Advances in seasonal thermal energy storage for solar district heating applications: A critical review on large-scale hot-water tank and pit thermal energy storage systems

Nowadays, buildings consume a large amount of conventional energy sources in European countries and subsequently they contribute significantly to fossil fuels emissions. Therefore, many European countries have introduced several policies to minimize this consumption by transitioning buildings into more energy efficient ones, whereas some other policies focus on integrating renewables into energy systems. In this context, solar district heating is one of the promising technologies that reduces the use of fossils and, thereby, leads to fewer CO2 emissions. The main drawback of solar energy, however, is that it fluctuates on daily and seasonal basis in which the highest heat availability is in summer, while the highest demand is in winter. Hence, a seasonal thermal energy storage (STES) is required to bridge the temporal mismatch between renewable energy availability and buildings' demand. Accordingly, this study reviews briefly the different seasonal thermal energy storage technologies that are feasible for district heating applications. Then, the paper focuses chiefly on large-scale hot water TES (tanks and pits). Construction (geometry and envelope), modeling and design of these TES systems are the primary focus. Next, system performance indicators are also reviewed. A synopsis of the current TES systems is eventually presented as well. The literature review reveals: (1) Tank TES (MS) and pit TES (PTES) are less subjected to hydro-geological conditions than aquifer TES (ATES) and borehole TES (BTES), (2) TIES and PTES require high construction cost compared to ATES and BTES, (3) TTES and PTES provide higher charging/discharging power than ATES and BTES due to higher operational temperature difference and flow rates, (4) in hot water TES, as the depth decreases, the more the stratification tends to degrade and, therefore, tanks are preferable over pits, (5) no established co-simulation platform between TES envelope and surroundings coupled to energy analysis models and (6) no effective approach or measure has been found to evaluate one TES to another.