This article presents a technical innovation consisting of replacing the piston-cylinder system by metallic bellows in free-displacer Stirling engines. It deals with the advantage of heat engines of low power, integrating this innovation with plane solar collectors. A theoretical study requiring notions of finite time thermodynamics determines the optimised dimensioning of such systems and makes them appear serious competitors, on the one hand to solar cells to exploit solar energy and, on the other hand, to power stations regarding kW h unit cost. (C) 2004 Elsevier Ltd. All rights reserved.