Tank size and operating strategy optimization of a stratified chilled water storage system

In the downtown area of Austin, Texas, United States, it is planned to build a new naturally stratified chilled water storage tank and share it among four separated chilled water plants in order to reduce the utility billing cost. Each plant is charged with a typical time-of-use utility rate including energy charge and demand charge. This paper presents the method of determining the optimal tank size as well as corresponding optimal operating strategies for this project. A simplified thermal energy storage plus four plants model is built based on some assumptions. Three conventional control strategies (full storage, chiller priority, and storage priority) with limitations on the maximum number of chillers running during the off-peak and on-peak periods are simulated. The results show that a 3.5 million gallon (13,249 m3) tank has the shortest simple payback time and the projected total capital cost is within the budget. Full storage strategy is selected for the summer months and storage-priority strategy is selected for the winter months. The annual billing cost savings are estimated at $907,231 and the simple payback time is 12.5 years.