Study the feasibility of the Rooftop panels on Schools in the provision of solar energy in Tripoli Abdalhadi Atia Alateki Giamal Mashina School of Engineering and Applied Sciences Libyan Academy Tripoli, Libya firstname.lastname@example.org The center for solar energy research and studies Tripoli, Libya email@example.com Abstract— There is significant potential for the use of the solar energy in Libya, which receive abundant amounts of solar radiation around the year. The schools building of on-grid PV solar plants can be contribute to the diversification, independence, ecological and economic sustainability of the national power supply system. them, their long axis facing to the southeast and another 40%, their long axis facing to the southwest. This paper studied the feasibility of using roofs of government schools at Tripoli city in the generation of electricity from solar energy to cover schools load and export excess power to reduce the loads on the general low voltage electrical network. SuqAltholatha School was selected to study how to take advantage of the implementation of the GCPV solar system. In this work, typical Poly-crystalline modules with capacity of 255 Wp are chosen in the analysis. The horizontal PV system proposed based on the available roof space was of a 73.4 kW. The array consists of 288 modules. The PV array is configured in a way that the string includes 24 modules connected in series with 12 inverters, the estimated specific energy productions is found to be 1399.09 kWh/kWp per year, the annual performance ratio is found to be 71.9%, the estimated annual energy production is found to be 103.05 MWh Fig 1. Rectangular roof (RR) of SuqAlthlatha School and its orientation The rest of them (20%), their long axis have completely southern facing as shown in fig 1. One of rectangular-shaped roofs (RR) school type is SuqAltholatha School. It is located in the east of the Tripoli city as shown in Fig 2. The results showed that it could benefit from the roofs of schools in the city of Tripoli to produce electricity from solar energy, thus reducing the load on the public network, especially in the summer time when the schools are not working.) Keywords— GCPV, roofs of schools, solar, PV array I. INTRODUCTION The electrical energy is the necessity for the economic development of Libya and most of the energy produced today comes from burning fossil fuels, which are limited and caused pollution. There is significant potential for the use of the photovoltaic solar energy in city like Tripoli, which receive plenty, amounts of solar radiation around the year. The roofs of school buildings in particular could be one of the best options for the production of electricity from the sun. There are a total of 380 schools in Tripoli , 50% of them can be classified into two categories based on the building’s roof form, the first category deals with a rectangular form (RR) and the second one in the form of (U). There are 110 of (RR) schools, 40% of Fig 2. SuqAltholatha School It has been chosen as a case study. Choosing this school (faced to the NW-SE direction) based on that it produces less energy compared with the other schools. II. SITE CHARACTERISTICS Tripoli's is the capital and largest city of Libya. It is a coastal city in the north west of the country. It is located on latitude 32°52′30″ N. and longitude of 13°11′14″ E. The climate of this area is typically hot semi-arid climate with long, hot and extremely dry summers with relatively wet and warm winters. Climate data for Tripoli city are clearly listed in the Table 1. . TABLE I. CLIMATE DATA FOR TRIPOLI III. THE AVERAGE YEARLY ENERGY YIELD OF SUQALTHOLATHA SCHOOL There are many of different size solar panels available. Specifications of solar panel are important for sizing the PV array, these specifications as shown in table II. have been chosen because of good warranty not high price and not bad efficiency. TABLE II. The daily average solar radiation rate on a horizontal plane of /d in in coastal region, with an average about 7.1 (KWh)/ sun duration of more than 3500 hours per year and 5.33 PSH. The variations of the clearness index values are not far apart, but in general, as shown in Fig 3. With low values characterized in winter season and high values in summer time . The solar data of plant location is assumed to be as of Tripoli and is adopted from NASA Satellite included in the database of software Homer. Fig 3. Tripoli global horizontal radiation In order to choose the optimum placement of solar modules on the rooftop of the school, it is important to consider and determine the free space of the shadows because of the surrounding obstacles, the structure of the roof, and due to the self-shadowing of the solar modules themselves during all hours of the day and throughout the year. The available roof free area of