Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.The energy storage units (ESU) described in this article are to be attached to the cold finger of a cryocooler with the objective of holding the low temperature environment constant while the cooler may be temporarily stopped to provide a totally vibration-free environment. Or, it may be used to damp out temperature fluctuations if a sudden cooling power increase is momentarily necessary. The developed ESU consists of a nitrogen cell coupled to a GM cryocooler by a gas-gap heat switch, and connected to an expansion volume at room temperature to limit the pressure increase. It was designed to store ≈3600 J between ≈65 K and ≈80 K. After condensing the nitrogen into the liquid phase, the heat switch is used to decouple the cell from the cryocooler, and a constant heating power is applied. During the liquid evaporation, the temperature drift obtained is very slow. In this paper, we present the tests performed using a 35 cm³ cell and an expansion volume of 6 litres or 24 litres. Applying 1 W to the ESU, about 4 kJ were stored with a slow drift from 76 K up to 80 K using the 24 litres expansion volume. Modelling of the experiment agrees within 5% with the experimental results. Software, written for sizing such an ESU, includes parameters for the ESU's stored energy, the cell and expansion volumes, and the cryogenic fluid used. An ESU using the liquid-gas latent heat leads to a slow temperature drift, while a triple-point cell keeps the temperature strictly constant. However, such an ESU stores a thermal energy one order of magnitude larger than a triple-point one for the same low temperature cell volume. Preliminary results for a gravity insensitive ESU are presented
