American Journal of Engineering and Technology Research
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Design & Implementation of an Improved Solar Charge Controller for Variable Range of Solar Panels Sabuj Das Gupta1, Md. AbidHasan2, Md. SajidHossain3, S. M. ArifulHaque4, Sadia Tasnim Mowri5 1 ECE Department, University of Victoria, BC, Canada. 2,3,4 Student, EEE Department, American International University-Bangladesh. 5 Student, EEE Department, Chittagong University of Engineering & Technology Email:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected] Abstract.Today the micro-grid for solar home system has turned out to be a very promising solution for remote places of Bangladesh where the grid supply has not reached yet. In this regard our conventional solar home system charge controller is asking for a solid and innovative solution to accommodate variety of solar panels with different voltage level for micro grid. So far the solar home system charge controller we have in the present market is customized for 15V panel mostly. Looking at this problem we tried to come up with a new charge controller, based on DC to DC flyback converter which enables the system to use any voltage level solar panel and charge the same 12V battery. One of the other important features of this charge controller is that, as it allows the system to use high voltage panel like 48V, the copper loss from panel to charge controller is much smaller compared to other system. As the charge controller remains with the battery, the panel to charge controller wire loss dominates the total system loss and therefore our proposed charge controller can make the conventional solar home system more efficient and more effective. In this paper, the proposed charge controller circuit have been designed, simulated and implemented our and also have connected it with a regular solar home system and charged a 7.2AH battery with 500mA charging current. We have presented the circuit diagram and simulation results of our proposed charge controller in this paper which is done in LT Spice platform. The layout of the charge controller PCB board, done in Express PCB, is also illustrated. The real system test results with the proposed charge controller are also added in this paper. Keywrods: Introduction Solar energy is energy that comes directly from the sun. The sun is a constant natural source of heat and light, and its radiation can be converted to electricity. This source will last for at least a billion years [1]. First photovoltaic (PV) solar panels have been designed and used mainly in space technologies, as the production costs of such panels were very high. As the time passes, the photovoltaic cells can be produced cheaper and cheaper and their efficiency is rising [2]. This is also a reason why they are being used much more frequently and it is not rare to see them on the rooftops any more. The future offers even bigger possibilities, as new thin plastic solar cells are being developed with prospects of cheap large scale production using printing technology [3]. Photovoltaic solar systems can be divided into two basic categories – grid connected and off-grid (also stand alone or isolated) solar systems. The grid connected systems feed the electricity produced by solar panels to the grid using an inverter. When the electricity is needed during night or periods with little sunlight, the energy is taken back from grid [4]. In isolated systems, the excess 65
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electricity is usually stored in batteries during the day and batteries are used to power the appliances in times when photovoltaic panels do not produce enough energy. A photovoltaic (PV) system may be a combination of several components such as a battery system, DC/AC conversion circuits and other power conditioning devices, in addition to the solar panels themselves [5, 6]. A lead-acid battery is an electrical storage device that uses a reversible chemical reaction to store energy. Using lead-acid battery charge can be stored more swiftly by grid connected system. A dc chopper converts directly from dc to dc and is also known as a dc-to-dc converter. A dc chopper circuit can be considered as dc equivalent to a transformer with a continuously variable turn ratio. Like a transformer, it can be used to step-down or step-up a dc voltage source. Here a dc-to-dc converter is used to charge the battery sufficiently. The information presenting in this body of work concentrates more on the electronic means of enhancing energy efficiency in a PV system as well as describing energy storage and power grid integration techniques. This branch of power electronics is generally called power conditioning and in the present case is used to describe the management of electrical energy to effectively charge batteries, draw maximum power from the solar panels or provide a high quality AC output. Literature Review Renewable energy is energy created from natural sources. This includes sunlight, wind, rain, tides and geothermal heat. With global warming, such as climate change, concerns linked to high oil prices, the drive for more renewable energy is being lobbied by local governments [6]. Not only would renewable energy help with the global warming concerns, but also may help turn around the recent economic crisis. This would mean less money spent on expensive fossil fuels and more focus on renewable energy sources. So world energy now requires renewable sources to sustain more power accuracy in future. Among the natural sources sunlight is more essential to mankind. Using this source power can be generated within cheap rate [9]. Power failure in Bangladesh is a major issue and this condition is degrading with time. People are making themselves dependent on power backup machines such as Instant Power Supply (IPS), generator, Uninterruptible Power Supply (UPS) etc. These machines have a high market value and besides they require expensive maintenance such as fuelling the generator at regular interval, high electricity bill for charging an IPS, checking the distilled water in IPS. Still there are many rural areas in third world countries where the power transmission lines have not been provided yet. As a consequence people leaving over those areas have no other way but to suffer from lack of electricity.Using the energy of sun, solar panels at home, is the best solution to all these problems. To make this system more efficient and cost-effective it is better to use a dc-dc converter to charge a battery instead of using a simple charge controller. There are some conventional charge controllers, which have some problems of charging system. So these charge controllers cannot charge batteries at some extent. To overcome these problems a new type of dc to dc converter, naming fly back converter has been introduced. A block diagram of the conventional charge controller is given below.
