The city of Frankfurt in Germany decided to reduce the operating costs of public buildings through less energy consumption and increased materials efficiency, taking into account an expected life cycle of 40 years. The municipality reduced the energy demand using the Passive House standards for new and existing buildings through “economic construction guidelines”, mandatory for every new public tender (new buildings and renovations). The guidelines are based on the buildings’ life cycle. The aim is to minimise capital, operating and environmental follow-up costs, all the way from the planning stage to demolition and disposal. All public construction projects and all contracts concluded with architects and engineers have been subject to the guidelines since 2005.
Ridelberg school (Figure 1) is an example of public building implementing the Passive house standard in Frankfurt. It was opened in 2004 after 14 month construction period and the cost was about 5% more expensive compared to the same building fulfilling the current energy efficiency German regulation. The surface of the school is 7670 m2, furthermore, a kindergarten and a sports hall are also present (Peper et al., 2007).
Figure 1: Ridelberg school, Frankfurt, Germany
High level of insulation is achieved thanks to a thickness of the walls of 30cm (Figure 2), leading to U-values of 0.1W/m2K.
Figure 2: Wall section at Ridelberg school
Indoor comfort during winter and summer were investigated, reporting low temperature differences among rooms (19.5-20.6ºC during winter), low relative humidity (however acceptable) and comfortable temperatures also during summer (about 23ºC). During summer the building is cooled down during the night thanks to natural ventilation (Figure 3).
Figure 3: Summer natural ventilation at Ridelberg school
Air quality results also being comfortable, with 16.4 m3/h/person, when the ventilating system is operated appropriately. The school has 6 heat-exchangers to recover the heat from exhaust air during the winter, providing new air into the building but reducing the energy demand for heating (Figure 4).
Figure 4: Exhaust air heat exchangers at Ridelberg school
Table 1 shows the main technical characteristics to assess the thermal envelope of the school. A technical and very comprehensive report of the performance of the school is given by Peper et al., 2007. Other characteristics are shown in Table 2.
Table 1: Technical characteristics of the thermal envelope of the Riedberg school in Frankfurt
Element |
Description |
Façade |
Modular, wood-aluminium substructure, U = 0.16 W/m2K |
Roof |
0.11 W/m2K |
Floor |
Frost barrier (20 cm of insulation extends 2m below the floor slab) U = 0.34 W/m2K (with a reduction factor of 0.22) |
Windows |
Triple glazed, U-value of 0.74 W/m2K |
Internal Loads |
25 students per room, where the internal gain is up to 2 kW per room. This reduces the need for insulation. |
External blinds |
Automatic control with a temporary manual switch |
Ventilation |
Central ventilation. 6 systems (3 of them are passive house systems and the other three are for the kitchen, cafeteria and sportshall) with a total capacity of 21700 m3/h (estimate of 20 m3/h per person). Heat recovery with an efficiency of 84 %. Consumption of 0.45 Wh/m3 (meeting DIN 13779 and Passive House standards) |
Air change rate |
At fully occupancy is 2/h; n50 value is 0.46/h |
Table 2: Other energy-related technical characteristics of the Riedberg schooling Frankfurt
Element |
Description |
Heating system |
10.5 W/m2; 2 automatic wooden pellet boilers of 60 kW each Heat demand coverage is made by radiators (stealing in the sportshall), individual rooms thermostat |
Primary energy demand |
59 kWh/m2yr |
Lighting system |
< 6 W/m2 (requirement of less than 2 W / 100 lux /m2) |
Others |
Users: 400 primary school students in 16 classes plus 100 – 125 kindergarten children and 50 adults. Volumetric flow regulator including CO2 and/or mixed gas sensors |
Figure 5 shows the seasonal consumption of the school in 2009 for every internal process (a), and the monthly variation from 2007 to 2009 of the space heating demand (b), and of the heating degree days (c). In the charts, air heating refers to the energy consumed to heat the air entering to different parts through the six air intake systems and heating is the energy consumed to heat all the radiators, except for those in the sportshall, as they are differentiated. As shown, the heating energy consumption is almost constant for every month (except for August) and the variation between the energy consumption for heating and the heating degree days is proportional, though with some exceptions, such as February 2007.
a)
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b)
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c)
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Figure 5: a) Monthly heating energy consumption at Riedberg school during 2009 for several internal processes. b) Monthly heating energy consumption (excl. hot water preparation) in 2007, 2008 and 2009. c) Monthly value of heating degree days in Frankfurt in 2007, 2008 and 2009. (Peper et al., 2007)
The overall energy performance of the Ridelberg school is outstanding with only 59kWh of primary energy consumption per square meter per year (without offseting the PV electricity generation), therefore the Passive House Standard energy requirement is more than fulfilled (120kWh/m2yr).