Enhancing chiller efficiencies via use of cold energy storage

The efficient planning and dimensioning of cold water supply systems is a complex task. Even in new buildings, chillers are often over-sized in order to accommodate for the warmest days of the year or to hold capacity available for future upgrading. The chillers are usually designed for maximum expected flow and pressure values in the cold water system and show low efficiencies in partial load times. A practical solution for enhancing system efficiencies is to integrate cold energy storage, which provides the possibility to decouple cold energy supply from demand. Thus, cold water production may be shifted to off-peak hours, when it is energetically more favorable. It is the aim of this paper to investigate the economic potentials of cold energy storage in combination with screw compression chillers by incorporating measured demand profiles from a cold water system at the IISB. The measurement data is used as input for simulations that evaluate different cold water storage and chiller dimensions with respect to energetic demands and energy saving potentials. The results show that cold water production costs can be reduced by up to 22% depending on the considered chiller unit and the chosen storage tank volume. Via application of thermal energy storage for reduction of peak cooling loads, the size of cooling components can be reduced and the chillers can be operated more efficiently. The comparison with saving potentials of a speed-controlled chiller motor shows, that the benefits of both technologies are similar over wide working ranges. A decision for one technology or the other has to be made individually based on the system boundary conditions.