Building of A Portable Solar AC & DC Power Supply

2014 Fifth International Conference on Intelligent Systems, Modelling and Simulation Building of A Portable Solar AC & DC Power Supply Luo-Qi Soh SIM University, School of Science & Technology, Singapore e-mail: lqsoh001@unisim.edu.sg Chee-Chiang Derrick Tiew SIM University, School of Science & Technology, Singapore e-mail: derricktiew001@unisim.edu.sg Abstract This paper presents the building process of a small scale, cost effective portable solar power supply. The end product comes with a solar panel to capture and convert solar energy to electrical energy. The electrical energy was stored in a rechargeable battery with a charge controller to regulate the charging process. A battery level indicator was in place to monitor the battery storage capacity. A low-cost square wave inverter was built to generate the AC power supply required for the operation of low power rated household devices. A voltage regulator was constructed to step down the 12V DC voltage to a regulated 5V DC power supply for the charging of handheld devices like smartphones and tablets. The final product carries a weight of 5.5kg that provides both simultaneously a portable 230V 50W AC power generator and a regulated 5V 1W DC power supply source in times of emergency. Keywords-solar panel; charge controller; inverter; voltage regulator; battery indicator; portable power supply I. INTRODUCTION With the improvement in technology over the years, consumption of power to support the standard of living has also increased tremendously. This led to the fact that we need to find a way to harvest energy from alternative methods since traditional methods like burning will deplete natural resources in time and also causes pollution. Solar power is a source of clean energy or green energy and together with its abundance, there is sufficient reason for us to continue to explore solar energy not just as an alternative energy source but as a potential foundation of energy creation. A solar cell is a basically a p-n junction device with no direct voltage applied. The fundamental idea of a solar cell is to convert light energy to electrical energy. Light energy, which comes mainly from the sun, is transmitted by photons, small packets or quantum of light. Photons of sufficient energy (greater than the bandgap of the material), creates mobile electron hole pairs in a semiconductor as shown in Fig. 1 [1]. Therefore, a solar cell converts light energy which is a flow of photons, to electrical energy which is a flow of electrons. This is known as the photoelectric effect. The photo-generated minority carriers will be collected at the p-n junction where the excited electrons will get swept to the n-region while the holes will get swept to the p-region to become majority carriers. Figure 1. Photogeneration of electron-hole pairs in a semiconductor When an external circuit is connected across the solar cell, the excited electrons will flow through the external circuit to recombine with the holes in the p-region as shown in Fig. 2 [2]. The flow of electrons through the external circuit constitutes the direct current which could be stored in a rechargeable battery. An inverter could then be used to convert the direct current (DC) to alternate current (AC) to power up electrical appliances that rely on AC power supply. An example is Tahsina H. Loba & Khosru M. Salim [3] who designed and implemented a micro-inverter for solar home system. Figure 2. Conversion of light energy to electrical energy The aim of this project is to build a portable AC and DC power supply using solar panel to harness solar energy since it is considered a form of renewable energy which is readily available. The power supply will be used for powering up some low power home appliances. As it is portable, it could also be used for outdoor activities. II. HARDWARE DEVICES A. System Architecture The system architecture of the portable solar power supply is illustrated in Fig. 3. The prototype consists of [4]: Solar panel for charging up the battery bank. 2166-0662/14 $31.00 2014 IEEE DOI 10.1109/ISMS.2014.82 445

Charge controller to prevent over-charging which is detrimental to the health of the battery. Voltage regulator for regulating a constant 5V DC power supply. Inverter for converting DC to AC power supply, and step it up to 230V by the use of a transformer to provide AC voltage for the operating of low power home appliances. Battery indicator to allow user to have an idea the amount voltage left inside the battery bank. A storage box to contain all the electronic circuits to prevent damage to components as well as the danger to personnel using it since high AC voltage is present. Figure 3. Integrated system of a portable solar AC & DC power supply Table I provides the overall system specification of the portable power supply. TABLE I. SPECIFICATION OF PORTABLE SOLAR POWER SUPPLY Descriptions Specifications Input voltage of solar panel Inverter output voltage (AC operations) Voltage regulator output voltage (DC operations) Battery bank Charge controller Battery meter 12 Volt 21 Volt 220-230 VAC 5 Volt 12 