Steam Storage for Flexible Biomass Power Generation
De Montfort University
Due to the widespread installation of renewable energy plants, with the aim of a decarbonised electricity supply, the proportion of this generation has increased significantly in recent years. However, these plants increase the variability of electricity generation. This variable power generation leads to challenges in the operation of the power grid, e.g. to local grid bottlenecks and grid overloads. To counteract this variable, and to a certain extent, unpredictable power generation, storage devices or flexible power plants are necessary. Several biogas plants have already been modified in such a way that demand-driven and flexible power plant operation is possible. In response to the need for flexibility, the presented research aims at flexible power generation for solid biomass-fuelled CHP plants. By integrating a steam storage device into the biomass CHP plant, the steam turbines should be enabled to operate with a flexible, demand-dependent steam mass flow in order to adapt their power output to the grid demand. The main research questions are, which storage system is most suitable, what the key parameters of the flexible plant are and what impact it has on the grid and markets. The storage system as well as utilization in biomass CHP plants are novel concepts. Especially the operation parameter for the specific boundaries of these plant technologies have not been investigated before. A combination of two or more technologies to separately store the latent and sensible energy of superheated steam was identified as necessary. A utility analysis, supported by a Delphi study with experts, was carried out. In this case, a steam accumulator (SA) combined with a solid thermal store (STS) has proven to be the most suitable storage device for the given requirements. A MATLAB/SIMULINK model for the flexible biomass CHP plant was developed and validated to investigate the proposed system. Parameter studies were conducted to determine the key values of the flexible plant, such as energy capacity, charge/discharge time and efficiency. A storage configuration consisting of a 100 m³ SA and 12.3 m³ STS is capable of reducing the electricity production by 3.5–3.7 MWh during charging. During discharge, an additional amount of 1.8–1.9 MWh is generated. A system efficiency of 76–92 % was achieved. Flexible operation depending on prices in the short-term electricity markets causes a reduction of between 0.5% and 2% of the total revenue of the plant of due to process losses related to the operating the storage facility. The feed-in tariff structure has had a significant impact on this revenue shortfall. The flexible operation allows a temporary peak reduction of 28% (from 19.2 MW to 13.7 MW). Compared to competing technologies such as pumped hydro, batteries or hydrogen storage systems, the proposed flexible biomass CHP plant system is competitive. It was shown that an operation similar to flexible biogas plants is possible. The operation of already existing flexible biogas plants can also be improved by using the proposed solution.