New Fraunhofer ISE Institute

Framework project EnBau

The institute moved into its new building in 2001. The view shows the front building and the main entrance area.

© Fraunhofer ISE

Building summary

Project status Optimized
LocationHeidenhofstraße 2, 79110 Freiburg, Baden-Württemberg
Gross floor area15,130 m2
Heated net floor area13,150 m2
Gross volume64,322 m3
Work places300
A/V ratio0.31 m2/m3
Key aspects

Project description

The new building for the institute is situated on the north-western fringe of Freiburg’s city centre. Because the urban realm here is characterised by a diverse range of different uses, the building also takes on an important function in urban design terms. The site is rather narrow and extends in a north-south direction. The new building was also designed to provide space for around 300 members of staff, whereby it was expressly specified that there should be high-quality workplaces, low energy requirements and high quality design. Two thirds of the main usable area are taken up by laboratories and workshops, while the rest is used for offices and meeting rooms. The building has enough space for around 300 employees. The design finally realised was chosen from three competing designs, which were evaluated in terms of their indoor environment, daylight and energy requirements.

Building concept

The building is comb-shaped, with a main axis running in a north/south direction with three wings. The main axis begins in a front building, which houses the central facilities. West of the main axis is the technical lab, the workshops and a 270 m² clean room for developing solar cells. The wings have three stories above ground, with offices on the south side and laboratories on the north side.

A characteristic feature of office workstations in research institutes is the high density of people and computer equipment. Even based on optimistic assumptions, this leads to thermal loads of around 15 watts per m². It is therefore only possible to dispense with active air-conditioning if the electricity consumption is rigorously sunk and all possibilities for passive air-conditioning are exhausted. Various measures have been implemented with this new institute building, such as tailoring glazing surfaces and types to the usage and orientation, external and automatically controlled solar shading, and intensive thermal coupling of the indoor air to the thermal building mass, etc.

The building’s use of wings increases the incident light and the chosen orientation reduces the number of east- and west-facing facades, which are disadvantageous in terms of glare. Combining the laboratories and offices together on each floor ensures very generous room heights of 3.3 metres, which combined with room depths of just five metres enables very good daylight utilisation.

Energy concept

The institute requires large amounts of electrical process energy and cooling. For this purpose, a gas-operated co-generation (CHP) plant is used in combination with an absorption cooling system for generating energy using combined power, heating and cooling. At the same time, it ensures interruption-free electricity supply should there be a power cut in the electricity grid. In winter, the waste heat is used for heating and in summer it is converted into ambient cooling required for the laboratories and special rooms. A gas boiler and vapour-compression chiller cover the peak requirements for heating and cooling. The waste heat from the CHP is also used for the sorptive dehumidification of the air supply for the clean room. Solar power systems for generating additional electricity and heat were also installed.


The building was monitored for several years. The evaluation focused on aspects such as the characteristic energy values, the combined heating, cooling and power system, passive cooling by night ventilation as well as daylight utilisation, electrical lighting and solar shading.

Characteristic energy value

With a maximum of 83 kWh/m², the primary energy consumption was well below the target value of 100 kWh/m². This takes into account additional electricity from the CHP operation. The heating consumption was somewhat greater than planned with 37 kWh/m². This is mainly due to a too low recovery efficiency figure of 0.46, which measures the effectiveness with which heat is transferred to the close-circuit interconnected system for recovering heat. The consumption values for electrical lighting are very low at 2.5 kWh/m² p.a. The largest proportion of the electricity required for the building’s technical equipment is used by the air conveyance system with 13.3 kWh/m² p.a.

Combined heating, cooling and power generation

The operation of the CHP plant led to considerable primary energy savings. The savings potential cannot be completely exhausted, however, because the absorption cooling system in the combined power, heating and cooling system does not work very effectively (COP = 0.5). The cooling is similarly ineffective as the utilised vapour-compression chiller (COP = 2.6) and does not meet the planned values. The main savings are therefore made solely through utilising waste heat from the building heating.

