Main content:
Office buildings as investment property
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As well as playing an important role in the energy concept, the atrium is also a design highlight.
© fbta, Universität Karlsruhe
There are often good tax and financial reasons for investors to build and then lease office buildings, even if the construction order comes from the future user. At the same time, the parties involved often have different interests. A new office building was built for employees of DB Netz AG in Hamm (Westphalia/Germany). The project also highlights the particular difficulties that an arrangement of this kind creates for an energy-optimised building. There are often good tax and financial reasons for investors to build and then lease office buildings, even if the construction order comes from the future user. At the same time, the parties involved often have different interests. Whereas low construction costs and timely completion are priorities for investors, low operating costs, a high level of comfort in the workplace and an attractive design are important to users. A new office building with 190 office workplaces, conference and function rooms, and a cafeteria was built for employees of DB Netz AG in Hamm (Westphalia/Germany). Following the completion of the design phase, the site was sold to an investor who arranged the construction of the planned building and leased it back to DB Netz AG. The project also highlights the particular difficulties that an arrangement of this kind creates for an energy-optimised building. The aim was for the new building to do without air conditioning or a complex ventilation system. Instead, internal and external thermal loads were significantly reduced and the building was made an integral part of the energy concept through its floor plan layout and the selection of materials. Workplaces are situated close to the facade, ventilation is provided via windows, and natural and artificial lighting can be individually adjusted. As a result, users experience a high level of comfort at work. A glazed atrium significantly reduces the building’s exterior surface area, which in combination with the building’s high insulation standard supports low energy requirements. A ground-coupled heat exchanger and a night ventilation concept are intended to minimise the risk of overheating.
Building summary
| Project status |  |
|---|---|
| Location | Wilhelmstraße 4, 59067 Hamm, Nordrhein-Westfalen |
| Completion | 1999 |
| Inauguration | 11/1999 |
| Building owner | Unternehmensgruppe Roland Ernst |
| Occupant | DB Netz AG, Niederlassung West |
| Heated net floor area | 5.974 m2 |
| Gross volume | 25.705 m3 |
| Work places | 190 |
| Usable floor area (according to EnEV) | 4.047 m2 |
| A/V ratio | 0,27 m2/m3 |
| Key aspects |
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Project description
The concept emerged from a competition by invitation that was organised by DB Immobilien AG. The commissioned architects worked in close collaboration with energy planners from the outset. From the design stage, the aim was to achieve a pleasant indoor environment using as little technology as possible. The future user (DB Netz AG) acted as building developer during the planning phase.
The energy targets were specified by the planning team before being tested and optimised using building simulations for the different variants. Several change requests from the user meant that new energy investigations had to be carried out.
Once the design phase was complete, the site was sold to an investor who arranged the construction of the planned building and leased it back to DB Netz AG. The construction contract was awarded to a general contractor. During this phase, DB Immobilien represented the interests of the building developer, while the architects were involved as general planners. The project also highlights the particular difficulties that are encountered with arrangements like this when it comes to successful energy management.
Since the new project partners were not involved in the target-setting stage, the energy consumption and indoor environment requirements were mainly conveyed by the functional specifications. Communication problems along with economic factors and time pressure meant that further decisions were mostly taken without sufficient attention to the effects on energy efficiency.
Building concept
U-shaped floors surround a large glazed atrium to form a compact building with a net floor area of 6,000 m². For urban planning reasons, the atrium and the main entrance are located to the east and adjoin an existing train station.
The ground floor, with service areas and offices, utilises the entire floor area. The western section has a basement level below it that accommodates storage rooms and technical rooms. On the four upper storeys, in each case offices are arranged on either side of a corridor on the north and south sides of the building. The middle corridors are designed as multifunction zones. The rooms that face the atrium are also fully illuminated and ventilated via the atrium. The western part of the building houses secondary rooms and infrastructure, which are arranged in one axis. There are also offices along the outer facade.
The support structure is a reinforced concrete skeleton construction, with the exception of the secondary rooms wing, which has a solid construction. The exposed areas of concrete can store thermal loads. Concrete elements with a thermal insulation composite system form the perforated facade of the office floors. The windows consist of thermally insulating double glazing in thermally separated metal frames. Their size is optimised for use of daylight and solar yield. Towards the atrium, the offices feature insulating glazing to ceiling height.
For the atrium glazing, thermal and lighting simulations for solar control glass yielded a good compromise between sufficient lighting for the offices that face the atrium and protection against overheating. A system of east-west orientated shading elements was selected so that the sun protection, which was additionally required, would not significantly worsen the lighting situation. Interior screen stores protect the workplaces that face the atrium from glare effects – especially in winter when the aim is to achieve solar yields in the unshaded atrium.
