Isobaric storage of compressed air: Introduction of a novel concept based on phase change materials and pressure equalizing modules

Abstract

In industrial compressed air systems (CAS), compressed air receivers have a significant impact on the overall energy efficiency. Isobaric compressed air receivers have the potential to increase the amount of stored air significantly and, hence, the energy efficiency can be improved compared to the usage of conventional isochoric compressed air receivers. In this work, we propose a novel concept of an isobaric compressed air receiver based on pressure equalizing modules (PEQM). These PEQM are filled with a condensable gas – in this work called phase change material (PCMl/g) – and are placed inside the receiver. During the charging and discharging of a compressed air receiver equipped with PEQM, the PCMl/g condenses or evaporates, respectively. Due to the large volume change during the phase change of the PCMl/g air can be stored isobarically. However, a proper selection of suitable PCMl/g is crucial for efficient and safe use. Therefore, we introduce a three-step selection process based on an adapted digital-logic method. In this selection process, qualitative (e.g. flammability) and quantitative (e.g., heat of condensation) properties are considered. As a result, R-1234ze(E) is selected as PCMl/g which owns a vapor pressure of 800 kPa at a temperature of 314.7 K. First experiments with the novel PEQM filled with R-1234ze(E) have been conducted in a lab-scale test plant and the results are shown in this work. The storage capacity of the test plant has been increased by 37.9 %. Additionally, the phase change of the PCMl/g during charging and discharging has been observed. However, the phase change is not isobaric yet. Temperature changes during the phase change inside the PEQM hinder the isobaric storing of the working gas. Therefore, proper heat management inside the PEQM is necessary, for example, by using a second phase change material for heat storage.