Olanrewaju L. Kadir 1, Okechi Onuoha 2, Nnaemeka C. Onuekwusi 3, Nwanyinnaya Nwogu 4, Uzoigwe U. Victor 5, Albert C. Agulanna 6

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1 Exloiting the Potentials of Photovoltaic Cells in the Extension of Lifetime of a 2.5 ka Power nverter for Stable Power Suly in a Develoing ation Olanrewaju L. Kadir, Okechi Onuoha 2, naemeka C. Onuekwusi 3, wanyinnaya wogu 4, Uzoigwe U. ictor 5, Albert C. Agulanna 6,2,4,5,6 Projects Develoment nstitute (PRODA), igeria 3 Federal University of Technology, Owerri (FUTO), igeria Abstract: The erennial yearn of develoing countries for stable electricity suly and individual efforts to overcome this challenge are well noted situations. This study basically resents an alternative ower generating system (ower inverter with Photovoltaic cells backu) that rovides electricity to household aliances during failure from national grid. This continuity of suly is achieved by converting direct current (D.C) from rechargeable batteries to alternating current (A.C) at a given frequency and sulying it to the load unit automatically without any noticeable interrution. The system switches over to the mains suly once the ublic ower is restored and back to the inverter during ower outage. For imroved efficiency and extended lifetime, the system consists of two comlementary charging sources that charge the rechargeable batteries. The two charging sources are solar hotovoltaic (P) cells and a rectifier circuit. Detailed design calculations and considerations are shown in the work. Switches are incororated in the system to switch the source of charging between the charging sources and to establish connection between the load and either the mains or the batteries. The different tests and analysis erformed on the system show that the P cells rovide significant imrovement on the battery charge (lifetime) of the inverter unlike when the system s only charging source is from the mains. Keywords: Photovoltaic backu, comlementary outut, inversion, low battery tri, eak inverse voltage.. TRODUTO Electrical energy lays an imortant role in the socio-economic and technological develoment of any country. While develoed countries enjoy stable electricity suly, develoing countries are still faced with the challenge of eiletic ower suly. To overcome this situation, electricity consumers in develoing countries seek alternatives mostly individually. As alternative ower suly technologies, the inverters are seedily gaining oularity and gradually relacing the generators because of their low cost of oeration, maintenance and environmental friendliness. Power inverter is an electronic system or circuit that converts direct current (DC) stored in a battery to alternating current (AC). t is used to suly continuous ower to the load connected to its outut. The inverter derives its ower from energy stored in batteries and erhas has its greatest setback emanating from its deendence on the eiletic ower suly from national grid to charge its battery. The exerience is that most times the inverters may not be readily available as a result Page 45

2 of limited lifetime determined by the battery charge. Although the lifetime of inverters is a function of other factors like the reliability characteristics of the inverter, inverter configuration and reair time [], however in this aer the lifetime is assumed to be only a function of battery charge. This aer is a develoment of our earlier work in [8] and is necessitated by our realization of the need for a reliable, effective and better system that will suly electric ower continuously to domestic house during ower failure from the national electric suly. n [8], the design of a low cost 2.5KA inverter was undertaken. However, the realization of its short lifetime as a result of low battery charge occasioned by the eiletic ower suly from the national grid roelled the need for a backu charging source exlored in this aer. Research efforts have shown that electric ower generated from hotovoltaic cells (Solar anels) can be used as a ready charging source for inverters [2-5]. Electricity generated from sunlight is called solar electricity and the rocess of converting solar light into electricity is known as the hotovoltaic rocess. Energy roduced by hotovoltaic rocess can solve the ower crisis exerienced by