Experimental and numerical analysis of a cement based thermal energy storage system with a helical heat exchanger

This study presents a combined experimental and numerical investigation of the performance of a new modular sensible heat storage system with a cement based, water saturated, porous storage matrix and a helical tubular heat exchanger during cyclic storage operation. To characterize the storage system with respect to achievable storage rates and storage capacity, two sets of dedicated and highly controlled dynamic charging-discharging cycles were experimentally conducted using a one cubic meter lab-scale storage unit prototype. A prognostic process based and high resolution 3D finite element model for this storage unit was developed, tested and validated by comparison to the experimental data. The overall model agreement with the experimental data is excellent, also for extended cyclic storage operations using up to nine charging-discharging cycles, with root mean square errors of temperatures within the storage unit smaller than 1.3 degrees C, heat balances within 3% of the experimental value and an average Nash Sutcliffe model efficiency index as high as 0.993. The numerical model could thus be used for application specific, simulation based storage design and dimensioning by simulating storage performance for modified heat exchanger geometries, component materials and operational boundary conditions. The simulation results indicate, that the heat transfer rate of the laboratory prototype can be increased by up to 150% for short-term (i.e. <1 h) and up to 90% for mid-term (<6 h) charging durations, respectively, by employing an elevated charging temperature, increasing the thermal conductivity of the storage medium and heat exchanger pipe, and decreased heat exchanger coil pitch height. The corresponding achievable short- and mid-term charging capacities can thus be improved by about 170 and 130%.