The new Bahnstadt district in Heidelberg (Figure 1) is one of the largest sustainable urban development projects in Germany and the largest Passive House settlement worldwide at present. The Bahnstadt is still under development; the first 2,500 tenants have already moved in, and infrastructure has largely been completed. In 2007, Heidelberg’s city council adopted a new energy concept focusing on energy efficiency and renewables as a special part of the urban development. The Passive House standard was adopted for all new buildings in Bahnstadt, thereby setting clear specifications for future heating energy consumption and primary energy consumption. The quality assurance process required for building permits ensures that Passive House goals are met and that the most important steps in construction are reviewed on the construction site. These targets are specified in urban construction contracts, purchase contracts and development plans. In addition to these clearly defined tasks, investors also receive consulting services from the city of Heidelberg and the Climate Protection Agency, and funding as a part of the Rational Energy Consumption programme.

Figure 1 Aerial view of the Bahnstadt, where the buildings reviewed here are located (photo: Kay Sommer; image rights: City of Heidelberg)

Consumption values are currently being monitored to determine the success of the Bahnstadt-Heidelberg construction project [Peper 2015]. Monthly meter readings are available for all heat consumption (space heating, hot water, losses, etc.) for entire complexes with more than a hundred residential units each. The data is analysed as part of a “minimal monitoring” process, in which heating energy consumption is quite accurately derived from the monthly averages. The procedure is described in [Peper 2012]. The interim report for consumption levels in 2014 showed the annual results; the consumption statistics for 2015 were compiled in early 2016 and were presented at the 20th Passive House Conference.

The consumption data for seven residential complexes (698 residential units, 61,981 m²) and student dormitories (564 apartments, 15,457 m²) are currently being analysed. The treated floor area is defined as the useful area in accordance with the PHPP living area; assessment of the findings must take into consideration the fact that the current energy-saving building standard in Germany EnEV defines the floor space AN for these buildings as being 28 % greater, so the specific consumption values are accordingly lower. The buildings under investigation have all had tenants for an entire year except for three complexes, which only became occupied in the course of the first quarter of 2014. It is unclear whether this partial occupancy led to lower or higher consumption; both outcomes are possible.

Based on the monthly consumption values, the expenses for hot water, distribution and storage in the core summer months can be determined irrespective of building heating. Here, it is assumed that no unplanned and undesirable space heating was used in the summer. To adjust for the effect of absence during the vacation period, the month with the lowest summer consumption is left out of the equation. Because the complexes contain a large number of residential units, it can be assumed that only a small number of tenants were on vacation at the same time. The average consumption of the view summer months June to September is calculated and the expenses without heating are used as the consumption figure for each month.. All consumption values above the base consumption in the other months are considered “energy consumption for space heating”. In this simplified approach, the heat output from the distribution lines is assumed to be constant over the year.

This type of calculation allows a value to be determined for space heating energy consumption from the small amount of measurement data available. However, this initial rough estimate is too high for various reasons: lower hot water consumption in the summer (overestimation: 1 to 2 kWh/(m²a); moderate approach 10 % overestimation from winter to summer), heating for underground parking driveways (estimation: 0.1 to 0.3 kWh/(m²a)), possible unintentional heating (such as in the month of May) because of improper use of controls (in May, between 0.4 and 1.2 kWh/m²; on average: 0.7 kWh/m²), and higher winter heat losses from distribution lines in the ground and basement (estimate: 0.2 kWh/(m²a)).

In total, these effects overestimate space heating consumption in the initial rough estimate (“base method”) by 1.4 to 2.5 kWh/(m²a), not including any “unplanned” space heating in May. The maximum value of 2.5 kWh/(m²a) and the project-specific heating consumption for May increase the overestimation to 2.9 to 3.7 kWh/(m²a). This consumption then has to be deducted in the second estimate to produce a more realistic value for district heat consumption. At this point, the main effects that lead to overestimation of heating energy consumption are taken into account. The values determined in the second estimate are indicated as “heating energy consumption” below. The average measurement deviation is likely to be ca. ± 4 kWh/(m²a). Even with this (relatively large, but quite small in absolute terms) tolerance range, the measured heating energy consumption is extremely low over the more than 75,000 m² of useful area.

In 2014, the specific total consumption values for district heat in the residential complexes ranged from 46 to 68 kWh/(m²a). The base consumption levels were 33.0 to 48.0 kWh/(m²a). Heating energy ranged from 9.3 to 24.2 kWh/(m²a), with the average area-weighted value being 14.9 kWh/(m²a). The level of accuracy that can be reached in this assessment of consumption data is limited, however. In the Bahnstadt, the ratio of space heating energy and other “base heat” for hot water, distribution and storage is similar to previously investigated Passive House projects.

An assessment of consumption values for heating energy in particular must include the consideration of the weather during the period under review and of the room temperature, which is chosen by the tenants. It cannot simply be assumed that the building’s planning value of, for example, 15.0 kWh/(m²a) will be exactly the consumption value. Furthermore, when a complex has a large number of dwellings, only the average value is reliable. For actual consumption, the actual weather conditions and real room temperatures must be taken into consideration, and they are not known during planning. The analysis of consumption data convincingly shows that the City of Heidelberg’s comprehensive efforts to design an entire district to ambitious energy targets using quality assurance provision were successful. With average consumption values for heating energy of 14.9 kWh/(m²a), the results of measurements taken mainly in the first year of operation – including the dormitories – are very good. Impressively, the number of dwellings is very large (far more than 1,000), as is the floor area investigated at more than 75,000 m². This large number of buildings shows that it is possible to successfully construct buildings with high energy efficiency with input from a wide range of stakeholders.