develoing nations [6]. A hotovoltaic or solar module consists of several interconnected solar cells that are embedded between two glass or lastic lates and are therefore rotected from the effects of the weather. As a rule, the modules are installed in a frame on a roofto or a suort mount [7]. The design of a 2.5KA inverter with P cells as backu for the charging of the inverter batteries is exlored in this aer. The aer comrises of five sections. Section one is the introduction while section two resents a brief survey of revious works on the theme of the aer. The design methodology in section three resents the block diagram, circuit calculation and functions of each block. Testing and erformance evaluation of the system are resented in section four while section five concludes the aer as well as makes recommendations for further works.. PREOUS RELATED WORKS The use of hotovoltaic cells (Solar anels) as a ready charging source for inverters has been exlored in a lethora of literatures. A market imact assessment of P inverter systems in the United States of America and by extension the globe was carried out in [7]. Results from this study show that the P inverter market is booming with significant growth in the residential sector. Research efforts in [9] resent the designing of a low owered (25-30 watts), ortable and cost effective solar micro -inverter wherein the single solar anel is able to run the AC loads along with DC loads. The system consists of a solar anel, DC - DC ush -ull converter, DC - AC inverter, LC filter and the test loads. A closed loo analysis was erformed on the system and the simulation results obtained show a good agreement with the theoretical analysis resented in the aer. The steady state error was reduced by using P controller. n a related work in [0], a hotovoltaic (P) based 500W solar inverter system is develoed which consists of P Array, battery bank andsolar inverter cum charge controller. The system works on both solar and AC mains ower deending on the energy requirement. The P based inverter system was designed, analyzed, develoed and simulated. The system was tested on resistive and inductive loads. oltage/current waveform analysis and ower quality analysis were carried out using ower quality analyzer as well as load sharing between the two schemes of P array and battery and then battery and mains suly. The 2.5KA ower inverter discussed in this aer extends the lifetime of the inverter by the incororation of P cells which serves as a backu to the mains suly to ensure steady charge in the inverter battery.the batteries will be charged by two comlementary sources, a rectifier circuit and solar energy. The rectifier circuit consists of a transformer, a bridge rectifier, a filter and a voltage regulator. The solar energy is obtained from solar (hoto-voltaic) cells. An automatic change over system will shift the charging source from national grid in case of the national ower suly failure to the solar source. Priority is however set that if there is ower suly from national grid the charging source will be from it.. DESG METHODOLOGY The system block diagram is shown in Figure 3.. t illustrates the solar anels and their connection to the inverter system. Descritions of the constituent units of the entire system as well as some analysis leading to the system realization are resented. The circuit diagram of the system is shown in Figure 3.2. Page 46

3 Solar Panel Solar Panel Rectifier Circuit Charging Controller Battery Control Switch Oscillator/ Driver Unit Transformation Unit AC Main Unit Battery Change Over Switch Load/outut Unit Fig. 3.: Block Diagram of P Power nverter Fig. 3.2: Circuit Diagram of P Power nverter Page 47

