Techno-Economic Planning and Exergy Analysis of Large-Scale Hot-Water Tank and Pits

Abstract

Large-scale seasonal thermal energy storage (STES) substantially facilitates a full exploitation of the local renewable energy sources (e.g. geothermal, solar, waste heat) potential in renewables-based district heating systems in order to mitigate CO2 emissions and the climate change. Large-scale seasonal TES systems store energy for lengthy timescales; therefore, it is essential to properly plan these structures in order to avoid high capital cost and/or performance below expectations. The STES planning phase includes a wide list of variables such as hydrogeological conditions (e.g. soil type, groundwater existence and/or flowing), TES geometry (e.g. tanks, conical pits, pyramid stump pits), TES construction (e.g. freestanding, partially or fully buried), system characteristics (e.g. operation temperatures), liners and insulation and others. Therefore, it is crucial to strive for an optimal TES selection in which a compromise between the technical performance and the economic investment is made. This work examines the planning of large-scale TES systems by means of numerical simulations. The models used are calibrated using measured data from the pit thermal energy storage in Dronninglund (Denmark). For the techno-economic assessment, different key performance indicators are used such as: energetic efficiency, exergetic efficiency, stratification efficiency and levelized cost of stored heat (LCOS). In this context, the investigation depicts that a hybrid TES arises as a promising option that combines the advantages of both tank and shallow pit. Accordingly, hybrid LCOS deems to be the lowest among other geometries. Further, the examination reveals that a tank has better technical performance and lower LCOS than a shallow pit under the same set of boundary conditions. Considering the transition to low-temperature district heating (DH) systems, the work further investigates the influence of DH temperature on TES techno-economic performance. Not only does the lowtemperature DH lead to an increase in TES performance but it also results in lower LCOS compared to its counterpart in a DH with high-temperature.