光信息科学与技术本科毕业论文外文翻译.doc

安徽工业大学毕业设计(论文)外文翻译Performance evaluation of concentrating solar photovoltaicand photovoltaic/thermal systemsAbstractHybrid conversion of solar radiation, which allows simultaneous conversion of sunlight into thermal and electrical energy in the photovoltaic/thermal collector, is one of the most promising techniques of solar energy exploitation. In this study, low concentrating photovoltaic (PV) and photovoltaic/thermal (PVT) systems were designed and tested for a given spring climatic condition of the Tunisian Saharan city Tozeur. The system is basically an asymmetric compound parabolic photovoltaic concentrator. As this systems performance deteriorates with rising the solar cells temperature, we proposed to convert it on a hybrid one in order to improve its electrical efficiency and to recuperate simultaneously thermal energy. The comparison of these systems operating conrmed the improvement of the electrical performance of the combined PVT system and its acceptable thermal energy production. A computational uid dynamics “CFD” model which interprets the PVT system was then developed and validated against the experimental results, proving the validity of the developed model use to identify numerically this system limitations and predict the possible improvements.2013 Elsevier Ltd. All rights reserved.Keywords: Concentrating photovoltaic system (CPVS); Concentrating photovoltaic/thermal system (CPVTS); CFD simulation; Performance roductionHybrid photovoltaic/thermal (PVT) solar systems are less expensive devices than the two separate units which can simultaneously provide electricity and heat with higher conversion rates of the absorbed solar radiation than standard PV modules. During the last two decades, the utilization of this solar technology was the subject of several theoretical and experimental studies, helping to sort out suitable products and systems with the best performance. Ibrahim et al. (2011) presented the performance of water, air and combination of water and air systems for a at PVT collector. This review has considered different designs and indicated that the most important factors that inuence the efficiency of the system are the area where the collector is covered, the number of passes and the gap between the absorber and solar cells. Similarly, Mishra and Tiwari (2013) studied the effect of the collector area covered by PV module on the performance of hybrid PVT water collector. They considered two congurations in which the collector is partially and fully covered by PV module and compared their results with those of a conventional at plate collector. Ghani et al. (2012) considered a PVT collector of various design, geometric shape and operating characteristics and discussed the effect of non-uniform ow distribution on the thermal and electrical performance of their solar system. Li et al. (2011a) characterized experimentally the thermal and electrical performance of a 2m2 PVT system. These experiments were done for three different types of solar cells and the optimal design was evaluated. In other investigation, Li et al. (2011b) evaluated the overall performance of a 10 m2 CPVTS and discussed the effect of the solar cell array on its thermal and electrical efficiency. In order to improve the performance of this system, a 2 m2 PVT system utilizing a mirror of higher reectivity was then built and its contribution to the effective operating of the PVT system was presented regarding its thermal efficiency. Ji et al. (2007) considered a solar system which consists of a at-box aluminum-alloy photovoltaic and water-heating system with single-crystalline silicon cells integrated. Dynamic simulations were performed and the effect of the PV cell covering factor and the glazing transmissivity on the overall system performance were discussed. Garg and Agarwal (1995) proposed a conversion of a conventional water heater into a combined system by pasting solar cells directly over the absorber plate. This study was done for different solar cell areas,mass ow rates and different water masses, and indicated the optimal ow rate for a maximum efficiency. Similarly,the effect of the mass ow rate on the thermal and electrical efficiency of a PVT air system was studied by Bambrook and Sproul (2012) who showed that their solar system performs better for high values of the air mass ow rate. Another integrated combined PVT solar water heating system was designed and tested outdoor New Delphi climate by Dubey and Tiwari (2008) who discussed the effect of the design and the climatic conditions on the system operation. Bernardo et al. (2011) proposed a complete methodology to simulate a PVT collector and presented a comparison of their system performance relatively to a standard PV module and a at plate collector. Cristofari et al. (2009) developed a simulation model of nite differences describing the operating of a hybrid PVT collector and noted the importance of the utilization of a copolymer for the total design of the solar collector. Tiwari et al.