Thermal energy storage systems for low-grade heat applications: Design and experimental testing of lab-scale prototypes

The main objective of this thesis is the development of prototypes of thermal energy storages suitable for coupling with low-grade waste heat (e.g. non-concentrating solar, industrial process heat) and their experimental testing. Benefits of thermal energy storages are several, but the experience in non-sensible heat storage is still limited, especially in the design of prototypes. High temperature heat storage (T >150°C) has been the subject of a quite extensive research, but low-grade heat sources are still not fully exploited, due to the competition with water, that is available at a risible cost. In the present thesis, two different technologies were investigated, latent heat and adsorption heat, by design and experimental testing of lab-scale storages. In particular, data from experimental testing on Phase Change Materials carried out at CNR-ITAE were used for the development of thermal energy storages using latent heat technology (with phase change materials). Since only limited data on PCM-based devices in the investigated temperature range were available, two approaches were followed: a custom fin-and-tube heat exchanger and a commercial plate heat exchanger were tested with the same PCM (a paraffin) and the results used for a design analysis. In order to complete the analysis, a simplified numerical model was developed through the commercial software COMSOL Multiphysics and validated against experimental results. The model was able to describe the behaviour of the fin-and-tube system with low computational effort, showing good possibility for a future design optimization and easy adaptability to different configurations. Measurement on adsorption equilibrium curves available for adsorbent materials, instead, were used as the input for the development of a thermal energy storage making use of adsorption technology. While the storage was designed to use the same heat sources as the latent thermal ones, different operating conditions on the user-side were considered, taking into account both cold or hot storage possibilities. The experimental measurements on both the technologies highlighted the good potential of the investigated systems and therefore that further research in the specific temperature range analysed is feasible and will allow overcoming the limitations that still exist. However, the intense research activity that is on-going in the field of thermal energy storage cannot preclude from a standardization, both in the definition of relevant indicators and the assessment of the systems. To this aim, an attempt has been made at comparing the developed storages (2 latent heat storages and 1 adsorption storage), by defining common performance indicators and evaluating whether they can be applied to such different cases, in terms of characteristics, sizes and application. Results obtained have shown that both technologies allow reaching a higher energy storage density than water, under all the examined conditions (i.e. charging temperature of 75°C to 90°C), with values up to 900 kJ/kg in the case of the adsorption heat storage. The operating parameters affecting storage operation were analysed as well: indeed, it was found that the performance of the storages is strongly dependent not only on the heat sources and external ambient conditions, but also on the control of the system (i.e. the flow rate imposed, the part load operation) and the construction features, such as the material used for the shells or the insulation. Finally, the methodology suggested for the evaluation of the storage could be successfully applied to all the systems, allowing a qualitative and quantitative comparison. The main outcomes of the work here reported can then lead the path towards the optimization of the heat storage systems, from lab-scale to pre-commercial ones, thus increasing the technology readiness level and making a step forward towards practical application.