4 3.Outut Unit/ Load Unit After the conversion of dc to ac by the inverter, the ac voltage roduced is connected to a socket outlet. All the loads (electrical aliances) are connected to the inverter through the socket. A voltmeter may be included in the outut to measure the outut voltage. 3.2 Change-over unit This unit is use to change the suly from either the battery or the mains suly. A relay switch is incororated at the outut to erform this function. 3.3 Transformation/Load Unit A transformer is a static device by means of which electrical ower in one circuit can be transformed into another circuit. t can ste-u or ste-down the voltage in a circuit with a corresonding decrease or increase in current. n its simlest form, it consists of two inductive coils (rimary and secondary coils) which are electrically searated but magnetically linked through a ath of low reluctance. n this design a 24/240 ste-u transformer (T ) is used. The transformer has a centre ta which is connected to the ositive terminal of the battery Determination of the number of turns of coil on each side of T windings Power rating of T is 2.5kA (Figure 3.). The volt er turn is given by Et 4.44 fbmax A s s where E t is volt er turn, f is the frequency in Hertz, B max, maximum flux density in Tesla, A is the lamination core area in m 2, is the rimary side voltage, is the number of turns on the rimary side, s is the secondary side voltage, s is the number of turns on the secondary side. The size of the transformer core must be determined based on the transformer s total ower. The area of the core should at least have the value according to the Equation (2). Therefore the area of lamination of transformer T is given as A 2 cm 2 ower Watt A m 3 ower Watt A cos 0 4 m 2 4 where Cosө is the ower factor. The outut ower of the inverter is 2500A. Choosing a ower factor of 0.8, then, Area A of lamination was obtained to be 4472 x 0-6 m 2 and E t was obtained to be.93volt/turn using Equations () and (4) resectively. The exected outut voltage of T is 240. Secondary voltage s of T is 240 The number of turns on the secondary winding ( s ) is given as s s turns E t 5 s 202turns Also, the numbers of turns in the rimary winding ( ) is given as E t turns 40turns 6 Page 48

5 3.3.2 Gauge Estimation Power (A) is given by Power rating of T is 2500A. Assuming 90% efficiency, then inut rating is P i P out eff 7 where P i is the inut ower, P out is the outut ower and eff is the efficiency T. The secondary voltage is 240. The secondary current is Pout s 0. 4Ameres s And the rimary current is Pi Ameres 3.4 Driver Unit This unit makes use of ower MOSFETs. The MOSFETs (RFP250) has a maximum current ( M ) and voltage of 30A and 200 resectively. Hence the number of MOSFETs, m required is m m 2 max m 8 Hence 2 MOSFETs can safely handle the exected rimary current but 4 MOSFETs is recommended for higher reliability Signal Generator (Oscillator) Unit This stage makes use of CD4047 C for ulse generation of 50Hz. A constant voltage was sulied to the C from the battery through the voltage regulator C 782. t is configured such that ins 4, 5, 6 and 4 were connected to the cc while ins 7, 8, 9 and 2 were connected to the ground (Figure 3.). The oscillating frequency (50Hz) was determined by R and C 2 network. Pin 0 and in were the outut (Q) and comlementary outut (Q-) resectively. The two oututs searate the signal into two channels. Each channel was connected to the gates of ower MOSFETs which was then connected to the ends of rimary side of transformert. An electrical signal indicator LED is connected to indicate when the inverter is on. Resistors R and R 2 serve as limiting resistors to limit the current entering the gates of the MOSFETS. The eriod T of the oscillator was determined using Equations (9) (). T 4.4R C and T f 2 sec s 9 0 f 4.4R C 2 Choosing C 2 to be 00nf (00 nano Farad), R was obtained to be 45.45kΩ. From manufacturer data sheet, when the voltage at in 4 is 2, the outut voltage and current at in 0 and in of C4047CD are 5.6 and 50mA resectively. The value of resistors R and R 2 were obtained using Equation (2). out at in 0 and in of C 4047CD are 5.6, gs = 0 and max =50mA Page 49

6 R out max gs 2 R 4 was obtained using Equation (3), which is given by cc R4 LED LED 3 Where cc is 2 from RG, LED is 2 and LED is 0mA from manufacturer secification. 3.5 Battery Control Switch Unit This unit is used to disconnect the battery from the inverting section whenever there is ower suly from the national grid. This unit makes use of relay switch. 