Whether old building or glass skyscraper: The energy efficiency of many existing buildings is "suboptimal" - to put it mildly

A look at the energy certificates shows: The average consumption per square meter in Frankfurt is still between 150 and 200 kWh/m²a. The EU Commission is aiming for around 30 kWh/m²a as the low-energy standard.

New buildings in Germany must not have more than a maximum of 55 kWh/m²a since 2020.

The 875,000 m2 bridge residential buildings as low-energy buildings require a total of approx. 26 GWh/a thermal energy for space heating

For all residential buildings on the bridges, the target value of the EU Commission of 30 kWh/m2 a is undercut. This also complies with the Building Energy Act of 2021.

This is possible because heat pump technology is used and the construction and energy technology of the buildings are optimized.

The residential buildings on the bridges have a heat demand of about 26 GWh/a, the heat demand for all non-residential buildings on Frankfurt bridges is 14 GWh/a in total

In the non-residential building stock of the bridge quarters, the specific heat demand has been calculated with the help of the "Announcement of the Rules for Energy Consumption Values" (BAnz AT 16.04.2021 B1) and with the help of comparative values from practice. In the calculations, it is assumed that the heat demand can be reduced to one third of the values stated in the announcement for already existing non-residential buildings. The specific heat demand for non-residential buildings changes with the building type. For the planned building types or usage distribution on the bridges, the total demand is about 14 GWh/a.

Almost 440 GWh/a of thermal energy is collected from three energy sources with the help of the Frankfurt bridges: Solar thermal energy, geothermal energy and waste heat from data centers - but only 40 GWh/a of heat is required on the bridges.

However, it must be taken into account: In the end, only 238 GWh/a of the 438 GWh/a of heat can actually be used: The rest is lost due to the losses typical for geothermal storage despite good insulation and short transport distances.

Since only about 40 GWh/a are needed by the bridges themselves, almost 200 GWh/a of the 238 GWh/a heat from the ground remain. This must be extracted and used along the bridges in winter, as the ground otherwise heats up over time: In the distant future, the collected surplus energy will be used to supply the adjacent residential and non-residential buildings along the bridges - as soon as they can obtain their space heating from heat pumps. Until then, other consumer solutions will have to be found.

Possible customers along the bridges can be swimming pools, for example, or greenhouses. There are a number of greenhouses in Frankfurt: one in the Palmengarten, the other in the Oberräder fields.

As soon as the building stock along the bridges is equipped with heat pumps and thermally activated surfaces, the Frankfurt bridges will be able to release their thermal energy to them. However, since the gas heating systems will only disappear after renovation and new construction cycles of 20 to 30 years, the surplus heat must be used elsewhere until then. If it were not withdrawn from the ground storage tanks in winter, the ground would heat up over the years, which would have negative consequences for the groundwater and possibly also for the geotechnical conditions.

Around half of all greenhouses along the Frankfurt bridges can be heated in winter with thermal heat from the ground

Greenhouses with a total area estimated at 85,000 m2 are located along the Frankfurt bridges. They consume significantly more heat per square meter in winter compared to residential or non-residential buildings. The average heat consumption for a greenhouse that needs to maintain an average temperature of at least (18 °C) in winter for tropical plants, for example, is over 400 kWh/m2 a. Greenhouses with European plants, on the other hand, for which the temperature must not fall below only 5° to 10 ° in winter, require 60 to 120 kWh/m a. 2

Greenhouses along the bridges can get half of their estimated heat demand of about 40 GWh/a from geothermal heat provided by the geothermally activated piles of the bridge columns; the other half of their demand can be met with the help of solar thermal PVT heat stored in the ground during summer BETS.

The space heating consumption on the bridges of 26 GWh/a by residential buildings and 14 GWh/a by non-residential buildings, and the consumption of half of the greenhouses along the bridges of about 20 GWh/a, can be met with the 35 GWh/a of geothermal energy and the 12 GWh/a of solar heat plus 13 GWh/a of heat pump energy

In addition to geothermal heat, about 220 GWh/a of heat from other sources is available in winter

This heat can be used to heat the other half of the greenhouses, whose heating needs are not met by geothermal energy. Defrosting roads or warming bus stops, on and under bridges where people have to wait, can also be potential consumers, as can swimming pools or large halls near bridges.

As soon as buildings along the bridges switch to heat pump heating, they will naturally receive priority heat from this large (albeit only low-temperature available) "heat stock".