Volt Able to charge 12 Volt rechargeable battery Display a range of voltage using LEDs B. Hardware Design Theory and Calculations Some calculations are needed to be carried out before proceeding to hardware design stage. According to Ohm s Law, the current flowing across an electronic component inside a circuit should be directly proportional to amount of voltage applied, while keeping the resistance constant. Using the properties of Ohm s law, we can now calculate the capacity needed to operate some low wattage appliances running at 230V AC supply. Table II tabulates the calculated power consumption for selected hardware devices. TABLE II. POWER CONSUMPTION FOR SELECTED HARDWARE DEVICES Description Ratings Usage Consumption (Watts) (Hours) (Watt Hours) Quantity Solar panel 30 - - 1 Battery 12 Volt, 40 AH - - 2 Fluorescent bulb 30 2 60 2 Filament bulb 50 2 100 2 Laptop 35 2 70 1 1) This calculation is based on a list of items stated above. The power ratings of some of the appliances were obtained through web resources. The fluorescent lights are rated at 15W each, so we based on operating 2 units to achieve 30W. The filament bulb is rated at 25W, so we based on operating 2 units to achieve 50W. As for the HP laptop, since we know that it is drawing a current of 0.15A from an AC voltage of 230V, thus it is operating at roughly 35W. 2) The number of watt hours required to operate them for a period of 2 hours is (30+50+35)*2= 230 Watt Hours. The figure is derived to determine when recharging of the battery is needed. 3) To run the appliances for a period of 2 hours each day for 2 days, power consumption is 230*2 = 460 Watt Hours. For this, we are assuming that the consumption is directly from the battery alone considering there is no solar power is available from the sun for that period of 2 days. 4) To prevent discharging of batteries to below 50% of their full charged value assuming no sun light is available for that period of 2 days, we therefore need to multiply the consumption for 2 days by a factor of 2. Total Watt Hours needed by the battery is thus 460*2 = 920 Watt Hours. This factor is important because lifespan and performance of the batteries can be increased if the battery is always operating at a charged value greater than 50% of the fully charged level. 5) For now, we need to determine the size of battery bank in ampere hours which is needed for this design. The battery we will be using is 12V, therefore the calculated ampere hour need by the battery is 920/12 = 77 Ampere Hours. If a 40 Ampere Hours battery is considered here, 77AH/40AH = 1.9 ~ 2 which means two batteries rated at 40AH are needed. 6) Finally, the last step is determining the rating of the solar panel needed. Since we are considering this design to be portable, we will limit the design to use of just 1 solar panel. We assume that we have around 8 hours of solar radiation daily. The daily consumption, which in this case is 230 Watts Hours. To find the wattage required of the solar panel, we take 230 = 8*(watts of solar panel), giving us a value of 230/8 = 28.75. Therefore, a 30W solar panel will be sufficient when considering this design. C. Selection of Solar Panel With the consistent improvement in technology and research development, there are many solar panels readily 446

available on the market. It is important to choose a solar panel which is able to meet the objectives and goals of this project. For the selection of solar panel, we look at the three important factors which are: Cost Open Circuit Voltage Size Cost Of the three different types of solar panel (i.e. monocrystalline, poly-crystalline, and amorphous), monocrystalline is most expensive but has the highest efficiency. Poly-crystalline is less expensive but its efficiency is lower than mono-crystalline. Amorphous type is least expensive but in terms of efficiency, it is also the lowest. So to get the best of both worlds, poly-crystalline type solar panel was selected for this project. Open Circuit Voltage (V oc ) Since we need the solar panel to be able to charge a 12V rechargeable battery, the open circuit voltage of the solar panel must be greater than 12V to be able to charge the battery efficiently. Size Since we need the design to be portable, we should not choose a solar panel that is too bulky. Although we can join small pieces of solar panels in parallel to increase the open circuit voltage, we should choose a single piece of solar panel which is framed to increase the overall durability. We decided to use MC-SP5.0-GCS [5] which is a 5W 17.5V solar panel (Fig. 4) instead since the size of a 30W solar panel is rather large and is not so suitable for a portable design. With the use of a 5W instead of a 30W solar panel, based on the calculation in Section II B, this means that we need to cut down the use of electronic devices proportionally. This solar panel was chosen for the following features described below: High efficiency High transparent low-iron, tempered glass Unique technique to give an aesthetic appearance Outstanding low light performance Figure 4. Picture of MC-SP5.0 solar panel D. Selection of Battery There are