Development Of Phase Change Materials And Application In Solar Greenhouses

Current greenhouses in Northwest China have low thermal effectiveness, and high costsand low efficiency in production. The phase change heat storage technique was used as thebasis for greenhouse wall design to resolve those issues. We identified optimal inorganic andorganic phase change heat storage materials, based on the characteristics of Northern Chinaclimate, solar greenhouse production, and requirements of growth of plants, with respect tothe phase change temperature and latent heat values. Inorganic phase change materials havelow phase change temperature and phase separation. We carried out various experiments withmodified materials, and two types of materials for high and low temperatures were identified.Encapsulated heat storage materials were used as greenhouse wallboards. Based onmeasurements of internal air temperature, humidity and ground temperature in the greenhouse,improvement in greenhouse characteristics was verified. A single kind of organic phasechange material cannot meet the requirements of all greenhouse productions, and solid-liquidmaterials have leakage problems during phase change. We studied compositions and shapingmethods for composite phase change materials, and the mechanical and thermalcharacteristics of the materials, and two different kinds of phase change material for heatstorage system suitable for solar greenhouses were identified, with different latent heat.Phase change blocks were produced by mixing these materials with wall building materials.Greenhouse environmental parameters and plant growth characteristics show that thegreenhouse based on phase change heat storage materials has excellent temperatureadjustment performance.The major conclusions of this thesis are as follows.（1） Na2HPO4· H₂O is a phase change heat storage material optimal for solargreenhouses in the north, but its low temperature and phase separation characteristics affectits performance stability. With identified nucleating and thickening agents frommodification experiments, the composition for phase change materials at about33℃isidentified as a mixture with mass ratio of Na2HPO4· H₂O: sodium silicate: graphite=100:3:3.Adding to this high temperature heat storage material system with mass ratio8%KCl toreduce the phase transition temperature, a low temperature （25²6℃） material can beobtained, but this causes over cooling and phase separation. Further modification experiments show that an optimal material with mass ratio of Na2HPO4· H₂O: KCl: sodium silicate:graphite=50:4:3:1has phase transition temperature of25²6℃as well. Heat storagematerials of the two temperatures can be used in solar greenhouses for different crops.（2） Special plastic packaging was used for the Na2HPO4·12H₂O phase change materialsystem, for embedding in greenhouse wall to form a phase change heat storage wall. Airtemperature, humidity, and low temperature data for typical days and continuousmeasurements for the coldest month show that the greenhouse built with the phase changematerial clearly increased the lowest and average indoor temperatures, reduces the highestindoor temperature. The materials also improved minimum and lowest indoor soiltemperatures, reduced highest indoor soil temperature. Corresponding to the indoortemperatures, the materials can reduce the minimum and average indoor humidity. Comparedwith control greenhouses with lime-sand bricks, phase change wall greenhouses haveimproved performance. This indicates that the phase transition materials have improved cropgrowth environment, winter performance and production efficiency of greenhouses.（3） Paraffin wax （PW） and n-butyl stearate （BS） have complementary thermalperformances, and good chemical compatibility and stability. The optimal ratio of compositeheat storage material is identified for paraffin wax: n-butyl stearate=1:1, based onexperiments on their thermal properties. A composite phase change material is made byvacuum adsorption by rice husk as BS/PW/rice husk phase change material. Differentialscanning calorimetry is used to determine the phase change heat transfer of phase change heatstorage materials, and the density peaks appeared at50.49C and49.54C, respectively; themelting and solidification latent heats, are63.32J/g and59.82J/g, respectively. After3000melting and solidification cycles, BS/PW/rice husk still showed good thermal stability, andinfrared spectral scanning showed that the components were in good compatibility andchemical stability. Scanning electron microscopy showed that most n-butyl stearate andparaffin composites were absorbed inside the rice husk, having good compactness. Withpolystyrene particles replacing rice husk with vacuum adsorption to form BS/PW/polystyreneparticle, a phase change material of higher latent heat value is realized, with its melting andsolidification latent heat of93.61J/g and95.58J/g, respectively. The thermal and chemicalstabilities and compatibility are good. Scanning electron microscopy showed that its densitywas better than the BS/PW/rice husk phase change material. Considering the phase changetemperature and latent heat value, the BS/PW/polystyrene phase change material is a goodheat storage phase change material.（4） Weak-alkalinized cement can be mixed directly with organic phase changematerials to make concrete blocks. Experiments show that the optimized mixing ratio of cement, fly ash, sand, gravel, ceramsite: lime, gypsum=20:25:17:10:25:1:2. Based on themechanical characteristics （minimum compressive strength and dry density, maximum waterabsorption and softening coefficients） and thermal indexes （maximum thermal conductivity,specific heat, coefficient of heat storage, phase change latent heat value）, polystyrene particlephase change blocks have better heat storage capacity, but the rice husk phase change blockshave better mechanical characteristics. The maximum mixture ratio of phase change materialsis restricted by the lower threshold of mechanical properties. We suggest that phase changerice husk content is not more than5%, and polystyrene particles content is not more than12%.（5） In the solar greenhouse built with concrete blocks of BP/BS/rice husk phase changematerial, we found that the measured indoor air temperature, variations in the typical indoortemperature were lower than the control greenhouse of common bricks, the minimumtemperature and average temperature increased, and the temperature difference betweeninside and outside the north wall is reduced. Jinpeng No.1tomatoes grown in theexperimental greenhouse have better early stage plant height, stem diameter, dry weight, freshweight, and later growth stage plant height, root length, stem diameter, leaf number andflower number, fruit number, weight, plant total weight, etc. The phase transition material hasa clear advantage for the experiment greenhouse than the control one.（6） Our experiments of inorganic and organic phase change material walls in solargreenhouses show that the new materials have good thermal effects for "cutting peaks andvalleys" in temperature control. The materials can improve the production environment andproduction efficiency of greenhouses. The results of our studies have great practicalapplication values.