Service life estimations in the design of a PCM based night cooling system

University dissertation from Stockholm : KTH

Abstract: The use of Phase Change Material, PCM, to change the thermal inertia of lightweight buildings is investigated in the CRAFT project C-TIDE. It is a joint project with Italian and Swedish partners, representing both industry and research. PCMs are materials where the phase change enthalpy can be used for thermal storage. The Swedish application is a night ventilation system where cold night air is used to solidify the PCM. The PCM is melted in the day with warm indoor air and thereby the indoor air is cooled. The system is intended for light weight buildings with an overproduction of heat during daytime. In the thesis, the results of experiments and numerical simulations of the application are presented. The theoretical background in order design the heat exchanger and applying the installation in thermal simulation software is presented. An extensive program is set up, in order to develop test methods and carry tests to evaluate the performance over time of the PCM. Testing procedures are set up according to ISO standards concerning service life testing. The tests are focused on the change over time of the Thermal Storage Capacity (TSC) in different temperature spans. Measurements are carried out on large samples with a water bath calorimeter. The service life estimation of a material is based on the performance of one or more critical properties over time. When the performances of these properties are below the performance requirements, the material has reached its service life. The critical properties of the PCM are evaluated by simulation of the application. The performance requirements of the material are set up according to general requirements of PCM and requirements according to building legislation. The critical properties of a PCM are the transition temperature, the melting temperature range and the TSC in the operative temperature interval. The critical property of the application is its energy efficiency.The results of the study show that the night cooling system will lower the indoor air temperature during daytime. It also shows that the tested PCM does not have a clear phase change, but an increased specific heat in the operative temperature interval. Increasing the amount of material, used in the application, can compensate this. Finally, the tested PCM is thermally stable and the service life of the product is within the range of the design lives of the building services. It is essential to for all designers to know the performance over time of the properties of PCMs. Therefore it is desirable that standardized testing methods of PCM are established and standardized classification systems of PCMs are developed.

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