Numerous measures can be taken to reduce the CO2 balance of Frankfurt's bridges to a quarter: The most important levers include the use of steel produced in an almost CO2-neutral manner, the favoring of climate-friendly vehicle propulsion energy and the generation of green electricity. Furthermore, where there are no buildings on the bridges, low-CO2 concrete can be used - as a showcase for innovation.
The first section shows the magnitude of CO2 emissions caused by infrastructure projects and how important the reduction is for ALL infrastructure projects in Germany and worldwide.
The second section presents the levers used to reduce the material-related CO2 footprint of Frankfurt's bridges.
The third section summarises how the whole of Frankfurt can achieve its climate target in the long term with the help of the Frankfurt bridges.
Compared to other infrastructure projects, the CO2 footprint for the Frankfurt bridges is significantly reduced through a variety of levers.
In addition, they differ from other infrastructure projects by their multiple functions in terms of environment, humane living space and, above all, in terms of research: Since concrete structures reinforced with steel are so CO2-intensive, a technology showcase like the Frankfurt bridges is urgently needed to demonstrate the innovative means by which infrastructure projects of the future can be designed to be significantly less CO2-intensive.
Although the Frankfurt bridges serve many positive purposes and are not expected to produce as many GHG emissions as tunnelling and deep laying, the emission value of around 1.5 million tons of GHG (CO2-equivalent GHG emissions*) from construction materials is still too high. All reduction options must therefore be explored and implemented.
In addition to carbon dioxide (CO2 ), probably the best-known greenhouse gas, there are also other greenhouse gases that have an impact on the climate, e.g. methane (CH4 ) or nitrous oxide (N2 O).
Since their global warming potential differs from that of CO2 , they are usually converted into CO2 equivalents (CO2 e). For example, one tonne of methane contributes 28 times as much to the greenhouse effect over 100 years as one tonne of CO2 , i.e. 28 tons of CO2 equivalents.
Since CO2 is not only the most relevant of the greenhouse gases in the construction industry, but also represents the largest share of the GHG mixture, many statistics speak of CO2 equivalent or even just CO2 emissions, even if the values often still subsume the other greenhouse gases.
If the Frankfurt bridges were to be built overnight, using conventional building materials and without taking into account the CO2 footprint of traffic and energy generation, the CO2 emissions of around 1.5 million tons generated for the production of the steel and concrete required would continue to contribute to global climate change - as has often happened, or threatens to happen, with other construction projects of this magnitude, which have often been harshly criticised. However, if all available savings potential is taken into account, the GHG emissions of the Frankfurt bridges can be reduced by around three quarters - i.e. more than 1.1 million tons of CO2!
Because they are a showcase of innovations for technologies and concepts to promote sustainability and humanity in the big city.
Autonomous driving traffic system
Low-packaging quarters through system innovation
Neighbourhood supply through photovoltaics and geothermal energy
Bidirectional use and thus storage of renewable energy in cars
Use of waste heat from data centres and industry
Percolation concept close to the city for groundwater recharge
Partial roadway construction with low CO2 concrete
Building construction with low CO2 building physics
Test tracks for the optimisation of drought-resistant urban greenery and low-plastic roof/façade greenery
Jobs with integrated further training
Decent accommodation structures for the homeless
Connection of senior gardens with toddler gardens
Work and earning opportunities for people who have completely fallen out of the social safety net
Affordable housing close to the workplace for people with social professions
Opportunities to study without educational qualifications/credentials
Revival of craftsmanship for resource-saving building and repair culture instead of throwaway culture
The main drivers of GHG emissions from reinforced concrete are the cement in the concrete and the steel that is either integrated into the concrete as reinforcement, i.e. for strengthening, or installed as steel columns in the infrastructure project in addition to the concrete. Unfortunately, neither component can be completely dispensed with: The steel provides stability and the cement is a kind of "glue" that causes the aggregate to stick together and not disintegrate.
Around 45 percent of cement emissions are caused by the need for heat and electricity: raw materials such as limestone, clay, sand and iron ore have to be dried and ground (the most important ingredient here is limestone, as will be explained later).
The ground raw material is then fired at 1,450 degrees Celsius - a very energy-intensive process, as one can imagine at these temperatures. But that's not all: the material burned to "clinker" must then be ground again, this time much finer than before. Only then is the clinker ready for use.