mainly three types of rechargeable battery which are available for use, i.e. Flooded lead acid, Gelled electrolyte sealed lead acid, and Sealed Absorbed Glass Mat (AGM) [6]. We decided to select sealed lead acid rechargeable battery which is rated at 12 V, 7.2 Ampere Hour as shown in Fig. 5. This type of battery is also not expensive and is easily available in the market. It has also been proven by electronic hobbyists that this type of battery is guaranteed to work in an inverter design. There are several advantages of using lead acid battery: Maintenance free: Unlike using flooded lead acid, this type of battery does not need to be filled with water to work. Environment friendly: Literally, this is no emission of harmfully gases since the gases produced are been absorbed by the battery itself which means it can be used anyway and does not require special ventilation process. Spillage free: It is termed sealed since we are unable to gain access to the electrolytes stored inside the battery which also means there is ease of installation rendering it ideal for portable design. The chance of getting acidic burns is also eliminated when compared to using flooded lead acid type. Figure 5. Picture of 12 Volt Sealed Lead Acid Battery III. CIRCUIT DESIGNS A. Charge Controller Fig. 6 shows the intended charge controller circuit which is used to charge the 12V battery [7]. This design is a simple linear regulator. Although it does not have a low voltage disconnect, it can prevent over charging of the battery by automatically reducing the charging current to a low level whenever the battery is fully charged. The input is connected to the solar panel and the output is connected to the battery. Figure 6. Charge controller circuit Operation of the charge controller: Resistor R2 and Zener diode D2 form a 6V voltage reference, Transistor Q1 and Q2 form the classical differential amplifier which is capable of amplifying the difference between the reference voltage and feedback voltage from the potentiometer R9. Output taken from the collector of Q1 drives the gate of the P-channel MOSFET M1. Differential voltage gain is in the range of between 100-200. As the feedback voltage increases at R9, Q2 will operate faster since the base is connected to R9 and will result in 447

stealing some emitter current away from Q1. Since the collector current Q1 follows the emitter current, this will cause a lower voltage drop across R4 which reduces the V sg of p-mosfet M1, thus turning it off. Capacitor C2 is needed for providing frequency compensation which prevents the amplifier from oscillating. Transistor Q3 acts as a reverse battery protection which does nothing unless the battery is connected in the wrong polarity. When this happens, Q3 will turn on and reduces the reference input voltage to zero which turns off transistor Q1 and p-mosfet M1and prevents the battery from damaged. D1 is a red light emitting diode which turns on to indicate the solar panel is in active mode and ready to charge the battery. D4 is a green light emitting diode which turns on to indicate the battery has reached a certain amount of voltage. It can be pre-set by adjusting the value of potentiometer R9 which we can set it to around 14V to indicate the battery is fully charged. B. Inverter Since we are designing the portable power supply for home and outdoor use, we need a design that is neither too expensive nor too complex. After much consideration, we decided to implement a square wave inverter since it is not too complex to design, cheap and able to operate resistive loads like light bulbs therefore making it suitable for simple home and outdoor usage. Fig. 7 shows the circuit diagram of the inverter [8]. Finally, outputs from the collectors of power transistors (Q7, Q8) are connected to the primary side of a centre tap transformer which would then step up the 12V voltage to 230V AC supply. The circuit was simulated with Circuit Lab. As could be seen in Fig. 7, the outputs from Q1 and Q2 produce a value of 30.96mA as displayed on ammeter AM1 and AM2. After going through the Darlington driver stage, the current is amplified to 342.5mA as displayed on ammeter AM3 and AM4. The two 2N3055 power transistors further amplify the current to 22.48A (as displayed on ammeter AM5 and AM6) which is required by the transformer to step up the required AC voltage from 12V to 230V. C. Voltage Regulator The input voltage supplied from the 12V battery is not stable enough for the charging of mobile phones. We need a DC-DC step down voltage regulator to regulate a constant voltage of 5V to render it suitable for charging. There are two types of regulator which is linear regulator and switching regulator. After considering the advantages of switching regulator over linear regulator, the chosen regulator is based on integrated circuit (IC) LM2575 chip. This IC chip requires only 3 additional external components (inductor, diode and capacitor) to work, and the output produced is very stable. Fig. 8 shows the circuit diagram of the voltage regulator [9]. Figurer 7. Inverter circuit Operation of the inverter circuit: Circuit consists of 8 