Optimisation of opaque building envelope components with Phase Change Materials

The objective of the present thesis is to provide a methodological approach for the design of responsive building envelope components through the application of optimisation analyses. In detail, this approach was applied to opaque building envelope components with Phase Change Materials (PCMs). Since multi-objective optimisation problems generally result in a series of trade-off solutions called Pareto-front, the main focus was to investigate which values assumed by the optimisation variables led to the optimal set of solutions. In this way, the optimisation analysis was used as a tool to gain knowledge on specific problems. After an overview on PCMs and on the application of optimisation analyses to the building envelope for improving the energy efficiency of buildings, three levels of analysis were explored; material level, component level and building level. At the material level, the optimisation approach was applied to estimate the temperature-dependent specific heat curve of PCMs through best-fit of experimental data. Given the measured surface temperatures of a sample as boundary conditions and the known thermo-physical properties of the materials to a numerical model, the curve which minimised the difference between measured and simulated heat fluxes on both faces of the sample was found. At the component level, “equivalent” parameters for the dynamic thermal characterisation of opaque building envelope components with PCM were proposed. Starting from the definition of the traditional dynamic thermal properties according to ISO 13786:2007, a monthly equivalent periodic thermal transmittance and the corresponding time shift were defined by imposing steady-periodic conditions with monthly average external air temperature and solar irradiance profiles while keeping a constant air temperature on the internal side. Then, the monthly equivalent values were synthesised in a unique yearly value by means of a simple average. A parametric model was subsequently developed to describe PCM-enhanced multi-layer walls with simultaneous use of at most two PCMs, and an optimisation analysis was carried out for three locations (Palermo, Torino and Oslo) to find wall layout and PCMs' thermo-physical properties (melting temperature, melting temperature range, latent heat of fusion and thermal conductivity) which minimise yearly equivalent periodic thermal transmittance, overall PCM thickness and thickness of the wall. At the building level, the investigations focused on the application of optimisation analyses for the energy retrofit of office buildings. Three retrofit options on the opaque envelope components were considered in the aforementioned locations; intervention either on the external side of the wall, on the internal side of the wall, or on both sides of the wall. Moreover, either the same retrofit solution for all the walls or a different wall solution for each orientation were considered. In both cases, a maximum of two PCM materials could be selected by the optimisation algorithm. With regard to the objective functions, the problem was faced under two points of view. On one side, optimisations were run with three objectives to minimise the building energy need for heating, cooling and the investment cost. On the other side, the optimisations were performed with two objectives to minimise primary energy consumption and global cost. Only for the climate of Oslo, where heating is mostly electric and no cooling system was adopted, the minimisation objectives were primary energy consumption, global cost and thermal discomfort. Even though a proper optimisation of the thermo-physical properties of PCMs was found to be especially advisable when the operation of the HVAC system implies a non-trivial solution, the results of these analyses allowed to propose a few design guidelines for PCM selection and application. However, for the analysed case studies, PCM prices need to be reduced in order to become a cost-effective retrofit option.