INTEGRATION OF A THERMAL ENERGY STORAGE MODEL WITHIN ENERGYPLUS

A module for ice-based thermal energy storage (TES) systems has been developed and integrated within EnergyPlus. The TES module uses BLAST models for two direct ice systems (ice-on-coil external melt and ice harvester) and one indirect ice systems (iceon-coil internal melt). The TES systems are integrated as part of the EnergyPlus cooling plant components and are able to operate for any charge/discharge rates provided as input data. In this paper, the structure of the TES module as implemented in the EnergyPlus is described. In addition, typical input-output variables from the added TES module are illustrated. Moreover, the operation of the TES systems is discussed for various conventional control strategies. INTRODUCTION Thermal energy storage (TES) is an electrical load management and building equipment utilization strategy, which can reduce utility electricity demand and equipment first-costs. Indeed, TES systems have been utilized as a demand-side management (DSM) strategy by several utilities to shift electricity use associated with cooling from on-peak periods to offpeak periods. For building managers and owners, TES systems are designed to avoid high utility demand and energy charges from cooling during onpeak periods associated with time-of-use (TOU) rates or real-time pricing (RTP) rates. In addition, TES systems have been promoted as a means to reduce installed chiller capacity. Typical applications of TES systems include medium-size to large office buildings, hotels, and retail stores. The main obstacle that hinders a wider acceptance of TES systems is the lack of understanding among HVAC designers and facility operators of the proper operation and control that improve the costeffectiveness of TES systems (Akbari and Sezgen, 1992 and Guven and Flynn, 1992). Several studies have proposed improved optimal control strategies for TES systems (Drees and Braun, 1996, Henze et al., 1997, and Gibson, 1997). However, almost all of these studies are based on either simplified TES models or sole analysis of the cooling plant without considering the impact of the entire building operating and design conditions such as building thermal mass and internal gains effects. In this paper, realistic models for TES systems are integrated within the state-of-the-art whole-building simulation program, EnergyPlus, to allow for future analysis of the performance of TES systems under various control strategies and design options. The TES model is based on a steady-state plant model developed by King and Potter (1998) using algorithms adapted from the building load and system thermodynamics (BLAST) energy simulation program (BLAST, 1995). The model was designed to meet building cooling load directly and was used in evaluating optimal control of ice thermal energy storage systems (Henze et al. 1997). The TES model was developed as a packaged unit system containing zone fan-coil unit, chiller, pump, and cooling tower. Unfortunately, the model cannot be used in EnergyPlus directly due to the optimal control methodology employed and the fan-coil unit system which is already contained in EnergyPlus. The new TES plant module, presented in this paper, is developed to work as an integral part of EnergyPlus plant equipment and to accommodate the entire continuum of charge/discharge rates given as user input data. This paper describes the structure of the TES plant module as integrated within EnergyPlus. In addition, typical input-output variables from the added TES module are illustrated. Finally, the operation of the TES systems is evaluated for conventional control strategies including chiller-priority and storagepriority using a small office building. TES MODEL DESCRIPTION As in BLAST, TES systems are modeled as heat exchangers with the charging/discharging rates as functions of the state-of-charge and the log-mean temperature difference between the ice and brine side. Eighth International IBPSA Conference Eindhoven, Netherlands August 11-14, 2003