(2009) were based on the energy balance and discussed analytically the performance of an integrated PVT solar system regarding the water temperature, the exergy and the thermal and electrical efficiency for different hot water withdrawal ow rates. Regarding the strategies proposed to improve these systems performance and in order to get more thermal and electrical energy, reectors were mounted by Kostic et al. (2010) in the PVT collector and their position was changed to evaluate the optimal one. Similarly, for additional power production, Kosmadakis et al. (2011) presented a feasibility study of a CPVTS in which the heat produced is recovered by an organic Rankine cycle. Results showed the effectiveness of the change regarding the electrical production increase, the PV cells cooling and the system electrical effciency improvement.A novel technology which consists of coupling a linear Fresnel concentrator with a channel PVT collector in order to increase the solar conversion efficiency was described by Rosell et al. (2005). Theoretical analyses of this solar system were presented, conrming the importance of the thermal conduction between the PV cells and the absorber plate. Another novel hybrid PVT system was studied by Rajoria et al. (2013) who were interested to the exergetic and enviroeconomic analysis. Tripanagnostopoulos (2007) presented an experimental study of a new type of PVT collector with dual heat extraction operation, either with air or water circulation. The most effective design was studied, and low cost modications were applied toimprove the system thermal and electrical energy output. Kostic et al. (2010) studied the inuence of reectance from at plate solar radiation concentrators made of Al sheet and Al foil on energy efficiency of a PVT collector. This work discussed also the optimal position of solar radiation concentrators and appropriate thermal and electrical efficiency of the PVT collector were determined. Tiwari and Sodha (2006) developed a thermal model of an integrated photovoltaic and thermal solar system. The numerical simulations were carried out for different climatic and design parameters, and a daily thermal efficiency of 58% was predicted. Corbin and Zhai (2010) proposed an experimentally validated computational uid dynamics (CFD) model of a novel building integrated PVT collector. They discussed the effect of active heat recovery on cell efficiency and studied the effectiveness of the device as a solar water heater. In this review, a new correlation which allows cell effciency to be calculated directly was also developed, relating electrical effciency to collector inlet water temperature, ambient air temperature and insulation. Another numerical model was developed by Ji et al. (2006) to analyze the performance of a hybrid PVT system. In this study, the combined effects of the solar cell packing factor and the water mass ow rate on the thermal and electrical effciency were investigated. Kalogirou and Tripanagnostopoulos (2006) used the TRNSYS software to simulate hybrid PVT solar systems for domestic hot water applications and showed the considerable amount of thermal and electrical energy produced by these systems.Despite the fact that all these authors are in agreement that the increasing cells temperature has an adverse effect on the PV system electrical performance and that the PVT system presents improved electrical performance in addition to its thermal energy production, there is limited experimental data on how much is the electrical and thermal gain reached by the PV system conversion on a PVT system. In this experimental study, the performance of a CPVS is rstly studied, this system is then converted on a CPVTS and the resulting thermal and electrical gain is evaluated. Similarly, Computational uid dynamics CFD was not used to model concentrating PVT systems and the few CFD studies encountered concern non-concentrating systems. In the numerical part of this study, the performance of a CPVTS is studied using the CFD package Fluent 6.3 and the developed CFD model is validated by comparing numerical results to the experimental data.This paper presents so experimental results of concentrating PV and PVT systems constructed and analyzed under a Tunisian Saharan climate. The rst system, which is the PV one, consists of the concentrator, the receiver which represents the PV module and the electrical energy output system. As the rising solar cell temperature causes a reduction of the system electrical efficiency in addition to the risks that the cells exhibit long-term degradation if the temperature exceeds a certain limit, we propose to convert this PV system into a combined PVT one. A rectangular conduct dimensioned according to the characteristics of the solar PV panel is so constructed and this system electrical performance is analyzed and compared to that of the PV system. The PVT system thermal performance is also evaluated for two water mass ow rates. A 3D CFD model describing the CPVTS operation is then developed and numerical results are validated against the experimental data.2. Experimental models2.1. CPVSThe CPVS test device includes the concentrator and the PV panel. The concentrator is made of stainless steel; it is 3.64 m long and 2 m wide. The PV panel is an STP020S12/cb panel of 18 single crystalline silicon solar cells and its specications are detailed in Table 1. The experiments have been conducted for a Tunisian Saharan climate, in the city of Tozeur and the CPVS was south facing and 34 titled above the horizontal.2.2. CPVTSThe CPVTS consists of a thermal unit for the heat extraction by the water which circulates through the rectangular pipe and the PV module which are mounted through the concentrating system. A rectangular conduct is so constructed and installed in contact with the PV cells, allowing simultaneously the PV cells cooling and the thermal energy production. The length of this conduct is 1.825 m, its width is 0.275 m and its depth is 0.05 m. The PV panel is positioned in the middle of the water conduct so that the ow is fully developed in the contact of these two surfaces. The black painted steel water conduct, the PV panel and the whole CPVTS are described in Fig. 1.2.3. Analyzed parameters and measuring instrumentsFor the calculation of the PV system electrical output,the current I and the voltage V are measured. Some meteorological parameters such as the ambient temperature Ta and the solar irradiance G are also experimentally determined in order to evaluate the system electrical efficiency which is function of the operating conditions. Regarding the CPVTS operation, these same parameters are measured, in addition to the water inlet and outlet temperatures which are used for the system thermal