shadows for SuqAltholatha School and the number of solar modules can be installed on its roof is estimated using the Skechup program. It is found that a 464 free roof top area and the required numbers of modules are obtained using the module area as follows: THE MODULE DATA The average yearly energy yield can be determined by using the following formula : = * * * * * = rated output power of the array under standard test conditions. The factor including derating due to manufacturers output tolerance ( , derating due to dirt ( ), and derating due to Temperature ). In SuqAltholata School, there are 288 ( modules and its PSH equal 5.33, the actual DC energy output from the solar array will be equal the derated output power from all modules multiplying the PSH. The DC energy will be reduced again due to the losses (3%) in the cable connecting to the inverter. By assuming that the inverter efficiency ( ) is 95%, the DC energy will be further reduced due to conversion of DC to AC. The AC energy will be reduced through the cable connecting to the grid. By assuming that the AC cables losses ( ) is 1%. The AC energy delivered to the grid will be: =255*0.873*0.95*0.95*288*5.33*0.97*0.95*0.99 =281.35KWh. The energy yield of the solar arrays over a typical year for SuqAltholatha School will be: = 365 * 281.35 =102693.9 kWh/y The specific energy yielded will be equal SY= = 102693/ 73.4= 1399.09 kWh/Wp-y The ideal energy can be calculated by using the following formula: = x Where The sunny time, 6 hours during the normal day, start from 10:00 am to 4:00 pm. The possible plant capacity is 73.4 KWp. = yearly average daily irradiation, in KWh/ for the specified tilt angle. Therefore the yearly PSH would be: =5.33*x 365= 1945.45 h/y. The ideal energy from the system per year would be: =73.4x 1945.45 = 142796.03 KWh/y. summer holiday period (June, July and August) is at the lowest levels. At this time, most of the electrical energy produced by The performance ratio is a reflection of the system losses. PR= =102693/142796.03 = 0.719= 71.9% Based on: manufacturer, dirt, DC and temperature losses for Tripoli, the inverter of GCPV system is chosen to be large enough to handle the total amount of Watts at one time, it is rated at 84 KW . When the temperature is high as in Tripoli during summer time, it is important that the maximum power (MPP) and voltage at Maximum Power ( ) of the array is not falling below the minimum operating voltage of the inverter. The maximum effective cell temperature will be 73°C. According to the module selected, the voltage would be = 48 x - 0.431=-20.69V The Fig 4. The scheme in the Homer tool (a) without and (b) with battery PV system will be transferred to the grid, which is support the local electricity network at its annually rush time. In addition, input Values as shown in Fig 5. , tell Homer that Load reach to 63.75KW. at 73°C will be = 30.8 -20.69 = 10.11 V. If the maximum voltage drops in the DC cables of 3% then the effective minimum MPP voltage input at the inverter for each module in PV system will be: =0.97 * 10.11 = 9.81 V If the minimum voltage window for an inverter is 180V and by taking in the account, safety margin of 10%, then the minimum inverter voltage should be equal 198V.The minimum number of modules in a string is =198 / 9.81= 20.18 ≈ 21modules. During the winter season, the temperature could reach 0°C with the open circuit voltage ( ) of 38.19 V and a voltage coefficient of (-0.431V /°C). Solar panel' specifications). Therefore, the would be increased by = (0-25) * (-0.431) = 10.78 V The at 0°C would be =30.8+10.775= 41.58V If the maximum voltage allowed by the inverter is 1000V. The maximum number of modules connected in series in each string will be: =1000 / 41.58 = 24.05 ≈ 24modules. Then the number of inverters will be 12 inverters of size 7 KW. IV. IMPLEMENTATION OF THE HOMER MODEL The grid-connected system was modeled using Homer software as shown in Fig 4.a. A GCPV system Composed of 73.4 kW of PV and 84 kW inverter, grid utility and primary load of school. On the schematic, it is could be noted that it is possible to add a backup battery bank for critical loads in case of off utility as shown in Fig. 4.b, the arrow connects PV system to the DC bus and shows the direction of the energy flow to the inverter. The load profile and the roof area is essential for the designing process, the average load of typical school in Tripoli has been inputted as AC load in the load profile table. Homer showed that the school load in the Fig 5. Load inputs In the fact it is impossible to operate all loads together from 8: am to 4: pm for school but there is a load factor of about 13.2% of the Load does not operate during all this period of time with annual average of energy consumption for this school was 363 kWh/d. In costs table horizontal PV system size of 73.4 KW