With variable renewable energy on the rise, large-scale energy storage is needed to assure grid stability. Thermal energy storage (TES) is a new player in the large-scale energy storage landscape, but might prove a cheap, safe and reliable option, with low permitting barriers. TES is not geographically limited, generally compact and has a low levelised cost of storage (LCOS) which comes at a cost of a lower round-trip efficiency compared to other grid-scale energy storage methods. Thermal energy storage is most suited for intraday and multiday storage.
Within thermal energy storage, several types can be distinguished based on the design of the thermodynamic cycle. In this thesis, these types are analysed and compared on basis of technology, cost, and miscellaneous characteristics. An overview of cost and characteristics of current relevant thermal energy storage (TES) storage materials and TES companies is given. A Rankine-type TES, as is used in the E2S project, is generally cheaper and easier to implement, at a cost of a reduced round-trip efficiency. Rankine-type TES have a round-trip efficiency of about 40% as it is limited by the Rankine-cycle efficiency.
The TES studied in the scope of this thesis relies on evaporation of water and a steam turbine to recover power. The work of the thesis comprises of a thermodynamic and hydraulic optimization study regarding the heat transfer during conduction, convection and boiling inside the TES. To this end, a steady boiling model is constructed based on empirical and phenomenological correlations. It is found that during steady state, boiling phenomena incur higher heat transfer coefficients for low to medium wall superheat in comparison to a model that does not regard boiling phenomena. Conversely, lower heat transfer coefficients are attained for high to very high wall superheat in comparison to a model that does not regard boiling phenomena. It is hypothesised that this phenomenon will incur higher overall boiling heat transfer during transient operation.
An optimization study is conducted with the aforementioned boiling model to find the optimum combination of TES parameters for a given power and capacity, with the optimization parameter being the combined material cost of steam generator tubing and storage material. Several design guidelines are concluded. Most notably is found that pressure drop plays a very minor role in TES optimization and therefore a lower pressure drop may be chosen. Furthermore, critical parameters and bottlenecks of the TES are identified in a sensitivity analysis.
Lastly, integration of the TES in a steam cycle is evaluated. The effect of variations in pressure and temperature output of the TES are analysed, and the total round-trip efficiency and its performance during part-load operation are determined. Various qualitative changes are discussed, and a quantitative estimate is made of the sizing of a start-up TES, which may be used for cold, warm and hot start of the steam turbine. As the TES is to be implemented in a thermal power plant, an estimate is made of the sizing and cost of such an implementation using the data from the optimisation study. Various aspects of this plant are evaluated such as system response, start-up time and a techno-economical analysis of multi-day operation.