From the point of economic, environmental and social view our current trends in energy supply and use are unsustainable. Our current energy and climate direction must be changed. Renewable technologies will play crucial role in this change. In order to address these challenges International energy agency (lEA) is developing number of roadmaps for different technologies that can have effects in C02 reduction. There are also roadmaps for renewable technologies. One of them is: "Roadmap for Energy Efficient Heating and Cooling" according to which 20% of increasing Coefficient of Performance (COP) in renewable technologies is planned by the year 2020 and global target in reducing carbon dioxide (C02) for 50% from current levels till 2050. (lEA, 2010) Phase change material (PCM) storages can represent a real challenge to design. One of the challenges is choosing a PCM that is compatible with the system that we want to optimize, and the another is choosing the exact heat exchanger that will give optimal surface area during charging and discharging, due to the fact that PCMs have low thermal conductivity. Clearly, water can be used as a storage medium, but it does not give us the freedom to maintain the system at the desired temperature. In order to gain maximum thermal power from their potential, proper geometrical design needs to be used for each kind of PCM thermal energy storage (TES). The Nusselt equations for heat exchangers used for air and water cannot be applied to PCMs. Extensive research therefore needs to be undertaken in order to investigate the different geometrical configurations of heat exchangers and their interactions with different kinds of PCMs. This thesis summarizes the results from 3 different types of experiments. The first type was with a single tube in water and PCM storage. The second type of experiment was with a multi-tube heat exchanger in PCM-TES. The third type of experiments was with a heat pump system coupled to water/PCM TES. In total 66 tests were performed: 6 experiments with single tube, 42 experiments with multi-tube and 18 experiments with heat pump system. The main contribution presented in this thesis was novel Nusselt number correlations for multi-tube heat exchanger, as heat transfer fluid refrigerant was used and as a storage medium PCM was used.