The amount of heat that can be stored by common heat transfer fluids, e.g. water or oils, depends on the useable temperature range which can be exploited. The efficiency of such heat transfer fluids is poor if an application allows only a small temperature difference. One solution to increase the efficiency of such applications is to use phase-change materials, the so-called PCMs. The energy that can be stored at the phase transition of some materials, e.g. from solid to liquid, is very high. Paraffin is a PCM that can be pumped through pipes when it is micro-encapsulated and dispersed in a carrier fluid, e.g. water. These mixtures of PCMs and carrier fluids are called phase-change slurries (PCSs). We are working on PCSs within a project which is supported by the European Commission. One of the aims of the project is to develop micro-encapsuled PCSs that are stable enough to withstand the harsh conditions in piping system including various commonly used components. To do the necessary experiments, a test rig was built to perform thermal cycling of different PCSs. The stability of the micro-capsules was also tested by exposing them to high shear stresses, that normally occur mainly in pumps. The compatibility of different components when used together with these PCSs was also examined. After several weeks of pumping, scanning electron microscopy (SEM) was used to control the state of the capsules visually. The thermal behaviour of the PCS was measured while it was pumped through heat exchangers. Also, a simulation tool was extended to compute thermal systems operating with these PCSs and to evaluate their potential in different applications. (C) 2005 Elsevier B.V. All rights reserved.