Design & Construction of Microcontroller Based maximum power point PWM Charge Controller for Photovoltaic Application Gazi Mohammad Sharif, S. M. Mohaiminul Islam, Khosru Mohammad Salim School of Engineering and Computer Science, Independent University, Bangladesh. email:
[email protected]
Abstract: Photovoltaic cell converts solar energy directly into electricity. This paper describes a design of a charge controller to get the maximum power by using the Pulse Width Modulation (PWM) technique. In this paper PWM is controlled by the PIC Microcontroller. The work of the paper is to charge a 12 volt battery by using a 50 watt solar panel with maximum power. In this paper how maximum power can be achieved from solar panel to charge the battery through PWM signal generated by microcontroller is discussed and the experiments by taking voltage and current’s values in different time interval are shown. Finally the experimental results are illustrated in this paper.
1. Introduction Photovoltaic production becomes double every two years, increasing by an average of 48 percent each year since 2002. For this reason it becomes the world’s fastestgrowing energy technology [1]. Photovoltaic efficiency is very important for solar application. There are three ways to increase the efficiency of a photovoltaic (PV) system. The first way is to improve the solar cell’s efficiency. The second method is a solar panel tracking system to increase the efficiency of a PV system. Progress of solar panel tracking systems has been continuing for a number of years now. The third way is to maximize the energy conversion from the solar panel. The paper focuses on maximum power point PWM charge controller for photovoltaic application. When a solar panel is under an open circuit is able to supply a maximum voltage and there is no current, whereas under a short circuit is able to provide a maximum current with no voltage. In either case, the amount of power supplied by the solar panel is zero. The target is to develop a method whereby maximum power can be obtained from the voltage and current multiplied together.
Fig.1 Illustration of a V-I Curve for a Solar Panel [2]
The “maximum power point” is demonstrated by through the voltage-current (VI) curve in Figure 1, the “knee” of the curve is the vital point [2]. The technology for solar photovoltaic battery charge controllers increased dynamically. PWM charging is very famous for its useful application. In previous days simple on-off regulators were used to limit battery out gassing when a solar panel produced overkill power. On the other hand, as solar systems developed it is clear now how much these simple devices hampered with the charging process. In previous, when the on-off regulators were used early battery failures, increasing load disconnects happened very frequently. For this reason, user frustrations were grown. In recent times PWM has played a significant role in solar battery charging. PWM solar chargers use the technology related to other modern high quality battery chargers. When the battery voltage reaches the regulation set point, the PWM algorithm slowly reduces the charging current to avoid heating and gassing of the battery, however the charging continues to return the maximum amount of energy to the battery in the shortest time. For that a superior charging efficiency, quick recharging, and a healthy battery at full capacity are got. The new way of solar battery charging guarantees many benefits for PWM technique. By using this technique it is possible to recover lost battery capacity and to increase the charge reception of battery. These includes the maintaining high average battery capacities (90% to 95%) compared to on-off regulated state-ofcharge levels that are typically 55% to 60%. The other benefits are balancing drifting battery cells, reducing battery heating, gassing and automatically regulating for battery aging, voltage drops and temperature effects in solar systems [3]. In this paper a charge controller is designed to get the maximum power by using the PWM technique. This PWM technique is employed by the PIC Microcontroller. Here a 12 volt battery is charged by using a 50 watt solar panel with maximum power through the PWM charge controller circuit. The hardware designs, software implementation, prototype construction, flow chart of the program are discussed here. How maximum power can be achieved from solar panel to charge the battery through PWM signal generated by microcontroller is explained and the experiments by taking voltage and current’s values in different time interval are also
described.
2. Hardware design and implementation A PWM Charge controller circuit is designed and developed to sense the voltage of the solar panel. This is a half bridge circuit. A half bridge circuit diagram is shown in fig 2. When there is enough sun shine, the panel voltage is around 20 volt. When the battery is charged, the voltage of the solar panel falls down. But the circuit will maintain the required voltage of solar panel by the program in microcontroller. The required voltage is determined from the experimental results. These experiments are done before designing the software and hardware.
voltage through its one pin and digitizes it. It compares the digitized value with the stored value of 14 volt. If it matches, microcontroller maintains the pulse. But if the getting voltage is greater than 14 volt, decreases the pulse width, hence voltage will be increased and vice versa.
Fig. 2 Half Bridge Circuit
Fig 3 shows the PWM charge controller circuit which is mainly responsible for getting the maximum power. A PIC microcontroller is the heart of the circuit. It senses the voltage of the solar panel and increases or decreases its pulse width for maintaining the required voltage for the maximum power. Fig. 4 Flow Chart of the PWM Charge Control program
3. Prototype, construction and experimental results A 50 watt solar panel was used for the experiments. The picture of solar panels is shown in fig 5. In a shiny day, the voltage and current were measured in different time interval using a voltmeter and an ammeter. Voltages and currents were measured from 12 pm to 4 pm in a day. Two wires (one is positive and another is negative) of the solar panel are connected with a Heater coil which was used as a load here. The wires and the load are connected in series with an ammeter.
Fig. 3 The constructed PWM Charge Controller Circuit
Fig 4 shows the Flow Chart of the PWM Charge Control program. According to the flow chart a program is written in assembly language and loaded the program in to the microcontroller. Here, the microcontroller initially set a pulse which is appropriate for 14 volt at where the maximum power point was achieved from the experiments before. Microcontroller gets the solar panel’s
Fig. 5 Solar panel used for experiments Fig. 7 Maximum Power Tracking Curve
Then different voltages from 18 volt to 11 volt are measured using a voltmeter by changing the load. The flows of current were changed according to the change of voltages. Voltage and current are shown in figure 6.
A 12 volt battery was charged during the experiments and the charging current was observed. Figure 8 shows the picture of the charging current when the battery is charged. The battery can be charged efficiently by ensuring the maximum power point for photovoltaic application.
Fig. 6 voltage and current are shown
The formula of power is; P = V*I. By calculating the voltages and currents, different powers were obtained. These are plotted in Microsoft excel. Fig 7 shows the graph which is maximum power tracking curve. From this curve, it is assumed that the maximum power obtained is 32 watt at 14 volt when the current was 2.28 ampere. So it was important to know the maximum power before software design, because microcontroller increases or decreases the pulse width according to the voltage whether it is less or greater than 14 volt, the target is to stable the voltage to 14 volt.
Fig. 8 Charging current is measured when the battery is charged
4. Conclusions The PWM charge control technique presented in this paper shows the assurance for charging the battery with the maximum power. Though the voltage of photovoltaic cell and the voltage of battery are different, the maximum power can be attained during battery charging from the photovoltaic cell through this technique. The voltages of the solar panel are different in different time and days. This depends on the solar cell’s consumed sun light. When the sky is cloudy or there is no enough sun light, the voltage of the solar panel becomes low. In that case, by increasing the pulse width, the battery is charged with maximum power. For this reason, the battery is charged quickly and efficiently.
References [1] Wikipedia, http://en.wikipedia.org/wiki/Photovoltaics [2]Robert Weissbach, Isaac Aunkst, AC 2007-1213: A MICROCONTROLLER-BASED SOLAR PANEL TRACKING SYSTEM, American Society for Engineering Education, 2007 [3]Why PWM, Morning Star Corporation, USA, www.morningstarcorp.org
