The challenge: generation of renewable energy is mostly decentralized and, moreover, extremely volatile

In central power plants with combustion technology, the amount of energy generated can be regulated higher or lower depending on demand, especially in the case of gas-fired power plants. In the case of photovoltaics, the amount of energy generated is irregular depending on the time of day, season or weather.

For the CO2-neutral city of the future, the lack of controllability and predictability means that electrical energy must be stored in large quantities by means of buffer storage so that it is available to the end consumer at all times.

The smart city of the future will have to cope with two tasks: creating a grid structure for decentrally generated energy and controlling the energy supply in the event of volatile energy volumes - this has been modeled for the bridges as an example

The energy concept of the Frankfurt bridges has been simulated using Polysun

Energy sources:

  PV modules: in 8 directions (south, south-west, west, etc.) with angles of 0, 37 and 90 degrees.

  PVT modules: in south direction with optimal angle

  Geothermal probes: for space heating

  Energy source: representing waste heat from data centers.  

  Grid: If the power demand is high, the grid power is consumed (if vice versa, the grid is an energy consumer)

Energy storage:

  Li-ion batteries: 320 MWh storage capacity.

  Redox flow batteries: 80 MWh storage capacity

  Geothermal probes: as BTES storage

Energy consumers:

Eectrical consumers: for residential and non-residential buildings, bridge infrastructure (lighting, irrigation, etc.), electric vehicles, hydrogen production

  Swimming pools: as thermal consumers

  Buildings: for space heating of residential and non-residential buildings and greenhouses

  Energy sink, source: representing the residential buildings during space cooling.

Facilities:

  Heat pumps: for space heating and cooling

  Combined heat and power plant: as fuel cells for the purpose of back-up for increased energy demand in winter

  Control: controlling of the whole systems

Parameters, boundary conditions and prerequisites for modeling the energy concept of the Frankfurt bridges

In order not to make the very complex simulation too complex, it has been carried out for 1% of the total energy of the bridges as a representative subsection for 100% of the energy. Therefore, the simulation is performed accordingly with only two supply centers, instead of the 200 supply centers that exist at the Frankfurt bridges.

The simulation time steps are 1 hour each, over the period of one year. The simulation lead time is used with 270 days to more accurately simulate storage effects for the BTES.

High temperature heat pumps coupled with data center waste heat and solar heat are used, while low temperature heat pumps are used for coupling with geothermal heat.

Heat from PVT modules and waste heat from data centers is stored in the BTES from Apr. to Sep. and extracted or consumed from Oct. to March.

In order to use heat from fuel cells more efficiently, the fuel cells are only in operation in winter when energy demand is high.

The swimming pools are mentioned as exemplary thermal energy consumers, since a multiple of the bridge consumption of thermal energy is generated or stored and buyers must be found for this, so that the stored heat does not accumulate in the ground over the years and heat it up as well as the groundwater. In the distant future, however, when the buildings along the bridges have gone through renovation cycles, this heat can then be directed to heat pumps in those buildings.

In summer, heat from cooling ceilings in the roofs of the residential buildings is sent down into the ground for regeneration. In winter, some of the excess thermal energy from PVT modules is also sent into the ground for regeneration, but only in a temperature-controlled manner when the brine temperature of the PVT modules is lower than 30 °C, since the geothermal storage tanks in the column piles (unlike the much deeper-reaching probe fields) are not allowed to heat up the ground and groundwater excessively, but only supply heat there for regeneration purposes.

The result: The Frankfurt bridges have a high self-sufficiency rate and almost 100 percent self-consumption ratio

In summer, when photovoltaic electricity is produced or collected in abundance with the help of the Frankfurt bridges, the degree of self-sufficiency of the bridges is almost 100%. Only in winter do the bridges rely on drawing electricity from the grid - but at less than 10% of their demand. The self-consumption ratio is almost 100% throughout the year, as the bridges always either consume all the energy generated themselves or supply it to other users in the vicinity for direct consumption.

The hydrogen produced can be used for the vehicles on and next to the bridges and also ensure electricity production in winter

The largest share of the electricity generated (approx. 171 GWh/a) is used for hydrogen production: On the one hand, this can be used to supply H2-powered vehicles on and along the bridges all year round. In addition, the surplus of generated electricity in summer is "stored" in the form of hydrogen, so that in winter, when there is less sunshine, there is always an energy source that can provide compensation.

3,240 tons of hydrogen are produced p.a. with the 171 GWh of surplus electricity: A storage volume of 135,000 m3 is required to store these quantities at 350 bar in underground tanks

In order to produce the 3,240 tons of hydrogen in PEM electrolysers, 52,000 m3 of water are required. About 3% to 4% of the rainwater collected or stored in the cisterns under the bridges can be used as a water source.

Since the space required for 3,240 tons of hydrogen is comparatively large even at 350 bar, 42 farmlands and sports fields with a total area of 840,000 m2 were identified next to the 7 arms of the bridges that could be considered for hydrogen production as well as storage - however, only 20,000 m2 are needed. The use of these areas for the installation of the water tanks including the PEM electrolyser at a depth of 2 to 3 meters will not be affected - accordingly, it is attractive for the land owners to "lease" their subsoil for such an infrastructure, of which they hardly notice anything.

In order to be able to convert 90% of the surplus electricity into hydrogen in summer, 5 electrolyser stacks with 5 MW each are required per hydrogen station on the outer arms of the bridges

Of the 133 GWh/a of hydrogen generated in summer, 71 GWh/a (approx. 53%) are stored for winter

The produced hydrogen is not completely stored once to be consumed then, but there is permanently a more (in winter) or less (in the transition months) intensive consumption during the year: Therefore, only 41%, i.e. approx. 71 GWh/a or 1,345 tons in summer months, of the approx. 171 GWh/a surplus electricity used for hydrogen production is stored for consumption in winter.

