| Fresnel solar collectors are mainly divided into Fresnel lens solar collectors andFresnel reflector solar collectors. In terms of the concentration methods, they are dividedinto point-focus Fresnel solar collectors and line-focus Fresnel solar collectors. Fresnellens is a concentration element of a transmission concentration system which consists ofconcentric grooves with wedges. The angle of each wedge changes gradually from thecenter of the lens to the edge of the lens. Then the incident solar rays from different radiusare refracted at different refraction angles and concentrating to the focal plane where solarthermal conversion is achieved by a solar absorber. Fresnel reflector solar collector refersto the use of line-focus reflector mirror arrays to concentrating sunlight to a fixed solarabsorber at the focal line to realizing solar thermal conversion. Fresnel solar collectorshave broad application prospects in the areas such as concentrating photovoltaic powergeneration, middle and high temperature solar thermal applications, solar refrigeration andair conditioning and concentrating solar power generation. In this thesis, the properties ofFresnel solar concentrators are considered and the matched cavity absorbers are analyzed.Theoretical and experimental analyses on Fresnel solar collectors using different cavityabsorbers are done. The specific works are as follows:Firstly, the thermodynamic analysis on solar thermal conversion process ofconcentrating solar collector using cavity absorbers is done. The optimal collectortemperature and cavity absorber thermal efficiency are derived under the conditions suchas a certain geometrical concentration ratio, optical efficiency, air quality and transparencyof the atmosphere and so on. Through the establishment of theoretical model of the cavityabsorber, the complete expression of exergy efficiency of concentrating solar collectorusing cavity absorbers based on the second law of thermodynamics is obtained. The exergyefficiency values obtained by the expression can reflect the thermal performance ofconcentrating solar collectors using cavity absorbers. At the same time, it can be used tooptimize cavity absorbers. High efficiency solar thermal conversion is achieved under high solar energy concentration. The theoretical optimal collector temperature is about775K,the optimal exergy efficiency is about0.50for line-focus solar collector using cavityabsorbers under the theoretical maximum geometrical concentration ratio. The theoreticaloptimal collector temperature is about2500K, the optimal exergy efficiency is about0.84for point-focus solar collector using cavity absorbers under the theoretical maximumgeometrical concentration ratio.Secondly, the optical performance of Fresnel solar collectors is studied theoretically.The theoretical expressions of geometrical concentration ratio of Fresnel lenses andFresnel reflectors are derived. The theoretical models of Fresnel solar collector using eightkinds of point-focus cavity absorbers and eight kinds of line-focus cavity absorbers areestablished. According to the results of simulation, for the point-focus Fresnel lens solarcollector, when the size of the point-focus Fresnel lens is1000mm×1000mm×3mm and thediameter of the cavity apture is60mm, the conical cavity absorber (the vertex angle ofcross section is60°) has the best optical performance, the theoretical optical eddiciency is89.95%, and the optical concentration ratio is318.12; for the line-focus Fresnel lens solarcollector, when the size of the line-focus Fresnel lens is400mm×320mm×2mm and thewidth of the cavity apture is80mm, the triangular cavity absorber (the vertex angle of crosssection is60°) has the best optical performance, the theoretical optical eddiciency is81.15%, and the optical concentration ratio is4.06. For the line-focus Fresnel reflectorsolar collector, when the single row size of the line-focus Fresnel reflector is6000mm×300mm×2mm and the width of the cavity apture is60mm, the arc-shaped cavityabsorber has the best optical performance, the theoretical optical eddiciency is81.67%, andthe optical concentration ratio is32.67. However, the energy distribution in the cavity ofthe triangular cavity absorber is uniformer than the arc-shaped cavity absorber and theenergy density on the inner surface of the cavity of the triangular cavity absorber is higherthan the arc-shaped cavity absorber and other line-focus cavity absorbers. Consequently,the line-focus Fresnel reflector solar collector using triangular cavity absorber has betterthermal performance. In addition, the characteristics of Fresnel solar collectors underdifferent incident declinations of sunlight are analyzed. The results show that for theFresnel lens solar collector using a conical cavity absorber, the tracking error should becontrolled within±1.8°; for the Fresnel lens solar collector using a triangular cavityabsorber, the tracking error should be controlled within±4.5°; for the Fresnel reflectorsolar collector using a triangular cavity absorber, the tracking error should be controlled within±8.0°. Then the focal spot of the above three Fresnel solar collectors can be ensuredto fall within the cavity absorbersThirdly, the experimental setups