However, this energy-devouring process only releases just under half of the total CO2 emissions. 55 percent of the emissions are not caused by the energy required for temperature and grinding, but by a chemical process during heating that produces CO2 as a final substance - for anyone who still remembers their chemistry lessons:
Lime does not occur in nature as pure Ca (calcium), but as calcium carbonate: CaCO3 . However, calcium oxide is required for the production of cement clinker: CaO (as an intermediate product - from which the actually important cement constituents, namely calcium silicates, are formed). If calcium oxide is to be obtained, the formula is:
CaCO3 → CaO + CO2
Institut für Werkstoffe im Bauwesen Uni Stuttgart - Green concrete LTC University of Bordeaux
The construction of the Frankfurt bridges would require around 50,000 cubic meters of reinforced concrete per kilometer for the entire bridge corpus, as the bridges are on average around 35 meters wide. By way of comparison, a two-track subway tunnel is about 15 meters wide and consumes an average of about 20,000 cubic meters of reinforced concrete per kilometer - about twice as much.
If the entire 60 km long section of the Frankfurt bridges were to be built in reinforced concrete (which is not planned -> see .... ), around 3 million cubic metres of concrete would be used, along with 365,000 tons of steel.
For comparison:
According to Tagesschau, the Berlin airport has swallowed 1.3 million cubic meters of concrete.
Stuttgart 21 will also use around 3 million cubic metres of concrete - not including the four additional tunnels that are now to be added.
According to the company's own information, only 3.2 million cubic metres of concrete are estimated for the Fehmarnbelt Tunnel, with only 360,000 tons of steel - however, an additional 2.2 million tons of granite must be delivered and used for the construction section in the breakwater.
Given their scale of construction, Frankfurt's bridges belong to the group of infrastructure projects that so often come under criticism for the exorbitant scale of their greenhouse gas emissions.
It is time that all projects of this magnitude are checked for their greenhouse gas benefit balance and that transparent, meaningful life cycle assessments are drawn up as part of planning approval procedures - which at the same time also show potential for reducing CO2 and other greenhouse gas emissions.
Frankfurt's bridges also need to be thoroughly assessed for their GHG benefit balance: If all the stops are pulled out, the greenhouse gas emissions from the construction of the bridges can be reduced or offset by around two thirds - through substitute materials in the construction of the bridges, compensatory functions of the bridges as a structure and energy-reducing processes on the bridges.
An adult human breathes out about 0.4 tons of CO2 per year. A car driving 10,000 km per year emits around 1.5 t CO2 per year. 80 beeches manage to bind about 1.0 t CO2 per year. This means that 120 beech trees are needed to absorb the 1.5 t CO2 emissions of a car with a combustion engine from the air.
Compensating for CO2 emissions by planting trees becomes very difficult if one considers the city as a whole: Frankfurt's total emissions are currently estimated at around 7,000,000 t CO2 equivalents per year, of which around 1,600,000 t CO2 e come from vehicle traffic.
So 1.5 million tons of CO2 equivalents is quite a lot, even if it is just a one-off and not an annual occurrence. For on the one hand, one could argue: What are around 1.5 million tons of CO2 emissions for the construction of a project if we in Frankfurt blow 1.6 million tons of CO2 into the air every year through our motor vehicle traffic alone? Well, this is exactly why we have a climate problem of drastic proportions, because all the CO2 - once blown into the air - is difficult to bind again.
To really understand how dramatic every million tons of CO2 released into the atmosphere is, you only need to go back to the beech trees: One would have to plant 800,000 beech trees at the start of construction to compensate for the CO2 emissions of the Frankfurt bridges, so that after 100 years 1 million tons would be taken out of the air.
Unfortunately, we do not have 100 years to deal with the consequences of climate change, so at least 2,000,000 beech trees would have to be planted in order to have bound 1 million tons of CO2 again after at least 40 years.
Just for comparison: Frankfurt as a whole has only 200,000 trees in its urban area. There would not be enough space in the entire Rhine-Main area for the 2 million required compensation trees.
Of the 739 million tons of greenhouse gases (CO2 equivalents) that Germany emits per year, by far the largest proportion is CO2 With its 7 million tons of greenhouse gases, Frankfurt contributes around one hundredth of the total German greenhouse gas emissions. GHG emissions - in line with its population of around 800,000, which also accounts for one hundredth of Germany's 80 million people.
With its approximately 675 million tons of CO2 p.a., Germany currently ranks 7th among the highest-emitting nations in the world and is responsible for approximately 1.8% of global CO2 emissions of around 35 billion tons of CO2. Thus, although its share of global emissions is low, it is still above average given that Germany accounts for only 1% of the world's population, which is also reflected in the comparison of CO2 emissions calculated on a per capita basis.
Universität Stuttgart - Institut für Werkstoffe im Bauwesen — Verband Deutscher Zementindustrie
The 11-lever concept, which aims to significantly reduce CO2 emissions from Frankfurt's bridges, is based on a combination of very different impact paths.
