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Stuttgart–Burgholzhof: 800 low-energy residential units with solar supported local district heating

Highlighted Case August 2010
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The Burgholzhof project is a good example for the development, realisation and monitoring of 800 residential units plus a school building all designed to be in low energy quality. The heating and domestic hot water supply was realised by a solar supported district energy network. The city of Stuttgart used in 1995 the instrument 'development measure' (Entwicklungsmassnahme) and bought a property, added the technical and social infrastructure and resold the ground to several private investors. The contract of sale included some commitments for the property and building owners such as improved thermal quality of the buildings (low energy) and realisation of the energy concept with solar supported district heating. The case study includes results and experiences made during the design, the realisation and the monitoring phase.
Project background
In 1995 a new urban planning instrument called “development measure” (Entwicklungsmassnahme) was used for the first time by the city of Stuttgart. Trustees of the city bought a property that was formerly used as housing areas by the US army. The city then added the necessary technical and social infrastructure and resold the ground to several private investors. The contract of sale included some commitments for the property owners and future building owners, such as:
  • Improved thermal quality of the buildings (30 % less heating energy need compared to the current German energy decree of 1995)
  • Realisation of an energy concept with solar supported district heating
  • Construction of financially supported multi-family houses on specified parts of the area
  • Consideration of social goals defined by the local council
The biggest part of the residential units has been privately financed. Urban planning focussed on the active and passive use of solar energy. Close cooperation between the city and its advisers and the builders (including several colloquia) made it possible to accelerate the process of building application. A low-energy elementary school was also erected on the location.
The proofs of compliance with the thermal insulation regulation (30 % better than the German requirements of that time) was checked by experts of Fraunhofer Institute for Building Physics, who also checked specific component joints in order to reduce thermal bridge effects, made visits to the construction sites and compared parts of the documented building material to the initially designed material defined in the building application.
The solar district heating system consisted of flat collectors mounted on three multi-family houses with a total surface area of 1750 m² feeding the heated water into a 90 m³ buffer storage tank in the central heating station below the school. Whenever the required temperature cannot be reached through the solar system alone, the 3.7 MW central gas condensing boiler combination increases the heating water temperature. All buildings are heated by this local district heating system, which also provides the domestic hot water. The plan was to supply about 11 % of the heating and DHW energy need by the solar system and the remaining 89 % by the boiler combination. 

Overall objectives

The overall objectives of the urban development project Stuttgart-Burgholzhof (besides the construction of 800 residential units in combination with an elementary school) were:
  • to realise the buildings on a low energy level which was defined to be 30 % better than the actual German energy performance requirements in force at that time, and
  • to present a good example for a solar supported local district heating system with planned share of 11 % solar and 89 % gas boiler generation of the heating and domestic hot water energy.



Milestones

  • commitments of the building developers to realise more energy efficient buildings than required by the German energy decree (contained in the contracts of sale)
  • realisation of the technical infrastructure including the solar supported local district energy network
  • submitted, checked and revised proofs of compliance with the thermal insulation regulation (tightened by additional 30 %) including submission of specific building component joint details that were also checked and partly revised
  • construction of the buildings including site visits by the surveying institution
  • monitoring of the buildings’ energy consumption and the ratio of the solar generated and boiler generated energy shares for heating and domestic water
  • final report of the project including the results of the surveys and the values of energy consumption and heat generation

Unfortunately, the monitoring was not realised in detail. However the consumption data per building could be gathered and compared to the calculated energy needs.

Energy use:

Residential units:

Space heating energy use measured: 58.2 kWh/m²a

Domestic hot water energy use measured: 15.5 kWh/m²a

Cooling energy use: 0 kWh/m²a

Electricity use: unknown

Elementary school building:

Space heating energy use measured: 35.8 kWh/m²a

Domestic hot water energy use measured: 0 kWh/m²a

Cooling energy use: 0 kWh/m²a

Electricity use: unknown


Energy need:

Residential units:

Space heating energy use calculated: 47.2 kWh/m²a     measured: 46.9 kWh/m²a

Domestic hot water energy use: 12.5 kWh/m²a*     measured: 12.5 kWh/m²a*

Cooling energy use: 0 kWh/m²a

Electricity use: unknown

Elementary school building:

Space heating energy use calculated: 50.8 kWh/m²a     measured: 22.4 kWh/m²a

Domestic hot water energy use calculated: 0 kWh/m²a     measured: 0 kWh/m²a

Cooling energy use: 0 kWh/m²a

Electricity use: unknown

* the specific energy need for domestic hot water was calculated using a default value of 12.5 kWh/m²a. The measurement did not distinguish between heating and domestic hot water (only one meter). Therefore the same value was used as for the calculation.