with lifetime of 25 years, capital cost of PV panels was 38,168 $ , Operation & Maintenance 300 $per year , 77 % derating factor , 15.5 % efficiency and it can operate in hot weather where the ambient temperature reach to C . Values tell Homer that the inverters of 84 KW with efficiency of, 96 %, cost 8400$ with lifetime of 15 years, that means inverters cost 16800$ for 25 years of the system lifetime where they are replaced after 15 years of operation.. V. RESULTS ANALYSIS The results show that the horizontal PV system installed on RR of SuqAltholatha School facing NE-SW can produce 103.05 MWh/y as shown in Fig 7. For this system, as shown in Fig 6. , it can use an inverter with capacity of 84 KW with maximum output of 58.1 KW and capacity factor of 14%, with losses of 4.3 MWh/y during 4384 hours. Fig 8. Energy purchased/sold between PV system and grid Figure 6. Inverter output Fig 7. shows that the seasonal peak sales to the grid during summer time when schools' load at the lowest level which is one of the advantages of using this system. In this study 3 KW load considered as a critical load, this load need number of batters can be calculated as following: In case of choosing a deep cycle battery of 105Ah and 12V DC and cost 233 $  the cost can be found as below: = 233*1.33= 310 LD for each battery. Now to cover this critical load of 3KW for at least 3.5 hours in the blackout periods, it is necessary to determine the suitable number of batteries. Where the inverter stop at 25% of battery discharging for battery safety then each battery can deliver only: = 105* 0.75= 78Ah The Batteries that is needed to install = (3.5 h *3000) / 12*78) ≈ 12 Figure 7. Annually electric production from horizontal PV system on RR faced NE-SW The results of Homer program as shown in table 3. idecats that SuqAltholtha School will produce annually an amount of emission approximately 57346 Kg/y due to burning fossil fuel to generate electrical energy, which is caused pollution . Using horizontal PV system of 73.4 KW, can save the environment from amount of 5070.5 Kg /y of emission (The percentage saved about 8.8 %) as shown in Table III. TABLE III. THE ANNUALLY AMOUNT OF EMISSION PRODUCED (FOSSIL FUEL) AND SAVED (PV SYSTEM) Although using the PV system, still some gases produced due to the consumption from grid’s fuel. Saved emissions means saved money. Where each barrel of oil can produce about 600Kg of , then the saved money over 25 years due to using GCPV system in SuqAltholata School, with assumption that the barrel cost of 20$, will be 4225.4 $. From the simulation results, it can be pointed out that annually school purchase energy of 47.31 MWh from the grid to operate its load and PV system sell energy of 61.66 MWh/y to the grid. For insuring a continuance for energy supply, backup system is needed to be installed. Then the cost of 12 batteries for each 5 years will be 3720 LD. VI. CONCLUSION Tripoli is the largest consumer of electrical energy in Libya is suffering from a shortage of electricity and forcing the distributor to practice regular load shedding. Therefore, decision-makers aimed to compensate for the lack of electricity through the introduction of solar energy technologies. The solar potential of the country is very good. The schools building of on-grid PV solar plants can be extremely contribute to the diversification, independence, ecological and economic sustainability of the national power supply system. A proposed PV system were planned to meet a part of the school's entire electric load and inject some of their high energy production to the low voltage electricity network, the systems were sized and simulated using SketchUp and HOMER programs. In our case (RR-rectangular school's rooftop), the system is composed of 73.4 KW of PV and 84 kW converters with average school's load consumption of 88695 KWh/y. The production of PV system was 103045 KWh/y, which is, covers all school's demand of electrical energy all over the year, producing additional energy transferred to the public network The results showed that it could benefit from the roofs of schools in the city of Tripoli to produce electricity from solar energy, thus reducing the load on the public network, especially in the summer time when the schools are not working.  REFERENCES     2015 سنة النشر, منشورات مكتب التعليم األساسي والمتوسط طرابلس Tripoli,”https://en.wikipedia.org/wiki/Tripoli ”, access for the site on 22/12/2016. M. A. Al-Refai Optimal Design and Simulation of a Grid-Connected Photovoltaic (PV) Power System for an Electrical Department in  International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering Vol:8, No:6, 2014 Solar Electric Photovoltaic, Solar Direct Modules, ”http://www.solardirect.com/pv/pvlist/pvlist.htm”, access for the site on 15/1/2016. 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