In view of rising energy prices and an increasing share of power generated by renewable energy sources, the importance of energy storage is growing. In the framework of this project, a thermal energy storage concept for solar power towers is being developed, in which quartz sand serves as a storage medium. Sand is suitable due to its properties such as high thermal stability, specific heat capacity, and low-cost availability. Compared with storages based on ceramic bodies, the use of sand promises to reduce costs of energy storage and thus to reduce the costs of electricity generation. In addition, the storage concept could be applicable in the steel industry. The central element of the storage concept is an air-sand heat exchanger, which is presently under development. This paper describes simulation results and measurements of the heat exchanger prototype. It includes sand flow behavior and experience with different porous walls as well as up-scaling options.