Energy storage is required for the mass implementation of renewable energy sources into our energy mix. Smart latent thermal energy storage (LTES) coupled to heat pumps has been identified by the European commission as a key enabling technology. Due to the high potential of LTES systems, a large amount of research has been performed on storage materials, heat transfer enhancement and implementation of LTES in specific applications. However, this large volume of research has not lead to design methods since a fundamental understanding of the operation of transient heat exchangers is lacking. The present proposal describes sensible and latent TES systems based on a decomposition in energy fractions and a novel integration method of the heat transfer behavior of a representative unit volume. The result of this approach is a unified theory for testing and designing both LTES systems and phase change problems under constant boundary conditions. The knowledge developed in this project will allow researchers to define design models for LTES heat exchangers, to compare LTES heat exchanger geometries and to implement LTES systems in software for energy system simulation for techno-economic and life cycle optimization. By enabling the implementation of LTES heat exchangers in the energy systems underpinning our society, the results of the project will be a major step forward toward a sustainable future.