When the building's use changed from a kindergarten to a high-tech training centre and engineer's office, local architect Hennig seized the opportunity to set new standards in environment-friendly roofing. The architect was looking at how it could minimise the building's consumption of electricity and water, and a new roof was seen as key to the scheme's success.
For the waterproof membrane, Hennig chose a new type of roofing material called Evalon-Solar, manufactured by German company Alwitra. This membrane combines a conventional roofing membrane with flexible photovoltaic cells to produce a roof capable of generating significant levels of environment-friendly solar power.
At the heart of the system is a series of 395 × 5490 mm photovoltaic modules made from flexible solar cells. The cells are manufactured by United Solar Systems, a joint venture between US company Energy Conversion Devices and Japanese office equipment giant Canon, using a process first developed to manufacture photocopier drums.
The cells consist of amorphous silicon deposited on a 5 mm thick stainless steel substrate encapsulated in weather-resistant transparent polymers. Three separate layers of silicon form each cell. Each layer uses a different light wavelength to generate electricity. The manufacturer claims that this gives the module a performance up to 30% better than that of standard crystalline modules, particularly when operating in cloudy conditions. The cells are then bonded to Alwitra's single-ply roofing membrane.
Installing the new system was no different to installing a conventional single-ply membrane. The material was delivered to site in rolls and fitted by approved installers using automatic fastening machines and seam welders. To allow the technology to be used, the manufacturer has positioned the strips of photovoltaic modules away from the roofing seams at the edge of the rolls.
The modules have been installed only on the central area of the roof at Jena-Lobeda; the perimeter and corner zones are covered in a standard roofing membrane. "Wind uplift forces are higher at the edge of the roof than in the centre," explains Alwitra managing director Michael Steinbach, "so a greater number of mechanical fasteners have to be used to hold down the membrane." With the high number of fasteners needed at the edge, the concern was that the photovoltaic cells would be punctured. Even so, the photovoltaics cover 40% of the roof area.
Electricity produced by the photovoltaics is carried by factory-fitted wires located beneath the membrane to protect them from the weather. The wires are gathered from up to six modules before being routed through the roof insulation and decking to an inverter unit mounted on the underside of the roof. The inverter converts the direct electric current generated by the cells to the alternating current used in the building. The distance between the cells and the inverter is kept to a minimum to reduce power losses in the wiring.
All electricity produced by the roof is sold to the local utility supplier. Under German law, power-supply companies must buy all the electricity a renewable producer sells "at a rate of 99 pfennigs [31p]/kWh of electricity produced", says Steinbach. But the real bonus for the electricity producer is that the cost of buying electricity from the supplier is much cheaper – "only about 24 pfennigs [7p]/kWh". Obviously, this will encourage the use of renewable energy – unlike in the UK, where producers get a much lower rate for selling electricity to a supplier.
The roof at Jena-Lobeda was completed in September 1999, and is now being monitored to see if its actual performance measures up to that predicted.
Although the roof at Jena-Lobeda is a flat, warm deck with the insulation installed below the waterproofing, the membrane has been designed to be suitable for both warm deck and cold ventilated roofs. "Ideal applications are pitched roofs, especially south-facing, northlight-type roofs," says Steinbach, although he is keen to point out that the flexible membrane can be used on most types of roof, including barrel-vault, duo-pitch and butterfly types.
"A south-facing 30° pitch is ideal for maximum power generation," says Steinbach, "but not essential," he adds quickly. As a single-ply roofing membrane, the system is equally appropriate for new-build or refurbishment work, where it can be laid directly over the existing failed roof.
Electricity is not the only green aspect of the roof: it is also used to collect rainwater for use in a greywater system. Rainwater is collected as run-off and stored in a large tank where it is used to flush toilets.
As for cost, Steinbach talks of the roofing membrane itself costing about £30/m sq, with the photovoltaics adding another £300/m sq. He admits that for many UK applications, this is prohibitively expensive in terms of payback. However, he takes the long-term view and appears confident that the ever-increasing need to generate power by environment-friendly means, to meet the government's targets for reducing carbon dioxide agreed at the Kyoto Summit, will stimulate the UK market.
"To get the economies of scale needed to bring down the cost of photovoltaics, some form of subsidy needs to be introduced," he says.
The system has just been launched in the UK and Steinbach eagerly awaits his first order.