Feasibility analysis of thermal energy storage: Differences using blended versus actual rate schedules

This paper reports on the results of analyzing yearlong monitored 15 min data from the cooling plant of a large university campus consisting of multiple chillers and multiple chilled water Thermal Energy Storage (TES) tanks. The objective of the analysis was to determine whether the addition of another TES tank would be economically justified under the present electric rate structure and cooling load demand of the campus. The analysis was done: (i) using blended on-peak and off-peak energy rates (an approach commonly adopted due to its simplicity for evaluating different system alternatives and operating strategies meant to reduce cost and/or energy use), and (ii) using the actual electric rate structure which includes energy and demand charges. The latter rate structure suggests a 42 year payback, while the former rates predicted a payback period of over 100 years. If the incremental avoided cost of an additional chiller (to meet anticipated increases in cooling loads) is included in the economic analysis, the payback will be greatly reduced from the 42 year payback, and make this option a design choice meriting further investigation. The study also suggests a way of generating indifference plots which provide insights into how future changes in the electric rate structure would impact the payback period. The methodology adopted in this study would serve as a case study example to energy analysts evaluating TES systems as a design option for meeting increasing cooling demand and reducing costs in an existing building or campus facility.