Thomas Thiis

A new photovoltaic system combining electrical power production with snow mitigation intends to reduce the snow load on flat roofs. Applying electrical power to PV modules causes heat production on the module surface, allowing the ablation of snow. This study combines measurements and theoretical analysis to investigate which conditions are favourable for snow load reduction and discusses the system’s feasibility to perform a controlled snow load reduction in a heavy snow load scenario for buildings with flat roofs. Both melting and sublimating of snow are investigated as means to reduce the load. The results show that the potential for load reduction is highly dependent upon weather conditions and snowpack characteristics during system operation. The refreezing of meltwater and water saturation of snow are identified as phenomena potentially preventing sufficient load reduction in cold conditions. Due to such temperature sensitivity, the system is likely to be more suitable for warm climates occasionally experiencing heavy snow loads than for climates with long and cold winters.

Keywords: snow, PV systems, load reduction, roofs, reliability, climate robustness

Studium wykonalności fotowoltaicznych systemów ograniczania śniegu dla dachów płaskich

Streszczenie

Nowy system fotowoltaiczny łączący produkcję energii elektrycznej z ograniczaniem śniegu ma na celu zmniejszenie obciążenia śniegiem na dachy płaskie. Zastosowanie energii elektrycznej w modułach fotowoltaicznych powoduje wytwarzanie ciepła na powierzchni modułu, umożliwiając ablację śniegu. Niniejsze badanie łączy pomiary i analizę teoretyczną w celu zbadania, które warunki sprzyjają zmniejszeniu obciążenia śniegiem i omawia możliwości systemu w zakresie kontrolowanej redukcji obciążenia śniegiem w scenariuszu dużego obciążenia śniegiem dla budynków z płaskimi dachami. Zarówno topienie, jak i sublimacja śniegu są badane jako sposób na zmniejszenie obciążenia. Wyniki pokazują, że potencjał zmniejszenia obciążenia zależy w dużym stopniu od warunków pogodowych i charakterystyki śniegu podczas pracy systemu. Ponowne zamoczenie wody morskiej i nasycenie wody śniegiem są identyfikowane jako zjawiska potencjalnie uniemożliwiające wystarczające zmniejszenie obciążenia w niskich temperaturach. Ze względu na taką wrażliwość na temperaturę system może być bardziej odpowiedni do ciepłych klimatów, czasami doświadczając większych obciążeń śniegiem niż w klimatach o długich i zimnych zimach. 

Słowa kluczowe: śnieg, systemy PV, redukcja obciążenia, dachy, niezawodność, odporność na klimat

Transport and deviation of snow by wind induce many constraints on buildings, vehicles and industrial systems. A selection of questions from snow-wind engineering are presented in the paper. The experimental method that was undertaken to investigate these questions makes use of a large climatic wind tunnel, partly designed to address snow engineering problems at full scale: snow penetration in buildings, into ventilation systems of buildings and vehicles and snow or ice accretions on structures.

In the search for new renewable energy sources, photovoltaic systems and solar thermal collectors have become more common in buildings. With increased efficiency and demand for energy, solar power has also become exploitable at higher latitudes where snow is a major load on buildings. For flat roofs, one usually expects approximately 80% of the snow to be eroded off the roof surface. Installing solar panels would change this since the flow pattern and wind conditions on the roof are affected by their presence. This study shows the erosion of sand particles from underneath solar panels of various configurations associated with different wind velocities. The pattern of erosion is used to determine the relative friction velocity, u*REL, of the wind on the roof. This value is the friction velocity on the roof relative to the friction velocity on a flat roof without solar panels. The experiments, conducted in a wind tunnel, showthat the area where u*REL is 0 and where it is expected that sand and snow will accumulate in case of an upwind particle source and decrease with increasing distances between roof and solar panel. It is also shown that a larger gap between the solar panel and roof surface creates larger erosion zones, where u*REL > 1 for both wind directions. Since the erosion is closely linked to the air flow under the solar panels, and that higher air velocity increases the erosion, it is likely that a larger solar panel, extending higher into the free air flow would be desirable to avoid snow accumulation on a flat roof with solar panels. If the solar panel has large enough dimensions, the solar panels can be used as a deflector to decrease snow accumulation on flat roofs. With solar panels of the size in the current experiments, a building with a length smaller than the equivalent of x/L = 0.3 would have u*REL > 1 on most of the roof surface and would thus likely have a lower snow load than an equivalent float roof without solar panels.