Heat Storage Performance of PCM in a Novel Vertical Pointer-Shaped Finned Latent Heat Tank

The heat storage performance of latent heat storage systems is not good due to the poor thermal conductivity of phase change materials.In this paper,a new type of pointer-shaped fins combining rectangular and triangular fins has been employed to numerically simulate the melting process in the heat storage tank,and the fin geometry parameter effects on heat storage performance have been studied.The results indicate that compared with the bare tube and the rectangular finned tank,the melting time of the phase change material in the pointer-shaped finned tank is reduced by 64.2%and 15.1%,respectively.The closer the tip of the triangular fin is to the hot wall,the better the heat transfer efficiency.The optimal height of the triangular fin is about 8 mm.Increasing the number of fins from 4 to 6 and from 6 to 8 reduces the melting time by 16.0%and 16.7%respectively.However,increasing the number of fins from 8 to 10 only reduces the melting time by 8.4%.When the fin dimensionless length is increased from 0.3 to 0.5 and from 0.5 to 0.7,the melting time is shortened by 17.5%and 13.0%.But the melting time is only reduced by 2.9%when the dimensionless fin length is increased from 0.7 to 0.9.For optimising the design of the thermal storage system,the results can provide a reference value.

Comparison of thermal and light performance in two typical Chinese solar greenhouses in Beijing

Solar greenhouses have been used for producing vegetables in northern China during early spring,late autumn or over-winter.To improve the thermal performance of solar greenhouses,a traditional type and a retrofitted design were comparatively evaluated.In the retrofitted design,three adjustments were incorporated:the material and structure of the walls,south-facing roof angle,and structure of the north-facing back-roof.The results indicated that the thermal and light performance of the retrofitted greenhouse was much better than that of the traditional greenhouse.Specifically,the daily mean temperature,minimum air temperature,and soil temperature inside the greenhouses after retrofit ting were increased by 1.3,2.4,and 1.9℃,respectively,meanwhile,the daily total solar radiation and PAR were increased by 28.2%and 9.2%,respectively.The wall temperature and its daily variation range were reduced with increasing depth and height.The characteristic analysis of heat storage and release indicated that higher locations have longer heat storage,and shorter heat release time in vertical direction,as well as a lower ratio of heat release to storage.In horizontal direction,the western wall has the shortest heat storage time but the highest heat release flux density.Altogether,the heat storage time of the wall is 1.5 h less than that of the soil.The heat storage flux density of the wall is 1.5 times of that of the soil,but the heat release flux is only 61%of the soil’s value.The total wall heat storage is half of that of the soil in the greenhouse;the total wall heat release amount is only a quarter of that of the soil.Therefore,the thermal environment of solar greenhouses can be further improved by improving the thermal insulation properties of the wall.

The influence of pore size distribution on thermal conductivity,permeability, and phase change behavior of hierarchical porous materials

Porous foams have been widely employed to improve heat storage rate and prevent leakage of phase change materials(PCMs).Actual porous foams have non-uniform or hierarchical pore size distribution, which is usually neglected in most researches.Here, we establish hierarchical porous models considering different pore size distributions by using Voronoi tessellations. Effects of pore size distribution on thermal conductivity, permeability, and phase change behavior of hierarchical porous foams/PCMs composites are investigated. Uneven pore size distributions are found to decrease the thermal conductivity of porous foams to some extent. On the other hand, the permeability can be reduced by 27.6%, and the heat storage rate can be improved by 7.7% by introducing moderate hierarchy without changing the total porosity. This work opens a new route to enhance heat storage performance of PCMs via leveraging hierarchy of pore size distribution of porous foams.