Figure 2 shows consumption of heating energy in the residential buildings by complex along with the average area-weighted values. Different boundary weather conditions in the next year under review are expected to produce slightly different results; furthermore, 2015 will be the first year of complete usage throughout the year, and the one-off effects from the first year will be over. One focal point will be on whether the two higher consumption values(20 and 24 kWh/(m²a), respectively) change under full occupancy. In any case, even these levels are very low and cannot be seen as problematic.

Figure 2: Annual consumption of heating energy in the Bahnstadt for residential buildings (including student dormitories) by complex (BS)

PHPP is the planning tool for all buildings in Bahnstadt. The City of Heidelberg also uses it to conduct quality assurance for planning. None of the buildings investigated here, however, received certification by the Passive House Institute or an accredited certification body. The correct and complete input of data in PHPP, including changes during planning and construction in particular, is decisive for a realistic calculation. If the data are accurate, PHPP has proven to map heating energy demand realistically in accordance with the boundary conditions used, such as climate data, occupancy, internal heat sources, interior temperature, etc.

Local weather data for the year being investigated were added to the PHPP calculations. For a lack of measurements for indoor temperatures, 21.5 °C was assumed based on experience. Balances were created based on minimal monitoring of the buildings investigated here for a total of 30 PHPP calculations. Because only one heat meter is available per complex, the demand values for heating energy were calculated from the individual PHPP calculations for each complex and weighted by area to produce a comparative value for the complex. It was not possible to review the PHPP calculations provided to the Passive House Institute as a part of this investigation. Nonetheless, a number of items were salient during the processing of the PHPP calculations, such as the addition of the weather data set for 2014. Some of them have a notable impact on heating energy demand and were therefore adjusted.

Figure 3 shows consumption values for heating energy in the complexes along with the PHPP results (for 20.0 and 21.5 °C).

Figure 3: Comparison of heating consumption data with PHPP heating demand values for the weather data set from Ludwigshafen/Heidelberg (LU/HD) in 2014 and two room temperatures for the seven complexes (BS) investigated in the Bahnstadt

The consumption data are best compared under the boundary conditions described (such as the usual indoor air temperature of 21.5 °C and actual weather data). Five of the seven complexes show very good overlapping, with deviations between 0.3 and 3.9 kWh/(m²a). Such deviations can be considered excellent for comparisons of consumption measurements, especially because the expected measurement deviations in such minimal monitoring are of the same size. The results of the PHPP calculations can therefore be assumed to be reliable. However, two of the projects studied (BS-07/08 and BS 13) deviate by 10.8 and 16.9 kWh/(m²a), respectively, revealing much greater differences between the measured consumption data and the PHPP calculations. These complexes were also the ones with the highest measured consumption values. There are a number of possible reasons for the considerable deviations: the buildings were fully occupied only later; changes during construction were not taken into account; user behaviour was significantly different (unlikely); or suboptimal settings and technical flaws could have played a role. Quite possibly, the real cause is a mixture of these reasons.

On average, the buildings investigated here (more than 75,000 m² of useful area) consume 55 kWh/(m²a), about a third the level of district heat for comparable existing buildings for space heat, hot water and losses from distribution and storage. The result of this minimal monitoring is close to the findings previously reached in detailed studies of Passive House buildings with district heat connections. On average, 15 kWh/(m²a) (± 4 kWh/(m²a)) of district heat is consumed – an excellent result for the first year of consumption. Higher levels were measured in two of the complexes where tenants were still moving in after the year started, probably stemming from move-in activities.

A comparison with PHPP planning values (recalculated with current weather data) reveals excellent correlation, with measurement/calculation deviations of less than ± 4 kWh/(m²a). These deviations are within the tolerance range aside from the two complexes that people were still moving into while measurements were taken. Continued monitoring of technical systems and settings is recommended, especially for the outliers. Despite the two outliers, 81 % of heating energy was saved even in the first year of operation in this project, very close to the target of 87 % in the Passive House planning relative to the average value of 112 kWh/(m²a) (according to the “Energiekennwerte 2014” study conducted by Techem on existing residential complexes with district heat).

[Peper 2012] Peper, S.: Messung zur Verbrauchskontrolle – Minimalmonitoring. In: Arbeitskreis kostengünstige Passivhäuser, Protokollband Nr. 45: Richtig messen in Energiesparhäusern
[Research Group Cost-efficient Passive Houses, Volume 45: Proper measuring in low-energy houses]. Passive House Institute, Darmstadt, 2012. Available on Passipedia
[Peper 2015] Peper, S.: Monitoring in der Passivhaus-Siedlung Bahnstadt Heidelberg, interim report 2014. Passive House Institute, Darmstadt, 2015

Detailed monitoring report: Energy monitoring of residential buildings in the Passive House city district of Heidelberg-Bahnstadt