| It is well established that atmospheric and environmental pollution as a result of extensive fossil fuel exploitationin almost all human activities has led to some undesirable phenomena that have not been experienced before in known human history and they are varied and include global warming, the greenhouse affect, climate change, and acid rain. On the other hand, worldwide this conventional energy sources are rapidly depleting. While population growth, increased expectations and means, and scientific and technological developments have dramatically increased the global demand for energy in its various forms. In order to resolve these problems, the two main alternatives are either to improve the fossil fuel quality thus reducing their harmful emissions into the atmosphere or, more significantly, to replace fossil fuel usage as much as possible with environmentally friendly, clean, and renewable energy sources. What this all implies is that the world is in the initial stages of an inevitable transition to a new energy system that, over time, will be less dependent on traditional uses of fossil fuels and increasingly dependent on renewable energy resources.Nowadays, solar energy, as renewable energy, is attracting a lot of attention, since it is clean, pollution-free, and inexhaustible. As we known, solar radiation incident upon the Earth is the primary energy source by which the life of mankind has developed and most renewable energies derive their energy from the sun. Solar energy is the most abundant, sustainable source of energy, which provides over150,000terawatts of power to the Earth. Only a small fraction of the available solar energy reaching the Earth surface would be enough to satisfy the global expected energy demand. Directly utilizing solar energy, one of the greatest scientific and technological challenges we are facing is to develop efficient ways to collect, convert, store, and utilize solar energy at affordable costs, Which is because that although the solar radiation is a high quality energy source because of the high temperature and exergy at its source, its power density at the earth’s surface makes it difficult to extract work and achieve reasonable temperatures in common working fluid. These problems can be better solved by concentrating solar thermal power technology (CSP) and for large-scale generation, CSP has been more common. It can be say that CSP with optical concentration technologies is important candidates for providing a major share of the clean and renewable energy needed in the future CSP systems use optical devices (usually mirrors) and sun tracking systems to concentrate a large area of sunlight into a smaller receiving area. The concentrated solar energy is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists. The main concentrating concepts are parabolic troughs, solar dishes, solar power towers, and Linear Fresnel reflector (LFR). The main purpose of the concentrating solar energy is to produce high temperatures and therefore high thermodynamic efficiencies. This thesis mainly researches LFR concentrating power system.Linear Fresnel CSP technology derives its name from a type of optical system that uses a multiplicity of small flat optical faces, invented by the French engineer Augustin-Jean Fresnel and it is the only one that was not built experimentally during the oil crises of the1970s.As the "youngest" of the CSP technologies, LFR technology relies on an array of linear mirror strips which concentrate light on to a fixed receiver mounted on a linear tower. The LFR field can be imagined as a broken-up parabolic trough reflector, but unlike parabolictroughs, it does not have to be of parabolic shape. every planar (or nearly planar) mirror elements are all mounted at the same height near the ground. They follow the position of the Sun by rotating around their long axes so that they point to a focus line at a height of10-15m, which remains fixed over time. Along this line, an absorber tube up to1000m long is mounted, and in it HTF is heated or water is directly vaporized. The steam from many parallel absorber tubes can operate a large turbine.This thesis mainly discusses three matters about LFR system. Firstly, solar position algorithms are researched.10classical solar position algorithms are introduced and accurate computed methods using astronomical almanac is presented. By improving the algorithms of day number of the year, the precision can be tremendously increased. Secondly, the LFR’s optic geometry characteristics and optical performance are investigated and optimal system structures are presented. Finally, solar tracking control system for LFR mirror field is detailed description.High concentration solar thermal power systems require the Sun to be tracked real time for improving efficiency of generating electricity, which needs to know the solar position. The accuracy of computing the sun position is more important of opening-loop tracking control system than that of the others. This paper presents some typical sun position algorithms of which the characteristics was summarized by comparing and analyzing and appropriate algorithms were commended for different solar thermal-power systems.Aimed at some typical solar position algorithms which have been broadly used and compares its’computational error and summarizes the detailed algorithm of day number of the year for every solar position algorithms.The calculating accuracy of solar position can be improved by using optimum algorithm of day number of years. The results indicate that the algorithm accuracy can be improved by30%at least by corrected algorithm of day number of years and computational error of solar azimuth and altitude angle is smaller than0.02°if combining Bourges’declination angle formula and Lamm’s EOT formula.For optic geometry characteristics of LFR system, This paper presents a vector algorithm of incidence angle and reflected position and sun tracking tilt-angle of linear Fresnel concentrating solar device (LFR). In the LFR systems, every mirror needs to track the Sun real time and reflects the beam radiation to fixed linear receiver, therefore some concerned quantities consequentially vary throughout the year, which make the calculation very complexity. This paper simplifies the processes of derivation via vector analysis and educes some necessary formulae fitted into the application of LFR and farther shows the curves of various quantities on average days for months in2009by computing based on idiographic example.In order to testify the validity of above derived formulae, the software simulation and experiments was done. To