Night ventilation

During normal operation, the night ventilation reduces the average indoor temperature by 1 kelvin. When there is a normal climate at the location, that corresponds to a reduction in the overheating hours (times with temperatures above 25 °C) from around 400 to 150 hours or from 16 to 6% of the operating time.

An unusually high number of tropical nights occurred in the extreme summer of 2003. During this year, it was only possible to maintain the comfort conditions for 80% of the operating time. The night ventilation is therefore only capable of ensuring sufficient comfort in typical summers, and even then the comfort criteria according to DIN 1946-2 cannot be continually maintained. The COP for the night ventilation is 4.5, requiring 0.49 W/m³ for the air conveyance.

Extensive air change measurements with tracer gas were able to confirm that there is uniform circulation through the offices, whereby the daytime heat removal amounted to 65 Wh/m². A major element in the hybrid ventilation concept applied at the Fraunhofer Institute is the manual operation of windows and skylights. By means of window contacts, it was possible to determine how frequently users open windows and thus activate the window ventilation. It was shown that the observed ventilation behaviour corresponds to the expectations during the planning. The windows are predominantly opened in accordance with the external temperature. Night ventilation and solar shading supplement each other and have roughly the same effect on the thermal building behaviour.

Ground-coupled heat exchanger

The ground-coupled heat exchanger is very effective with this building. It supplies a high thermal output, which is not least due to the very good thermal transfer between the ground and the air. Expressed in figures: With an air volume flow of up to 11,000 m³/h, the in-flowing external air is cooled by up to 12 kelvin, which amounts to 76% of what can be theoretically achieved. The maximum thermal output was measured in winter, which with a value of 46 kW (thermal capacity) is slightly higher than in summer (43 kW thermal capacity).

Daylight utilisation and electrical lighting

The electricity requirement for the lighting in the offices is very low at 2.5 kWh/m² p.a., which is partly due to the high daylight autonomy of 75% and partly due to the low installed output of 5.8 W/m² for the general lighting.

The electric lighting is used as expected and, in qualitative and quantitative terms, shows a similar dependence on the level of daylight as has been observed in scientific studies. The evaluation of corresponding measurement data shows that the solar shading for this building is not consistently used in summer, even with high external temperatures. And, largely irrespective of the blind control system’s pre-settings (closed during summer mornings, opened during winter mornings), there was always a similar daily profile for the solar shading in accordance with the sun’s position. From this it can be concluded that, firstly, visual contact is particularly important for some users and that, secondly, there is a preference for using solar shading to prevent glare from direct sunlight.

Optimisation measures and possibilities

The combined heating, cooling and power system was optimised during the ongoing building operation. With a slight increase in the energy utilisation requirements, it was possible to reduce the primary energy consumption from an initial 18 to 14 gigawatt-hours a year. It would also be possible to optimise the cooling production with the absorption cooling system even further by using a larger cold water storage tank. This would enable continual operation of the CHP plant, which in turn would create a greater spread of supply and return temperatures for the cold water and enable higher output figures for the thermal chiller.

Construction costs and economic viability

The construction costs were similar to comparable laboratory and office buildings with considerably reduced primary energy requirements.

Key energy data

Energy indices according to German regulation EnEV (in kWh/m2a)
Heating energy demand41.20

Implementation costs

Costs of implementation in €/m2
Construction (KG 300)757
Technical system (KG 400)567

These figures represent calculated costs
Net construction costs (according to German DIN 276) relating to gross floor area (BGF, according to German DIN 277)

Zusätzliche Informationen:

dissing+weitling architecture
Construction management, Cost management
Weinbrenner, Single, Arabzadeh Architektenwerkgemeinschaft
Theobald + Partner Ingenieure
Building services equipment
Rentschler und Riedesser
Energy concept, Building physics, Simulation
Fraunhofer ISE
Hochschule Biberach
  • Für den Neubau des Fraunhofer-Instituts für Solare Energiesysteme in Freiburg zeigt das Bild ein Energieflussdiagramm für das Jahr 2004.