreme heat wave in 2018.

They handled the numerical evaluation with the Weather Research and Forecasting (WRF) model, coupled with building effect parameterisation (BEP) and building energy model (BEM).

“Forced by the atmospheric conditions on the lowest level of the WRF model, the system computes the surface momentum, heat, humidity and turbulent kinetic energy fluxes induced by the underlying urban surface and feeds back these fluxes to the atmospheric dynamic governing equations,” said the researchers.

The academics analysed 12 simulations with different percentages of rooftop coverage and panel conversion efficiency against a control case. They assumed that the PV arrays were installed in parallel and at a certain height away from the roof. It was also assumed that every building in the city used an air-conditioning system during the modelled heatwave conditions.

The control case had an air-conditioning system installed, but no rooftop PV. The 12 scenarios consist of rooftop PV coverage levels of 25%, 50%, 75%, and 100%, with conversion efficiencies of 10%, 20%, and 30%.

The results showed that an increase in covered area and conversion efficiency contributed to strong cooling effects. Rooftops with 100% PV coverage and energy efficiency of 30% could lower air temperatures by 0.4 C to 0.7 C. The combination reduced air-conditioning energy consumption by 14.74%.

“Such results indicate that the cities with a lower albedo and emissivity could benefit more from [rooftop PV] implementation in urban heat mitigation, along with an increasing urban effective albedo,” said the researchers.

Based on the 12 scenarios, the average daily cooling energy consumption reduction ranged between 1.29 MWh and 6.58 MWh per square kilometre. That represented an energy reduction of 2.89% to 14.74 %, relative to the control scenario. The 25% rooftop coverage and 10% efficiency scenario showed negligible thermal impacts, “suggesting a minimal threshold of [rooftop PV] coverage in terms of improving thermal environment,” the researchers concluded.

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