For example, the CO2 balance of the bridges is improved by using building materials that are less CO2 intensive, either naturally or due to the processes used.
On the other hand, the bridges function constructively as components of building envelopes, for which material consumption and thus CO2 emissions are eliminated elsewhere.
In addition, the Frankfurt bridges are also the initiator of structural changes in the transport and energy sectors, which can be used to pursue a permanent and supra-regional CO2 avoidance strategy.
For each of the 11 levers, the corresponding mode of action is explained and the CO2 savings to be assumed in each case are quantified.
For example, by dispensing with conventional, high-emission building materials and switching to other materials. This can be achieved via three levers:
Lever 1 - eco-concrete: The amount of cement in concrete is reduced by innovative material concepts, so-called eco-concrete.
Lever 2 - Steel replacement: Replace the steel content in the concrete and/or the steel in the other structure with carbon fibres, bamboo, etc.
Lever 3 - Concretereplacement: You replace the concrete completely, e.g. with natural stone or wood.
There are numerous promising research approaches to reduce the use of cement. Nevertheless, conventional cement is still used for the most part.
There is no lack of good ideas, but rather it is difficult to get the solutions found approved because their use - by definition - could not be tested for years on large buildings in advance.
The cement industry is also trying to improve its CO2 balance, but more by working on process optimisation to save electricity and fuel for heating and not so much by using fundamentally different material mixtures.
The reason why neither research nor industry come up with completely new innovative super solutions is quite simple: the buildings produced with the help of cement must meet the highest quality requirements as well as strict safety and warranty aspects, because the stability of buildings always involves human lives.
And even if this were not the case, it should be borne in mind that the investment involved in any major construction project is extremely high - so it is not possible simply to test entire series of innovations on a trial-and-error basis.
Last but not least, it should also be taken into account that it is not enough to construct a building with innovative concrete and have it be stable and last for a few years. The real goodness of a concrete or even reinforced concrete innovation only becomes apparent after decades.
The formulation change can work with substitute materials, enable changed mixing ratios - through additives or process changes - or also bring about more favourable proportions of the input materials by changing grain sizes through finer grinding processes, so that less cement is required.
The cement - mixed with water - works like an adhesive (glue) that wraps around the aggregate in the concrete mix and then hardens (crystallizes).
Option 1: Optimize the aggregate of the concrete mix so that less cement is needed to "stick":
either by using finer, denser grit, or by changing the surface of the grit, or by choosing a type of rock on which the adhesion process works well.
The cement itself is already a mixture, but it consists mainly of calcium silicates.
Option 2: Reduce the calcium oxide required in cement, which is responsible for 55 % of its CO2 emissions, by adding other hydraulic binders in parts that require less lime and energy.
Possibility 3: The cement mixture with a modified composition is refined in such a way that so-called "eco-micro-fillers" can be added: finely ground materials from the region.
However, in order to start building with these modern formulations worldwide, much more intensive research would have to be carried out into how durable these new concretes are in real life. Prototypical structures are missing. And this is where the Frankfurt bridges come in . . .
Universität Stuttgart - Institut für Werkstoffe im Bauwesen — TU Braunschweig - Institut für Baustoffe, Massivbau und Brandschutz — TU Darmstadt - Institut für Massivbau — Universität Hannover - Institut für Baustoffe — Universität Kassel - Institut für konstruktiven Ingenieurbau — TU Graz - Institut für Materialprüfung und Baustofftechnologie — Universität der Bundeswehr München - Institut für Werkstoffe des Bauwesens — Fraunhofer-Institut für Bauphysik
Carbon concrete consists of two components: Concrete and reinforcement, only in this case not steel, but carbon fibers in the form of mats and rods.
Carbon reinforcement material has two major advantages compared to steel reinforcement: It has 5 times the tensile strength of steel, so less reinforcement material is needed in comparison. In addition, carbon reinforcement is chemically inert to the stresses in construction and does not have to be protected against corrosion by a concrete cover several centimetres thick like steel reinforcement.
For components made of carbon concrete, material can thus be saved and they can be made significantly thinner. The CO2 saving when using carbon reinforcement instead of steel is estimated at 30 %.
Of the 1,500,000 tons of CO2 emissions from Frankfurt's bridges, around one third, i.e. 525,000 tons, come from steel. If one fifth of this is replaced by carbon, then the 105,000 tons of CO2 emissions are reduced by around 35,000 tons - not including the indirect reduction through reduced concrete consumption.
Instead of reinforced concrete, some sections of the Frankfurt bridges can also be built using regional natural stone or wood.
