(554e) Modeling and Optimization of High-Temperature Thermal Storage (HTTS) Integrated with Fossil-Fueled Power Plants

Increasing integration of renewable energy sources into the electric grid is causing high variability and uncertainty in energy supply, which, in turn, is forcing many fossil fuel-based conventional power plants to ramp their load rapidly to ensure the stability of the power grid. The complex dynamics and the long time delay of the energy transfer from the fuel to the working steam in thermal power plant often result in significant loss of energy. Frequent cycling also leads to a decrease in the overall efficiency and an increase in the failure rates of high-temperature units in the long term [1]. To mitigate these challenges, high temperature thermal energy storage (HTTS) systems integrated with the thermal plant have high potential in improving the efficiency of the power plant and decreasing its ramp rate. Furthermore, such integration can synergistically exploit the existing equipment items in the thermal power plants for power production thus greatly reducing the capital cost compared to the centralized HTTS systems. HTTS systems also have long lifetime and high efficiency.

While the concept of thermal storage has been widely applied for buildings, chemical processes and solar plants in the past [2-5], integrating HTTS with a large-scale fossil-fueled power plant has not been addressed rigorously. Several studies have focused on the impact of thermal storage charging and discharging strategies on the power plant [6-7]. However, exergy loss, efficiency and the cost of HTTS systems depend on the design and integration of generation and storage systems. The choice of storage medium, charging, and discharging strategy also vary with different energy inputs. Here we propose a HTTS system that synergistically extract steams at different pressure and temperature levels from the steam turbine and recover energy from the storage systems at different exergy levels in order to minimize the exergy loss due to the storage. Specifically, we have developed a “superstructure” configuration of the HTTS system, which allows the system to adapt different charging, discharging and integration strategies. Both phase change materials (PCMs) and molten salts are considered as storage medium in the form of latent and sensible heat, respectively. While the PCM chambers are the primary storage media, the molten salt tank is incorporated into the superstructure to investigate the most cost-effective design with highest efficiency and exergy rate. We also consider both direct and indirect charging. Heat transfer directly from steam to storage medium reduces the energy loss and the cost of the heat exchangers. On the other hand, the use of a heat transfer fluid allows the storage system to operate more flexibly. Based on the superstructure, we develop a mixed-integer nonlinear programming (MINLP)-based optimization framework to determine the most promising charging/discharging strategies and HTTS designs for a given power plant considering various tradeoffs between ramp rates, life span and capital costs, and load balances. Through a case study, we illustrate how the proposed modeling and optimization framework provides the target HTTS configurations, capacities and operational decisions that contribute to reducing the overall cost of energy storage for a conventional power plant.

References:

[1]Kumar, N., Besuner, P., Lefton, S., Agan, D., & Hilleman, D. (2012). Power Plant Cycling Costs. doi: 10.2172/1046269

[2]Edwards, J., Bindra, H., & Sabharwall, P. (2016). Exergy analysis of thermal energy storage options with nuclear power plants. Annals of Nuclear Energy, 96, 104–111. doi: 10.1016/j.anucene.2016.06.005

[3]Richter, M., Möllenbruck, F., Starinsk, A., Oeljeklaus, G., & Görner, K. (2015). Flexibilization of Coal-fired Power Plants by Dynamic Simulation. Proceedings of the 11th International Modelica Conference, Versailles, France, September 21-23, 2015. doi: 10.3384/ecp15118715

[4]Miró, L., Gasia, J., & Cabeza, L. F. (2016). Thermal energy storage (TES) for industrial waste heat (IWH) recovery: A review. Applied Energy, 179, 284–301. doi: 10.1016/j.apenergy.2016.06.14

[5]Zhu, Y., Zhai, R., Peng, H., & Yang, Y. (2016). Exergy destruction analysis of solar tower aided coal-fired power generation system using exergy and advanced exergetic methods. Applied Thermal Engineering, 108, 339–346. doi: 10.1016/j.applthermaleng.2016.07.116

[6]Wojcik, J., & Wang, J. (2017). Technical Feasibility Study of Thermal Energy Storage Integration into the Conventional Power Plant Cycle. Energies, 10(2), 205. doi: 10.3390/en10020205

[7]Li, D., Zhang, W., & Wang, J. (2019). Flexible Operation of Supercritical Power Plant via Integration of Thermal Energy Storage. Power Plants in the Industry. doi: 10.5772/intechopen.79735