Because human population is growing at such a high rate, as well as the energy consumption per person, new ways of preserving and efficiently using available energy must be explored. Until new and abundant energy sources are found and means of their exploitation developed, research attention should be focused on finding ways of proper and safe use of what is nowadays available. In that sense, storage of thermal energy plays an important role both in heating and/or cooling applications, such as in residential or commercial buildings, and in industrial processes. Some of the technical solutions are based on phase change materials (PCM) which can help to preserve and to increase the efficiency of energy use when used at right temperature levels at which PCMs change their phase. Thus in just a few degrees of temperature difference, a large quantity of energy can be stored. PCMs might find their application in fixing the energy storage problems in different fields, for they represent a kind of thermal battery. PCM may allow keeping the temperature of the room stable because of their high density of energy storage. PCMs can help keeping the temperature level of a water tank at a certain point. The successful usage of PCMs is not only a question of energy storage density, but on the other hand it is a question of proper charge and discharge of the energy stored with power suitable for the desired application . The aim of this work is to design and analyze up to three different systems for thermal energy storage based on application of PCMs and, where applicable, examine their performance in comparison with water used as a storage fluid. In order to do this simulation program TRNSYS is used. TRNSYS model TYPE 840 is used for representation of water storage tank with PCM material included. Model TYPE 840 is validated in the work of other authors . Different experiments are simulated in order to investigate feasibility of the use of PCM modules in these systems. The following applications of thermal storage are considered in this work: • a tank with an electric heater, • a tank in a solar thermal application, and• a simple heat exchanger with a PCM module inside, that might be used as a portable thermal accumulator (battery). For the first and second system, simulation in TRNSYS is done to evaluate the features of PCM incorporation in a water tank and for the third system, a simulation is also done to show the characteristics of the heat exchanger designed.