A cryogen-free cold source for temperature below 6 K without mechanical, thermal and electromagnetic perturbations would be welcome in many sensitive applications. This article describes such a device (Energy Storage Unit-ESU) built to store 36J between 3 K and 6K. This ESU consists of a solid state enthalpy reservoir connected to a cryocooler by a heat switch. Its different parts as well as the experimental results are presented. The choice of Gd2O2S (GOS) as high specific heat solid material for the enthalpy reservoir is discussed. Tests in different conditions were performed. A very good agreement was found between the experimental data and those predicted using the heat switch characteristics and the specific heat measurements of the GOS used in this experimental set-up. A stable 6 K temperature was maintained during more than 50 min in a completely silent environment A semi-continuous operation for this cold source was successfully tested during 2.5 h. (C) 2009 Elsevier Ltd. All rights reserved.