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What’s That on the Roof?

Steinbeis introduces a solar panel inventory to analyze the energy potential of universities in Hesse

The experts at the Steinbeis Transfer Center for Geoinformation and Land Management have been dedicated to the potential of solar energy for years. The center in Weikersheim has been assessing the solar energy potential offered by roofs on behalf of local authorities, district authorities, and entire states in Germany. It is now using data to draft a rooftop solar panel inventory. The Steinbeis experts started pulling together the inventory for universities in Hesse in 2017, on behalf of the Hessian Ministry for Science and the Arts. The aim was to show how the techniques of geoinformatics can be used to highlight the potential to reduce the carbon emissions of governmental buildings.

As part of a neutral carbon footprint initiative launched by the Hessian state government, the Steinbeis experts first conducted a pilot project, looking at the roofs of ten university buildings to examine their potential to generate solar electricity. Information on the photovoltaic potential of university buildings and thus the potential to reduce carbon emissions is based on a solar energy inventory for Hesse, which was pulled together by the Hessian Ministry of Economics, Energy, Transport, and Regional Development. The inventory was put online in September 2016, and the Steinbeis Transfer Center in Weikersheim has regularly updated the list ever since.

“The Hessian solar inventory is based on extremely high-resolution data, which is available for the whole state.” Explaining the sources of the inventory, Prof. Dr. Martina Klärle, director of the Steinbeis Transfer Center and professor at the Frankfurt University of Applied Sciences, says: “The information was derived from the official land survey register, 5,600 aerial digital images, and a comprehensive laser scan of Hesse based on at least four readings per square meter, which were obtained through an airborne survey.” By combining a 3D cloud of points provided by a surface model with the land survey data and a simulation of exposure to sunshine, not just on individual days but also for the whole year, the anticipated solar energy or electricity yield can be calculated precisely for any given area.

The information takes the solar inventory into account in combination with the intensity of solar radiation in Hesse. Even more importantly, it considers partial sections, making it possible to assess even the smallest structures on roofs, such as chimneys, dormers, or skylights. It even works out how these objects result in shade or how shadows are cast by neighboring trees or even distant mountains (close/distant shadow effects), and all this information is included in the calculations.

Aside from making it possible to calculate the potential to save energy and reduce carbon footprints, the solar inventory allows users to gauge the individual economic efficiency of a photovoltaic or thermal energy unit. Individual parameters can be adjusted such as power consumption, consumption profiles, the different orientation or angle of modules (on flat roofs), the different levels of efficiency of units, when units come into operation, system outlays, ongoing costs, available capital, the size and terms of loans, interest rates, electricity storage, current electricity prices, and developments in electricity rates.

So what does a calculation provided by the inventory actually look like in terms of understanding potential savings? To show how the system works, the Steinbeis experts use a university building in Kassel as an example. The building has a flat roof, which is broadly suitable for use by mounting elevated solar panel units. By setting the equipment at the optimum angle of 30° facing to the south and leaving the right spaces between each row, a system could achieve an output of 60 kWp.

To calculate commercial viability, the Steinbeis experts selected the entire suitable roof area, excluding an area covered by a variety of roof mountings. Out of the potential overall area of 1,054 square meters, units inclined at 30° and spaced in rows could be mounted on a southfacing area measuring 471 square meters. Using this total potential roof area would thus offer a carbon dioxide saving of 36,187 kg per year. Applying the experts’ calculation to the building, the optimum level of domestic consumption would be 48% of the generated electricity; this would be enough to cover 59% of the required electricity for the building. The other 52% of the electricity could be forwarded to the electricity grid and generate income under German renewable energy laws. Extrapolated to a 20-year period, the amount of electricity used in the building would cut bills by € 155,267 with a further € 72,800 on top for electricity added to the grid. Based on an initial outlay for the equipment of € 66,341, the units would pay for themselves after just five years. After 20 years, the panels would generate revenues of € 128,559. The Steinbeis experts have every confidence that these are the sort of numbers many people would enjoy reading.