Amandla Aluhalaza. Andrea P. Solano, Juan J. Valera, Manuel E. Keesee, Randall Lay

Transcription

1 Amandla Aluhalaza Andrea P. Solano, Juan J. Valera, Manuel E. Keesee, Randall Lay School of Electrical Engineering and Computer Science, University of Central Florida, Orlando, Florida, Abstract This paper presents the design, methodology and intension to create an efficient, robust and low cost charge controller with a maximum power point tracking algorithm. Acknowledging the high demand of alternative energy sources, research in such areas have rapidly emerged and become critical matters. The design presented in this paper demonstrates an effective and reliable integration of both major energy sources, solar and wind power generations. In addition, monitoring devices will record current, voltage and temperature of both input to charge controller and battery bank, and finally will be available to the end user directly to power outlet sockets. Index Terms DC-DC power converters, energy efficiency, photovoltaic systems, power electronics, solar energy, wind energy I. INTRODUCTION Nowadays, the renewable energy industry has increased due to higher concerns on environmental and global warming issues. Every day, the world is striving on creating a superior and more reliable source of energy that will not only benefit society, but will protect the environment as well. Hence, the market for solar, wind, hydro, and geothermal power is rapidly growing and potentially becoming a major source of energy in the world. With that in mind, the goal of the project is to implement a method of integration and control for a renewable energy system, which is able to take two different energy sources at different times, such as solar and wind energy respectively. This project will help provide electricity to support energy needs of the Pomolong Township community center located near Harrismith, South Africa, using an eco-friendly energy platform. The system will be portable enough to be transportable outside the United States of America due to the final prototype will be shipped to Harrismith, South Africa. The goal is to create an efficient, robust and low cost charge controller with a maximum power point tracking (MPPT) algorithm to obtain the highest possible power output from the photovoltaic panels and in this way, increase the overall efficiency of the system. The method of implementation consists of bringing two renewable power generations sources; solar and wind energy, into a single integrated hybrid energy system. Components such as an inverter will be used to convert DC power to AC. A storage system made of batteries is used to distribute energy when the output power of the solar or wind source is not significant enough to maintain the energy demanded by the end user. II. SYSTEM OVERVIEW In order to implement a stand-alone off-the-grid power system, there are six major components that will be used. First, the solar panel and the wind turbine acting as the energy sources, then the charge controller which will read in the data from voltage and current sensors to keep track of the overall charging stage of the system and use the MPPT algorithms programmed in it to compute the maximum power point output from the photovoltaic panels. The charge controller is followed by the batteries which allows for energy storage, then the power inverter which provides AC current to the American standard outlet. Finally, a transformer is connected to the power inverter which will step up the voltage to 240V and will supply the power to the European Spec outlet. In order to implement maximum power point tracking (MPPT) into our system, the SM72442 integrated circuit from Texas instruments is embedded into the charger controller. This integrated circuit has its own proprietary MPPT algorithm that uses the Perturb and Observe technique. The charge controller will read the data from the incoming current and voltage from the photovoltaic panels as well as the voltage from the battery to calculate what amount of power should be delivered to the battery bank. The charge controller will need to not only find maximum power point but also to control the DC to DC converters output voltage that will be delivered to the battery bank. If there is a trend of some abnormal voltage the controller will be able to open the circuit connecting the solar or wind power source and save the internal electronics.