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Fig-1: Conventional charge controller. The conventional charge controller of solar home system has the following disadvantages: It provides inadequate current during low sunlight. It cannot support a solar panel of high voltage. In the conventional charge controller excessive power loss occurs in the transmission line between solar panel and charge controller. In the conventional system the panel gets shorted when the battery is completely charged, which affects the panel longevity. In this circuit MPPT (Maximum Power Point Tracking) cannot be introduced. To overcome these shortcomings, a new type of dc to dc converter (fly back converter) is introduced in this paper. The advantages of using this converter over the conventional charge controller are: It can take variable panel voltage. Its efficiency is better than the conventional one as it reduces wire loss. It is suitable for micro-grid. No panel- short circuit occurs here. It is possible to add MPPT (Maximum Power Point Tracking) with our system The main objective of this paper is to improve the performance of solar home system by replacing the conventional charge controller with a dc to dc converter. Main aim is to supply constant voltage at the output though the sunlight (input voltage) is not sufficient. For that reason, a dc-dc fly back converter is introduced to give the proper charging system. It is to be applied on 12V batteries, because this voltage is most commonly used in isolated solar systems. A solar panel, 12v lead-acid battery, fly back dc-dcconverter is needed to make this system. Using this, an improved version of converter is to be made which has proper efficiency to charge up a battery. The steps had been taken are given below:
DESIGN a fly back converter as the charge controller of solar home system. SIMULATE this DC to DC converter circuit in LTSPICE simulation platform. Then IMPLEMENT it in the PCB board. PCB board is designed in EXPRESS PCB platform.
For easy and better understanding a simplified block diagram of this system is illustrated below. 67
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Fig-2:Simplifiedblock diagram of the model. Circuit Design In the proposed circuit, to reduce wire (I2R) loss and to give a sufficient voltage to the battery even at low sunlight some advanced modification was taken. The circuit diagram for this model is shown below.