Where the bridges pass over old trees, viaduct-like ashlar structures or masonry viaducts could be a solution: they could wind their way on slender columns as a five-meter-wide ribbon through the Senckenberg complex, for example. Suitable natural stones for such ashlar structures are red Main sandstone and Taunus quartzite, which are quarried in regional quarries near Frankfurt. The red Main sandstone in particular is not only suitable for artistic designs, but also as a protective covering for the concrete of the bridges to protect it from corrosion.
The Master Craftsman Academy is supposed to provide the professional capacity for this, because hardly anyone can do it today. But not only the craftsmen, also the structural engineers are in demand: After all, many buildings have lasted for centuries, but they cannot be calculated with modern post-war DIN standards.
The rediscovery of traditional construction methods with low CO2 materials will therefore be a challenge. The use of wood in construction is desirable anyway as a renewable raw material - but in the case of the Frankfurt bridges it is only possible in certain sections: suitable here are primarily stretches on which there are no buildings, but only footpaths and walkways, as this entails a significantly lower load.
Fraunhofer-Institut für Bauphysik — Fraunhofer-Institut für Holzforschung — Deutsches Institut für Bautechnik — TU Braunschweig - Institut für Baukonstruktion und Holzbau — FH Aachen - Institut für Baustoffe und Baukonstruktionen — Hochschule Bremen - Institut für Experimentelle Statik — ETH Zürich - Institut für Baustatik und Konstruktion
Natural stone can be used to span old tall trees where large massive columns can be founded. In addition, bridge sections on high "stilts" look more beautiful, if they are masonry viaduct style.
Timber is used in places where there are no carriageways on the route or where there are carriageways but they are not heavily frequented. Compared to natural stone, wooden structures have the advantage that they can span wide sections even without high wall arches.
Eco-concrete can be used in exactly the same way as conventional concrete, but it is also only used in places where there are no buildings on the bridges, as there is still no long-term experience with these materials and in the event of renovation in a few decades no other structures would be affected.
Many stretches of the Frankfurt bridges are not built on with houses, but serve as traffic links, walking paths or green spaces.
These areas are suitable as test sections where the bridge is built from eco-concrete, which is mature but has not yet been tested on a large scale and for extended periods of time.
The areas must be monitored and checked by research and industry, and regular tests and evaluations must be carried out to ensure that any need for remediation is identified at an early stage. If there are no buildings on them, then potential remediation, although tedious, can be carried out more quickly and without too much effort.
Alternatively, the use of brick or natural stone masonry or timber should be considered on such sections.
Green hydrogen plays a key role here, making it possible to greatly reduce the release of the climate-damaging greenhouse gas into the atmosphere. As such innovative processes are increasingly coming into focus in both cement and steel production, two further levers are being added to the CO2 balance sheet of the Frankfurt bridges.
Lever 4 Concretecarboncapture in cementproductionCapturing and reusing the CO2 produced in cement production makes it possible to use more climate-friendly concretes for Frankfurt's bridges.
Lever 5 Steel - CO2savings in steelproductionwiththeaidof hydrogen: CO2 emissions for steel can be drastically reduced through an innovative process for substituting coke with hydrogen as a reaction partner in the extraction of iron from iron ore.
In cement production, the actual production process is preceded by an electrolysis process in which water is split into oxygen and hydrogen with the help of wind or solar energy. The pure oxygen can then be fed to the rotary kiln instead of "normal" air (oxyfuel). This has the advantage that the CO2 produced during the combustion process remains free of impurities and the greenhouse gas can be captured (carbon capture). Subsequently, the captured CO2 is converted together with the hydrogen from electrolysis into other raw materials, such as synthetic fuels. In this way, the climate impact of a large proportion of the CO2 emissions generated in the cement industry can be prevented. Since the process described is currently being tested on a large scale and its increasing implementation in the manufacturing process can be assumed for the next few years, the Frankfurt bridges will also benefit from this.
Projektwebsite Westküste100 — FH Westküste - Institut für die Transformation des Energiesystems — Projektwebsite CEMCAP — SINTEF — European Cement Research Academy — Universität Stuttgart - Institut für Feuerungs- und Kraftwerkstechnik — Verband Deutscher Zementindustrie — TNO — ETH Zürich - Institut für Energie- und Verfahrenstechnik
The savings potential for CO2 emissions is particularly large in steel production. The current blast furnace route, in which the iron ore is reduced to pig iron with the aid of coal or coke and enormous quantities of greenhouse gases are released, can be replaced in the near future by a new hydrogen-based process. Here, too, green hydrogen is first generated by means of electrolysis, which can then be used to extract the iron from the iron ore in a direct reduction plant before it is processed into crude steel in an electric arc.