Energy concept
The atrium has a major influence on the building’s energy characteristics and it is a central element in the ventilation concept. It supplies fresh air for the adjacent offices, which, as is also the case for the exterior-facing offices, simply enters the rooms through opened windows. A ground-coupled heat exchanger with a length of 1.8 km preconditions the outdoor air for the atrium and the offices. In the event of extremely high outdoor temperatures, all offices are supplied with precooled air. In the conference rooms this is supplemented by peak load cooling using compression refrigeration units. The multifunction zones are mechanically ventilated. The exhaust air from these zones is recovered in a closed-circuit interconnected system.
The systematic reduction of internal and external loads in combination with thermal masses and a night ventilation concept enables this cooling system to be largely passive. During summer nights, flaps in the atrium and office skylights are opened via an electromechanical system to allow cold night air into the building to cool down the thermal masses. If the indoor temperature is high, the night air is conducted through the multifunction and office zones at an increased volume flow. This forces warm air outside, or into the atrium where it is discharged via ventilation openings in the roof.
In winter, if it is still necessary to increase the air temperature after it passes through the ground-coupled heat exchanger, this is ensured via the heat recovery system and a backup heat exchanger. There are also static radiators in all office zones which enable individual adjustment of the room air temperature. These are supplied by a gas condensing boiler.
The shallow room depth of approx. 4 m favours largely natural lighting in the offices. For design reasons, however, the windows on the external facade are implemented with a lintel and wide, dark window profiles, which reduces the daylight yield and users’ visual comfort. The artificial lighting in the corridor zone is controlled via a bus system based on daylight conditions and presence detection.
Performance
In the construction phase, the high requirements for airtight implementation without any thermal bridges were not sufficiently communicated or achieved despite the planners’ efforts. The somewhat unconventional approaches to controlling the building services equipment also caused implementation and commissioning difficulties. The shortcomings which arose here have considerable impacts on energy consumption. In 2001, total primary energy consumption for building services equipment was 136.5 kWh/m² p.a., which is significantly higher than the target limit of 100 kWh/m² p.a. although compared to conventional new office buildings at the time this is still very low.
The problems are in the details, even though these can have large impacts in some cases. For example, a faulty temperature sensor was detected, as a result of which the heating system had only rarely been operating within its most efficient range. There were also problems with the exhaust air system and the heat recovery equipment. Optimisation approaches were also found for the lighting control system.
A special feature of the project is that measurement data is presented and accessible via the Internet. A special analysis and visualisation tool was developed for this purpose. In this way, the data is stored centrally but is also freely available to the various parties involved and interested third parties.
Optimisation measures and possibilities
The building concept is fault tolerant. However, a number of problems were encountered in the control system for the technical equipment that had to be overcome in order to get closer to the anticipated energy consumption. Since the investor went bankrupt in the intervening period, it was not possible to implement the most important optimisation proposals until the period from October 2002 to June 2003. In the main, these comprised measures for night ventilation such as the switch-on characteristics of the heat recovery system and the control system for opening the flaps in the atrium for natural ventilation, as well as the operational management of the ground-coupled heat exchanger.
All the parties involved learned a great deal from the project. It shows that a concept involving high energy efficiency standards can also be realised in an investor model. Yet it also became clear that some of the planned qualities were lost as a result of the discontinuities between the planning phase and the execution phase. Thus it is important that all parties are involved from an early stage, if possible, so that they understand the way in which the energy measures interact with each other and support the decisions that are taken. It is helpful to specify precisely in writing the targets that are to be met, with an explanation of the physical and technical relationships. This can also form a contractual basis. During the construction phase, it is important that there is intensive support for the planners and companies that are carrying out the work (especially regarding building services equipment) as the installation and adjustment of certain components in the energy concept, some of which may be unfamiliar, is a demanding task. The operation of the innovative system also needs to be explained both to users and system managers as they will need to change their habits in some cases. Precise metrological monitoring of innovative projects like this helps to identify problem points in planning, execution and operation. It also helps energy efficiency become a more natural aspect of the construction process.
Construction costs and economic viability
Measured on the building costs index, the construction costs are relatively low. However, at 1,130 euros per m² for cost groups 300+400, the real construction costs (established costs) were somewhat higher than the cost estimate at the beginning of the project (see cost table below).
Key energy data
| Energy indices according to German regulation EnEV (in kWh/m2a) | |
| Heating energy demand | 65,20 |
|---|---|
| Measured energy consumption data (in kWh/m2a) | |
| Site energy for heating and domestic hot water (dhw) | 62,00 |
| Source energy for heating and domestic hot water (dhw) | 68,00 |
| Total source energy | 226,00 |
| Energy office lighting | 5,70 |
| Energy exhaust air unit | 7,20 |
| Energy supply air unit | 2,70 |
| Energy cooling server | 3,80 |
| Energy main pumps | 1,00 |
| Energy elevator | 1,00 |
Implementation costs
| Costs of implementation in €/m2 | |
| Construction (KG 300) | 654 |
|---|---|
| Technical system (KG 400) | 297 |
These figures represent estimated costs
Net construction costs (according to German DIN 276) relating to gross floor area (BGF, according to German DIN 277)