of Fresnel lens solar collector and Fresnel reflectorsolar collector are built. The stagnation parameters, the instantaneous efficiency curves andthe overall heat loss coefficient of Fresnel solar collectors are obtained. The results showthat the point-focus Fresnel lens solar collector using conical cavity absorber has astagnation parameter which is about0.55(m~2·K)/W, the highest collection temperature isabout250℃, the solar thermal conversion efficiency is about35.40%and the overall heatloss coefficient is about255W/(m~2·K). The line-focus Fresnel lens solar collector usingtriangular cavity absorber has a stagnation parameter which is about0.33(m~2·K)/W, thehighest collection temperature is about180℃, the solar thermal conversion efficiency isabout28.70%and the overall heat loss coefficient is about50W/(m~2·K). For the Fresnelreflector solar collector using tube bundles triangular cavity absorber, the stagnationparameter is about0.36(m~2·K)/W. When the collection temperature is about90℃, the solarthermal conversion efficiency is about45.20%and the overall heat loss coefficient is about8W/(m~2·K). When the collection temperature is about120℃, the solar thermal conversionefficiency is about40.10%and the overall heat loss coefficient is about12W/(m~2·K). Whenthe collection temperature is about150℃, the solar thermal conversion efficiency is about36.60%and the overall heat loss coefficient is about17W/(m~2·K). Compared with theoverall heat loss coefficient of the line-focus Fresnel lens solar collector using triangularcavity absorber, in the same range of collector temperatures, the overall heat losscoefficient of the line-focus Fresnel reflector solar collector using tube bundles triangularcavity absorber is smaller. This is mainly because the cavity aperture of the former isupward and the convection heat loss is larger, while the cavity aperture of the latter isdownward which has advantageous in suppressing the convection heat loss, and thus theoverall heat loss coefficient is smaller. Under given conditions, the thermal performance ofthree kinds of Fresnel solar collectors is better than evacuated tube solar collector. If theheat loss is controlled much better, the thermal performance still has larger rising potential.Finally, on the basis of theoretical analysis and experimental investigation, the heatremoval factor theory of Fresnel solar collector using cavity absorbers is established. Thethermal performance of Fresnel solar collector using eight kinds of point-focus cavityabsorbers, eight kinds of line-focus cavity absorbers and there improved line-focus cavityabsorbers is analyzed comparatively. The optimized analysis results show that for the Fresnel lens solar collector using conical cavity absorber, the optimum aperture diameter ofthe cavity, the optimum inside diameter of the receiver tube and the optimum vertex angleof the cross section through the symmetric axis of the receiver are80mm,15mm and60°,respectively. For better thermal performance, the geometrical concentration ratio of thestudied Fresnel lens solar collector should be more than500, the convection heat transfercoefficient of receiver tube and heat transfer fluid should be more than1200W/(m~2·K) andthe overall heat loss coefficient is less than100W/(m~2·K). At the same time, for the Fresnellens solar collector using triangular cavity absorber in this paper, the optimum aperturewidth of the cavity, the optimum inside diameter of the receiver tube and the optimumvertex angle of the cross section through the symmetric axis of the receiver are50mm,18mm and60°, respectively. For better thermal performance, the fin efficiency should bemore than0.8, the geometrical concentration ratio of the studied Fresnel lens solarcollector should be more than55, the convection heat transfer coefficient of receiver tubeand heat transfer fluid should be more than600W/(m~2·K) and the overall heat losscoefficient is less than25W/(m~2·K). Based on the results of optimization, Fresnel solarcollector using three kinds of improved triangular cavity absorber is proposed and thetheoretical and experimental analysis is done. Results show that the experimentalefficiency factor and heat removal factor using rectangular pipeline as the absorber plateare improved by7.21%and6.67%, the experimental efficiency factor and heat removalfactor using tube bundles as the absorber plate are improved by5.89%and5.95%and theexperimental efficiency factor and heat removal factor using integrated sheet-tube as theabsorber plate are improved by8.41%and7.89%. Consequently, when the systemoperation pressure is not very high (e.g. lower than0.20MPa), the line-focus Fresnel lenssolar collector using optimized rectangular pipelines triangular cavity absorber design canbe adopted. However, it is suggested that the line-focus Fresnel lens solar collector usingoptimized tube bundles triangular cavity absorber or integrated sheet-tube triangular cavityabsorber design should be adopted when the system operation pressure is higher (e.g.higher than0.30MPa) to enhance the thermal performance of the system and meet therequiements of middle and high temperature solar thermal conversion spplications. |
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