ASPECTS REGARDING TO ENERGY EFFICIENCY OF STAND ALONE PHOTOVOLTAIC SYSTEM USED FOR ANTI HAIL STATIONS

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1 ASPECTS REGARDING TO ENERGY EFFICIENCY OF STAND ALONE PHOTOVOLTAIC SYSTEM USED FOR ANTI HAIL STATIONS Laurenţiu ALBOTEANU, Gheorghe Manolea, Constantin ŞULEA Faculty for Electromechanical, Environment and Industrial Informatics Engineering, University of Craiova 107, Decebal Bl., , Craiova, Tel , Fax , ABSTRACT This paper presents aspects concerning to increase the energy efficiency of stand alone photovoltaic system used for anti hail stations. Are shown ways to increase energy efficiency of PV system. For a method, the theoretical concepts presented are substantiated by numerical simulations, with HOMER specialized program and by experimental results. Experimental results were obtained on an orientation system for PV panel type pseudo-equatorial. Keywords: energy efficiency, photovoltaic system, anti-hail, solar irradiation, panel orientation, 1. INTRODUCTION The use of alternative energy sources is now one of the main world topics due to the fact that the World Energy Council estimates that the primary energy consumption will rise 40% by the end of This rise will be possible mainly by using these alternative energy sources; also it has to be taken into account that the world reserves of oil and natural gas are estimated to finish off in tens of years and the coal reserves in 200 years. More over, the nuclear energy has taken almost 30 years of research and 20 years of implementation in order to satisfy nearly 10% of the world demand in primary energy. In this sense it has to be mentioned that the nuclear energy production encounters a very strong opposition from public opinion in almost all developed countries. Watching to the actual solar energy utilization level it seems that in Romania are in fact very few significant and functional projects having real practical applications in the domain. So, the approach of the analyzed theme and generalization of results applied to the national anti-hail system are really justified. Recently released estimations reveals that the electric energy provided by alternative sources will reach some 8.3% of the entire national electric energy consumption by the end of The importance of the theme is related to the utilization of solar energy as well as to the destination of equipments driven by this energy (isolated ant-hail stations). On one hand, the installed power of these equipments is small indeed and the utilization period of time can reach 6 months, but, on the other hand, the alimentation network construction spending is quite big. More over, some of the isolated stations have to change their locations every year. Also, we have to consider that the active exploitation of the anti-hail stations during a year starts early April and ends late September and that coincides with the maximum solar radiation period of time during the same year. It turns out that the utilization of the solar energy represents a really successful solution and is important to the growth of National Anti-Hail System both in Moldova and Oltenia. This theme is also important because 60% of the storms on the Romanian territory (during the march-september period of one year) are accompanied by hail falls and 40% of these hail falls lead to significant or even total losses in crops. So, the present work is related to the new and actual research domain and its main goal consists in building up a study concerning the existing technical solutions and exploring some new ways in order to solve the identified problems. Recent researches, made in different countries, reveal that considering the price/efficiency ratio, the most part of anti-hail systems works by using rockets as carrying vector of the condensation germs. But in order to launch these rockets an electric energy source is needed in order to power the rockets fuses. Security reasons require that the launching stations of rockets have to be placed far out of reach of towns or villages, so, these launching stations simply cannot access the public electric energy network. At least for now, the powering of rockets fuses is made using electric accumulators that are periodically recharged using a Diesel generator; sometimes the accumulators are transported to the command centers. In this way the topic discussed is of extremely importance. The utilization of photovoltaic module to directly power the accumulators will significantly reduce the spending level on liquid fuel as well as the number and size of those accumulators.

2 2. ELECTRIC ENERGY CONSUMER AND STRUCTURE OF PHOTOVOLTAIC SYSTEM The needed daily electricity for consumers of a antihail station is 1004 Wh/day. Consumers Nominal voltage, [V] Radio station Nominal power, [W] Main users and their characteristics are presented in Table 1. Positioning of anti-hail missile launch ramp is made with DC motors with low power consumption. Lifetime, [h/day] Electricity needed, [Wh/day] Warhead missile launchers 24 1,2x6 0,3 2,16 Servodrive of PV panel orientation ,2 12 Servodrive of ramp orientation 1 2 2x300 0,2 120 Development system with microcontroller Lighting 220 2x TV Radio Total electricity needed [Wh/day] 1004,16 Tabel 1: Electric consumers of a anti-hail station The ensemble consisting of photovoltaic modules (PV), electronic power conditioning, battery and protective factors is called photovoltaic system. Typically, a PV system includes the following components: PV modules, power conditioning devices, charge and discharge controller, converter DC-AC, PV module support structures, wiring and power distribution boxes, elements of protection, anti diodes - return, bypass diodes or by - pass, automatic switches or fuses illegible, batteries to store energy. In figure 1 is shown the block diagram of stand alone PV system for anti hail station. Figure 1: Bloc diagram of Stand Alone PV System To ensure high performance as the photovoltaic system, it is necessary to be considered technical conditions: a) Universality - the system must provide electricity as more consumers characterized by different nominal supply voltages; b) Robustness, low mass, the possibility of traveling with the most common means of transport, including manually; c) To ensure a high degree of autonomy during cloudy or at night; d) To monitor operational parameters and the available energy; e) To ensure a high efficiency. 3. METRHODS OF INCRESE ENERGY EFFICIENCY OF PHOTOVOLTAIC SYSTEM Requirement imposed to the photovoltaic system is that to convert sunlight as much as possible, in the desired energy form and can be used by consumers, with minimal losses. For radiation receptor (PV module), this means that: - the full spectrum of solar radiation to be absorbed completely. - all energy obtained from each absorbed photon will be converted only in the form of energy consumed by the user. The fulfill of these requirements not only depends on the system s quality. In most cases, losses arising from fundamental physical reasons limit energy conversion efficiency. Convert radiation into electricity involves: - according with the material properties of solar cells, only part of the solar spectrum will be absorbed (because solar cells have a certain colors depending on their type).

3 -only a fraction of energy absorbed by the cell is converted into electricity, a significant proportion is converted into heat causing heating modules during operation. Quality conversion of solar radiation in the total energy used is described by the value of process efficiency (yield): E E 2 η = (1) 1 where: E 2 - is the total energy used; E 1 - is the energy absorbed by photovoltaic panel This quality of conversion should take into account all the losses occurring in the system. The power processing systems have also a decisive influence and affect the whole system performance (eg transport of energy losses, low efficiency of electronic components that have partial load, etc.). Since PV modules have a relatively low yield (up to 30% in laboratory conditions) the aim to optimize their energy. 3.1 Increase of energy efficiency by orientation of photovoltaic panels The conditions imposed of the solar radiation receiver also play an important role in energy production. As the sun changes with the seasons and over a day, the amount of radiation available for the conversion process depends on the panel orientation. A method of optimizing available solar energy conversion with real possibilities of implementation is the use of orientation systems. Literature [6], shows that the use of orientation systems increase from 20% to 40% the amount of energy produced by converting (fig. 2). Figure 2: Explanatory regarding of energy efficiency of photovoltaic system Ideally, a PV panel should follow the sun so that sun rays fall perpendicular to its surface, thus maximizing solar energy capture and thus we obtain the maximum output power. In practice are two kinds of orientation systems: - passive orientation systems; - active orientation systems. Passive orientation systems follow the sun without a motor drive. The system consists of gas-filled tubes located on both sides of the panel. When the sun warms the gas found in the first tube, it relaxes and empties into the second tube. System changes its equilibrium position and the panel bowed to the sun automatically. Usually these systems are used frequently in equatorial areas, since they have a structure with a single axis orientation system, which leads to maximum efficiency only in those areas. Active orientation systems are used to drive actuators panel. There are two distinct modes of active orientation of a solar energy conversion system: after a single axis of rotation and after two perpendicular axes of rotation. If after two axes orientation are distinguished 3 types of systems, depending on how the axes are located and how the two movements are entered into the system [5]: azimuth systems, the equatorial system and pseudo-equatorial system. 3.2 Increase of energy efficiency using maximum power point tracing method One experimentally, observed that the PV cell present the great oscillations of output electrical power according to the solar radiation intensity and to the meteorological conditions. In addition, when they debited on the electrical charge on can observe certain problems, and power transferred to the charge, rarely correspond to the maximum power transferred by PV panel. This method is called Maximum Power Point Tracing (MPPT) and it forces the PV system operates in the maximum power point[4]. When a power source is connected to a charge, the work point is found to the intersection between (I-V) characteristics. This point is permanently modified, because of the power source or the charge is modified permanently. For this reason, it does not work in MPP, and the power furnish to the charge is less than maximum power which could be emitted. The principles of the regulators MPPT are frequently based on the Power-Volts (P-V) characteristic the elbow [9]. It s a method based on explore, as we can see in figure 3. Being in a certain point on the curve (X 1 ), one can see if the power value in the next point is higher or not than first one. If yes, the work point is moved in next point (X 2 ), until the next value (X n ), become lower than the previous one value (X n-1 ). In this moment one take into account the smaller interval between the known and specified

4 points and one can also start again going on from (X n-1 ) until the MPP is reached. When the radiation intensity is modified, from E 1 to E 2, with E 2 >E 1, the P-V characteristic modified by it self. The point (X), which has been MPP until now, becomes an untrue work point in the new conditions as one can see in figure 4. Another point is MPP, noted (X'). As in the linear regulator case, the control is based on an adjusting system which has the input output variable (X i ) respectively (X o ) (figure 5). In the most of the adjusting systems, one needs only a measurement for finding out the report between X o and X i. This one isn t available any more in an individual system in which this report depends on the time. This permanently evolution of X i determines some permanent oscillations around the maximum value. 3.2 Increase of energy efficiency by reducing the temperature photovoltaic cells The energy efficiency of a cell depends on the lighting and temperature. Temperature is an important parameter, because the cells are exposed to solar radiation, their heating is possible. In addition, part of the energy absorbed is converted to electricity: it dissipates as heat. For those reasons, cell temperature is always higher than ambient temperature. To estimate the temperature of cells (T c ), knowing the ambient temperature (T a ) may be used expression: Em T c = Ta + ( Toc 20) [ºC] (2) 800 where: E m - is the medium lighting; T oc - is the temperature operating cells. In figure 6 is presented the influence of temperature on the characteristics of USP 150 photovoltaic module with monocrystalline silicon cell at a solar irradiation of 1000 W/m 2 [1]. Figure 3: The principle MPP search Figure 4: The resultants solar radiation modified about MPP Figure 5: Classical scheme of MPP regulator The modification points can be assimilated with a perturbation in the maximum adjusting control system. As a consequence, if one knows the derivate sign X o and if this one show that X o deviates from the maximum, the regulator is changes the sign and direction of X i for finding out once again maximum. Figure 6: I-V curves of USP 150 PV module for different temperatures at 1000 W/m 2 In the figure is apparent that the cell temperature has a great significance on electrical performance. With both the temperature is lower, with both the cell is more efficient. Each degree of warming of the cell causes a yield loss of around 0.5%. The empirically found that photocurrent increases less with temperature (in the order of 0.05% / K for silicon cells). It is also apparent that the maximum power point can have significant variations.

5 4. SIMULATION OF STAND-ALONE PV SYSTEM In order to determine the energy production and the energy efficiency of stand alone PV system, it was developed the modeling and the simulation of this system. It was used a specialized software for simulation of renewable sources named HOMER [8]. HOMER simulates the operation of a system by making energy balance calculations for each of the 8,760 hours in a year. For each hour, HOMER compares the electric demand in the hour to the energy that the system can supply in that hour, and calculates the flows of energy to and from each component of the system. For this system that includes batteries, HOMER also decides for each hour how to operate the generators and whether to charge or discharge the batteries. In figure 7 are presented HOMER components of stand-alone PV system. The simulation results give information about the work of system and the energy production. In order to evaluate the energy production of PV system were analyzed two cases: in the first case the system uses fixed panels while in the second case uses panels oriented. The graphical of monthly average electrical energy production by fixed PV system, using HOMER program simulation is presented in figure 9, and monthly average electrical energy production by oriented PV system is presented in figure 10. Figure 9: Monthly average electric production by fixed PV system Figure 10: Monthly average electric production by oriented PV system Figure 7: HOMER components of PV system For simulation of PV system must be made an introduction of input dates in system. Using weather data provided by ESRA program for Craiova location [1], results graphics simulation of PV system analyzed. In figure 8 are presented the dates regarding the daily radiation and clearness index. Figure 8: Solar resource input of PV system Graphs of simulations show that the annually average energy production by the fixed system is 401 kwh/year and 560 kwh/year by the oriented system. The oriented system are better than the fixed system because the first mentioned produces substantial energy achieved during April-September and coincides with the active period of anti-hail stations. 4. EXPERIMENTAL RESULTS The experimentation of pseudo-equatorial orientation system was realise in location Craiova in June month, between and hours with a 150W PV panel. The experiments was aimed at studying the efficiency of a 2 axis orientation system. Using a pyranometer it measured solar radiation on a horizontal plane, a plane inclined at an angle of 45 degrees respectively, on an inclined plane at an angle of 25 degrees. Angle of 45 degrees is considered appropriate for optimum yearly fixed angle mounting photovoltaic panels.

6 Angle of 25 degrees is the optimum angle of inclination of the photovoltaic panel in June, the month in which measurements were made. Also was measured current and voltage of photovoltaic panel oriented after two axis and was calculated power provided by photovoltaic panel. In figure 11 is shows the graphs of output power of photovoltaic panel for the two cases analyzed. Using an oriented system after two axes, for photovoltaic panels, it is increased the energy efficiency with about 30%. Analysis of graphs obtained from measurements result: - Higher values of power supplied by photovoltaic panel tilt angle of 25 degrees; - Photovoltaic panel conversion efficiency is much higher if the panel is oriented at an angle of 25 degrees from where it is oriented at an angle of 45 degrees; - Panel achieves maximum conversion efficiency of 15% during the afternoon, conversion efficiency give by the manufacturers for mono-crystalline silicon cells constituting the panel. All these considerations justify the need for an orientation system for PV panels. References Figure 11. Output power of PV panel To study the influence of the orientation for photovoltaic panel on the process of converting solar energy was calculated the conversion efficiency of the photovoltaic panel for the two cases analyzed. In figure 12 is shown the conversion efficiency of photovoltaic panel for the two cases analyzed. Figure 12. Conversion efficiency of PV panel 5. CONCLUSIONS Software HOMER program, including any modules and subprograms, data, and information are provided as a renewable energy source. HOMER, the micropower optimization model, simplifies the task of evaluating designs of both offgrid and grid-connected power systems for a variety of applications. The simulation results give the information about the system s work of and about the energy produced by this. [1] Alboteanu L., Cercetări privind utilizarea energiei fotovoltaice pentru alimentarea staţiilor antigrindină izolate, Teza de doctorat, Universitatea din Craiova, [2] Alboteanu L., Manolea Gh., Ravigan Fl., Nour A., Strategy Of control For Solar Panels Possitioning Systems, Annals of the University of Petrosani, Electrical Engineering, vol. 9, 2007, ISSN , pp [3] Alboteanu L., Novac Al., Ravigan Fl., Gh. Manolea, Automation and supervision for orientation of the autonomous photovoltaic panels, Buletinul Institutului Politehnic Iasi, Tomul LIV,2008, pp , ISSN [4] Alboteanu L., Manolea Gh., Ravigan Fl., Positioning systems for solar panels placed in isolated areas, Proc. of the International Conference of Applied and Theoretical Electricity, oct., Baile Herculane, 2006, Ed. Universitaria 2006, ISSN pp [5] Comşiţ M. Mecanisme de orientare specifice sistemelor de conversie a energiei solare, Teza de doctorat, Universitatea Transilvania din Braşov, [6] Messenger R, Ventre J, Photovoltaic System Engineering, CRC Press, 2005, Sec. edition. [7] Perez Richard, To Track or not to Track, Home Power, no 101, june & july, [8]***, National Renewable Energy Laboratory, Getting Started Guide for HOMER Version 2.1, April [9] ***,