Stratified Low-Temperature Fluid Thermal Energy Storage (TES) in a Major Convention District— Aging Gracefully, as Fine Wine

摘要

A 10-year-old thermal energy storage (TES) installation continues to perform efficiently and economically at the heart of a district energy (DE)/combined heat and power (CHP) system in Chicago, Illinois. The thermal end-users include a major conference/exposition facility, a high-rise luxury hotel, and nearby public and private building facilities. The TES system is one of the largest in the world with a TES capacity of 123,000 ton-hours and a peak design discharge rate of 25,000 tons. The system employs a single welded-steel, above-ground storage tank with stratified low-temperature fluid as the storage medium. The TES system has exhibited the flexibility to continually evolve over time with the growing DE system. Initially, the TES system operated at conventional supply and return temperatures, in conjunction with existing CFC-based centrifugal chillers. As the DE system grew, the TES system was operated at an expanded ΔT of 24℉ (13.3℃) (30℉ [-1.1℃] supply and 54℉ [12.2℃] return) in conjunction with ammonia-based screw chillers and single-stage steam absorption chillers. The various system customers accept supply temperatures to suit their specific needs, from 30℉-34℉ (-1.1℃-1.1℃)for use with low-temperature air distribution systems to 42℉ (5.6℃)for conventional existing systems. Peak contract cooling load has already grown in several phases, from 8,000 tons in 1994 to well over 20,000 tons in 2000, with the potential for much higher load in the future. Initially operating in a full shift mode, as the DE system expands, the TES can be employed in a partial shift mode on peak days while remaining in a full shift mode during low-load days. The TES system has accommodated various electric utility rate structures and enabled the economic deployment of cogeneration. The TES system had very low initial capital cost and has provided dramatic reductions in operating energy costs. The TES system was initially specified and designed for an operating life of at least 29 years without any scheduled maintenance requiring draining of the tank. To date, the TES system has required near-zero maintenance. Meanwhile, the use of the low-temperature fluid has eliminated the need for conventional ongoing water treatment for corrosion inhibition and microbiological control. The DE developer has subsequently employed similar large, stratified TES installations at several additional DE developments around the country. Those installations include one that entered initial operation in 2003 rated at 160,000 ton-hours in chilled water service (with pre-designed expandability to 250,000 ton-hours when ultimately converted to low-temperature fluid service).