Control Optimization for a Chilled Water Thermal Storage System Under a Complicated Time-of-Use Electricity Rate Schedule

The existence of a 1.4-million-gallon chilled water thermal storage tank greatly increases the operational flexibility of a campus-wide chilled water system under a fourprice time-of-use electricity rate structure. While significant operational savings can be expected, the complication in the rate structure also requires more sophisticated control over the thermal storage tank charging and discharging processes. A chiller start-stop optimization program was developed and implemented into the Energy Management and Control System (EMCS) to determine the number of chillers that need to be brought on line and the start and stop times for each chiller every day, based on the prediction of the campus cooling load within the next 24 hours. With timely and accurate weather forecasting, the actual tank charging and discharging process closely matches the simulated process. The chiller plant’s operating schedules are dynamically optimized to deliver required cooling capacity at lowest possible operating costs. INTRODUCTION Thermal storage is economically attractive under a timeof-use electricity rate schedule. A typical use of the chilled water thermal storage tank is to charge the tank during offpeak hours when utility costs are relatively low and discharge the tank during on-peak hours to reduce operating costs. Equally significant savings may be achieved if a facility is subject to demand charges, in which case the thermal storage tank is used to “level-off” the peak demand in order to avoid hefty demand charges. Another less obvious benefit of the thermal storage is the decoupling of the thermal load profile from the operation of the equipment, adding an element of flexibility and reliability to the system [ASHRAE 2003]. While it is conceptually simple to take advantage of the thermal storage tank, the actual control strategy for a chilled water system with a thermal storage tank may turn out to be rather complicated, especially if multiple chillers are involved and the facility is under a complicated utility rate schedule. Even when the capacity of the thermal storage tank is big enough to serve the cooling load for the entire on-peak period, needs for operational optimization still exist and the tank should be used only to the extent “necessary” since unnecessary production of the storage almost always introduces extra cooling load [Tamblyn 1985]. Many papers have been published in the last two decades on the subject of thermal storage, including several which emphasize the operation and control issues [Shavit 1985 and Fiorino 1991]. Few sources, however, discuss in detail the optimization process for facilities under a complicated time-of-use utility rate structure. Unsuccessful thermal storage projects are seldom documented but do exist. Liu [1999] described how rehabilitation was able to improve the performance of an unsuccessful storage system and reduce the operating costs. Many other case studies can be found in literature, some of which details a step-by-step control optimization procedures for a specific electricity rate schedule [Wei 2002]. A complete control strategy for a thermal storage system describes the sequences of operation under all possible operating modes, such as charging storage, charging storage while meeting load, meeting load from discharging, meeting load from discharging and direct equipment operation, demand-limiting control, etc [ASHRAE 2003]. The most economic operating schedule has to be determined with many factors in mind, such as load prediction, utility rates, demand charges, capacities of the chillers and the thermal storage tank. J. Zhou is a research associate, G. Wei and S. Deng are assistant directors of the Energy Systems Laboratory, and W.D. Turner and D.E. Claridge are professors in the Department of Mechanical Engineering, all at Texas A&M University, College Station, Tex. O. Contreres is HVAC supervisor of Texas A&M University at Corpus Christi, Tex. ©2005 ASHRAE. ESL-PA-05-02-01 Vol. 111, Pt. 1, pp. 4759 4770