3.6 Battery Bank A battery is an electrochemical device that converts electrical energy to chemical energy during charging and chemical energy back to electrical energy during discharging. A battery bank is needed in solar design to store the energy generated by the solar modules and then used when there is no sunlight (at night) or when there is no ower suly from the national grid. Dee cycle battery tye is recommended in solar design because they are secifically designed for charge and discharge for a longer time. The battery should be large enough to store large amount of energy to oerate the household aliances for redicted or exected number of days of no light from the national grid. Two 2/200Ah dee cycle batteries connected in series will roduce an outut of 24 when fully charged. The batteries will be charged by two comlementary sources which are the rectifier circuit and solar modules. But the riority was that when there is suly from the mains, the source of charge should be from it. Measurements were taken from battery bank to know how it is being charged and discharged. 3.7 Charging Controller The rate at which electricity is been added or drawn from the battery needed to be controlled. The charging controller is used to rotect the battery from over-charging and over-drainage. The charging controller consists of solar charge controller and low battery voltage tri Solar Charge Controller To control the rate at which the solar modules charge the battery, a solar charge controller is required. The solar charge controller used here was bought and incororated in the design. The characteristics of the solar charger are as shown in Table 3.. Table 3.: Solar Charge Controller Model Maximum P array Power Oerating oltage Maximum Current ominal oltage ASC-MPPT W A Low Battery oltage Tri The inverter obtains its energy from the battery(s). f there is a rolong drainage from the batteries without charging, the life san of the batteries will be affected and therefore require a rotection. The low battery voltage tri erforms this function by disconnecting the oscillating section from the battery when the voltage across the series connected batteries has reduced to certain level. LM324 comarator (C 2 ) and zener diode ZD were used as shown in Figure 3.. Pin is the outut terminal, in 2 is the non-inverting terminal and in 3 is the inverting terminal. Pin 2 of the o-am is set to a constant voltage of +0 by ZD. Pin 3 is set at a lower voltage of 9 through otential divider formed by R 5 and R 6. As the batteries are discharging, the voltage at in 3 will be reducing. When the voltage at in 3 becomes lower than the voltage at in 2, it results to a high voltage at in which will bias the transistor Q 9 through resistor R 8. This triggers the relay connected to it thereby disconnecting the batteries from oscillator. At in 2, the inverting terminal, Page 50

7 R6 R6 R R 5 cc where cc2 is the battery voltage, R6 is the voltage across the inverting terminal set to 9 by otential divider formed by resistor R 5 and R 6. Choosing R 5 to be kω, R 6 was obtained to be kω from Equation 4 At in 3 (non-inverting terminal), the zener diode Z D, is set to a voltage of 0. R cc 2 7 Z D Z D 5 where ZD is the zener diode voltage, 0 and ZD is the zener diode current, 0mA. R 7 was obtained to be kω from Equation 5. The transistor C85 (Q 9 ) has the following secifications from the data sheet. cmax = 0.5A, BEmax = 5, β = cc Outut voltage of the o-am = 3 Where cc3 is the voltage regulator outut voltage to ower the o-am. Outut at in = Thus, BB cc R 8 and B C BB B BE 6 7 For LED 2 R 9 cc LED2 LED2 8 Where cc = 2, LED2 = 2 and LED2 = 0mA. From equations 23, 24 and 25 R8 and R9 were obtained to be 2kΩ and kω resectively. Diode D 3 is t has a eak voltage of 50 and a maximum current of 7A, it can safely rotect the relay from inverse voltage. 3.8 Rectifier Unit Battery(s) or a battery bank is needed in the design to store the energy from the P modules or from the suly mains. The ower from the suly mains is an AC suly which cannot be used to charge the batteries directly. Therefore a rectifier circuit is needed to change the ac suly to dc suly to charge the batteries. The rectifier consists of a 220/30 transformer (T 2 ) and the bridge rectifier consisting of four diodes, D D 4. The diodes convert the ac voltage at the rimary side of T 2 to dc voltage at the secondary side. The diodes have a eak inverse voltage of 00, and a maximum current of 30A which can safely handle T 2. The caacitor C is used to remove the riles from the rectified voltage. Equations (9) and (20) were used to obtain the value of C (6000μF). c C dv dt c 9 t f 20 Page 5

