Office building in Aachen

An electrically driven ground source heat pump system is used for heating and cooling an efficient office building with flexible room concept built in the year 2002 in Aachen. The heat source consists of 28 borehole heat exchangers with a depth of 42 meters each. In summertime the building is conditioned by free cooling via the borehole heat exchangers. The control concept was optimized continuously over a five years monitoring period. In the year 2007 a Seasonal Performance Factor (SPF) of 4.3 was reached.

Project description

Building, overall energy concept

Photo: The new office building lies on the edge of a "solar housing estate" in the German town of Aachen and follows the energy specifications for the residential buildings of the estate.

The compact, four-storey office building without cellar has a north-south orientation. The large surfaced floor plan without load-bearing interior walls allows a flexible subdivision. Five parties currently use the building with approx. 100 workplaces in total. In theory, up to eight units would be possible per floor.

The reinforced concrete skeleton construction with load-bearing precast concrete facades was, for the most part, prefabricated. The building shell was therefore finished within 8 weeks and the whole building within 9 months. The high standard of insulation is in line with the construction method of a passive house but only reaches an average U-value of 0.48 W/m²K. This is, among other things, a result of the glazing in the slightly-heated staircase which, however, is located inside the insulated envelope, with a U-value of 1.6 W/m²K. The outer walls are insulated with 20 cm mineral wool and faced with square tiles ventilated at rear. The windows in the office zones consist of triple glazing in thermically separated aluminium frames (UW=0.80 W/m²K). The windows account for 41% of the facade surface area. Because of the glazing's comparatively low g-value of approx. 50 % and cooling using thermally activated concrete surfaces, exterior shading was not installed. The annual heating requirement was calculated at 39 kWh/m² p.a. The air-tightness test of the building envelope produced an n50-value of 0.3 per hour.

Heat pump system

An electrically driven ground source heat pump is responsible for heat generation. Each of the 28 borehole heat exchangers is 42 meters deep. The heat pump supplies a buffer storage tank (900 liters) which feeds the concrete core temperature control (CCTC) with a maximum supply temperature of 28°C. The CCTC is divided into two control circuits (north and south) on each floor.

In summer, the heat is extracted from the rooms via CCTC and released into the earth by means of the borehole heat exchangers. The minimum supply temperature in summer is 18°C. In summertime the heat pump is not in operation (free cooling).

The energy requirements for cooling and heating balance each other out - the temperatures in the soil therefore do not build up during the course of the year. The borehole heat exchangers' water circuit can also be used for cooling and heating the supply air by means of an additional heat exchanger.

Operation experiences

• The planners' idea was a success. The operation of the building is both economically viable and energy efficient. The heat pump system can reach a high coefficient of performance because the temperature difference between the heat source and the supply is small, even with low outside temperatures. The energy equilibrium in the soil from the removal of heat in winter and the depositing of heat in summer also has a positive effect. However, evaluating several hundred measurement points over a five-year period was necessary to find the best control concept for operating the heat pump efficiently. For this, the accompanying EnOB monitoring was extremely important and will be continued.
• User satisfaction in the building is very high. It must be kept in mind, however, that today's users were both the contractors and the planners. On the other hand, this has the advantage that they have first-hand experience of the problems occurring in the operation of the building and can optimise control parameters without great dispute about responsibility or financing and without having to schedule a lot of meetings. The idea of the building going into "series production" is today being put into practice. The same building is currently being constructed in France. A variation on the design is planned for further locations.

Costs, economic efficiency, incentives

• When constructing the building, high-quality, durable materials were used and importance was also attached to the comfort of cooling and mechanical ventilation - not necessarily the norm in offices. Nevertheless, with a net figure of 1.125 /m² NFA (cost groups 300 and 400), the investment costs are below the average costs according to the German building cost index (BKI). This was reached because additional costs, e.g. arising from the use of geothermal energy, were compensated for by savings in other areas such as exterior sun protection. Prefabricated elements also reduced construction costs.
• Monthly energy costs for heating, cooling, air conveyance, lighting, water heating and the lifts were amounted to 0.21 /m² for 2005. In comparable new office buildings these costs are between 0.80 €/m² and 1.50 /m².
• The monitoring campaign started in summer 2002. It was sponsored by the German Federal Ministry of Economics and Technology within the scope of the key research area, EnOB - "Energy-Optimised Construction".

Regulations, guidelines, benchmarking

At 89 kWh/m² p.a., the primary energy characteristic value is below the target value of 100 kWh/m² p.a as defined in the incentive programme.

References

Efficient office building with flexible room concept. BINE projektinfo 13/07, FIZ Karlsruhe 2007

¹ measured in 2006