Winter power from house facades

The potential for renewable energy is not just on Swiss rooftops. House facades can be transformed into solar power plants. According to government estimates, some 17 TWh of electricity could be generated with this technology. Particularly in winter, solar plants on house facades can deliver more power because of the sun's low position. In other words, precisely when Switzerland's consumption is at its highest.

How will Switzerland replace the virtually CO2-free electricity produced by nuclear power plants once they go off grid?

Photovoltaics is the buzzword. Solar energy is renewable, and becoming more and more cost-efficient. Switzerland has a great deal of potential for producing solar energy, namely on about 9.6 million buildings. Over half of their rooftops are suitable for photovoltaic systems according to a new study by the ETH Lausanne. They could produce about 24 terrawatt-hours (TWh) of solar power, which is ten times more than today. This volume corresponds to about 40 per cent of Switzerland's current annual power consumption. A study by the Swiss Federal Office of Energy (SFOE) is even more optimistic and estimates that up to 50 TWh could be generated on Swiss rooftops with solar cells.

Lower yield

Switzerland has 9.6 million buildings. Their facades could also be used for solar energy generation. According to the SFOE study, a production of up to 17 TWh could be possible.

The most clearly visible difference between rooftop solar plants and facade units is the angle of inclination because the PV modules are mounted perpendicular to the building. A solar facade yields about two thirds of what a common, diagonally-mounted PV system with a suitable roof orientation can generate. It is also important that the plant is not located in shade coming, for example, from another building – and that the power-producing modules are well ventilated so that they do not overheat. High temperatures hinder solar panel performance.

Winter advantage

Solar facades have various advantages in comparison to rooftop systems. For example, the building wall does not necessarily have to face directly south in order for a solar facade to produce an optimal yield. On the contrary: The sun's position in the morning and evening is lower. With deviations of 30 degrees to the east or west an even higher yield can be expected in contrast to a southern exposure.

In addition, power production throughout the year is more balanced, snow cannot accumulate and rain drips off the panels. Because the sun's position is lower, solar facades get more sun in the winter half-year versus rooftop systems, and, as a result, yield is higher in the winter – precisely when Switzerland is consuming the most, and when power has to be imported from foreign nuclear or coal-fired power plants.

PV systems on facades could be easily used at higher altitudes says Jürg Rohrer, Professor and Head of Renewable Energy Research at Zurich University of Applied Sciences (ZHAW): "Thanks to the sun's flat angle and the Albedo effect (sunlight is reflected by the snow, producing higher yields; editor's assumption) solar modules on house facades make sense in the mountains."

Costs and profits

The negative aspect is the higher investment because more solar modules are needed for the same yield. In addition, mounting is more expensive in comparison to rooftop systems. How this could look in practice can be seen in the following calculation example on energieheld.ch: A solar facade with an annual yield of about 9000 kWh requires a rooftop PV system with a peak output of 10 kWp generated on a solar module surface of 60 m².

Because the square-metre yield of solar facades is lower (70 % – owing to the angle of incidence), nearly 86 m² of facade surface is required for the same yield, which results in a peak output of 14 kWp.

The estimated total investment for a plant like this (minus subsidies) is nearly CHF 30,490. Additional costs are 50 % higher than for rooftop systems (CHF 20,200).

Economical plants

However, common house facades also have a price. Solar facade profitability is on the rise. "Investments for a suspended solar facade are certainly higher than those for a suspended glass facade. But, the solar facade supplies power and, as a result, it provides a return of investment. Additional costs can be recouped. In this respect, these facades are already economical today,” comments Hartmut Nussbaumer, Head of the Research Group for PV modules at ZHAW, in an interview with "solarspar.ch".

Everything is possible

Aesthetics plays an important role when it comes to house facades. In the meantime, particularly in the area of glass facades, an increasing range of colours and designs have become available. Development continues says Karl Viridén, dipl. Architect FH, in the CKW-Blog: "In the future there will be no detectable, visible difference between common facades and those that produce power. The facade can look like glass, fibre cement or whatever."

A solar facade can even be integrated into the local landscape when mounted on a chalet in the mountains: "Active glass modules that look like a wooden facade are already available," explains Viridén. That confirms that diverse solar facades can already be put to use today.

More information on the potential of solar facades in Switzerland can be found on the Swiss Federal Office of Energy website under www.sonnenfassade.ch