The following graduation thesis "A change of state: A thermodynamic and cost-effective optimized Trombe wall based on latent heat storage (LHS) for year round application” aims to investigate and optimize the thermal energy performance of a Phase change material (PCM) Trombe wall to create an economically feasible product. This passive system reduces the total cost of ownership of the energy system, inside an office building located in The Netherlands, by reducing the energy demand and the maximum peak-loads from the mechanical system. In the European Union (EU) buildings are responsible for almost 40% of the total energy consumption, the energy efficiency of these buildings needs to increase by at least 32,5% by 2030 according to the Sustainable development goals (SDGs) from the EU. Research has shown that the information on the economic feasibility and optimization of PCM within the built environment is somewhat limited, this research will give an in-depth insight in the actual performance on thermodynamics and the cost-effectiveness. The PCM Trombe wall will be optimized by means of the Research through design - method, using different design strategies based on the knowledge of the thermodynamic principles of the PCM. An initial simulation model is used for the energy performance calculations, this model is extended and developed according to the parameters defined for the optimization. A combination of MATLAB and Simulink, a simulation environment based on textual and graphical programming, together with modeFRONTIER is employed. The results from these simulations are first validated with DesignBuilder Software Ltd to verify the legitimacy of the results from the added components and the changed location input data. The actual performance of the PCM on the reduction of the heating and cooling energy demand, the maximum power-load and the investment cost for the product are determined according to dynamic set-point calculations. A detailed study together with an yearly performance analysis is used, the results from all the different simulations and optimizations are summarized in a design guideline. This guideline gives a clear indication on the differences between the input parameters and the results from the performance of the system. In the end, these results are all brought together in an adaptive and integrated design solution for the application in an office building in the Netherlands.