8 3.9 Solar Panels There are many sources of renewable energy that are available in the universe. These include Wind, Solar, tides, waves and geothermal heat. These renewable energy can be converted to other forms of energy. Among these numerous sources, solar energy occuies a very imortant lace. The solar energy can relace convectional fuels used in electricity generation. Solar ower system (Solar hotovoltaic system) consists of hotovoltaic cells (P cells or modules) which are light sensors that generate electricity when illuminated. The electricity generated can either be stored or used directly. Solar P system is very reliable source of electricity that can suit many alications including residential buildings. Solar ower system is more exensive to install, but it is cheaer than mechanical generator due to low or no maintenance. Solar owered systems have a technical life time of 20 30years and they could rove much more reliable than oeration on diesel generator system which needs regular maintenance and re-fuelling. n terms of the environmental imact, the alternative source (Solar) roduces no noise, CO 2 emission or smell, thereby reducing environmental ollution. The Pcool-SP95 solar modules were used. Two modules were connected in series to increase the outut voltage while the current remains the same. Table 3.2 shows the characteristics of the solar module. The modules were taken outside where they can receive direct energy from the sun and then connected to battery bank.aroriate measurements were then taken. Table 3.2: Characteristics solar module. Module tye Pcool-SP95 Maximum Power, P max 200W Maximum Power oltage, m 8.7 Maximum Power Current, m 0.8A Oen Circuit oltage, oc 22.5 Short Circuit Current, sc.32a Length, L 037mm Width, W 527mm Thickness, T 54mm 3.0 Power Consumtion Demand for Household At household level, electricity is used for services such as lightings, heating, cooling cooking and for electronic aliances. Cooking and other ower consumtion equiment consumed a lot of ower and their inclusion in design will increase the ower required from the system which will also results in high cost of using the system. Therefore most ower consuming equiment are not considered in this design. Table 3.3 deicts tyical ower consumtion of demands of a household. Table 3.3: Tyical Power Consumtion Demands for Household Comonents Power Rating (W) o. Used Total Power (W) Lightings Ceiling fans Televisions Home theatres Decoders Comuters Refrigerator PERFORMACE EALUATO To ascertain if the desired objectives were achieved, the ower inverter was tested. The system outut voltage was measured at no load as shown in Table 4.. Page 52

9 4. Outut oltage at no Load 4.2 Charging Tests Table 4.: Outut oltage at no load Measurement Secified () Achieved () Correlation (%) Remarks Outut oltage (inverter Satisfactory mode) Outut Main Suly Satisfactory Though dee cycle batteries were roduced to undergo charging and discharging for long eriod of time, the batteries should be rotected from overcharging to rolong their life san. Overcharging test was carried out to ensure that the batteries were not overcharged by the charging sources. The test results are as shown in the following tables. Table 4.2: Charging Test Battery oltage () ndicator Remarks 8 O Satisfactory 20 O Satisfactory 22 O Satisfactory 24 O Satisfactory 25 OFF Satisfactory 4.2. Solar Charging Test The test was carried out to know the effectiveness of solar charging source. This was carried out during the day time. The solar anel was mounted to receive direct energy from the sun. The reading was taken from 8am to 6m for three days. The result is as shown in Table Table 4.2.: Solar Charging Test Time of Charging Charging Charging Mean the day Current Day Current Day 2 Current Day 3 alue AM.42A.44A.45A.44A 0.00 AM 2.54A 2.59A 2.63A 2.59A 2 OO 3.53A 3.59A 3.62A 3.58A 2.00 PM 4.3A 4.26A 4.33A 4.29A 4.00 PM 3.88A 3.94A 4.00A 3.94A 6.00 PM.84A.88A.90A.88A t will be observed from Table 4.2.; the solar charging source was very effective. The charging current from the solar modules deends on the time of the day. The solar charging source thus has the maximum charging current between 2 PM noon and 4.00 PM. The charging current from suly mains was also measured and it was found to be 6.54A at 220. Though the charging from the suly mains is more effective than the solar source but the current fluctuates due to fluctuation in the suly voltage from the main suly. The change-over between the charging sources was also tested. The test was carried out during the day time (2.00PM) when the sun is high and there is availability of suly from main (national) grid. t was observed that the charging source was from the suly mains. When the main suly is switched off the charging source automatically changed to solar source. 