The influence of incoming water temperature and solar radiation in flat-plate solar collectors thermal energy storage systems equipped with metal foams, nanoparticles and/or wavy walls

Latent heat storage may greatly minimize the issue of solar energy intermittent and discontinuous supply since they allow to store large amount of thermal energy without any temperature gradients. In this category, the most popular solar thermal application of a latent thermal energy storage system, often equipped with PCMs, is the flat-plate solar collectors (FPSC). In this application, it is important to underline that, due to the low thermal conductivity of most PCMs, improving thermal conductivity is a significant topic to enhance storage capabilities. As a result, porous metal foams (PMF), geometry modifications like finned surfaces, and nanoparticles (NPs), have been commonly used as thermal conductivity enhancers of PCM-based energy storage systems. The primary objective of this study is to develop a highly-efficient thermal energy storage system for a solar collector for use in residential areas and industrial applications, where particular emphasis is given to the effects of both working fluid inlet temperature and incoming heat flux. To model the PCM melting, an enthalpy-porosity approach is employed, where governing equations are numerically solved with a finite volume approach. This model incorporates nanoparticles, when these are employed, and the PCM in a single-phase equivalent medium, with water serving as the working HTF. When the porous foam is included, a two-energy equations local thermal non equilibrium model is employed. Besides, one could also consider here for the sake of comparison a mix of all these solutions by utilizing wavy surfaces too as extended heat transfer surfaces, in combination with the aforementioned PMFs, and NPs. Results here shown suggest that with adding nanoparticles (NPs) and metal foams (MF), the heat transfer rate augments and the melting time reduces, where the wavy wall helps in reducing the melting time. The results show that, if one constraints the HTF temperature at 90 °C and the incoming solar radiation with a value of 900 W/m2, with addition of NPs with a volume fraction of 5%, the melting time is reduced by 14.037%, compared to the FPSC without NPs. The melting time for the case with wavy wall, dispersed NPs and MF, in comparison with FPSC without NPs and MF, provides a further reduction of about 85.5%. This time for FPSC-wavy wall with NEPCM/MF has decreased by 26.07% and 40.86% when inlet water temperatures increase from 87 °C to 90 °C or 93 °C, respectively. Furthermore, this time has a slight effect depending on the applied heat flux. Finally, it is shown that the average storage heat rate (pL) for FPSC with NPs and MF (141.29 W) is higher than Pure PCM (20.49 W), while the average storage heat rate (pL) for HTF with Ste = 0.136 (176.56 W) is higher than that with Ste = 0.102 (141.29 W) or Ste = 0.068 (104.45 W), respectively. Results here shown would be useful to appreciate if and how inlet temperature and applied heat flux would affect the PCM performances