Optimizing night cooling for two systems in Stockholm
Abstract: Buildings in the commercial sector in Sweden accounts for around 12 % of the final energy consumption of the whole country. Utilizing energy efficient methods for heating, cooling and ventilation without compromising the indoor environment is therefore important. The incentives for energy conserving techniques are not only related to cost but also to lower the environmental impact. The cooling demand is highest during summer and to meet the demand indirect or direct cooling methods could be utilized. Night cooling is an indirect method and operating a night cooling system could be an option to lower the cooling demand. Utilizing night cooling means that the building is ventilated with cold air during nighttime, when the outdoor air temperature is lower than the indoor air temperature. The thermal mass of the building and equipment is therefore cooled down and could then act as a heat sink during daytime. The night cooling system is controlled with several conditions that needs to be fulfilled to allow the system to activate. Several previous studies and experiments have concluded that utilizing night cooling results in energy and cost savings, but some studies have established the opposite. Discomfort in the indoor environment has also been reported when operating night cooling systems in colder climates. Since the prediction of the performance also depends on several parameters the decision to install a night cooling system is not obvious. This study investigates two existing buildings in Hammarby sjöstad, Stockholm with night cooling systems installed. The aim of the study is to establish if night cooling should be utilized in the two facilities and discus if the obtained results are relevant for night cooling systems in general. To determine the performance of the systems the energy demand, the energy cost, the indoor environment, and the environmental impact will be analyzed. The method in the study has been to create building models of the two buildings in a building simulation performance software called IDA ICE. The initial information of the two buildings had different levels of detail. Information about one of the buildings was well documented from previous projects, while the information about the other building was limited. The missing information was compensated with standardized values provided from research on existing buildings in Sweden. In IDA ICE the buildings’ information and details was then imported to create the building models. A parametric study was then performed to test the effect of the night cooling. The results from the parametric study concluded that operating night cooling during more hours resulted in an energy decrease compared to when no night cooling was utilized. Operating night cooling with this condition also resulted in increased fan energy of the air handling units. The total energy cost was mostly decreased when night cooling was utilized, but the environmental impact became higher. On the other hand, the indoor environment was improved with night cooling systems installed.
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