By avoiding the use of fossil fuels, up to 95 % of CO2 emissions can be directly avoided. If the entire steel requirements of Frankfurt's bridges were covered by steel produced in this way, around 500,000 tons of CO2 -equivalent GHG emissions could be saved.
Frankfurt's bridges also create a lot of potential for indirectly reducing CO2 emissions. For example, the concrete and steel used fulfill several structural functions at the same time, for which construction material does not have to be used again elsewhere.
Lever 6 - Bridge corpusasfoundationslab: The concrete and steel used assume the function of foundation slabs for the buildings on the bridges, so that new buildings can be dispensed with elsewhere.
Lever 7 - Bridge Arch Buildings: In the distant future, the Frankfurt bridges can take over the function of the supporting structure for bridge arch buildings below it, which means that less building material is needed for these buildings.
The bridge corpus can be used for a variety of structural purposes. This includes many of the functions and uses of the Frankfurt bridges already presented.
Finally, the bridges create a new piece of Frankfurt, with an area of around 2 million square metres, on which buildings can be constructed without having to make foundation slabs for them.
The buildings on the bridges have a total construction area of around 450,000 square metres.
For this, around 180,000 cubic metres of concrete (approx. 450,000 tons of concrete) would have to be placed on the greenfield site, which would be eliminated by using the bridge corpus as a foundation slab.
Wuppertal Institut — Fraunhofer-Institut für Bauphysik — Fraunhofer-Allianz Bau — International Resource Panel — World Resources Forum
In addition, for a period of 100 years or more, a further function must be provided for and planned in structurally from the outset:
With massive reductions in traffic through optimized autonomous driving traffic systems, a city's formerly four- or six-lane entry roads can be reduced to two vehicle lanes and two bike lanes.
The space freed up under bridges (two or more lanes, 6 or more metres wide) can be used in places by converting it into living space:
This living space then already has supporting columns (the supports of the bridge), possibly already wall areas (if there was a bridge-bearing central wall on the median strip before) and a "roof" (the bridge corpus).
Most of Frankfurt's bridges run over major four- or six-lane traffic roads. If a city were to have exclusively autonomously controlled traffic, the number of all vehicles could be significantly reduced (some forecasts expect up to 80 % fewer vehicles). Some lanes could then be eliminated, and space would be freed up under the bridges that could be converted into building space, since supports and roofs are already in place.
Since all the bridge supports are geothermally activated, the "buildings" under the bridges can also be heated in an energy-efficient manner. Due to the connections to the supply centers, which were installed from the beginning and are planned every few hundred meters along the bridge, the supply with electricity, drinking water, etc. is already available.
The areas with potential for bridge arch buildings are primarily along the ring road and the beginnings of the outer arms - but you don't know what the bridge network will look like in 50 years: There may also be further opportunities for fixtures under the bridges elsewhere.
It is estimated in the planning that around 20 % of the line could be built half-way underneath: i.e. up to 12 kilometres of continuous building lines with a depth of around 7 metres would be created.
This means that around 84,000 additional square metres of building space can be created in this way, for which the load-bearing structure is already in place thanks to the bridges, so that their construction or expansion hardly requires any concrete or steel.
Due to the long time horizon, the estimation of CO2 savings for the bridge arch houses can only be made roughly:
The bridge corpus, which serves as the ceiling of the bridge arch houses, has a thickness of 0.5 m. With a total area of 84,000 square meters, the volume of the reinforced concrete serving as the slab thus amounts to 42,000 cubic meters.
On the other hand, greater concessions will have to be made for the piers. This is because the approximately 3,000 columns that would be affected by the bridge arch installation are oversized in terms of their dimensions for the intended use of the later bridge arch houses. Therefore, if the material of only 1,000 columns is taken into account, about 20,000 cubic meters of reinforced concrete are added.
More than 60,000 cubic meters of reinforced concrete could thus be put to dual use in the distant future.
Bundesinstitut für Bau-, Stadt- und Raumforschung — TU Dresden - Institut für Stahl- und Holzbau
The load-bearing function of bridges for autonomous traffic comes into its own.
Lever 8 - Optimisedtrafficflow on the "secondlevel“
The bridges carry "second-level" traffic that travels on a proprietary route. This makes an efficient autonomous driving system in the middle of the city possible for the first time.