4.3 Over-Drain Test To rotect the battery from too much drainage, low battery voltage tri was incororated in the design. Over-drainage test was carried out to confirm the function of the low battery voltage tri. After the batteries were fully charged, the charging sources were removed and the system was owered O. The oscillator and the system outut voltage were measured at different battery voltage. The results are shown in Table 4.3. Page 53

10 Table 4.3: Over-drain Test Battery oltage () Oscillator oltage () System Outut oltage () Remarks Satisfactory Satisfactory Satisfactory Satisfactory 4.4 Outut Power/Load Test The duration of suly from the ower inverter is a function of total load connected to it and the ower rating of the battery. The series connected batteries were rated 24, 200Amere-hour (24/200Ah). The ower inverter was tested with all the aliances shown in Table 3.3. The duration of suly from the battery was found to be 3hrs, 3 minutes. This imlies that the duration of suly from the battery can be imroved by connecting more battery to the system.. COCLUSO The solar modules served as an alternative source of charging to batteries. t was observed that the charging from the sun is more effective between 2 noon and 4.00 PM. The change-over between the charging sources was also effective as the riority was given to the main suly to charge the batteries if the two charging sources are available. The ower inverter outut was usable for all the connected household aliances. All the tests carried out on the system were satisfactory. The flexibility and well automated nature of the design eliminates the need of human oerator after been set u, thereby facilitating efficient and reliable use of batteries and the inverter. Since the ower inverter does not deends only on the suly from the national grid to charge the batteries, the reliability and availability of the system has been imroved. However, further research work need to be done on the use of wind as third source of energy. This energy can also be made to charge the batteries to further increase the reliability of the system. Also the caacity of the system can be increased so that more equiment can be owered by the system. REFERECES [] A. Pregelj, M. Begovic and A. Rohatgi, mact of nverter Configuration on P System Reliability and Energy Production 2003.htt://www2.ece.gatech.edu/research/UCEP/aers/inv_imact.df. [2] W. Bower, C. Whitaker, W. Erdman, M. Behnke, and M. Fitzgerald, Performance Test Protocol for Evaluating nverters Used in Grid-Connected Photovoltaic Systems. 20, 2008.htt:// /equiment/documents/ _test_protocol.df. [3] California Energy Commission and California Public Utilities Commission, Go Solar California, 200. htt:// [4] F. Ahammed, D. A. Taufiq, Alications of Solar P on Rural Develoment in Bangladesh, Journal of Rural and Community Develoment, vol. 3, , 2008 [5] M. Shiao, The Global P nverter Landscae 203: Technologies, Markets, and Survivors, GTM Research, 8 Aril 203, htt:// [6]. Chowdhury, S. E. Reza, T. A. itol, A.. Mahbub, Present Scenario of Renewable Energy in Bangladesh and a Proosed Hybrid System to Minimize Power Crisis in Remote Areas, nternational Journal of Renewable Energy Research-JRER, vol. 2, no. 2, , 202 [7] Energy Star Market and ndustry Scooing Reort Solar December, Solar P nverters, 203.htts:// [8] O. L. Kadir, O. Onuoah,. C. Onuekwusi, U. Onochoja, C.. Udezue, Develoment of Cost Effective 2.5kA Uninterrutible Power Suly, American Journal of Engineering Research,ol. 5, ssue 2, 25 35, February, 206. [9] A. Menon, R.Madhumitha, Design andsimulation ofsolar Micro-nverter withmultile Loads, nternational Journal of Advanced Research in Electrical, Electronics and nstrumentation Engineering ol. 3, ssue 5, May204. [0]. Kulkarni, R. ehete, Simulation and Analysis of Photo-oltaic (P) based Solar nverter System, nternational Journal of Soft Comuting and Engineering (JSCE),olume-3, ssue-6, January 204. Page 54