transistors, 8 resistors, 2 variable resistors and 1 centre tap transformer. Transistors (Q1, Q2) are being used here as an astable multi-vibrator which creates the alternation of current needed for the transformer input. The frequency can be adjusted to 50 Hz by adjusting the variable resistor R2 & R3. The complementary outputs which are obtained from the collectors of transistors (Q1, Q2) are then fed to the base of transistors (Q4, Q3) respectively. Transistors (Q3, Q4, Q5, Q6) form a typical PNP Darlington driver stage which is used to boost the current from transistors (Q1, Q2) to a higher amount as needed. Outputs from the Darlington drivers are then fed to a pair of 2N3055 power transistors (Q7, Q8) which provides the required push-pull operation at the output stage. Figurer 8. Circuit diagram of LM2575 Voltage Regulator D. Battery Level Indicator Since the battery will be depleted whenever the power supply is being used, it is important to have some indicators to allow the user to have a rough idea the amount of voltage left inside the battery bank. Fig. 9 shows the circuit diagram of the implemented battery level indicator drawn using Circuit Wizard software. Figure 9. Battery level indicator circuit 448

Operation of the battery level indicator: The Zener diode D6 forms the voltage reference. Variable resistor VR1 and resistors (R2, R3, R4, R10) will be used to set the various fixed voltage levels as required. Resistors (R11, R12) form a voltage divider which reduces the battery voltage by a factor of 3. IC LM339 chip is a comparator that compares the voltage difference between the voltage divider and different voltage levels formed by the resistors [10]. The open collector output from the comparator functions like a switch to turn ON the LEDs (D1-D5) accordingly. Simulation results show that when the battery is at a voltage of greater than 12V, all the LEDs will light up. When the voltage of the battery drops to ~10.8V, only 3 LEDs are lit up as shown in Fig. 10. Once the battery is less than 9V, only the red LED will light up, and this means that it is time to charge the battery prior to further operation. After adjusting the potentiometer R9 (Fig. 6) on the charge controller, a 14.6V was obtained as shown in Fig. 12. Figure 12. Calibration of the charge controller 14,6 Volt After sunlight was intentionally blocked, a 12.3V was obtained at the output (Fig. 13) which means that the output voltage is dependent on the amount of light. In order for the charge controller to function properly, sunlight must be directed on the solar panel without blockage. Figure 13. Calibration of the charge controller 12.3 Volt Figure 10. Simulation of battery level indicator with 10.8 Volt battery voltage level IV. HARWARE TESTING & RESULTS This section will discuss the testing and results of the hardware in building the solar power supply. Before the components are soldered onto the veroboard, breadboard was used to test each circuit design to ensure they are working properly. A. Testing of Solar Panel Under a bright sunlight, the solar panel is able to achieve a voltage of about 19.9V as shown in Fig. 11 which is quite close to the value stated in the design specification (Table I). We could then proceed to the next stage of hardware testing. A current of 102.6mA is flowing into the battery during charging process as shown in Fig. 14. The battery shows a voltage of 11.52V at the start of the charging process and 11.57V after a period of 20 minutes. This indicates that the charge controller is able to charge the battery but at a slower rate since the charging current is only about 100 ma. Figure 14. Current produced by the charge controller C. Testing of Inverter The inverter circuit was breadboard and tested with a 230 V 0.25W neon light. The neon light was able to operate smoothly and the output shows an AC voltage value of 265V as shown in Fig. 15. This value is higher than the intended 230V since the neon light is of low wattage. Figure 11. Open-circuit voltage of solar panel 19.9 Volt B. Testing of Charge Controller To be able to charge the 12V battery, we need to obtain a voltage of ~14.6V since it is understood that we need a potential higher than 12V to successfully charge the battery. Figure 15. Inverter operating a neon light 449

Next, the inverter was tested with a 230V 25W filament bulb and it was able to power up successfully. The output value of the square wave inverter drops to a value of 195V (Fig. 16) as the bulb consumes high wattage when operating. The voltage regulator is able to charge a Samsung S3 mobile phone connected to it as shown in Fig. 20. Figure 20. Voltage regulator charging a mobile phone Figure16. Inverter operating a filament bulb The inverter was then tested with a 230V 5W fluorescent and it was able to power on successfully with an output AC voltage of 236V as shown in Fig. 17. The voltage drop is lesser due to the reduction in the wattage of load. The charging process draws a constant current of 0.08A which is around 80 ma as shown in Fig. 21. Although the charging current is quite small as compared to USB charging on normal desktop, a smaller