Traffic on Frankfurt's bridges in vintage look and with luxurious interiors will significantly increase the acceptance of the use of non-owned passenger cars. Autonomously driving traffic will also make it much more attractive to do without one's own private vehicle, as all the worry and expense of owning one's own car will be eliminated by this form of "chauffeur-driven transport". Studies show that 90 percent fewer private cars are needed for complete coverage through car sharing. A centrally controlled system of luxurious autonomously driving vehicles will thus also gradually lead to comfortable "car sharing" on the roads.
The resulting CO2 savings from a reduction in new vehicles to be produced in Germany can only be estimated within the framework of the feasibility study. More precise simulations of this must be carried out as part of the planning phase for the Frankfurt bridges. The Frankfurt bridges are expected to handle around 50 million passenger journeys per year.
This is because the Frankfurt bridges collect solar energy on a large scale, which is made available to vehicle owners in Frankfurt in the form of electricity or after conversion into hydrogen.
Lever 9 - Acceleratingthemoveaway from the internal combustionengine
If, thanks to a dense network of inexpensive refuelling options, more vehicle owners in Frankfurt switch to clean drive energy earlier than they had planned, this will mean an immediate saving in vehicle-related CO2 emissions.
A car with an internal combustion engine that travels 10,000 kilometres per year currently emits an average of around 1.5 tons of CO2 per year. The nationwide target is to stop registering cars with internal combustion engines by 2030. The penetration rate of cars with hydrogen or electric drives is therefore likely to rise to up to 80 percent in the next 20 years.
After completion of the Frankfurt bridges in about 15-20 years, there will be at least seven more hydrogen filling stations close to the city centre and charging stations at all bridge pillars near the car parks, providing a very attractive additional range of filling options for hydrogen and e-cars.
Conservatively estimated, this should lead to an increase in the penetration rate of vehicles with clean drive energy of around 10 percent. With 386,000 vehicles in Frankfurt (as of 2020) and 400,000 commuter vehicles, approximately 78,000 vehicles could switch to clean energy up to three years earlier.
The urban energy turnaround can be realized on Frankfurt's bridges:
From photovoltaics to solar thermal and waste heat to geothermal energy: the city's complete potential for renewable energies can be used and optimally balanced. This will significantly reduce or replace CO2 emissions from coal and gas combustion over many years.
Lever 10 - Photovoltaics
The bridges are an exposed suspension surface for photovoltaics. The body of the infrastructure project is used to generate renewable energy. In the case of the bridges, the surfaces (aesthetically beautiful or invisible) serve as a photovoltaic park. In addition, due to their grid structure, they can also receive solar power generated along the bridges and transmit it to consumers.
135 GWh of electricity can be generated annually by photovoltaic modules on the body of the bridge. Of this, only 115 GWh of electricity is consumed on the bridges themselves. This leaves 20 GWh of residual energy available to the city in the form of electricity.
On the city side, the production of 135 GWh of electricity p.a. is substituted, for the generation of which around 37,000 tons of hard coal or 25 million cubic metres of natural gas are burnt in Mainova's current power plants.
The complete variety of photovoltaic systems, including (still) expensive systems from research and development, is presented at the bridges like in a "showcase of innovations" for other potential users and further tested in its long-term effect. The bridges are thus an application platform for the further development of the world of photovoltaics.
Another innovative feature is the optimized control of all energy components in the bridge district by means of an integrated, comprehensive AI system: The district is thus self-sufficient and virtually follows the functional principle familiar from smart homes on a smaller scale. This also serves as a model for other districts.
Frankfurt aims to become almost CO2 neutral by 2050 and to reduce electricity generation through the burning of fossil fuels as far as possible. The annual energy production from the renewable energy of the Frankfurt bridges should make a substitution contribution to this over a period of at least 5 years.
This will reduce CO2 emissions over many years, which come from burning gas to heat homes.
Lever 11 - Geothermal energy
A large part of the 15,000 pillars of Frankfurt's bridges are used to generate energy by activating them geothermally, which allows them to heat and cool the buildings on the bridges.
The use of near-surface geothermal energy is planned directly during the construction of the bridge: geothermal probes are inserted into the 15,000 piers of the bridges during construction. In principle, the subsequent integration of geothermal probes into a structure involves extremely high costs, which is why heating with fossil fuels will be important for much longer than coal or gas-fired power generation.
In addition to the use of near-surface geothermal energy, the waste heat from the data centres to the right and left of the bridges is used by the geothermal pipe system connecting the piers.
The consistent equipment of all buildings on the bridges with surface heating and cooling ceilings fulfils the prerequisite for the use of this low-temperature energy.
In winter, the pipes in the earth piles transport their brine liquid upwards, which is up to 14 degrees warmer than the outside temperature. The brine fluid transfers its heat from the ground to a heat exchanger, where a heat pump can be used to raise the heating water for buildings to a supply temperature of 50 degrees.