Fig-3: Circuit Diagram of the model. In this abovecircuit the input is given from the solar panel. Usually solar panel gives maximum (17v-21.5v) output from 12.00-3.00 pm (as per the data obtained from the datasheet) then it starts to decrease. To give a constant voltage at the battery (load), a fly back converter at the front of the battery was placed. This fly back converter is boost type which increases the voltage from low level to high level. In this case fly back converter will always regulate a constant voltage level. A 12v Battery was used in this purpose. If the input is less than that, it will give a feedback to the current controller. Correspond to that value the current controller mode UC3843A will 68
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output a pulse wave at its pin 6 to balance the voltage by simultaneously on and off the converter. The duty cycle of the pulse was 96% . Duty cycle= A switch is connected to pin 6 of the controller, as a consequence the switch will be on only when the pulse of the output pin is high else it will remain low. When the pulse condition is low, there is no current flowing through L3. So no voltage will be induced across L1.Again when the pulse is high, current flow through L3 and a corresponding output voltage will be obtained across L1.The capability of charging current of the system was measured by varying load. For different load following data table shows the load regulation characteristics. Table-1:Load Regulation Table obtained by varying load
R(Load)
I(mA)
Vout(Volts)
270 Ω
51.5
14.26
120.5 Ω
112.4
14.26
81.9 Ω
162
14.25
41.5 Ω
300
14.25
26.5 Ω
534
13.05
15 Ω
764
10.3
A. Calculation of R1 resistance: In order to obtain a sufficient power supply at the battery terminal, the calculation of resistance R1 is an important task. The current flowing through this resistance is sensed by the PWM (UC3843) at pin 3. The peak current through the primary coil of transformer depends on the voltage at compensation (pin 1). The peak current is calculated by using following formula[7]. (
)
(
)
(
)
The obtained parameters from the circuit are: I (peak) through primary coil L3 = 28.28 mA V (pin 1) at compensation pin = 1.43 v Applying the corresponding values in the above equation, the value of the resistance R1 is obtained
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R1 = 0.386 Ω. B. Calculation of RT/CT: In the Schematic diagram provided above,Rt and Ct is indicated by R3 and C1 respectively. The selection of Rt and Ct is done on the basis of “Timing resistor Vs Oscillator Frequency” curve provided in the datasheet of UC3843.
Fig-4: Rt, Timing Resistor Vs Frequency. The simulation was done for a frequency of 544.66 kHz. As a result, the value of R t and Ct obtained using the above curve are 14.8K and 100pF respectively. Simulation In normal condition solar panel gives 25V~21V input. The experiment is performed assuming input voltage as 25V and the output voltage is found 14 volts. The output graph is given below.
Fig-5: Input and output voltage of the system at normal condition. The standard reference voltage of current mode controller (UC3843) is 5V. This provides the charging current for capacitorCt through resistor Rt. Here the feedback voltage is also very important. It is the inverting input of the Error Amplifier. It is connected to the switching power supply (load battery) output through resistor (R7) divider. It is measured that the feedback voltage is 2.5 volt. The following graph is the representation of feedback voltage.
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Fig-6: Feedback voltage of the current controller. The Oscillator frequency and maximum Output duty cycle are programmed by connecting resistor RT to VREF and capacitor CT to ground. Operation to 600 kHz is possible. The calculation of output duty cycle and oscillator frequency has shown before. From the following figure-8, the duty cycle is 1.8μsec so frequency at Rt/Ct is 555.55 KHz .The corresponding Rt and Ct for the frequency was determined from timing resistor vs. oscillator curve of UC3843.
Fig-7: Pulse obtained at Rt/Ct. The output is taken from the pin 6 of the UC3843. The graph is presented below. This output directly drives the gate of a power MOSFET. Peakcurrents up to 1.0A are sourced and sunk by this pin. In our circuit a MOSFET switch is connected to pin 6(output pin). The switch become open at the time the pulse is high and at low it remains close.
Fig-8: Output (at pin 6) pulse of the UC3843. Our major objective of this project is to obtain a constant voltage level at extreme condition. Fig shows us the output voltage 14 volt (average) for 40 volt input(which is not desired for the solar home system). For any reason if such amount of voltage appears in our system the current controller will give an output pulse corresponding to the input to control the MOSFET switch. And finally will give an average voltage level (14volt) to the load (Battery).
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Fig-9: Output of the system when input is 40 volt. Similarly, the circuit can also perform at an under voltage. The below simulation result shows that the system can give an average voltage level (14 v) at a low input voltage for example 10V. It can be concluded that, the system is capable of charging a 13.6 volt battery at low sunlight as well.
Fig-10: Output of the system when input is 10 volt. The capacitor C8 plays a vital role in this circuit. The charging and discharging of this capacitor is illustrated below.