efficiency calculation. The measuring equipments characteristics are listed in Table 2.The conversion relation of the solarimeter used in these experiments is: 100 mV=1000 W/m2.2.4. Experimental results2.4.1. CPVS performanceThe current U and the voltage I delivered by the PV panel were measured and the CPVS electrical power P which is dened as their product was then calculated.The uncertainty in the experimental values is calculated as proposed by Kratzenberg et al. (2006) and the electricalefficiency is evaluated as demonstrated by Kalogirou and Tripanagnostopoulos (2006). These experimental investigations were undertaken on the 31st of May 2012 and measurements have been continuously monitored each h,from 6 am to 6 pm. The climatic conditions described by the solar irradiance and the ambient temperature were also monitored. During these experiments, the highest intensity of incident solar irradiance was of about 850Wm2, the maximum average ambient temperature was of 38 and they were respectively measured at 12 h and 16 h.The temporal evolution of the CPVS electrical power as well as this system electrical efficiency is shown in Fig. 2.Amaximum power of 13.8 W is measured at about 13 h 30 and the system maximum electrical efficiency which is of 0.095 is measured at about 14 h . CPVTS performanceDue to the negative temperature coefficient which characterizes the photovoltaic cells, the increase of the PV cells temperature causes a reduction of the system efficiency. This problem can be limited by the application of a heat extraction mode which allows the solar cells cooling. The application of a suitable cooling system is so the next step of this work and its contribution to the effective system operation is discussed regarding its electrical performance, in addition to the thermal efficiency evaluated as proposed by Li et al. (2011a). Measurements have also been performed each h, from 6 h to 18 h and that during two successive days, June 4th and 5th and for two different water mass ow rates, respectively 0.05 and 0.0187 l/s.Fig. 3 shows the effect of the water mass ow rate on the daily variation of the electrical power delivered by the CPVTS and its electrical efficiency. From the presented results of this gure, one can see that with a mass ow rate of 0.05 l/s, the electrical power is higher than that for a mass ow rate of 0.0187 l/s. Regarding the effect of the water ow rate on the CPVTS electrical efficiency, the cell efficiency can be seen improved in stages as the mass ow rate increases. Indeed, the maximum electrical efficiency achieved by the CPVTS is respectively of 9.8% and 10.02% for the corresponding water mass ow rates of 0.087 l/s and 0.05 l/s, whereas that of the CPVS is of 9.4%.In addition to the climatic conditions described by the ambient temperature and the solar irradiance, the water inlet and outlet temperature was also measured in order to evaluate the system thermal efficiency. The thermal efficiency of the CPVTS is illustrated for the considered water mass ow rates in Fig. 4 and this CPVTS thermal efficiency can be written as a function of the temperature rise DT and the solar irradiance G as follows:CPVTS with a water mass flow rate of 0.0187 l/sCPVTS with a water mass flow rate of 0.05 l/s :reached 10.02% for a water mass ow rate of 0.05 l/s.Experimental results also showed that this solar system is more efficient for high water mass ow rates. In addition to the electrical performance considerations, experiments are considered encouraging for the PVT conversion of this system regarding the relatively important thermal energy output. Indeed, a thermal efficiency of 16% is obtained and a combined (thermal and electrical) efficiency of 26% is reached in spite of the simplicity and low cost of the change.3. D CFD modelingThis paper describes a rst detailed simulation of a CPVTS. Based on the experiment tests, the studied solar system is dened, the mesh is generated, the boundary condition are identied then introduced for the CFD simulation and numerical results are compared to experimental data. 3.1. Geometry description and meshingThe same dimensions, materials and properties of the experimentally investigated PVT system are introduced for the CFD simulation. The mesh generated in the entire eld of this system consists of tetrahedral cells in the solar cells and PV panel and of hexahedral cells in the rest of theld. A grid independent study was also carried out and the optimum mesh size which was obtained with 604836 cells was used.3.2. Assumptions and boundary conditionsThe experimental validation of the PVT model has been focused on the system thermal and electrical efficiency. The performance of the developed model has been tested by simulating the PVT system over the 12 daily hours of daylight and for the experimentally investigated water mass ow rates. The same conditions as those of the experimental tests were considered and a tilt of 34 above the horizontal was simulated in CFD by considering a rotation of the gravitational forces by this angle in the south direction. The simplifying assumptions considered in this analysis are:(1) incompressible uid;(2) climatic conditions corresponding to fair weather conditions;(3) in the analysis of natural convection ows, the uid properties can be assumed constant except for the density change with temperature which is considered for the air, described by the Boussinesq approximation, and expressed as follows:Boundary conditions: the experimental results of the meteorological data collected on the global irradiance and ambient temperature were used as input for the CFD simulation. The external sky temperature Te required to estimate the radiative exchange was introduced as a user dened function UDF and its variation according to the ambient temperature is expressed as follows:In addition to these parameters, the temporal evolution of the water in