To prevent the soil from cooling down when heat is extracted each winter, the soil around the piles must be thermally "regenerated" in summer: This is done by reversing the process described above: The brine fluid flows through solar panels on surfaces exposed to the sun in the summer, returning to the ground warmed. This allows the soil to recover from the heat extraction during the winter period and prepares it for the next winter.
The levers for CO2 savings do not all take effect at once, but at different points in time or over different periods of time.
In addition, there is another time aspect when considering the extent to which CO2 savings can be maximized in the future for an infrastructure project such as the bridges. By the time the Frankfurt bridges are built, after several years of planning, infrastructure projects in Germany will generally produce fewer CO2 emissions: on the one hand, through progress in research and development for all materials, and on the other hand, through the ever-advancing expansion and optimized use of renewable energies.
With the help of the Frankfurt bridges, Frankfurt has the opportunity to achieve its ambitious CO2 reduction plans by 2050.
This is the result of the rough initial assessment from today's perspective, which is based on the rough and partly abstract quantification of all conceivable savings options. The 11 levers at a glance:
On some sections of Frankfurt's bridges, steel and concrete can be partially or even completely dispensed with. Where structural requirements allow, regionally available materials such as natural stone and wood can be used, resulting in significantly lower CO2 emissions. If a mix of non-reinforced concrete materials is used in all suitable sections of the bridge, CO2 emissions can be reduced by a further 55,000 tons.
New formulations in the cement industry will be able to reduce CO2 emissions from concrete worldwide in the future - provided they are tested under real conditions. This is precisely where the Frankfurt bridges come on the scene - as a showcase for innovations. The bridges therefore make more of a contribution to the global reduction of cement-related emissions than they are likely to make in themselves. For this reason, a saving of only 20,000 tonsis assumed here.
If the reinforcing steel in concrete is replaced by carbon, a large proportion of the CO2 emissions generated by the reinforcement can be avoided. As soon as carbon reinforcement is no longer produced primarily with petroleum, but there are more sustainable input materials for it, it will be able to make a significant contribution to CO2 reduction worldwide. So far, only partial substitution has been estimated for the Frankfurt bridges, which is why a saving of only 35,000 tonshasbeen calculated for this.
Geothermal probes are integrated in advance into a large part of the piers of the Frankfurt bridges, so that they are geothermally active and supply clean energy for heating and cooling the buildings on the bridges. In this way, up to 15 GWh of thermal energy from the combustion of natural gas can be substituted annually. As the conversion to geothermal systems for Frankfurt's building stock will still take a long time, Frankfurt's bridges will still be able to provide a compensation service for other heating systems for at least 10 years, which is why a saving of around 30,000 tons of CO2 seems plausible.
In times of the traffic turnaround, the Frankfurt bridges with their seven hydrogen filling stations and countless charging stations will be an additional stimulus in terms of clean drive technologies. This will lead to an accelerated shift from the combustion engine to hydrogen and electric cars. Calculated over two years, the Frankfurt bridges can be credited with a potential saving of around 240,000 tons of CO2
The autonomously driving traffic on the Frankfurt bridges will lead to a significant reduction in private vehicles, so that fewer cars will have to be manufactured in the long term. Due to the uncertainty about the extent of the effect, the CO2 saving was described with a very conservative value of 50,000 tons.
Building areas can be created under the bridge arches (in the distant future). For these buildings, structural components made of steel and concrete are not required, as ceilings and piers already exist. Therefore, 30,000 tons can be deducted from the CO2 footprint of the bridges.
The steel used for the Frankfurt bridges should ideally come from manufacturing processes in which hardly any CO2 emissions are produced thanks to innovative hydrogen-based processes. It is true that steel produced in this way will be increasingly available in the next few years. In view of the expected supply bottlenecks for green steel, the CO2 savings for the Frankfurt bridges have been limited to 265,000 tons as a precaution.
By using the oxygen obtained by electrolysis to capture the CO2 during cement production, the climate impact for a part of the greenhouse gases produced can be prevented. This gives the concrete a better CO2 balance. As the process is currently still in the trial phase, a conservative saving of 50,000 tons of CO2 was assumed for the Frankfurt bridges.
The body of the bridge fulfils the function of foundation slabs for the buildings on the bridges. If the housing were built on a greenfield site, the concrete would have to be consumed there. Thus, around 60,000 tonsof CO2 of the bridge corpus are to be credited to the buildings - and not to the Frankfurt bridges.
