Air conditioning and heat supply from wastewater for a new ministry building in Stuttgart

Central facility control room with wastewater shaft (RoK 4 in tank) and wastewater heat exchanger in the background
Central facility control room with wastewater shaft (RoK 4 in tank) and wastewater heat exchanger in the background

It is certainly nothing special if a German Federal Land builds a completely new ministry building. But is is definitely something special if they erect a building which not only meets very high energy efficiency requirements but uses a completely new method of air conditioning and heat supply. The energy source used in this project is the wastewater flow in the Nesenbach canal which passes directly in front of the building and is therefore a continuously available energy source and energy sink.

A new ministry office has been erected in the heart of the city of Stuttgart in 2012. The new building houses the ministry of the interior and parts of the ministry for rural affairs and consumer protection and of the ministry of the environment. A total of about 610 employees will work there. Other areas in the building must meet maximum safety requirements, such as the situation rooms of the Federal State government, the police and disaster control management departments. The building also provides enough room for a conference and event area, a day care facility for children and a canteen. Due to a flexible room concept the building can be used as required for the most different purposes without the need for structural alterations.

Exterior view of the facility control room
Exterior view of the facility control room

The "Staatliche Vermögens- und Hochbauverwaltung" is responsbile for all services relating to the administration of the property and building construction projects of of the Federal State of Baden-Württemberg. As experts in organising building projects they have also been responsible for the erection of the new ministry. The owner of the building is the Baden-Württemberg Stiftung GmbH. The Federal State of Baden-Württemberg has rent the house.

The energy efficiency requirements for the building which have to be met are very high. A maximum total primary energy demand of 120 kWh/m²a and a maximum heating energy demand of 30 kWh/m²a is required. A thermally optimised building envelope and modern, energy-efficient process equipment are applied to meet these requirements.

The innovative heating and cooling concept uses the municipal wastewater from the sewer Nesenbach canal as an energy source. The Nesenbach canal is one of the most important main sewers in Stuttgart with a flow of 170 l/sec under dry weather conditions. Under storm conditions, the flow rate can suddenly rise to several cubic metres per second. We carefully reviewed the edge conditions and after a number of meetings we presented our concept for heating and cooling with wastewater which was further optimised in the course of time. The basis of our concept is once more the HUBER ThermWin® system which has been installed in many successful projects already.

Two RoWin® Heat Exchangers size 8 and three heat pumps ensure that the ministry staff can work in a comfortable climate on every winter and summer day. To achieve this, up to 420 kW thermal energy is extracted from the wastewater and, through the use of heat pumps, about 530 kW of energy is delivered to the building. A district heating connection ensures the supply during peak load periods and serves as a redundancy system. Due to the use of favourable and high-efficiciency primary energy factors – district heat and heat from wastewater – the consumption of primary energy for heating can be reduced to below 30 kWh/m²a, which is equivalent to an energy consumption of 3 l fuel oil per year and square metre. A low temperature heating system is used to distribute the heat energy inside the building. To cover the basic heat load the heat is delivered to the offices via concrete core activation. Water-carrying pipelines running in the concrete ceilings and floors are used as heat and cold storage facilities. The concrete core activation system is also used for cooling during summer: About up to 600 kW heat are extracted from the building and 730 kW given off to the wastewater flow by means of the heat pumps. Due to the large effective surfaces a very pleasant indoor climate can be achieved with low temperature differences even on hot summer days. As the building can be heated with regenerative energy to a large extent it provides maximum comfortableness for the users at minimum energy costs and with maximum energy efficiency.

One of the two HUBER RoWin® Heat Exchanger units being lifted over the ministry building
One of the two HUBER RoWin® Heat Exchanger units being lifted over the ministry building
The first heat exchanger is in place inside
The first heat exchanger is in place inside

The installation of the equipment was a big challenge for the HUBER technicians but they managed it successfully due to their work experience and extraordinary work commitment. Due to space constraints the heat exchangers and all the other equipment had to be lifted over the building with a heavy-duty crane and immediately afterwards put down on the intended place of installation within the building. This had to be managed without interrupting the construction work on site. We would like to take this opportunity to give special thanks to our installation technicians under the direction of David Böhm and Thomas Götz.

The start-up of the wastewater systems supplied by HUBER took place in autumn 2012. After a test operation phase of several weeks the equipment was officially commissioned in December 2012. Presently, the equipment is in the adjustment phase and gradually optimised by the building operators. The new ministry building in Stuttgart represents a milestone and role model of energy-efficient public buildings.

Plants for the recovery of heat from wastewater are not available as “one-fits-all“ solution. Engineering knowhow combined with creativity is needed to plan them and provide tailor-made solutions that meet specific local requirements. This was also the case in Stuttgart where we have used the extensive experience we have gathered from previous projects.  Man-made climate change is one of today’s greatest challenges. Global warming must be limited to the maximum of 2 °C, otherwise life on earth as it is now will be impossible in the future. It must therefore be our goal to establish sustainable energy supply solutions. Laws and state programs can help but what is also needed is public acceptance and the readiness of each individual citizen to rise to the challenge and act accordingly. Unfortunately, the latter is frequently still negatively influenced by the focus on economic efficiency from the financial point of view. This wastewater heat recovery project in Stuttgart reveals the chances we have today and will have tomorrow to contribute to achieving the climate protection goals. The utilization of this regenerative energy source can significantly reduce primary energy consumption and thus CO2 emission, both in an economical way and without any sacrifice. The utilisation of heat from wastewater cannot completely replace presently used energy carriers but it is a reasonable completion in the sense of long-term perspectives in energy supply.


Facts and figures:

  • 2 HUBER Heat Exchangers RoWin®, size 8
  • Max. needed wastewater flow: 60 l/s
  • Heating:

    • 420 kW withdrawal from wastewater
    • 530 kW submittance to building

  • Cooling:

    • 580 kW withdrawal from building
    • 730 kW submittance to wastewater

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