Fig-11: Charging and discharging mode of capacitor C8. The current flowing through inductor L1, makes the diode forward biased and charges the capacitor C8. This operation occurs during the ‘ON’ condition of the switch (UC3843). Again when the switch is turned ‘OFF’, the current flow through inductor L1 becomes zero and the capacitor C8 discharges. In this case the diode gets reverse biased and the charge will not flow in the backward direction and eventually will not supply power to the battery for charging purpose. The following simulation result shows the charging and discharging mode of capacitor C8 correspond to the output pulse of the current mode controller to keep a constant average voltage at load side.
Fig-12: Output Voltage at Node 002 and Pulse obtained at Node 007(blue). 72
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Implementation EXPRESS PCB software was used to Design the PCB layout of the proposed charge controller circuit. The layout is provided below:
Fig-13:PCB Layout of the charge controller circuit. A pictorial representation of the practical implementation of this model is shown below.
Fig-14: PCB Implementation of the project circuit (right) and Lead Acid Battery (left). Epilogue The project is all about improving the solar home system by means of employing DC-DC flyback converter so that it can support solar panels of different power ratings.During the implementation of this model one of the mainproblems was, the charge controller circuit failed to 73
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supply current more than 50(mA). In order to address this situation the CS resistance was decreased from 1.8Ω to 0.5Ω. This resulted in a great improvement of the charge controller circuit, making it capable to produce up to 500(mA) charging current. Obviously, this model will greatly encourage the use of solar home system at the rural areas of Bangladesh. Solar is also very effective keeping in mind that, energy produced by the solar panel depends on the irradiance of the sunlight and thus it implies that a higher energy can be generated near the equatorial region than the polar region[7].Energy crisis is one of the great troubles in the recent world. Renewable energy is one of the viablesolutions to address this situation. Again among all the renewable energy sources, solar energy is abundantly available in nature and can be used without any cost. To grab this free energy in an efficient and smart way, different techniques can be used. The proposed charge controller is one such attempt of these techniques. MPPT (maximum power point tracker) using microcontroller along with some specific algorithms can be used to increase the efficiency of the charge controller. Reference [1]Energy resources and their utilization in a 40-year perspective up to 2050. A synthesis of the work done by the Energy Committee at the Royal Swedish Academy of Sciences. Energy Resources 3 May, 2010. [2] Nese and Grenci, The Global Ledger of Heat Energy, From "A World of Weather," 5th ed. © 2011 Kendall Hunt Publishing Co. www.kendallhunt.com/grenci-nese. [3]A White Paper on Solar Energy: Economic and Eco‐Systems Considerations, December, 2010.William T. Coyle, Fumiko Yamazaki and Mechel S. Paggi. [4] AlekIndra, Fiber Optics in Solar Energy Applications, White Paper. Avago Technologies. [5] Piao, Z.G.; Park, J.M; Kim, J.H.; Cho, G.B.; Baek, H.L. “A Study on the Tracking Photovoltaic System by Program Type,” Electrical Machines and Systems, 2005. ICEMS 2005. Proceedings of the Eighth International Conference on Vol 2, Pages: 971 – 973, Sept 2005. [6]Ch. Brunner, “IEC 61850 Process Connection—A Smart Solution to Connect the Primary Equipment to the Substation Automation System,” ABB Switzerland, Zurich, Switzerland. [7] Armstrong, S.; Hurley, W.G. “Investigating the Effectiveness of Maximum Power Point Tracking for a Solar System.” IEEE, Power Electronics Specialists Conference, 2005. [8] L. Andersson, Ch. Brunner and F. Engler, “Substation Automation based on IEC 61850 with New Process Close Technologies,” IEEE Powertech Bologna, June 2003. [9]M. Gohul, T. Jayachandran, A. Mohamed Syed Ali, T.G. Raju, N. Santhosh Kumar, M.R. Saravanan. A New Design of Grid Tie Inverter for A Grid Interactive Solar Photovoltaic Power Generation – An Innovative Option For Energy Conservation & Security. Iject Vol. 2, Issue 3, Sept. 2011, ISSN : 2230-7109(Online) | ISSN : 2230-9543(Print). Correspondence Author
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