In conventional solar water heaters, the thermal energy storage is accomplished by increasing the sensible heat in a fluid. Therefore, the accumulation capacity of sensible heat is proportional to the mass storage and the increase of temperature, so that an increase in the requirements involves a bigger tank volume. Phase change materials (PCM) stored energy at constant temperature (or at least in a fairly narrow range of temperature) while the phase change is produced, they are presented as an alternative to compensate the solar heat supply periods and the thermal demand with a better heat accumulation per volume unit. In contrast, these systems require more complicated thermal analysis and designs than the traditional systems by sensible heat with a single phase. The selection of PCM, its content and location on the device will have a determining effect on the overall performance of the solar collector. This implies that the heat exchanger must be designed for each specific application.
Currently, there are no commercial devices for heating water by solar energy using thermal accumulation with PCM. However, preliminary studies in lab scale have shown significant increases in efficiencies and supply capacity. Several authors have been performed experimental and numerical studies in solar collectors including PCM technology, but, due to the complexity of the phenomena and the high consumptions of resources for both approaches, it has not been possible to evaluate different configurations that lead to optimized designs for selection, location and amount of PCM. This fact shows the need to develop simplified models that consider the main physical phenomena in the operation, in order to support the experimental and numerical techniques to determine the comprehensive thermal behavior. This kind of models can be used to estimate the performance for different configurations and boundary conditions in a fast way, to make possible in a posterior stage a detailed evaluation with numerical analysis or an experimental technique.
In this paper, a simplified comprehensive model for assessing thermal performance of a flat-plate solar collector with PCM is presented with incorporation of specialized semi-empirical correlations. The model takes into account the main thermodynamic and heat transfer processes in the device, including the internal and external convection effects, conduction, solar radiation analysis, radiation, losses and interactions between surfaces, material solid-liquid phase change and conjugated problems in gas-liquid-solid zones.
Due to the numerous existing design alternatives, consideration of an excessive number of options in the final design can lead to long development times and process inefficiencies. Therefore, a methodology of design that includes fast calculations of the main thermal parameters is highly regarded, since this can reduce the number of study cases and thus obtain optimal configurations from the simplified models.
The performance of the reduced model, including a sensibility analysis of several input data, is compared qualitatively with results obtained in a traditional collector for a typical cycle available in bibliography. Integrated simplified models are developed to perform a coarse preliminary design of flat solar collectors with incorporation of PCM technology, and thus serve as a pre-evaluator of the different configurations.