Approx. 4,250 m2 are required for the hydrogen storage tanks, and approx. 450 to 500 m2 each for the PEM electrolysers - in total, this results in a space requirement of less than 5,000 m2 per station at the end of each bridge arm.

This "peak storage volume" has a space requirement of 56,000 m3 . Although a small part of the hydrogen can be stored directly on site at the bridges in the 200 supply centers located there: Each has a hydrogen tank with a volume of 3.5 m3 in one of its basements, so that of the total 56,000 m3, a total of about 4,900 m3 of hydrogen can be stored locally at 200 bridge points.

But most of the hydrogen, about 51,000 m3, is stored in the end sections of the 7 bridge arms in the 7 hydrogen stations -also underground-.

The tanks at the 200 supply centers are comparatively handy and compact at 1.5 m in diameter and 2 m in length, but the tanks at the seven hydrogen stations are much more complex to handle at 3.6 m D and 15 m L, especially since an average of 49 such tanks are needed for each of the seven stations.

Nevertheless, the space required for underground storage per hydrogen station remains manageable at around 4,250 m2 , especially since only a further 450 to 500 m2 are required for the electrolysers at each end of the bridge arm.

Of the 3,240 metric tons of hydrogen, around a quarter will be used to operate fuel cells, which can then also provide electrical and thermal energy in winter

Even in winter, when photovoltaically generated electricity as well as heat generated by solathermal means are significantly lower, the supply of electricity and heat must be ensured. This is done by using approx. 823 tons of hydrogen to operate fuel cells. These are primarily operated in winter to efficiently use the thermal energy generated at the same time.

Fuel cells with an output of 100,000 kW in total are required for the entire bridges: For this purpose, 200 fuel cells with an electrical output of 500 kW each are assumed, which are distributed to the 200 supply centers.

While the hydrogen and the fuel cells provide a summer-winter balance of the volatile generated PV energy, the balance for shorter sunless phases and the night is provided by batteries

On the bridges, there is a total of 400 MWh of battery capacity in the supply centers. Of this, 320 MWh are Li-ion batteries and only 80 MWh are redox-flow batteries, as the latter take up more space relative to their energy efficiency. However, the battery concept of the Frankfurt bridges could be modified in the future when organic flow batteries are ready for the market.

The Li-ion battery capacity is distributed over 70 of the 200 supply centers (VZ). On average, Li-ion batteries with a total capacity of 4.6 MWh and a space requirement of approx. 33 m2 are to be assumed per VZ. The Reddox batteries can be found in all supply centers: In 60 VZ there are 2 each with 300 KWh and in the remaining 140 VZ there is one each with 300 KWh.

Innovative energy supply infrastructure also takes into account the shortest possible transport routes between decentralized energy producers and consumers

With the bridges, Frankfurt is taking the first big step from centralized supply through power plant combustion to decentralized supply through renewable energies. Here, it is important that generators and consumers communicate with each other through a control system, a closed "Internet of Things," and that the energy is always consumed as close as possible to where it is generated:  Distances of heat and power transport are thus shorter and conversion losses due to transformers or thermal losses due to line distances are minimized.

On the bridges, for example, all surfaces covered with PVT modules are energy-generating units. The electricity they produce is always first transmitted to the next supply center, where it is used in an optimally controlled manner.

Another area of research to increase efficiency: avoidance of conversion losses. Theoretically, direct current energy generated by photovoltaics can be used directly to recharge e-vehicles without having to convert the direct current generated by photovoltaics into alternating current with losses. Only the voltage has to be adjusted.

Due to the storage landscape of the bridges, the demand on the Frankfurt grid from grid injection and withdrawal remains very low

In the case of the Frankfurt bridges, the exchange of electricity between the external and local grids is minimized: Surplus electricity is either stored in batteries or hydrogen is produced with it - both of which significantly reduce grid feed-in. Similarly, batteries as well as fuel cells reduce the amount of power drawn from the grid when there is a power deficit.  Only in the months of September to February is the amount of electricity drawn from the grid slightly increased due to reduced solar radiation - but in total it is still comparatively low at 2% of the electricity collected or generated by the bridges.

In the field of space heating and cooling, heat pumps are an important part of the bridge infrastructure

In the bridge quarters, heating and cooling is done with the help of heat pumps. They are installed in the supply centers when these supply several smaller buildings. Larger buildings on the bridges such as apartment buildings, retirement homes, kindergartens, etc. have their own heat pump. The system includes low-temperature (NT) and high-temperature (HT) heat pumps: the heat source for NT heat pumps is geothermal heat with a temperature of approximately 14 °C. Approximately 100% of the space heating and all space cooling is provided with the help of NT heat pumps. The buildings on the bridges and the greenhouses along the bridges require 100 NT heat pumps with about 290 kW heating capacity at W15/W35.

The heat sources for HT heat pumps are solar thermal from the PVT modules on and along the bridges and waste heat from data centers with a temperature above 20 °C.

In the first years, the HT heat pumps mainly provide heat for defrosting the roads on and under the bridges or swimming pools, etc., until the existing buildings along the bridges have switched from gas to heat pump heating systems in the course of renovations and the solar thermal heat and data center waste heat stored in the ground can also be used for these buildings. The COP of all heat pumps is between 4 and 7, depending on the water temperature.