| Chinese solar greenhouses(CSG) are predominantly used in Northern China as the significant infrastructures of agricultural production. Not only does it effectively provide a steady supply of vegetables during slack season in cold region, but also increases the income of farmers, which is playing an important role in the process of agricultural development. The cold and freezing injuries of CSG crop, however, are frequently happening in the practical production during winter, causinghuge financiallosses tofarmers. Phase change materials(PCM) will be efficiently storing/releasing huge energy when the phase state is changing. Hence, PCM wallboards, with both effects of heat preservation and accumulation, are considered as the better replacement of normal north walls in CSG. Thermal environment of CSG will be highly improved if the function of heat adjustment of PCM can be fully realized, including the increase of use ratio of solar radiation energyand the lowest indoor temperature, the decrease of day and night indoor temperature difference and the heat loss. Computational fluid dynamics(CFD) as an extensively and high-reliably used simulation tool will be successfully dealing with complex problems of phase change. It is adopted to model and analyze the myristic-lauric-capriccompoundfatty acids(MA-LA-CA/EG) wallboard in this research. The major contents of this paper are as follows: 1) Thermos-physical properties of PCM and sealing material of wallboard are individually measured using the Hot Wire method, the Adiabatic Calorimetry method, and the Laser Flash method. Equivalent specific heat isintroduced to replace specific heat of PCM during phase changing state. The result shows that the heat conductivity coefficients of PCM are different under solid(1.745 W m-1℃-1) and melted(1.975 W m-1℃-1) states, and obviously are in direct proportion to the temperature of PCM. The specific heatand heat conductivity coefficient of sealing material of wallboard are respectively 1.43 kJ kg-1℃-1 and 1.09 W m-1 ℃-1. The latent heat of PCM decreases by 47.9% after 1500 accelerated cooling and heating recycles. The hot box principle is adopted to test the heat charging/discharging process of phase change wallboard, and the result shows that heat charging stage of PCM wallboard lasts for 3 h, and the highest and lowest temperatures are respectively 62.5 ℃ and 31.2 ℃. Temperature distribution is non-uniform. Heat discharging stage of PCM wallboard lasts for 21 h with the highest temperature difference of 0.8℃. Temperature distribution is relatively uniform. Heat charging stage is faster than heat discharging stage with the time proportion of 1:7. Simulation tool of Fluent?, ANSYS 14.5 is employed to study the heat charging/discharging process of phase change wallboard with validation.The result shows that the simulated temperature at each measurement position is in good agreement with the experimental one with the same increasing and decreasing trends over the measurement period. The simulated values, however, are generally higher than the experimental values. The average absolute error, average relative error, maximum variance, index of agreement(IA), and mean squared deviation(MSD) are individually 1.80, 1%, 3.77, 0.96, and 2.54. 2) The heat charging/dischargingefficiency of normal north wall/PCM wallboard and CSG are separately calculated and analyzed. According to four aspects of limiting factors, grouping design and some discussions are made. This research combines CFD method,weighted entropy and fuzzy optimisation methods based comprehensive evaluation system to optimize the wallboard material, structure, and dimension with simultaneously considering the cost. The result shows that the theoretical accumulation of heat charging of unit area of PCM wallboard under sunny and cloudy day are respectively 3.95 MJ m-2 and 1.58 MJ m-2, which is 1.76~3.75 times of normal north wall in CSG. At the same time, the theoretical value is generally 2.56 times of practical value. According to measurements and calculations, the average practical heat dischargingratio of PCM wallboard is 51.65%. The experimental CSG withperforated brick-expanded polystyrene board(PB-EPS) combined north wall is provided to test the practical accumulation of heat charging during winter. The result shows that the practical accumulation of heat charging of normal CSG and PCM-CSG are separately 108.27 MJ and 196.67 MJ, and the latter is 1.82 times of the former one in sunny days. In cloudy days, above-mentioned values are changed as 43.31 MJ and 146.37 MJ, and the latter is 3.38 times of the former one. Therefore, it is necessary to optimize the material and structure of PCM wallboard basing on simulation tool and evaluation system. The result shows that the heat transfer efficiency of cases M5(steel slab) and M4(sandstone block), cases TH1(PCM thickness with 25 mm) and TH4(60 mm), cases TH5(sealing material thickness with 1 mm) and TH8(4 mm) are individually considered as the best and the worst models in group 1#, 2#, and 3#. The heat transfer rate and uniformity of wallboard will be decreasing and the heat charging/discharging efficiency will be increasing when the thicknesses of PCM and sealing material are increased. According to the comprehensive evaluation system, cases M6(asphalt sheet), TH4, and TH8 are separately considering as the optimized sealing material, PCM thickness, and sealing material thickness. CFD application will be significantly improving the heat transfer performance ofwallboard, shortening development cycle, and decreasing investment at the beginning of adesign research, that making products are more suitable for the targeted study objects. Two key factors including the comprehensive thermal performance and the cost of PCM wallboard are simultaneously balanced with introducing the weighted entropy and fuzzy optimisation methods based comprehensive evaluation system. This paper has the potential toprovide some valuable information for a better selection of materials, structures, and dimensions of PCM wallboards in CSG. |
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