Passive Cooling

The ambitious limit for the primary energy demand does not allow active cooling for most of the floor space. Therefore various passive cooling strategies have been ap­plied in the demonstration buildings. Common features are moderate glazing propor­tions in the facades, exterior shading systems (total energy transmission < 15%) and low internal loads of less than 190 Wh m-2d-1. Uncovered concrete ceilings serve as mass storage for heat loads during the days, with different elements attached to the ceilings or the walls to compensate unsuitable reverberation properties of the rooms. The heat removal from the ceilings is realised either by night ventilation or by an inte­grated piping system run with ground water (five projects).

Night ventilation can be achieved with a mechanical ventilation system. This guaran­tees a good control of the air mass flow but requires additional electric energy. A low pressure drop along the air path and high temperature differences are advantageous for high cooling efficiencies which lay between 8 (warm nights) and 24 (cold nights) in the Pollmeier building. In the FhG-ISE building the mean temperature level could be lowered by approx. 1.2 K with mechanical night ventilation only during the second half of the night until the early morning.

The mass flow in natural ventilation concepts is determined by the temperature dif­ferences between indoor and outdoor, the difference in elevation of the air inlet and outlet and wind induced pressure differences on the building surface. In the Wagner building an air change rate up to 1.2 h-1 was monitored during the night. Cross venti­lation increases the air change rate; up to 8 h-1 have been measured in hot periods in the FH Bonn-Rhein-Sieg building.

Another component of passive cooling concepts are earth-to-air heat exchangers which take advantage of the heat storage potential of the ground. While playing only a minor role for preheating air in combination with a heat recovery system, the pre­cooling can be essential for achieving comfortable indoor air temperatures. Different types (concrete or plastic tubes) with different diameters and lengths have been used either with mechanical or natural ventilation.

Figure 4 gives an evaluation of the passive cooling strategies of three projects. From the great number of measurements it can be concluded that discomfort can be avoided if the limit of 25 °C is not exceeded by more than 10% of the attendance time. Rooms with two differently oriented glazed facades have to be treated very carefully.

Lighting

Based on a total primary energy demand of 100 kWh m-2a-1 the electricity demand for lighting accounts to approx. 30%. The monitored projects covered a range between

3.7 and 18 kWh m-2a-1; the differences mainly result from the daylighting supply in the buildings, the applied glare protection/shading system, the electric power of the artifi­cial lighting, the control strategy and the user behaviour. In buildings with a high day­light autonomy the electric power demand shows a clear dependence on the global radiation: in the Lamparter building the daily mean electric power demand decreases below 1 W/m2 (installed power: 12 W/m2) with a global radiation of more than 100 W/m2. Here, sophisticated control systems show only little energy savings. In some of the buildings a rather high consumption (in correspondence with high cooling loads) was measured in corridors.

Conclusions

The funding programme with its realised demonstration buildings is an important step towards an environmental sound and resource-related evaluation of the (total) energy consumption of buildings. A corresponding EC directive on the total energy efficiency of buildings has to be incorporated in national codes within the next two years. The results of the programme show that a primary energy consumption of less than 100 kWh m-2a-1 can be achieved with investment costs that are comparable to con­ventional projects.

While the low energy and passive building standards seem to be transferable to commercial buildings without major problems, the extension of the scope to the sec­tor of electric energy is a real challenge for the planning of HVAC and lighting sys­tems. Passive cooling strategies showed promising results in terms of energy con­sumption and comfort. However the robustness of the concepts has to be improved because no back-up is available when disturbances occur. A better quality assess­ment of the planning and building process as well as of the operation of the building has to be achieved to keep up a maximum of workspace quality. New simulation tools incorporating models of the user behaviour in terms of ventilation, operation of shading systems etc. could improve the quality of decisions.

On the other hand comfort regulations and codes have probably to be revised in or­der to meet the new dynamic indoor climate situations due to passive cooling. Finally,

a number of prices and acknowledgements show that ambitious energy targets can go hand in hand very well with high quality architecture.

Acknowledgements

The work is funded within the project SolarBau:Monitor by the German Ministry of Economy and Labour (BMWA) under the reference number of 0335007C since 1995 and will end in December 2005. The authors also appreciate the support from the ministry’s project co-ordinator PTJ in Julich.

References

1. Voss, K.; Lohnert, G.; Wagner, A.: Energieeinsatz in Burogebauden, Bauphysik, part 1: Heft 2, S. 65 72, 2003; part 2: to be published in Heft 5, 2003

2. http://www. solarbau. de

3. Energy and Buildings — Special Issue on Thermal Comfort, volume 34, nr. 6, 2002

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *