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RESEARCH ON IMPROVING UTILIZATION EFFICIENCY OF PHOTOELECTRIC CELLBY USING FLUORESCENCE MATERIALLong TengfeiCollege of Mathematics and Computing Science, Guilin University of Electronic, Guilin 541004, Chinagpglltf@Ding XuanhaoCollege of Mathematics and Computing Science, Guilin University of Electronic, Guilin 541004, ChinaCai Ruhua (Third Author)College of Mathematics and Computing Science, Guilin University of Electronic, Guilin 541004, ChinaLu WenquanGuilin Institute of Optical Communications,Guilin 541004, ChinaABSTRACTThis paper proves that it is practicable to build a new power model which can work efficiently at night by using a type of fluorescence material. The utilization efficiency of photovoltaic cell is improved by prolonging the actual working hours of the cell within 24 hours. Both theoretical analysis and experiment on this model are carried out, and base on which the current-time curve (I-t curve) is depicted. It shows that the model which applies the fluorescence material to photovoltaic system is available. According to the analysis and comparison between the attenuating curve of the material and I-t curve of the power system, we can conclude that the weak illumination and the mismatch on wavelength are primary reasons for the low current. Finally, several improved measures of this power model for practical application are given, and it is expected to provide a new thought for photovoltaic study.1. INTRODUCTIONIt requires massive manpower and great deal of material resources to excogitate the high efficient photovoltaic cell. But it can’t fully exert its potential as the influence of all kinds of ambient conditions, and which make the utilization efficiency be low in practice. The most prominent problemis that the actual working hours are short. The actual working hours of present photovoltaic system are less than 12 hours which make it can’t work normally after the sun fades away. It will make great improvement of the utilization efficiency of photovoltaic cell if let it generate electricity at night. It is necessary to study a new kind of power model which can work normally not only in day but also at night.The most part of sunshine is not used when it shines to the earth and if these energies can be stored in day and used at night just as the sun radiates energy, the utilization efficiency of sun would be greatly improved. Base on introducing the fluorescence material which can store energy in day and emit it at night , this paper introduces a way to prolong the efficient working hours of photovoltaic cell and build a new model of photovoltaic cell which can generate electricity at night, and this paper has dual confirmations of theory and experiments.Corresponding author’s E-mail: gpglltf@Guangxi Science Foundation Project (0542046)the Graduate Student Innovation Project of Guangxi, China. (20061059 50701MO3)Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement 11042. THE CAPABILITY OF FLUORESCENCE MATERIALThe fluorescence material is also called solar energy-storing material, which does not need electrical source, and what only need is to absorb the natural light. The process of absorbing—storing—radiating could circulate illimitably and could be used forever. Take the series of aluminate and silicate fluorescence material as an example, it could be radiating continually for more than 12 hours in darkness after exposed outdoor for 10~20 minutes.As the continuously improvement of the fluorescence material capability, it can be used to store the sunshine in day and emit the energy to generate electricity at night. It plays the role of “night sun”.3. THE THEORY FEASIBIL ITY OF FL UORESCENCE MATERIAL’S APPL ICATION IN PHOTOVOL TAIC SYSTEM3.1 Conditions of Producing Photoelectric EffectPhotoelectric effect is a kind of physics phenomenon which the objects’ inner electrons emit out from the surface when the light irradiates the objects in the form of photons. Every object has its own response light-wave called threshold wavelength. The photoelectric effect only happens when the wavelength of incidence ray is less than the threshold wavelength of the irradiated object.3.2 Theory Analysis About the Feasibility ofNight-Working ModelFirstly, to check that if the emission spectrum of fluorescence material matches the silicon material. Actually, the main emission wavelength range of fluorescence material, which was produced and developed at home or abroad currently, is 400 nm ≤ λ ≤ 600nm; the response range of silicon cell is about: 400nm ≤ λ ≤ 1100nm[2,3]. According to the conditions of producing photoelectric effect, the emission spectrum of fluorescence material matches the threshold wavelength of the silicon cell.Secondly, to check that if the excitation spectrum of fluorescence material matches the sun spectrum. The range of excitation wavelength of fluorescence material is 200nm ≤ λ ≤ 450nm. And the wavelength range of sun radiation which absorbed by the earth is about 150nm ≤ λ ≤4000nm. Therefore part of the sun radiation could be absorbed by fluorescence material and become the most popular excitation source.Fig. 1 (a) and (b) show the excitation spectrum and emission spectrum [1] of a type of fluorescence material SrAl2O4: Eu,Dy, Fig. 2 (a) and (b) show the sun radiation and the silicon material’s response spectrum to different wavelength light. According to the analysis and the Fig. 1 and Fig. 2, it is feasible in theory to make the fluorescence material act as “night sun” and generate electricity byphotoelectricity.Fig. 1: (a) The excitation spectrum of fluorescence material;Fig. 1: (b) The emission spectrum of fluorescence material.4 PV TECHNOL OGIES, SYSTEMS AND APPL ICATIONS1105Fig. 2: (a) Sun radiate spectrum;Fig. 2: (b) Silicon cell response spectrum.4. EXPERIMENTIn the experiment, the fluorescence plate which was produced by a factory in Dalian China (specification: 30cm ×20cm. Emission wavelength: 470nm—550nm) is put under the sun for 6 hours, and then is moved to a darkroom. Then put it onto the silicon cell plate whose maximum output power is 4 W, short circuit current is 0.5A, open circuit voltage is 9V under standard condition. The Fig. 3 shows the relation of the changing short circuit current and the time (I-t curve).5. DATA ANALYSISFrom the experimental I-t curve, the current generated by the night-model is low because of the following two reasons: compare with the radiating intensity of the sun, ①the intensity of the energy-storing material used in laboratory is weak indeed, so is the output current; the②Fig. 3: I-t curve.light wavelength from the energy-storing material (470 nm ≤ λ ≤ 570nm)isn’t the best matching of the best response range of silicon cell’s spectrum whose optimized response range is from 800nm to 950nm[4]. The current is stronger relatively at the beginning, but it goes down rapidly 30 seconds later and stabilizes until reach the value of 0.8μA (only the data gained in anterior 50 seconds is shown as the limited space). That because the material has two attenuations: rapid-attenuation and slow-attenuation. The intensity of fluorescence material’s eradiation weakens rapidly during the rapid-attenuation process and which leads to the weakening current (from 4μA to 1μA); and then comes to the slow-attenuation process whose weaken intensity leads to low current by 0.8 µA.6. CONCLUTION AND FUTURE WORKIt has been already proved that it is feasible to use fluorescence material to generate electricity in both theory and experiment. The low current is due to the weak intensity of laboratorial fluorescence material’s eradiation and the mismatch between fluorescence material’s eradiating wavelength and silicon material’s exciting wavelength. In the unsaturated conditions, the storing energy capability of the fluorescence material is in proportion to the irradiated time and the intensity of excitation source. So put the fluorescence material in sunny days, more energy could be stored, the eradiating intensity will be stronger, the lasting time of radiation will be longer, which bring more output power than that in cloudy days; as the response wavelength of Se is about 550nm which is more adjacent to existing fluorescence material’s eradiatingProceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement 1106wavelength (400 nm≤ λ ≤ 600nm) than silicon’s exciting wavelength. It will produce more power if replace silicon cell by Se cell in application, because the spectrum characteristic and the irradiancy plate’s emission spectrum of Se is more suitable than that of silicon. If a new energy-storing material whose eradiating intensity is stronger, lasting time is longer and radiation wavelength is longer is developed, it could be popularly applied in the photovoltaic system. At the same time, a new study subject is pointed out to the field of energy-storing material’s research.7. ACKNOWLEDGMENTSThis work was supported by Guangxi Science Foundation and the Graduate Student Innovation Project Foundation of Guangxi, China. The authors would deeply like to thank the colleagues in College of Mathematics and Computing Science, Guilin University of Electronic Technology for their help on the preparations for the physics experiment.8. REFERENCES(1) L i Guolong, Tan Jingming etc.. Study of L anthanon’sRadiation Capability [J], SiChuan chemical industry, 2006.9(4), p8~10(2) An L iuying, L iu Jifang etc.. Photoelectron Technology[M], Electronical Industry Press, 2002, p132(3) Zhang Jixiong, Optoelectronics Course [M], SouthChina University of Technology Press, 2001, p235 (4) Zhao Zhengming, L iu Jianzheng etc.. Solar energypower generating and Its Application[M]. Science Press 2005, p32~33。