charging current will prolong the lifespan of the rechargeable battery. Figure 17. Inverter operating a fluorescent light When the inverter is not powering any load, the current drawn is about 2.08A as shown in Fig. 18. This indicates that the square wave inverter has high idle power. Hence, the inverter needs to be switched off even when not in use. Fig. 18 also shows that the current drawn by the inverter will increase when powering a load. Figure 21. Current drawn during charging process E. Testing of Battery Level Indicator The battery indictor was tested with a 12V and 9V battery. When battery voltage is less than 9V, only the red led is lit up, and this indicates battery charging is needed. When the battery voltage is around 11.35V, 3 LEDs are lit up. When the battery is at a voltage level greater than 12.7V, all LEDs are turned on, and this indicates the battery is at full charge. Fig. 22 shows the testing results of the battery level indicator. Figure 18. Current drawn by the inverter D. Testing of Voltage Regulator The voltage regulator using IC LM2575 chip is producing a constant 5V required as shown in Fig. 19. Figure 22. Testing of the battery indicator circuit The current drawn by the circuit is around 33mA when all the LEDs are lit up as shown in Fig. 23. A toggle switch is in place to power down this circuit since continuous operation of this circuit tends to waste additional power. Figure 19. Output voltage produced by the voltage regulator Figure 23. Current drawn by the battery indicator circuit 450

V. SYSTEM INTEGRATION All the circuits, i.e. charge controller, inverter, voltage regulator and battery level indicator, were installed on a storage box as shown in Fig. 24. Figure 26. Testing of the portable solar power supply end product Figure 24. Installation of circuits in a toolbox In order to dissipate heat generated from the circuits more efficiently, two small fans were installed in the box in addition to a heat sink as shown in Fig. 23. Few tiny holes were drilled along the rear top edge of the box to allow any hot air to dissipate to the surrounding atmosphere. The exterior of the storage box features a 3-pin power socket to provide 230V AC supply, an USB port to provide 5V DC power supply, LED lights for battery level indicator and operational status indicators, and switches for turning on/off part of circuits for power savings. A solar panel is mounted on top of the box as shown in Fig. 25. Figure 25. Exterior view of the portable solar power supply end product Fig. 26 shows the end product of the portable solar power supply powering up a fluorescent bulb and charging a Samsung Galaxy mobile phone at the same time. This shows that the portable solar power supply has met its intended objective and is capable of operating some low power rated appliances. The end product carries a weight of 5.5kg which is reasonably portable. It could serve as a versatile backup power source in times of emergency. VI. CONCLUSION A portable solar power supply was successfully built to the specification. The product is able to support simultaneous operation of low-power rated electrical appliances and charging of mobile phones. The product makes use of a 17V solar panel to capture the sunlight and convert it to electrical energy. A charge controller is in place to regulate the charging process of the 12V rechargeable battery. A low cost square wave inverter was built to generate a 230V AC power supply from the 12V battery. To support the charging of handheld devices such as smartphones and tablets, a voltage regulator was implemented to step down the 12V battery voltage level to a regulated 5V. The final product features a 3-pin power socket to provide a 230V 50W AC supply, and a USB port to provide regulated 5V DC 1W power supply. ACKNOWLEDGMENT The authors would like to express gratitude and appreciation to School of Science & Technology, SIM University for the provision of budget and support of this project. REFERENCES [1] Armin Aberie, Silicon Solar Cells, SERIS, National University of Singapore, April 2009. [2] ETAP, Photovoltaic Array Fundamentals. URL: http://etap.com/renewable-energy/photovoltaic-101.htm [3] Tahsina Hossain Loba & Khosru M. Salim, Design and Implementation of a Micro-Inverter for Single PV Panel based Solar Home System, 2013 International Conference on Informatics, Electronics and Vision (ICIEV 2013). [4] Michael Boxwell, Solar Electricity Handbook, Greenstream Publishing, 2013 Edition. [5] Multicomp MC-SP5.0-GCS Solar Panel Datasheet. [6] Wikipedia, VRLA Battery (valve-regulated lead-acid battery). URL: http://en.wikipedia.org/wiki/vrla_battery [7] Electroschematics.com, 12V LDO Solar Charge Controller. URL: http://www.electroschematics.com/6899/12v-ldo-solar-chargecontrol/ [8] Electroschematics.com, 12 Volt Inverter for Soldering Iron. URL: http://www.electroschematics.com/3542/12-volt-inverter-forsoldering-iron/ [9] Texas Instrumens LM2575 Datasheet, LM1575/LM2575/LM2575HV Simple Switcher 1A Step-Down Voltage Regulator. [10] Texas Instrumens LM339 Datasheet, LM139/LM239/LM339/LM2901/LM3302 Low Power Low Offset Voltage Quad Comparators. 451