July/August 2020

www.drivesncontrols.com July/August 2020 43 RENEWABLE ENERGY Folding solar roof goes where others cannot A Swiss start-up has developed what it claims is the world’s first foldable solar photovoltaic roof built over a sewage plant. The 5,800m 2 roof at a sewage treatment plant in the Swiss town of Chur can be retracted at night, or for protection in bad weather, allowing the solar array to be lighter and less expensive than a permanent structure. A series of distributed geared motors with built-in inverters and PLCs folds the roof autonomously if there is a threat of bad weather, or if there are problems such as communications or power failures. The solar roof, consisting of 2,120 photovoltaic modules, is expected to produce about 550MWh of electrical energy a year – enough to meet about 20% of the sewage plant’s auxiliary power needs. DHP Technology, the company behind the Horizon solar roof, believes that it could open up new applications for solar power where space is limited or access is needed to the area below the array. Sewage treatment plants cover large areas and consume large amounts of electrical energy. It is not practical to cover the entire area with conventional photovoltaic systems because the settling basins need to be accessed from above. “By retracting the foldable roof groups, we can ensure access to the settling basins so that maintenance can be performed without restriction,” explains DHP’s co-founder, Andreas Hügli. The 5.5m-high roof has widely spaced supports to ease access. Covering the basins with a roof reduces algae growth and the need for maintenance. The foldable roof operates autonomously and adapts to ambient conditions automatically based on the weather. When the sun rises, the panels unfold automatically and, when it sets, they return to their protective garage. The roof also folds to this position automatically in the event of bad weather. “Thanks to this protective position, the entire photovoltaic system need not be designed to withstand extreme weather,” Hügli explains. “Instead, we can make the modules lighter and then protect them against wind, snow, and hail in the garage.” The arrays – which cover the sewage plant’s pre-treatment, secondary (biological) treatment, and tertiary treatment basins – are retracted and extended using a cable system, with each of the 53 folding roof groups having its own drive. Four of the drives are linked via CANbus, with one acting as a master. The drives, supplied by Nord, consist of a worm geared motor with a brake, magnetic encoder and motor-mounted Nordac Flex SK 205E frequency inverter. The fact that the inverters are mounted on the motors simplifies installation. The drives are located in a protected position on the supporting structure and can be addressed separately, allowing maintenance personnel to retract the folding groups individually to perform maintenance on specific basins. PLCs in the inverters control the automatic extension and retraction operations. “The PLC reads local and external weather data and processes this data using a generic algorithm to issue the ‘extend’ or ‘retract’ commands,” explains DHP Technology’s system engineer, Philip Racine. Rotary encoders ensure that the position of the panels is known at any time. Identical ramp functions, consuming the same amount of energy, are used to extend and retract the roofs. The ramps start slowly, accelerate in the middle, and slow down again when the roof reaches its garage. The fact that the foldable roof can be erected over areas that are already in use, avoids the need to construct new buildings, or to restrict the use of the area below the array. As well as saving space, DHP believes that its technology will make large decentralised solar power plants possible where electricity is needed – for example, to power EV charging points in car parks – as well as providing shade. DHP and Nord are working to develop the technology further, for example, incorporating communications failure detection in the inverter PLCs, and adopting preassembled connectors to simplify installation. n A folding photovoltaic array built over a Swiss sewage plant is thought to the first of its type. It promises many potential benefits and can be built where normal arrays would be impossible. Its autonomous operation relies on distributed gearmotors with on-board inverters and PLCs. The foldable solar roof over the Swiss sewage plant is the first of its kind worldwide and allows for an intelligent dual use of industrial sites.