Thermodynamic analysis of stratification in thermal energy storages implemented in cogeneration systems

• Published in: Applied thermal engineering 2023-09, Vol.232, p.121015, Article 121015
• Main Authors: Thomas, B., van Schalkwyk, P.D., Engelbrecht, J.A.A., Haase, P., Maier, A., Widmann, C., Booysen, M.J.
• Format: Article
• Language: English
• Subjects: CHP unit; Cogeneration; Exergy analysis; Stratification indices; Thermal energy storage; Thermal stratification
• ISSN: 1359-4311
• DOI: 10.1016/j.applthermaleng.2023.121015

In the course of a more intensive energy generation from regenerative sources, an increased number of energy storages is required. In addition to the widespread means of storing electric energy, storing energy thermally can contribute significantly. However, limited research exists on the behaviour of thermal energy storages (TES) in practical operation. While the physical processes are well known, it is nevertheless often not possible to adequately evaluate its performance with respect to the quality of thermal stratification inside the tank, which is crucial for the thermodynamic effectiveness of the TES. The behaviour of a TES is experimentally investigated in cyclic charging and discharging operation in interaction with a cogeneration (CHP) unit at a test rig in the lab. From the measurements the quality of thermal stratification is evaluated under varying conditions using different metrics such as normalised stratification factor, modified MIX number, exergy number and exergy efficiency, which extends the state of art for CHP applications. The results show that the positioning of the temperature sensors for turning the CHP unit on and off has a significant influence on both the effective capacity of a TES and the quality of thermal stratification inside the tank. It is also revealed that the positioning of at least one of these sensors outside the storage tank, i.e. in the return line to the CHP unit, prevents deterioration of thermal stratification, thereby enhancing thermodynamic effectiveness. Furthermore, the effects of thermal load and thermal load profile on effective capacity and thermal stratification are discussed, even though these are much smaller compared to the effect of positioning the temperature sensors. •TES capacity depends on positions of temperature sensors for controlling a CHP unit.•One sensor should be placed outside of the TES to maintain proper stratification.•Variation of thermal load profile is of minor effect on thermal stratification.•MIX, ST-factor, Ex, Ex-efficiency are well suited metrics to evaluate stratification.