The urban energy revolution is taking place on Frankfurt's bridges. With the help of solar modules for photovoltaics and solar thermal energy alone, up to 135 GWh of electricity from fossil energy sources can be substituted annually. Against the background of Frankfurt's climate goals of becoming climate-neutral by 2050 and dispensing with the burning of hard coal and natural gas, the Frankfurt bridges are likely to continue to play a compensatory role in the area of energy supply for at least five years. A total CO2 saving of 300,000 tons is therefore realistic.
If one adds up the carefully estimated savings of all 11 levers, it is also clear: 395,000 tons of CO2 remain - albeit at the 2022 level, with technologies from 2022. Research and developments for CO2 reduction in construction are progressing rapidly.
By the time construction of Frankfurt's bridges begins in 2027, other technologies could have matured that could help the bridges become climate-neutral, perhaps even climate-positive.
This has not been taken into account in the CO2 savings levers: The levers only mention potentials that are related to the bridge concept.
Infrastructure projects usually source their concrete from the region in order to keep the transport costs for the material masses as low as possible. If the cement is produced in the regionally based concrete plants with the help of surpluses from regionally generated renewable energy, "green cement" is produced: However, this requires power lines to be laid from solar and wind farms to the plants. Since this requires a considerable investment in line infrastructure, such a regional measure is only worthwhile if projects of the magnitude of the Frankfurt bridges or, for example, the Frankfurt long-distance rail tunnel are in the pipeline. And even then, power line routes are costly. Moreover, the electricity from wind and solar parks is usually sold for years even before the parks are built. The cement for the Frankfurt bridges should therefore be produced with the help of "energy belts: These conduct electricity generated photovoltaically along highways to industrial companies - for example, to the plants of HeidelbergCement.
Photovoltaic ribbons can be set up along federal roads and motorways, whose electricity can be fed directly to the respective consumers: These can be industrial plants, charging stations for e-cars or water filling stations etc. in the Rhine-Main area. Surplus electricity from the energy belts is stored in underground hydrogen tanks on the right and left of the roads.
The sustainability of the bridge concept(s) should not only be considered in isolation, but also in its impact on the rest of the city:
If photovoltaics are installed on Frankfurt's bridges, for example, they must be aesthetically pleasing or invisible close to the city centre, as this acts like a showcase for other homeowners to encourage more photovoltaics to be installed in the established urban area. Or, if people first use autonomously driving vehicles on Frankfurt's bridges without hesitation, they will also find it much easier to get into autonomously driving vehicles on the road at some point. In this way, the innovative approaches on the bridges help to realize these innovations in the existing buildings next to the bridges.
In very concrete terms, the bridge's energy generation can help supply consumers in the city's stock: the surplus electricity on sunny days can be made available for electric cars to charge at the bridge's pillars; the geothermal energy, like its conduction system, can also be made usable for buildings to the right and left. This direct contribution of the bridges to CO2 savings for Frankfurt can potentially be expanded even further.
A ground-level geothermal system not only supplies the bridges and residents' buildings (with activated building surfaces) with heating and cooling, but also serves as a conduction system for waste heat from computer centres and other heat sources from the Frankfurt waste heat register.
Photovoltaics are invisibly integrated everywhere on new buildings and urban areas, and intelligent control systems reduce electricity demand throughout the city.
It also reduces storage losses through intelligent peak load utilization and bidirectional electricity use with vehicles.
The Frankfurt Bridges pillar landscape, with its offer of thousands of chargingoptions at the parking spaces next to the pillars, have led to a high penetration of electric cars. The 8 hydrogen fillingstationsof the bridges in all directions have also caused the number of hydrogen cars to grow. Vehicles with internal combustion engines are almost non-existent.
Autonomousdriving has been introduced in the city. When vehicles arrive from outside the city, they connect to the central control system upon entering the urban area, and the driver behind the wheel can sit back and relax.
Green areas in the city, made possible by the irrigation system of the Frankfurt bridges, reduced the CO2 content of the air by up to 200 tons per square kilometre p.a. Of the 250 km2 urban area, it was possible to unseal and green 25 % of the traffic routes (50 km2 ) in particular. A further 10 square kilometres have been added by greening facades and roofs.
As with other major infrastructure projects, the construction of the Frankfurt bridges also releases large quantities of CO2, mainly during the production of the concrete and steel required.
In order to keep the harmful effects on the global climate as low as possible, all available options (levers) are therefore being considered, through which the potentially emitted greenhouse gas of around 1.5 million tons of CO2 can be reduced by around three quarters to 395,000 tons of CO2.
The total savings of all CO2 saving options for the Frankfurt bridges therefore amount to around 1.1 million tons of CO2.