The increasing number of fluctuating renewable power producers in the electricity grid leads to several challenges in the grid's infrastructure and its operation. Biomass combustion plants, however, can be modified to increase the flexibility of power production by integrating steam storage devices. In this paper, the available storage systems are evaluated, considering the boundaries of the plants and the requirements of power grids and markets. An objective result is generated by conducting a Delphi study using energy experts. The utility-value-analysis method is used to identify the most promising storage concept. Using this methodology, the resulting storage concept is found to be a combination of a steam accumulator and a solid storage.