Thermochemical storage for long‐term low‐temperature applications: a review on current research at material and prototype scales

Thermochemical heat storage has the potential to store large amount of energy from renewables and other intermittent distributed sources, ideally without losses typical of sensible heat storage. owever, in order to have a commercially attractive system able to compete with conventional storage technologies, research at material, reactor, and ultimately at system level is still required. The aim of this work is to investigate the current state of the art research at the abovementioned scales, which can then be used to investigate the performances of long-term low-temperature thermochemical storage systems integrated in the energy grid. Regarding materials, focus is on pure salt hydrates, adsorbents, and composites for solid/gas reactions. Concerning reactors and systems, a review on existing prototypes based on solid/gas sorption reactions for low-temperature thermal storage is performed. At material level, pure salt hydrates have hydrothermal instabilities, kinetics, and corrosiveness issues. Composites are largely investigated because of their potential to overcome issues of pure salts but have various implementation problems. Amongst them, reduced mass transport within matrix pores, salt overhydration, and possible leaking of active material are still to be solved. Especially for open systems, the choice of zeolites rather than pure salts as active materials is prominent due to their better stability. However, high material costs and desorption temperatures coupled with lower energy densities decrease their commercial attractiveness. Beside research at material and reactor levels to overcome technological challenges, integration of thermochemical storage at grid level has to be investigated to assess its techno-economic feasibility based on performance and interactions with production and consumption technologies. In order to investigate a thermochemical storage incorporated in an energy grid, the most promising materials and reactors characteristics are evidenced with the aim to realize a system for that purpose, in which system reliability and cyclability will have priority over high energy density

Using representative time slices for optimization of thermal energy storage systems in low-temperature district heating systems

4 th generation district heating and cooling networks (shortly THERNETs) are often coined as a crucial technology to enable the transition towards low-carbon smart energy systems. Most importantly, they open perspectives for integration of low-grade residual heat from industry, renewable energy sources (such as geothermal heat and cold and solar thermal collectors), more efficient energy conversion units (such as collective heat pumps), while thermal energy storage (TES) systems increase system flexibility. In order to optimize design and control of such complex systems, a toolbox modesto (Multi-objective district energy systems toolbox for optimization) is under development. However, the representation of seasonal heat and cold storage systems on an annual basis requires large computational power. In an attempt to decrease computational cost, a technique with representative time slices (inspired by and combining aspects from optimization studies of electrical energy systems, unit commitment problems, thermal systems with short term energy storage and smaller scale industrial thermal systems with longer term energy storage) is developed and tested. The aim of this study is to investigate the applicability of such representative time periods to optimize seasonal TES systems in THERNETs. To this end a full year optimization is compared to one with representative time periods for a realistic case study that uses demand profiles from the city of Genk (Belgium) and energy system parameters from Marstal (Denmark). This comparative study shows that modelling with representative periods is sufficient to mimic the behaviour of a full year optimization. However, when curtailment of solar heat injection occurs, not all representations yield the same results. It was found that for the studied case, a selection of 12 representative weeks performs best, while all reduced optimizations result in a substantial reduction (speed-up of on average x4.8 to x7.7) of the calculation time

Investigation of a household-scale open sorption energy storage system based on the Zeolite 13X/water reacting pair

Sorption thermal energy storage is a promising concept for seasonal heat storage. Advantages of sorption heat storage are high energy storage density (compared to sensible and phase change heat storage) and negligible energy losses during storage over long time periods. In order to investigate the potential of sorption thermal energy storage, a high power open sorption heat storage system has been designed and built for household space heating applications. In this paper, the characteristics of the open zeolite 13X/water sorption energy storage system will be presented. The setup consists of four segments with a total capacity of 250 L of zeolite. A segmented reactor has been designed to reduce the pressure drop over the system, which results in less required fan power. This configuration also decreases the response time and makes the system scalable. Dehydration of the reactor is performed by supplying hot air to the zeolite bed. Hydration is performed by supplying humidified air to the bed. In all the segments, the pressure drop, temperature, and humidity are monitored. Furthermore, inside one of the reactor segments, the temperature is monitored at different locations in the zeolite bed. Several tests, using different mass flow rates, have been performed. During the tests, a maximum temperature step of 24 °C was realized. The maximum delivered power was 4.4 kW and the obtained storage capacity was 52 kWh. The reactor efficiency was 76% taking into consideration the conductive heat losses through the reactor wall and the sensible heat taken up by the thermal mass of the solids. Furthermore, it has been noticed that the flow through the bed was not completely uniform. This has a negative influence on the performance of the system