Annulus eccentricity optimisation of a phase-change material (PCM) horizontal double-pipe thermal energy store

The application of phase-change materials (PCMs) has received significant interest for use in thermal energy storage
(TES) systems that can adjust the mismatch between the energy availability and demand. In the building
sector, for example, PCMs can be used to reduce air-conditioning energy consumption by increasing the thermal
capacity of the walls. However, as promising this technology may be, the poor thermal conductivity of PCMs has
acted as a barrier to its commercialization, with many heat-transfer enhancement solutions proposed in the literature,
such as microencapsulation or metal foam inserts, being either too costly and/or complex. The present study
focuses on a low-cost and highly practical solution, in which natural-convective heat transfer is enhanced by placing
the PCM in an eccentric annulus within a horizontal double-pipe TES heat exchanger. This paper presents an
annulus-eccentricity optimisation study, whereby the optimal radial and tangential eccentricities are determined
to minimize the charging time of a PCM thermal energy store. The storage performance of several geometrical
configurations is predicted using a computational fluid dynamics (CFD) model based on the enthalpy-porosity
formulation. The optimal geometrical configuration is then determined with response surface methods. The horizontal
double-pipe heat exchanger studied considered here is an annulus filled with N-eicosane as the PCM for
initial studies. In presence of N-eicosane, for the concentric configuration (which is the baseline case), the charging
is completed at Fo=0.64, while the charging of optimum eccentric geometries with the quickest and slowest
charging is completed at Fo=0.09 and Fo=2.31, respectively. In addition, an investigation on the discharging
performance of the studied configurations with N-eicosane shows the quickest discharge occurs with the concentric
annulus case at Fo=0.99, while the discharge time of the proposed optimum annuli is about three times this
value. In other words, the proposed optimum geometry with the quickest charging time charges ~7.1 times faster
but also discharges ~3 times slower, which is ideal for a TES, especially when used as passive thermal storage
systems in nearly zero-emission buildings. Complementary studies demonstrate that the proposed optimum configuration
improves the TES performance also when employing other PCM types as well as various shell-to-tube
diameter ratios