A PV system rack-mounted above a pitched roof will produce only 3% more energy yield per year than a system utilising in-roof mounted modules, according to a new study by Viridian Solar.
The company’s researchers conducted tests to determine how much temperature affects the performance of PV modules on rooftops. According to Viridian, the gap in performance between roof-integrated PV systems and rack-mounted rooftop systems “is almost certainly less than you thought it was”.
Viridian Solar researchers collaborated with the engineering department of the University of Cambridge and micro-inverter manufacturer Enphase Energy to devise an experiment using five 240Wp monocrystalline photovoltaic panels affixed to test rigging.
Researchers then installed the test rig in five commonly used roofing situations, determining the changes in temperature caused by the ventilation available in each situation and assessing how much difference this made to the power generated:
- Free standing – the PV was installed, free standing on an aluminium angle framework, providing full ventilation to the rear of the panel.
- Above tiled roof – the PV was affixed to a pair of beams, supported on four roof anchors and installed above grey interlocking concrete tiles. The system was then tested with ventilation space of between 170mm to 190mm between the panel and the tiles.
- Integrated to tiled roof, cold roof construction – A VAT flashing kit was used to integrate the PV onto a roof of concrete interlocking tiles. Below the tile battens, a roofing membrane was draped by 20mm between the rafters.
- Integrated to tiled roof, warm roof construction – as above, but with 100mm of polyurethane insulation board between rafters, leaving a ventilation gap of 25mm below the top face of the rafter.
- Shingle roof, no ventilation – the PV was fixed directly to an 18mm thick layer of plywood sheeting over the rafters, so that the rear of the PV panel was fully enclosed.
All panels were mounted at a 35 degree angle, facing south. The area behind the structure was boarded, enclosing the area behind the roof. Space behind the tiles was enclosed, but eaves were left open. Modules were then wired to Enphase M215 Micro-inverters.
The temperature of each PV module, with the ambient temperature subtracted, was plotted against solar irradiance levels, to produce what Viridian calls “a characteristic temperature-irradiation curve” for each of the above installation types.
Interestingly, Viridian found that there was little difference in the temperature response of the cold and warm roof installations, around 0.1%. Ventilated batten space appears to offer a similar level of cooling irrespective of the presence of insulation.
The overall result showed that a free standing PV module with open rear would generate around 6% more energy than an in-roof system, while a system above a pitched roof would yield only 3% more. The in-roof module fixed to plywood sheeting produced only 1.1% less energy than the cold roof installation.
According to Viridian, little research has been done in this area so far. The company considers that as the value of property can be linked to aesthetics, it will become increasingly important to give “credible advice” on the performance of integrated systems for residential users.
Stuart Elmes, the company’s chief executive said: “Clearly, for some situations an extra 0.3% return on investment due to energy yield will matter, but for many customers minimising the visual impact on their building will be more important. As an industry at least we now have facts to present to potential customers so that they can make an educated choice.”
The full briefing can be viewed here.