The Berlin Solar Atlas, Developed by Dorothea Ludwig at the University of Applied Sciences (Fachhochschule) in Osnabrück, Germany, the assesses the suitability for solar energy generation of every roof surface over a wide area, and calculates the overall potential energy yield.

Results so far show that approximately 20 per cent of the collective building roof area in Berlin is suitable for solar use, and that this proportion could supply as much as 100 per cent of private electricity consumption in Germany.

Solar-potential roof maps have been available to the public in municipalities across the country for almost four years, with rooftop maps produced as part of council climate protection plans.

An online solar atlas that maps Berlin’s solar roof potential is also freely available, and is geared towards homeowners.

Data collection

The data for the solar-potential roof maps is obtained through high-resolution LiDAR (light detection and ranging) remote-sensing technology. Light reflection data from laser beams is recorded via a scanner mounted on the fuselage of a plane. By collecting at close intervals the values of elevation above mean sea level of the structures on the earth’s surface (buildings and vegetation), the programme can generate a high-precision image of the built-up area in the form of a three-dimensional surface model.

So detailed is the data that it captures even the smallest structures on roofs, such as chimneys and dormer windows. A second set of data using building outlines is used to locate the exact positions of the roofs.

After the surface model has been constructed using LiDAR data, geographic information systems (GIS) come into play to determine (automatically) how much of each roof area is suitable for the installation of solar panels. Five factors are taken into consideration:

• 1. Roof area
• 2. Roof orientation
• 3. Roof slope
• 4. Radiation energy
• 5. Shadow movements.

The last process of the methodology generates polygons which represent homogeneous non-clouded areas of the roof both in slope and orientation.

Solar Potential Classification

Parameters of solar energy potential are calculated for each suitable area.

These parameters are:

• Insulation
• Roof size
• Potential electricity generation
• Potential CO2 savings
• Power in kilowatts (kW)
• Investment volume.

Appropriate roof areas for using photovoltaic (PV) technology:

• Have access to 75 per cent–100 per cent of the maximum possible local radiation energy
• Are not affected by shadow
• Are at least 10 m2 (pitched roofs) and
• 25 m² (flat roofs).

It is desirable to install solar panels on a flat roof with some slope. In this case, a smaller surface area is to be used (about 40 per cent of the roof area). Solar thermal technology uses smaller surface area (smaller than 10–25 m²).

This makes calculating the solar energy potential for the whole region or city possible. Global radiation is defined as the sum of direct, diffuse and reflected radiation. The total solar radiation is therefore the radiation energy received on the earth on a horizontal surface. Germany has global radiation values of 850–1,100 kWh per square meter.

The solar roof website

The full value of a solar-potential map is realised through the awareness this creates. Councils create solar-mapping pages on their websites that allow homeowners to search for their houses, by street and house number, zooming in on the data for their own houses and retrieving suitability information for PV and solar thermal generation. Linked information pages with tips about solar installations, contact details for contractors and grant or subsidy options, make it easier for the interested person to carry out the project.

Solar-potential maps also help local authorities locate roof areas for solar installations without leaving their computers. They can find publicly owned buildings with suitable roof areas or carry out targeted evaluations of industrial and commercial companies, since these often have large roof areas which are very well-suited to solar panel installation.

A case study

The town of Osnabrück was the first town in Europe to get a solar-potential roof map. What started with an online presentation in November 2007, led to the installation rate of PV modules more than quintupling in less than four years. Meanwhile, the website is still in demand and being used extensively.

Osnabrück proved to have great potential for rooftop energy generation. A suitable area of just 2 km² can produce 249,131 megawatt hours per annum, or 109 per cent of the private power consumption in Osnabrück (235 kilowatt hours per year in 2006).

In Berlin, the calculation was implemented first in two pilot areas with a total of 19 km2. 14,300 roofs in a central area of the city around Friedrichstraße and a residential area in Lichterfelde were evaluated for solar suitability. The calculation in the two test areas was conducted to check basic information – in particular, the LiDAR-Data from Berlin. In the Friedrichstraße test area, they considered the Television Tower of Berlin, with a height of 368 m and the effect at shadowing. They also calibrated the method for Solar Irradiation calculation.

The results of the test calculation of the test areas were:

• The LiDAR-Data are of good quality, and captures even the smallest structures on roofs
• The Television Tower caused by its height shadow casting of 1,000 m around the tower.

For Berlin’s solar potential, the result was: approximately 3 million MWh per year electric current can be produced from about 220,000 building roofs from a total of 560,000 buildings in Berlin.

Future prospects

Publishing area-wide solar potential maps allows councils to promote solar energy production on roof areas and to involve citizens in climate protection – but it can also inform future development.

A combination of urban planning and the results of a detailed analysis of existing roof areas would give the approving authorities the tools to decide in each individual case whether a project is reasonable from a sustainability perspective.

The sustainability of this method is ensured by the fact that the method is easily transferable to other regions. The method has been adapted to more than 230 European environments – including Austria, Switzerland and the Netherlands – and for South American environments, specifically conditions in Chile.

Dorothea Ludwig developed the roof-mapping method as a research project for her doctorate and now heads the related branch at IP SYSCON GmbH – a software and system vendor in geographical information systems for the public and private sectors – in Osnabrück, Germany.