(230c) Optimal Scheduling of Pumped Thermal Energy Storage Plants for Multi-Vector Energy Provision

Industrial decarbonization has major obstacles arising from the lack of high-grade heat supply from conventional renewable sources. Additionally, renewables have inherent intermittency issues. Advanced energy storage technologies are required to solve the latter problem. In this paper, we present a method to supply high-grade heat from renewable sources coupled with a storage technology to address these two problems. The underlying technology is a pumped-thermal energy storage (PTES) system, capable of charging and discharging with electrical power and discharging high temperature heat. To accomplish this, we propose an optimization-based state-of-charge (SoC) management solution. PTES systems have been the focus of several studies recently, such as [1] investigating the off-design operation of the system or [2] investigating the various layouts and conducting a technology review. The layout and design of the PTES system in this study was informed by these recent studies and built upon by the authors.

The PTES technology in question has a charging/discharging cycle based on a supercritical CO₂ recuperative Brayton cycle (SCBC) with molten salt as hot storage and water as cold storage [3]. The underlying technology was analyzed, and a model built in previous work [4]. In that study, in addition to the model building, an in-cycle SoC management algorithm was developed. This ensured that in either the charging or the discharging cycle, the two storage media would follow the same SoC trend. This allowed for consistent operation of the system as management of only one side of the cycle was adequate. The current study builds on this previous work by the authors to extract the model data as a function of the electrical power as well as the intended heat supply rate. A SoC balancing heat exchanger and a utility heat exchanger are integrated to the base design to facilitate high temperature heat supply. With these functions, an optimization problem is formulated for the cycle-to-cycle SoC management. Cycle-to-cycle SoC management involves controlling the stored energy of the system and balancing between the charging and the discharging cycles. Inherently, SoC management is also a scheduling problem, as it requires managing power for use in a given timeframe. To do this, the system is discretized into hourly segments to which the problem assigns charging/discharging power. An electricity and heat supply profiles are used in the objective function, maximizing the operating profit of the system.

We conducted three case studies with the optimization problem. First case study was a base case scenario, without heat supply. This was used as a benchmark for the other two studies. The second study was a flexible, variable heat supply scenario and the final one was a continuous heat supply scenario in which the system delivers constant heat to a coupled process. The economic and thermodynamic results of the study are presented in Table 1 and show that the proposed approach to the PTES heat supply can consistently be optimized while keeping within the SoC constraints under various scenarios. The resulting SoC profiles are shown in Figure 1 and the charge/discharge power levels are shown in Figure 2. While the heat supply scenarios outperformed the base case both economically and thermodynamically, there is a tradeoff between heat arbitrage and constant heat supply. The results highlight the potential of integrating PTES system with industrial processes and the importance of SoC management algorithms.

References

[1] F. Frate, L. Paternostro, L. Ferrari, U. Desideri, Off-design of a pumped thermal energy storage based on closed brayton cycles, Journal of engineering for gas turbines and power (09 2021). doi:10.1115/1.4052426.

[2] J.D. McTigue, P. Farres-Antunez, C. N. Markides, A. White, Pumped thermal energy storage with liquid storage, Elsevier eBooks (01 2021). doi:10.1016/b978-0-12-819723-3.00054-8.

[3] M. Marchionni, G. Bianchi, S. A. Tassou, Review of supercritical carbon dioxide (sco2) technologies for high-grade waste heat to power conversion, SN Applied Sciences 2 (03 2020). doi:10.1007/s42452-020-2116-6.

[4] A. Albay, Z. Zhu, and M. Mercangöz, “State-Of-Charge (SOC) Management of PTES Coupled Industrial Cogeneration Systems,” in Turbo Expo 2024, ASME, Jun. 2024.