By 2050, Germany wants 100% electricity from renewable sources – because electricity generation is responsible for 40% of CO2 emissions in Germany

Renewable energies already account for more than one-third of gross electricity generation.

In Frankfurt, wind and solar energy account for only one-eighth of electricity generation - as of 2019

Frankfurt has the structural disadvantage that wind power cannot be expanded because of its proximity to the airport. In addition, the city's area is comparatively small, so no huge solar parks could be built on it without sacrificing green spaces or land needed for housing.

Accordingly, the focus for the generation of more sustainable electricity has so far been mainly on technological optimization with regard to the energetic use of waste and biomass.

Dams and wind turbines could not bring Frankfurt to urban energy transition, but electricity generation from photovoltaics is a viable solution for the city to meet 10% of Frankfurt's electricity needs, according to a study by Frankfurt University of Applied Science

The problem of aesthetics

No matter whether slate roof or roof with beautiful red tiles: The usual photovoltaic modules are generally not an optical gain for a building.

However, they are not optimized for aesthetics, but efficiency is clearly the main focus: Black is the best color for photovoltaics to absorb as much light as possible. And the eye-catching grid lines that crisscross the panels also have a purpose: they capture the generated electricity as closely as possible.

The industry has long recognized the aesthetics problem, and accordingly, there are now also colorful panels that are not crisscrossed by conspicuous metal grids.

The only drawback is that they are usually more inefficient than the conventional unsightly panels.

The motto for Frankfurt's bridges is therefore: better aesthetically pleasing or invisible photovoltaics with less efficiency than no photovoltaics at all with a lot of efficiency - at least in the inner city area. Efficiency-optimized photovoltaics, on the other hand, will be installed on the outer arms of the bridges, where they cannot disturb anyone.

Photovoltaic on the roof should look in the best case like the roof without photovoltaic

Even though many suppliers are working on it: There are still no roofs that have been covered with an authentic-looking red tile or a convincing photovoltaic slate imitation. Deceptively real looking imitations are not yet on the market. And the innovative products that have already been developed still have comparatively low efficiency.

Unfortunately, the costs per element are (still) relatively high because they are not (yet) mass products. It is simply not yet worthwhile for building owners to rely on beautiful but expensive aesthetic solutions with a low return on investment.

This is to change with the Frankfurt bridges: All "invisible" photovoltaic elements, which are already available in Europe and do not even look like photovoltaics, are to be used on the roofs of the bridges, as the bridges also serve as a "showcase of innovations" for this purpose. Accompanied by research institutes, regular evaluations aim to further optimize the products. And every visitor to the bridge gets an up-to-date overview of applied innovations, which are thus given a chance to become mass products. A permanent expo - which is also permanently updated or expanded.

On Frankfurt's bridges, not only the roofs but also the facades of modern buildings are equipped with photovoltaics - with aesthetically suitable or inconspicuous modules.

The simplest solution when there is not enough space on the roof:

The photovoltaic modules are attached to the facade. Especially in modern architecture, the mirror effect of the glass layer above the photovoltaics can be used to elegantly integrate shiny black modules into the architecture.

The efficiency of such black monocrystalline panels is now over 20% - at least if the modules do not have to be bent. Bendable photovoltaic surfaces unfortunately have a lower efficiency than straight surfaces.

With selective scattering filters, even white photovoltaic surfaces can be created

A selective scattering filter reflects the visible light with a multitude of layers. In this way, the visible light is still perceived by us humans, while the infrared radiation is directed to the solar cells. The reflection of the white light component is achieved by an additional microstructure on the back of the film. However, as expected, this technology entails a loss of efficiency, so it generates somewhat less energy.

More than a third of the buildings on Frankfurt's bridges consist of modern architecture, which, in addition to elegant, smooth, dark facades, also likes to work with light, plain facades: as shown here in a visualization of a single-family house on Hanauer Landstrasse.

The public area can also be used: Small walls on the bridges are equipped with photovoltaics, which is not recognizable as such - but also has lower efficiency

Institutes, companies and start-ups around the world are working hard to find alternatives to conventional black photovoltaic modules. Like Frankfurt, many cities and municipalities have already calculated how much electricity they could generate through photovoltaics on their surfaces and by when they can become CO2-neutral. If surfaces in public areas are renewed, as for example in the visualization on the chart, small walls can also be provided with photovoltaically activated surfaces to support the adjacent street lighting.

For example, some modules with a stone look do not reveal the photovoltaic technology behind them: the technology and the photovoltaic cells are hidden from view behind a printed front glass.

In addition to brightly colored printed front glass over the actual photovoltaic cells, there are now also technologies that can actually be used to produce colorful photovoltaic modules

While the colors of photovoltaic modules used to be rather muted and always had a gray or black cast, there is now a technology that the Fraunhofer Institute ISE from Freiburg has brought to the market: The so-called "MorphoColor modules".Here, the cover glasses of the photovoltaic modules are not colored with color pigments, but rather the physical effect of a butterfly wing is imitated: butterfly wings have a micrometer-fine surface structure that specifically reflects a color,. The Fraunhofer Institute has applied a similar surface structure to the back of the solar module glass layer. Depending on the structure, up to 7% of the incident light is reflected, causing the cover glass above the photovoltaics to be perceived as blue, red or green.

Elements in the public space can also be photovoltaically activated on the bridges and look very attractive in the process

Another way to enable optical diversity: curved photovoltaic surfaces instead of rigid flat modules. But here, too, efficiency losses must be accepted: The efficiency of this so-called "thin-film technology" is still below 15% today.

