Two tank storage systems using molten salt represent today's state of the art in energy storage for concentrating solar power (CSP) plants. This concept shows a limited potential for further cost reductions, since the capital costs are dominated by the expenses for the salt inventory. The application of solid storage materials represents a promising approach to reduce capital costs. While this approach avoids also the risk of freezing and lessens corrosion problems, the efficiency of the heat transfer between the heat transfer fluid (HTF) and the solid storage medium is crucial. This paper introduces the CellFlux concept, which uses an intermediate closed air loop to transfer energy between the HTF and the solid storage material. A modular concept is chosen to optimize the size of the air flow channels. An initial project will provide the fundamentals needed to design a CellFlux storage unit. The feasibility will be proven by a 100 kW/500 kWh pilot storage module.