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Highlights • Vertical fins contribute to melting by enhancing natural convection mechanism. • Extending fin length is more beneficial than increasing number of fins. • The cases with longer fins could have a higher overall heat transfer coefficient. • The compromise in energy density could be minor due to sensible energy gain. Abstract This paper investigates the effect of vertical fins, as an enhancement technique, on the heat transfer rate and energy density of a latent heat thermal energy storage system. This contributes with knowledge on the interaction of heat transfer surface with the storage material for optimizing storage capacity (energy) and power (heat transfer rate). For the assessment, numerical modeling is employed to study the melting process in a two-dimensional rectangular cavity. The cavity is considered heated isothermally from the bottom with surface temperatures of 55 ˚C, 60 ˚C or 70 ˚C, while the other surfaces are insulated from the surrounding. Aluminum and lauric acid are considered as fin/enclosure material and phase change material, respectively. Vertical fins attached to the bottom surface are employed to enhance the charging rate, and a parametric study is carried out by varying the fin length and number of fins. Thus, a broad range of data is provided to analyze the influence of fin configurations on contributing natural convection patterns, as well as the effects on melting time, enhanced heat transfer rate and accumulated energy. The results show that in addition to increasing the heat transfer surface area, the installation of vertically oriented fins does not suppress the natural convection mechanism. This is as opposed to horizontal fins which in previous studies have shown tendencies to reduce the impact of natural convection. This paper also highlights how using longer fins offers a higher rate of heat transfer and a better overall heat transfer coefficient rather than increasing the number of fins. Also, fins do not only enhance the heat transfer performance in the corresponding melting time, but also maintain similar total amount of stored energy as compared to the no-fin case. This paper discusses how this is the result of the enhanced heat transfer allowing a larger portion of sensible heat to be recovered. For example, in the case with long fins, the relative mean power enhancement is about 200% with merely 6% capacity reduction, even though the amount of PCM in the cavity has been reduced by 12% as compared to the no-fin case. Although the basis for these results stems from the principles of thermodynamics, this paper is bringing it forward with design consideration. This is because despite its importance for making appropriate comparisons among heat transfer enhancement techniques in latent heat thermal energy storage, it has not been previously discussed in the literature. In the end, the aim is to accomplish robust storage systems in terms of power and energy density. [ABSTRACT FROM AUTHOR]

In this paper, enhancement for convection heat transfer of turbulent flow in a solar central receiver tube with porous medium and non-uniform circumferential heat flux was numerically investigated. A new method was introduced to build different porous medium configurations in a unified grid system. Four kinds of enhanced receiver tubes (ERTs) with different porous insert configurations were modeled to optimize the performance of ERT. Furthermore, parameters including filling ratio of porous medium, thermal conductivity ratio (thermal conductivity of porous medium versus that of working fluid), porosity and Reynolds number were analyzed. The results showed that ERT partially filled with porous medium has better heat transfer performance than that fully filled with porous medium. The configuration of porous insert for optimal thermal or thermo-hydraulic performance is interactively affected by all the parameters discussed in this paper. The thermal conductivity ratio is the most crucial parameter to the thermal or thermo-hydraulic performance of ERT. The value of thermal conductivity ratio should be greater than 100 to obtain a good thermo-hydraulic performance. The ERTs with horizontal cylindrical segment shaped porous inserts and hollow cylinder shaped porous inserts are proposed because they can obtain optimal thermal or thermo-hydraulic performance. [ABSTRACT FROM AUTHOR]

In this paper, a dynamic heat transfer model based on thermal-electrical analogy has been built to compare convective and radiative heating systems for intermittent heating. An experiment has been set up to validate the model we built. The RMS Error was within 0.5 °C, confirming the accuracy of the model. The validated model was then applied to compare four typical space heating systems, namely all-air, radiator, in-slab floor heating, and lightweight floor heating systems. The concept of “ineffective heat” has been proposed to explain why the intermittent heating could reduce heating load. Two ideal characters of heating systems have been identified for intermittent heating, i.e. high heating capacity and low thermal mass, both of which are crucial to shorten the preheat time and to reduce the ineffective heat. The results indicate that convective heating systems were more comfortable (more rapidly raise the indoor temperature) and more energy efficient (lower heating load) for intermittent operation than radiative heating systems. The performance gap between radiative and convective heating systems would be enlarged in colder conditions. Among the four heating systems, convective heating system was the most suitable one for intermittent heating, and floor heating was the most unsuitable one. Though belonging to radiative heating systems, the radiator performs differently from floor heating for its potential to enhance heating capacity and relatively low thermal mass. Additionally, outdoor climate and building thermal insulation level also affect the choice of the most suitable heating system and heating mode. The findings of this research might shed some light on the debate of the choice of space heating systems in areas with similar climate conditions with China’s Hot Summer Cold Winter region. [ABSTRACT FROM AUTHOR]

• A novel radiant cooling technology is described. • The technology consists of a foldable surface with integrated microfluidics. • The thermal performance is characterized as a function of the surface folding angle. • Experiments confirm increased rates of heat transfer and cooling. • The technology may help reduce the lifecycle emissions of HVAC installations. Design and fabrication methods translated from microfluidics, microelectromechanical systems, origami, and biologically inspired engineering may offer efficiency improvements for water-based thermoregulation in buildings. This paper introduces foldable radiant cooling devices that are fabricated by lamination with integrated microfluidic water-circuits. These devices produce more surface convection because of their geometry and because they have more surface area than flat panels. As a result, lukewarm water instead of chilled water may be sufficient for cooling a room, leading to potential savings in primary energy use and lifecycle greenhouse gas emissions. Analytical models and physical experiments show a 55–67% improvement in thermal performance when comparing these novel folded surfaces to flat surfaces, mainly due to the improved convection heat transfer. [ABSTRACT FROM AUTHOR]