The cooling of the residential buildings simultaneously serves the regeneration of geothermal energy

To provide space heating for the bridge buildings, heat must be extracted from the ground in winter. To prevent the earth from cooling down over the years, the heat extracted in winter must be "regenerated". This is realized mainly in midsummer with the help of thermal energy sent down into the ground from the buildings for cooling purposes.

If heat were only ever extracted from the earth without sending new heat down, it would cool down over time and could no longer be used for heating in winter. On the Frankfurt bridges, however, about 26 GWh/a of heat is extracted from residential buildings in the course of cooling buildings in summer and sent underground by means of heat pumps for the purpose of regeneration.

The buildings on the Frankfurt bridges will achieve very low thermal energy consumption

On the residential buildings of the bridges, the EU target of 30 kWh/m2 a will be met. Several factors affect this value, but the most important are:

Construction

-Construction type: single-family house vs. multi-family house, compact construction vs. dissected construction, row house vs. detached building

-Building physics: building materials (brick, concrete, wood, clay, etc.) and insulation materials

Power Engineering

-Energy source: gas, oil, pellets, (air) heat pump

-Sealing: especially windows and doors

-Utilization of exhaust air heat/cooling

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On the Frankfurt bridges there are manifold construction methods with different construction and insulation materials.

The energy sources used are heat pumps, supported by ground-source geothermal energy, and electricity for heating water.

Energy consumption is optimized by appropriate seals, in parts by the (filter-free) thermal use of exhaust air, and by appropriate control systems.

On the Frankfurt bridges, both are innovatively combined: humane beautiful architecture with optimized energy efficiency

Monotonous architecture depresses, while beautiful, successful architecture is loved by people: it has a comforting and cheering effect - almost everyone has experienced this.

Innovative energy technology makes it possible to realize the full range of housing options on Frankfurt's bridges that different groups of people prefer: whether old buildings or modern architecture - everything is designed to be energy-efficient and humane.

In addition to the type of construction, the building materials used play a major role in the energy consumption of buildings

There are four criteria for construction materials to be used on the Frankfurt bridges:

1.They must have good insulating properties

2.They must be sustainable, taking into account the entire ecological rucksack

3.They must not present a fire hazard or increase the risk of fire

4.They must be comparatively light because of the statics of the bridges

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Each building material has its strengths in a combination of the four filter criteria, and therefore completely different materials are also used on the bridges, true to the concept of providing a showcase of innovations.

Some of these are conventional, tried-and-tested building materials and construction methods (e.g. timber framework), while others are modern, innovative building materials (e.g. Rabbitz constructions made of round iron, wire mesh and lime plaster as well as lime-cement plaster).

Which materials are used where is decided section by section, according to the architectural style in the respective section (for example, it does not make sense to want to build ultra-modern spherical buildings from classic half-timbering) and also with regard to the locality. The design of the bridge itself per section is also a determining factor in the selection of the building material: if the bridge is founded by more columns at one point, for example, a building above it can sometimes be made of tuff - although this is heavier as natural stone than, for example, timber framing, it is still comparatively light, very easy to work with craftsmanship, ecologically sound and does not burn.

Various types of "aerated concrete" are one of the most important groups of building materials for the buildings on the Frankfurt bridges due to their lightness, good designability and thermal insulation properties without additional thermal insulation measures

Aerated concrete belongs to the group of lightweight concretes, even though, strictly speaking, it is not concrete at all, since it does not contain aggregates.

Autoclaved aerated concrete has excellent thermal insulation properties due to the many air chambers inside. Therefore, aerated concrete can be used to build exterior walls that meet the low-energy house standard without additional thermal insulation measures.

It is just perfect for the decoration of the most diverse architectural styles and especially for the processing by artisans.

The relatively low density only has a disadvantageous effect on the sound insulation properties, and the moisture compensation behavior is comparatively poor due to the many pores.

However, there are already numerous innovative solutions for the disadvantages in terms of sound insulation - or even moisture protection - so that autoclaved aerated concrete is a suitable material for the Frankfurt bridges in its most diverse variants, especially for the neighborhood sections in the old building style.

