When considering the deployment of renewable energy sources in systems, the challenge of their utilization comes from their time instability when a mismatch between production and demand occurs. With the integration of thermal storages into systems that utilize renewable energy sources, such mismatch can be evened out. The use of phase-change materials (PCMs) as thermal storage has a theoretical advantage over the sensible one because of their high latent heat that is released or accumulated during the phase-change process. Therefore, the present paper is a review of latent thermal storages in hydronic systems for heating, cooling and domestic hot water in buildings. The work aims to offer an overview on applications of latent thermal storages coupled with heat pumps and solar collectors. The review shows that phase-change materials improve the release of heat from thermal storage and can supply heat or cold at a desired temperature level for longer time periods. The PCM review ends with the results from one of the Horizon2020 research projects, where indirect electrical storage in the form of thermal storage is considered. The review is a technological outline of the current state-of-the-art technology that could serve as a knowledge base for the practical implementation of latent thermal storages. The paper ends with an overview of energy storage maturity and the objectives from different roadmaps of European bodies.

One of the great challenges in the energy sector represents retrofit of residential buildings where 3/4 of buildings in Europe are residential. To reduce energy consumption and increase the use of renewables in existing residential buildings a holistic approach of retrofit with interconnected technological system is needed. In the present paper energy toolkit based on the synergetic interaction between technologies integrated in the system for holistic retrofit of residential buildings which is under development within HEART project (HORIZON 2020) is presented. In this project step towards self-sufficient heating and cooling of building is made with an increase in on-site consumption of self-produced energy in PV from solar energy, where produced electrical energy is used also for heat pump operation. In this case thermal energy storage plays an important role for storing heat or cold for time when solar energy is not available. Improvement of sensible thermal energy storage with implemented cylindrical modules at the top of the heat storage tank and filled with phase change material is investigated experimentally. 43 litres of paraffin with phase change temperature between 27 °C and 29 °C was used in a system, what represented 15 % of total volume of heat storage tank. The results from experiment shows that thermal energy storage unit with integrated modules filled with phase change material can supply desired level of water temperature for twice as long at smaller temperature level as sensible thermal energy storage what is the consequence of higher energy density that can be stored during phase change. The advantage of phase change materials is in thermal energy storage for applications that needs narrow temperature range of supplying and storing thermal energy what is the subject matter of consideration in the case of HEART project.