In a regenerative thermal storage system, thermal energy is transferred from a hot working fluid to the storage unit core elements during charging, and from the core elements to the cold working fluid during discharging. Regenerative thermal storage systems are used in many traditional applications, e.g. in steel-making processes. Supply of solar energy is uncontrollable due to cloud passages. The projected 10 MWe solar power plant PS10, to be built in Sanlucar la Mayor, Spain, is based on the atmospheric air circuit technology. In order to qualify for a special energy price benefit, the plant must operate with solar energy input only. For a successful operation of the plant, the energy to the steam system must be controllable. During solar transients, a regenerative thermal storage unit is to supply the hot air to the steam generator, until the solar power returns or a controlled plant shutdown is performed. New tools for the design of efficient storage units specifically for solar applications permit the definition of core elements that exhibit favourable heat transfer and pressure drop characteristics. With them, good use of the storage materials and consequently efficient storage unit designs can be achieved. This results in a drastic storage cost reductions compared to the earlier projects like PHOEBUS. (C) 2003 Elsevier Ltd. All rights reserved.