optimize and design a concentration solar power system it is essential to know the performances of the subsystem formed by the receiver and the concentrating mirror field by master user-friendly modeling tools. Soltrace is an NREL-developed ray tracing code for optically modeling all types of CSP technologies. Soltrace is flexible and comprehensive allowing the user to easily generate complex geometries for troughs, linear fresnel, dish and tower systems and analyze its optical performances, furthermore assess the available flux distribution of receiver. This paper particularly describes the features of Soltrace and presents the using methods. At the same time, LFR system modeling is done as a illustrative example of application.It is necessary for analyzing the concentration ratio for designing of concentrating solar thermal power system. Every mirror segment or row of LFR needs to track the Sun real time and reflects the sunlight to fixed linear receiver. therefore, the band width on the absorber illuminated by every mirror row varies throughout the year, which makes the analysis of concentration ratio very complexity. About concentration ratio analyses, firstly, the formulae of projected angle of incidence and reflection solar rays, tracking inclination angle and band width illuminated on the flat plane absorber were obtained by a two-dimensional optic analysis. Secondly, the expressions of geometric concentration ratio of LFR were derived and the correlation between ideal geometric concentration ratio and number of mirror slats and relative distance were analyzed. And then, optimizations of mirror field width and tower height are analyzed based on analysis above. Finally, the effects of angular size of sun’s disc on geometric concentration ratio were illuminated.At the aspect of radiation algorithm of LFR mirror field, this paper firstly utilizes the equations of incidence angle and tracking inclination angle to computed the instantaneous solar beam radiation of all mirror elements. And then, cumulated irradiation of whole mirror field during the available work was obtained. Finally, the algorithm of optimal tower highness of LFR mirror field was derived by analyzing excursion of reflecting sunlight.It is necessary for analyzing and comparing the optical performance of different mirror field distributions when concentrating solar thermal power systems are designed. This paper analyzed and compared the different optical efficiencies including cosine factor, atmospheric attenuation and end spillage loss by making use of the formulae of incidence angle, tracking angle and the reflected vector for north_south and east_west aligned LFR mirror field. The reasonable LFR mirror field layout mode was recommended in order to fit different application, which will provide important reference and instruction for large scale LFR technology application.One difficulty with the LFR technology is that avoidance of shading and blocking between adjacent reflectors leads to increased spacing between reflectors. Shading is the loss of illumination on a given mirror due to the interception of the incident sunlight by a neighboring mirror element. Blocking is the loss of illumination on the receiver due to the interception of reflected sunlight by some other neighboring mirror. Shading and blocking effects play a major role in limiting the cost effectiveness of the LFR system. These effects depend sensitively on the arrangement of the mirror element in the horizontal plane and on the position of the sun. Consequently it is necessary to have an axact geometrical analysis of shading and blocking. Then with the help of a computer we can begin to construct the appropriate averages and to seek an optimization of the mirror array. This paper firstly got shading and blocking distribution of analyzed mirror element by coordinates transform and ray-tracing method, and then annual average shading and blocking efficiency of LFR mirror field was analyzed by statistic means. Finally, the effect of shading and blocking efficiency of LFR mirror field due to different linear absorber high, spacing between adjacent mirror element and width of mirror slat was analyzed and fitted calculating model of shading and blocking efficiency of LFR mirror field was presented.Advances in the algorithms of sun tracking systems have enabled the development of many solar thermal and photovoltaic systems for a diverse variety of applications in recent years.Compared to their traditional fixed-position counterparts, solar systems which track the changes in the sun’s trajectory over the course of the day collect a far greater amount of solar energy, and therefore generate a significantly higher output power. It has been shown that these sun tracking algorithms can be broadly classified as either closed-loop or open-loop types, depending on their mode of control. Because, on the one hand, the closed loop system can not fit some bad weather conditions and the accuracy of tracking is poor, on the other hand, the open loop system can eliminates many of the problems and costs associated with closed-loop tracking, the current trend in solar concentrator tracking systems is to use open-loop controllers that compute the direction of the solar vector based on location and time. To keep down the price of the tracking system, the controller is based on a low-cost microprocessor.The last chapter of this thesis includes the design and implementation of a microcontroller-based LFR single-axis solar tracking control system. This tracking control system consists of an8-bit microcontroller (AT89C51RC) with32K byte of internal EEROM and512byte static RAM, Real Time Clock (PCF8563T) and chip for keyboard and LED display (ZLG7290)This tracking system is able to follow the sun with a certain degree of accuracy, return the collector to its original position at the end of the day and also track during periods of cloud over, protect the mirror element by face down and allow manual control of the array for repair, testing and cleaning. Considering all above aspects of this tracking system, a computer software has been developed to calculate solar position based on installation position, the start time and correct initial alignment, and compute the tracking angle, and send corresponding signal to stepper motor driver, so that, the stepper motor drives mirror element to rotate and achieve the tracking task. it can be concluded that, it is a flexible tracking system with low cost electromechanical set-up, low maintenance requirements and ease on installation and operation. |
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