In addition to its flexibility, however, it has another advantage: the modules are significantly lighter and can therefore be used or installed in a variety of ways where the static requirements are not met for heavier modules.

For seating, pavilions or canopies in public spaces, many surfaces can be photovoltaically activated using thin-film technology.

Even the windows on the bridges are photovoltaically activated without the viewer noticing it

A new approach in the photovoltaic industry works with the principle of so-called "wave guiding". This is used on bridges, for example, for windows of buildings.

With wave guiding, the incident radiation is directed into the edge of a window with commercially available glass panes. Electrical energy is then generated in the edge via photovoltaic modules.

So the windows are their own little energy generators.

The photovoltaic harvest from so many surfaces on the bridges can only be used optimally with the help of sophisticated control systems

The new Frankfurt Bridges neighborhood can show homeowners how beautiful and, above all, invisible rooftop photovoltaics can be and how efficient it is for all aspects of life, including mobility.

An intelligent control system is being developed on the bridges for this purpose. In this way, an entire neighborhood can be supplied with the cheapest electricity, both for its infrastructure and for modern "luxury processes", such as moving garbage cans, bridge vehicles on call, automated carrying service for groceries (no more lugging) and much more - without "wasteful" use of electricity.

As soon as the electricity demand on the bridges is covered, surplus electricity produced by the photovoltaic modules is first stored in batteries or offered to the electricity-powered vehicles next to the bridges, which can "refuel" with electricity at the charging stations on the bridge piers. In addition, excess electricity generated is used to produce hydrogen. The green hydrogen is consumed by the H2 -powered vehicles on the Frankfurt bridges, and surpluses are stored to be used in winter for power and heat generation by means of fuel cells.

Only when this demand on and along the bridges is also covered, all the bridges' energy storage systems are filled and there is still surplus electricity, will it be fed back into the grid of the local electricity supplier Mainova: There are therefore no individual invoices per building with the supplier Mainova, but only after the bridge-internal "netting" has taken place is there a bridge balancing with Mainova.

The photovoltaic modules on the bridges produce direct current (DC), which, however, cannot be used directly by the electricity consumers on the bridges, but must either be converted into suitable direct current with a different voltage or - for some types of end consumption - converted into alternating current (AC). In the case of so-called stand-alone solutions (generating and consuming elements form an "island" - e.g. in the case of street lighting with integrated photovoltaics), this is done without detours; however, the majority of the electricity produced on the bridges is first diverted to bridge-internal supply nodes, the "supply centers", where it is converted on a larger scale and centrally before being sent to the consumers.

The bridge quarter gets supply centers along the bridges - in them take place storage, conversion and processing of energy, but at the same time they serve as infrastructure for drinking water, firefighting water, communications, etc.

The energy from photovoltaic systems is not used directly for buildings, lanterns, etc. on the bridges, but is first routed to the so-called supply centers: approximately every 500 to 1000 m, there are centers to the right or left of the bridges, where many lines converge and the excess energy is distributed intelligently and efficiently.

At pedestrian level, the utility centers are kept extremely slim and space-saving - but in return they are up to two basement levels deep underground

Each supply center is stylistically adapted to the surrounding buildings: If the surrounding area is characterized by old buildings, then the supply centers dress up as Wilhelminian villas; in modern surroundings, on the other hand, they are ultra-modern, but artistically designed: through lighting effects, graffiti painting, modern art, natural stone cladding or other stylistic means of art.

Moreover, the distribution of excess electricity can take place not only on the bridges within the neighborhood, but also, for example, by charging electrically powered cars on and next to the bridges.

When all consumers on and along the bridges are supplied and their own storage facilities are full, they have to feed into the network of the municipal utility: This means that the bridge residents or users are not billed individually by Mainova, the supplier, but rather that internal netting within the neighborhood is the number one priority, followed by the filling of the bridge's own storage facilities and the supply of vehicles along the bridges. Only at the very end is there netting with Mainova via the bridge-internal supply nodes, the "supply centers".

The Frankfurt bridges are almost self-sufficient: both the degree of self-sufficiency and the self-consumption ratio are over 90% and almost 100% respectively

The degreeof self-sufficiency describes the ratio between self-supply and total consumption. The Frankfurt bridges can cover almost all their electricity needs through self-supply.

In this way, power exchange with the urban power grid is minimized: Surplus electricity is either stored in batteries or hydrogen is produced with it - both of which significantly reduce the grid feed-in. This leads to a high self-consumptionratio: self-consumption is covered by self-power production to almost 100%.

On the Frankfurt bridges, not only electricity from photovoltaics is used, but also heat from solar thermal energy

Solar thermal systems use the sun's heat to generate energy. To do this, however, you need relatively continuous, intense sunlight if you want to heat an entire house with it. Accordingly, solar thermal energy in Central Europe is usually only used alongside other energy systems, i.e. to support normal heating in the basement - including on the Frankfurt bridges.

The majority of the solar modules on the bridges are designed as so-called hybrid collectors: They can generate electricity through photovoltaics on their surface and at the same time collect thermal energy through cables underneath. In summer, the heat is stored underground in the so-called Borehole Thermal Energy Storage (BTES); It is used in winter by supporting the heat pumps with the (rather lukewarm water).

Heat surpluses in summer can be stored in the ground with the help of probe fields along the Frankfurt bridges

A potential storage of excess thermal energy is near-surface geothermal probe fields, which will be installed during the construction project where the road surface has to be renewed anyway for the construction of the bridges.