Contribution of the solar array to the local district energy system:

Heating and domestic hot water energy use by the Burgholzhof quarter (total of all energy uses measured at the transfer stations) calculated: 6221 MWh/a     measured: 5126 MWh/a

Losses by the district heating system calculated: 380 MWh/a (6 %)     measured: 1783 MWh/a (26 %)

Contribution by the solar array calculated: 720 MWh/a (11 %)     measured: 490 MWh/a (7 %)

Contributiuon by the gas boiler combination calculated: 5881 MWh/a (89 %)     measured: 6419 MWh/a (93 %)

The useful solar gains of the collector were 490 MWh/a or 280 kWh/m²a which is rather low.
The full load hours of the gas condensing boiler combination can be calculated to 1735 h/a. This is a good value for the utilisation of the installed capacity.


Description of calculation or measurement methods applied
The data was calculated on the basis of the actual energy decree of 1995 which gives as result the energy need of the building’s space heating. The domestic hot water energy need was assessed according to the revised energy decree of 2002, which contains a default value for the domestic hot water energy need of 12.5 kWh/m²a.
The measured data was based on the energy bills of the different houses. Additionally, the solar input to the local district heating system and the contribution by the boiler combination were monitored in detail.


Comparison of the specific energy demand to the national standard
The residential buildings met the targeted heating and DHW energy need, which was calculated to be 30 % lower than the requirements for new buildings at that time. The measured value (based on energy bills) was even lower with an average with 46.9 kWh/m²a compared to a planned value of 47.2 kWh/m²a.
The school building resulted in significantly lower energy demands than originally predicted. At that time, the heating energy need for non-residential buildings was calculated with the same method as for residential buildings. The different user profile for elementary schools therefore led in reality to a lower value of the heating energy need.

The specific space heating energy use for non-residential buildings can be compared to the current (2009) energy performance benchmarks as listed in the guidelines of the German Ministry of Transport, Building and Urban Development. For regular schools the benchmark value for heating energy (space heating + hot water) is 90 kWh/m²a.
For residential buildings there is no single benchmark value but a range of values that are indicating the energy performance level of dwellings.

Multi-family buildings:

New multifamily buildings (benchmark for buildings constructed in 2009): ~ 60 kWh/m²a

Average for existing residential buildings (benchmark): ~ 240 kWh/m²a

Burgholzhof buildings (constructed in 1999): 60.7 kWh/m²a

Elementary school:

Regular school building (benchmark for buildings constructed in 2009): 90 kWh/m²a

Burgholzhof school building (constructed in 1999): 35.8 kWh/m²a 

Though the residential buildings were built in 1999, their energy performance is still in the range of new buildings today. In the meantime, there were 2 tightenings of the energy performance requirements for buildings in Germany.
The heating energy use of the school building is lower than half of the benchmark value for regular school buildings. It has to be considered though that the school is an elementary school which usually has a shorter usage time than other school types.


Information on the specific tertiary uses and their partition of the site area
Besides the newly built elementary school, which was also designed to consume 30 % less heating energy than stipulated in the energy decree, an existing building, the so-called “Gutshof” (manor) was renovated and converted into a children's day-care centre and some small shops. The building has a separate heating system and its energy consumption is not part of the analysis presented in this document.

Installed energy conversion system:

District heating station: gas: 3.7 MWth

Solar thermal collectors: 1750 m² with 490 MWh/a useful collector gains

Central energy storage: 90 m³ volume

The total length of the district heating energy network is about 2.0 km. The supply temperature in winter is 80 °C, the return temperature 45 °C. In summer the supply/return temperatures are set to 70 °C/45°C. The buffer storage has a volume of 90 m³. The heat density of the area is 35 W/m². The average heating load of the buildings amounts to 45 W/m².


Measures for saving energy or increasing energy efficiency

The increased insulation measures at the buildings’ envelopes led to the following insulation thicknesses and U-values:

  • Walls: mostly composite thermal insulation systems with 16 - 22 cm insulation thickness and 0.2 - 0.3 W/m²K as resulting U-values
  • Roofs: polyurethane insulatiuon on top with 15 - 32 cm insulation thickness and < 0.2 W/m²K as resulting U-value
  • Ceilings and walls to the unheated cellar: mineral wool or polystyrene insulation on the exterior (cold) side with 5 - 12 cm insulation thickness and 0.3 - 0.4 W/m²K as resulting U-values
  • Windows: Low-E-coated double glazing in plastic or wooden frames

Each building had its own design and its own concept of the thermal insulation in order to meet the tightened requirements. The values given above show the range of the used insulation thicknesses and U-values.


