Numerical investigation of nano-PCM latent-sensible thermal energy storage enclosures

To provide consistent energy supply, solar water heating systems often incorporate both sensible and latent heat storage. The current study corresponds to a numerical investigation of a combined latent-sensible thermal energy storage enclosure using a phase change material (PCM) and water. The PCM domain (latent section) contains a coiled heat transfer tube that provides heat input (simulating solar radiation) and is separated by a solid copper wall from the surrounding water (sensible section). We examine the effect of dispersing copper nanoparticles in PCM on its melting performance. The Simulations are performed using the Galerkin finite element method to couple the heat transfer and fluid flow, using the enthalpy-porosity formulation to model the PCM phase change. An iterative scheme couples the PCM and water domains through the copper interface, and overall energy balance is verified at each time step. All external walls are insulated, and a time-dependent thermal input at the internal tube mimics diurnal solar heating. Results show that the PCM region achieves a significantly higher energy storage capacity than an equal mass of water, underscoring the advantage of latent heat storage. The water's average temperature increased almost linearly during charging, while the PCM's temperature remained nearly constant during the phase change. Adding copper nanoparticles (up to 2% by volume) to PCM provided only a modest improvement in melting rate, and higher nanoparticle concentrations offered no significant enhancement. These findings show that excessive nanoparticle loading yields diminishing returns in PCM thermal performance, consistent with prior studies. Overall, the results demonstrate the effectiveness of the dual PCM-water storage configuration.