Exergoeconomic analysis of glycol cold thermal energy storage systems

Abstract

An exergoeconomic analysis of glycol cold thermal energy storage (CTES) is reported. Glycol CTES is an application of sensible heat storage where the temperature of a storage material changes in order to store cold energy, usually generated from electricity when its cost is low. Exergoeconomic analysis combines thermodynamic analysis based on the first and second laws with principles of economics, mostly cost accounting. Exergy analysis accounts for energy quality and irreversibilities, and provides more meaningful and useful information than energy analysis about efficiency and losses. A storage tank with a capacity of 350 000 kg is considered for this investigation and a water solution based on ethylene glycol is used as the storage medium. Several thermodynamic system factors are analysed, such as change in storage temperature, coefficient of performance (COP) of the chiller, heat losses from the storage tank, and the mass flow rates. Simulation results indicate that the system exergy efficiency is much less (∼45%) than the energy efficiency. The average exergy efficiency of the storage tank is determined to be 35% and the average energy efficiency 80%. The system exergy efficiency is determined to be 30% and 40% at 45 and 25°C ambient temperatures, respectively. The chiller COP is observed to be strongly related to storage temperature, and to vary approximately between 2.4 and 5.8 at a 35°C ambient temperature. As ambient temperature decreases, COP increases. The exergoeconomic analysis indicates that the ratio of exergy-based thermodynamic loss to capital cost of the glycol CTES ranges from 0.00233 to 0.00225 kW $−1 at a 35°C reference environment temperature, and from 0.00235 to 0.00227 kW $−1 at a 25°C reference environment temperature. The reference environment temperature affects significantly exergy destruction and efficiency, e.g. a 10°C change in ambient temperature causes a 37.5% change in exergy efficiency. This result implies that cold energy is more valuable at higher ambient temperatures. Heat loss from the storage tank exhibits a mild dependence on ambient temperature, e.g. a 10°C increase in ambient temperature causes a heat loss increase of 7.1%. Copyright © 2007 John Wiley & Sons, Ltd.