Applied urban planning policies/instruments
The instrument “Entwicklungsmassnahme” (development measure) was used. The property was bought from the US army by the city (or its trustees, respectively) and then the technical and social infrastructures were installed. Afterwards the grounds were sold to different building developers. In the contract of sale the city committed the building developers to connect the buildings to the local district heating system, but also to design and realise buildings that needed 30 % less heating energy (for space heating and domestic hot water) than required by the German energy decree that was in force at that time.

Applied energy models/tools
German energy decree of 1995 (calculation method for the heating energy need as result of a balance between transmission losses, ventilation losses, solar gains, internal gains and heating energy need).
For the planning of the solar supported district heating energy system the simulation program TRNSYS was chosen.

Tools used for the energy monitoring
Energy bills of the different buildings.
The central heating unit was monitored with several heat meters.

Ownership structure of the project
During the development and construction phase, the ground and the buildings were owned by different private investors. After the construction was completed, parts of the apartments were sold, others were rented and some were used for social housings.

Financial set-up / incentives
Costs of the property for the city/its trustees: 36.7 million Euros. This is exactly the same amount as the total receipts from selling the ground to the building developers and from funding (~1.9 million Euros).

Sale of the ground and buildings
Two thirds of the residential units were built without funding. The other third received funding of in total 18.3 million Euros from the federal state and 4.3 million Euros from the city of Stuttgart. The funding was given according to:

  • social apartment buildings: 13.0 million Euros
  • staff apartment buildings (for the local hospital): 2.1 million Euros
  • housing programme “Low-cost proprietary”: 7.5 million Euros

The ground for the buildings that were publicly funded was also sold at reduced prices. This subsidy amounted to about 6.1 million Euros.

The solar thermal collector fields have been financially supported by the German Ministry of Research (BMBF) and the local energy supply company (TWS at that time, nowadays EnBW).


Acknowledgements

  • Management of the project and property developer: Landeshauptstadt Stuttgart, Hochbauamt/Amt für Umweltschutz
  • Planning of the solar supported local district heating network: Steinbeis-Transferzentrum, EGS-plan
  • Surveying organisation for the low-energy buildings (proof of compliance with the thermal insulation regulation, specific component joints, construction sites visits, etc.): Fraunhofer Institute for Building Physics

Acronym of the case

Stuttgart-Burgholzhof

Author(s) information

Name

Heike Erhorn-Kluttig

Address

Fraunhofer Institute for Building Physics, Nobelstr. 12, D-70569 Stuttgart

Name

Hans Erhorn

Address

Fraunhofer Institute for Building Physics, Nobelstr. 12, D-70569 Stuttgart

Lessons learnt

The use of the new instrument “development measure” proved to be successful. The properties could be sold, including additional energy efficiency commitments of the building developers. On average, the thermal quality and the energy need of the buildings met the requirement of being 30 % better than stipulated in the national thermal insulation regulation. The intensive surveillance exerted by the involved research institute was not always appreciated by the different building developers but helped to reach the goal. The surveillance also gave insight in possible and recurring mistakes made by architects and engineers while performing the calculations for the proof of the compliance with the national thermal insulation regulation. Additionally, experience was made with frequently occurring problems associated with designing building joints. The collection of material information on the building sites, but also by gathering delivery receipts showed that not always the correct material was delivered and mounted on the construction. The experience gained from this building project led to the first energy edict of the city. For more than 15 years now the city of Stuttgart has required all building owners buying property from the city to meet energy efficiency targets that are about 30 % tighter than the current national requirements. This procedure also applies to all buildings owned by the city. The energy edict is being updated with each revision of the national energy decree.

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Start date - End date

Sunday, 1 January, 1995 to Tuesday, 31 December, 2002

Operational date

Tuesday, 31 December, 2002

References

[1] Landeshauptstadt Stuttgart, Stadtplanungsamt: Wohnen auf dem Burgholzhof: Ein neuer Stadtteil entsteht – Dokumentation der Entwicklungsmassnahme (1998) (in German). [2] Erhorn, H. and Kluttig, H.: Baumassnahme Burgholzhof Stuttgart mit ca. 800 Wohneinheiten in Niedrigbauweise – Ergebnisse des wissenschaftlichen Begleitprojekts. Bericht des Fraunhofer-Instituts für Bauphysik (2002) (in German). [3] Erhorn, H. et al.: Energiesparendes Bauen in der Praxis – Ein Erfahrungsbericht ueber die Entwicklungsmassnahme Burgholzhof in Stuttgart mit ca. 1000 Wohneinheiten in Niedrigenergiebauweise. WKSB Journal, volume 43 (1998), issue 42, pages 45-52 (in German).