The cooling load calculation according to VDI 2078 is based on a defined heat period of 19 days with increasing temperatures up to the maximum design temperature on the actual design day, the CDD (Cooling Design Day). The annual simulation, on the other hand, uses realistic weather patterns of a whole year for calculation, i.e. the hour-based test reference year data (TRY) of the German Meteorological Service (Deutscher Wetterdienst). The weather data from the TRY differ significantly from the solar radiation values and outdoor temperatures during the defined heat periods of the cooling load calculation (see Fig. 1a and 1b). Due to the different basic conditions, the results of the highest cooling load differ between the cooling load calculation and the annual simulation. In addition to the highest cooling load, the annual simulation can be used to calculate the annual energy demand for heating and cooling, as well as the highest heating and cooling load and its timing. Furthermore the annual simulation provides the operating hours of the heating and cooling system for the entire building or individual rooms.
Among other things, the results of the annual simulation can be used to design heating and cooling systems with the highest possible base load and the lowest possible peak load. This additional information from the annual simulation is available without additional effort, since both calculations are performed with the same settings for internal loads and temperature profiles.
With help of a sample project, the differences between the results from the annual simulation and the dynamic cooling load calculation are to be clarified. For comparison, a corner room on the second floor with south-west orientation is examined (see Fig. 2). The building is located in Aachen, Germany and the internal loads correspond to the values from the previous parts of the article series and remain unchanged. In this article, the factors of operation, control concept and weather data are considered as variable parameters. Cooling load calculations are repeatedly performed with varying parameters and their influence on the results is discussed.
Influence of the operating time of the cooling system
When observing the influence of operating times, a continuous operating time of the cooling system (24 h operation on 7 days per week) is compared with a reduced operating time (11 h operation on the 5 working days). A cooling load calculation and annual simulation are performed for both operating periods. The curves on the design day of the cooling load calculation and the day with the highest cooling capacity from the annual simulation are shown in Fig. 3. In addition, the annual cooling energy demand of the two annual simulations is provided here. The maximum required cooling capacity is indicated for the “11h operation”. This behavior is mainly due to the storage capacity of the components. In “24h operation”, the solar heat stored during the day is discharged via the cooling system at night. - In "11h operation", the temperature of the components is higher at the beginning of the operating time and the components can store less heat during the day, which leads to a higher cooling capacity. If, in parallel to the maximum cooling capacity, the annual cooling energy demand from the annual simulation is considered, it becomes clear, although the “11h operation” results in higher maximum cooling loads, the annual energy demand is lower than for the "24h operation". Thus, the operating time of the cooling system not only influences the cooling load, but also the annual energy demand. Extending the operating time lowers the required maximum cooling capacity and the cooling energy demand increases. This effect increases with increasing effective heat storage capacity.