However, the Frankfurt bridges also rely on traditional sustainable building materials

Wood and clay are both sustainable, comparatively lightweight in certain designs, insulate well, and (with proper treatment) are flame retardant. Without complementary materials for insulation or structural improvement, neither material can reach its optimum. On the Frankfurt bridges, the aim was to implement a wide variety of construction methods and combinations of conventional and sustainable building materials in their buildings - depending on the architectural style in a section of the quarter, a different construction method would then prevail. Accompanied by research and science, the best construction methods in terms of CO2 emissions and sustainability are to be identified over the years. For example, the bridge section above the Deutsche Bank Arena parking lot in the south of Frankfurt is suitable for half-timbered houses, where a homage to Frankfurt's old town is being created on an area of 27,000 m2 . Some of the houses in Frankfurt's old town were masterpieces of half-timbered architecture, which can still offer an excellent living atmosphere today.

Further new territory: so-called rabbit constructions consisting of round iron, wire mesh girders, a plaster base and lime gypsum on the inside and lime cement plaster on the outside.

Rabbit structures are ideal for buildings on the bridges because their main building material, gypsum, is a multi-talent: as a purely natural product, it contains no pollutants, is associated with low CO2 emissions, and can also be recycled indefinitely. In addition, gypsum is not flammable thanks to the enclosed water and even has a fire-retardant effect. Due to its low thermal conductivity, gypsum retains heat for a long time - a big plus when it comes to heating. Its porous surface absorbs moisture well and can thus regulate the indoor climate.

There is only one drawback: gypsum is partially soluble in water. Therefore, in the interior use lime gypsum, and in the exterior - lime-cement plaster. In between there is room for insulation, which can be made of various materials (grass, hemp, etc., or even perlite).

The necessary U-values* for the goal of getting below 30 kWh/m2 a heating energy can be achieved with the right combination of building materials and construction technology with the materials presented here

The Frankfurt bridges are also a showcase of innovations in terms of modern building physics, so that a wide variety of building materials, combinations and insulation systems can be tried out and further researched in long-term tests.

No matter what type of construction and building material: The decision whether to heat with oil and gas or with a heat pump system has the greatest impact on the EU target of 30 kWh/m2 a for residential buildings

Conventional heating processes work by burning something, be it oil, gas or even pellets.

Against the background of CO2 reduction measures, the heat pump has now become the means of choice: Here, nothing is burned, but a principle is used that works virtually the other way around like a refrigerator.

The heat pump uses a small amount of electrical energy and 75% of thermal energy (in the case of COP=4) from the environment, e.g. air, earth or brine from solar systems.

Compression processes are used to raise an environmentally friendly refrigerant to a higher temperature level so that it can be used to heat the water in the heating pipes.

On the Frankfurt bridges, all buildings are equipped with thermally activated surfaces

Whether wall, floor or ledge in front of the window, the heating systems also have small disadvantages: With the wall, you have to know where heating loops are inside so as not to accidentally hammer nails into them; floor heating systems are comparatively sluggish: After all, you do not want to start heating already at noon, when you come home only in the evening at 20:00. And baseboard heaters, from which warm air blows, can also raise an unpleasant amount of dust, similar to air conditioners, and are a problem for allergy sufferers and asthmatics in particular.

In order to compensate for the negative effects of these modern and climate-friendly technologies on luxury, small conventional radiators that run on high-temperature heat (e.g. generated with fuel cells) are also installed redundantly in the buildings in isolated cases. These radiators are only used in special situations, for example, when heat is only needed for one or two hours in the evening during the transitional period and the underfloor heating, once started, reacts slowly and takes far too long to reheat; or as a backup in the event of a strong heat demand on a cold winter's day.

On the Frankfurt bridges, all buildings are equipped with thermally activated surfaces

Whether wall, floor or ledge in front of the window, the heating systems also have small disadvantages: With the wall, you have to know where heating loops are inside so as not to accidentally hammer nails into them; floor heating systems are comparatively sluggish: After all, you do not want to start heating already at noon, when you come home only in the evening at 20:00. And baseboard heaters, from which warm air blows, can also raise an unpleasant amount of dust, similar to air conditioners, and are a problem for allergy sufferers and asthmatics in particular.

In order to compensate for the negative effects of these modern and climate-friendly technologies on luxury, small conventional radiators that run on high-temperature heat (e.g. generated with fuel cells) are also installed redundantly in the buildings in isolated cases. These radiators are only used in special situations, for example, when heat is only needed for one or two hours in the evening during the transitional period and the underfloor heating, once started, reacts slowly and takes far too long to reheat; or as a backup in the event of a strong heat demand on a cold winter's day.