2 Figure 1 System Overview III. POWER COMPONENTS The power components of the system include the photovoltaic panels and the wind turbine acting as the sources, the battery bank that will store the harvested energy, the inverter, the transformer and the designed charge controller that will be attached in between the battery bank and the photovoltaic panels, in this way, the charge controller supplies the exact amount of power needed to the batteries by taking control of the DC-DC buck and boost converter. A. Solar Panel The solar panel used to prototype is provided by the Florida Solar Energy Center, located in Cocoa, Florida. There are two mechanical engineering senior design teams working on conjunction to integrate the two initial sources, wind and solar, to the battery bank of this system. Research on data collected will be conducted by one of the engineering team as accordance of the use of the solar panel module. The Solairedirect SD module utilizes 60 polycrystalline solar cells to deliver power to the integration system. This module has an open circuit voltage of 36.6 V and has been factory configured for 30V use. It has a short circuit current of 8.6 A, therefore high current parts have been acquire for the system. As with most other solar units, efficiency is inversely proportional to the temperature. As the temperature of the panel changes, the maximum power point will also move accordingly. This is the advantage of the design of a MPPT charge controller to dynamically adjust to these changes of conditions, and will generates the most efficient way of charging the battery bank. Even though the mechanical design team chose the PV panel according their specifications, this system needed to accommodate all the components to satisfy the input current and voltage and maintain a constant charging system with an overall goal of protecting the life span of the batteries. B. Battery The batteries are the central focus of the system. It is important to check on them often, especially when charging and often when the system is not charging, due to batteries can be damaged if there is any overcharging. Choosing the right battery is fundamental for this design, since a lot of factors must be considered; cost, efficiency, capacity, voltage, safety, durability (number of deep cycles), CCA (cold cranking amps), C-rate, weight and practicality. Multiple researches were conducted in the different types of batteries including describing leadacid (Pb-acid), lithium-ion (Li-ion), molten salt, nickelcadmium (NiCd), nickel-metal hydride (NiMH), lithiumpolymer (Li-poly) and zinc-air. These were compared and analyzed in order to choose the correct battery for the system. Out of all the batteries researched it was concluded that deep-cycle batteries are the most effective and lower cost for this specific system. The Intimidator AGM Deep Cycle Series provides an ideal solution for heavy marine house power, renewable energy powered equipment, portable power needs, golf cars and other types of electric vehicles. Completely spill-proof and maintenance-free AGM technology eliminates watering and unnecessary maintenance. Intimidator Deep Cycle batteries spend less time on the charger and more time in service by actually recharging faster than conventional batteries. A high deep discharge abuse tolerance provides added resiliency for dependable deep cycle service. [1] As shown in Figure 2 Lead-acid batteries are sluggish to charge, usually taking between 12 and 16 hours to reach full capacity. This is an advantage for solar cell applications. The sun usually shines for many hours during a given day, delivering power at a rate that is slow enough to allow efficient charging of lead-acid batteries. Lead-acid batteries also have a high overcharge tolerance compared to lithium-ion. For instance, a lead-acid left at an absorption level voltage (~14.3V) for extended periods of time after reaching full charge will not destroy the battery like it would in Li-ion chemistry. Figure 2 Lead-Acid Charging stages

3 Intimidator AGM deep cycle series has extra protection against deep discharging. Ultra-deep discharging is what causes life-shortening plate shedding and accelerated positive grid corrosion, which can destroy a battery. Intimidator deep cycle batteries are designed to use the optimized amount of acid. This means that the power in the acid is used before the power in the plates. This design, along with the enhanced durability in the glass mat and plate construction, protects the internal components from ultra-deep discharges. This enhanced electrolyte suspension system absorbs more electrolyte, protects internal components and enhanced micro-porous glass separators prevents acid spills and terminal corrosion. Figure 3 shows the average lead-acid AGM deep cycle batteries discharge percentage over number of cycles. Figure 3 Average Lead-Acid AGM Deep Cycle Batteries In addition it has lower internal resistance which ensures higher discharge rates, also extra deep discharge protection that withstands damaging ultra-deep cycle service two times the cycle life of traditional batteries. Most importantly this battery model requires less charging time for more optimized battery use. In the project two 12 Volts AGM Intimidator 31M battery will be used, meaning the charging algorithms will increment by a product of two, since instead of having a 6 cell battery, the system will have an equivalent of 12 cells since two 6 cell batteries will be connected in series. C. Inverter Facilitating the access of power from the batteries; the use of an inverter is essential. This design will provide the user to acquire power straight from an outlet socket. Research was conducted as well to choose the right inverter that will deliver power meeting all the design specifications. Due to the high power system, it is essential that the inverter can withstand more than the maximum power deliver to it. The AIMS 2.5K Watt inverted was chosen. This specific inverter is designed to receive Volts input and provides 2500 Watts to the user via the standard 115 V AC. In addition to the American standard 3-prong AC outlet there is also Volt meter on front panel to monitor the battery voltage, which it can help for immediate battery level check. As mentioned above, there are external outlet sockets attach on the enclosure for the user to plug their devices on it. A transformer will be used to provide both standards, American and European, to the end user. D. Transformer The reason this system includes a power transformer is the need of providing the user both voltages for a worldwide use. After the power has gone through the inverter, a transformer will be placed, thus the power can be used at two different operating outputs; 110Volt and 220 Volt. Nowadays, many devices are not affected by the cycle or frequency change on the outlet, due to an internal converter they claim from the manufacturer. Products such as analog devices can interfere with the frequency and might not operate properly. Therefore, users purchase a voltage transformer to step down or up the voltage on the outlet, depends on the requirement of the device. Researching various types of transformers the SevenStar THG-1000 Watt transformer with model B003XM374I is the best option for the design. Having the lowest price, and approximately the same dimensions and weight compared to the other transformer found, this transformer will be suitable to our design specifications. The dual step benefits the design because it adds higher dynamic to the system, giving the user more freedom when utilizing such transformer, and it also comes with 3 or 2 US and one European plugs. The reason this project focus on a voltage transformers that are over 1000 Watt of power, due the inverter will be supplying approximately 2000 Watt power out for the consumer. Even though the user will not use as much power as the specification of the transformer, the design it will provide a power that exceeds the maximum amount. IV. CHARGE CONTROLLER During the absorption stage of the battery the maximum power point tracking needs to be found to quickly charge the battery. This again needs to be found as quickly as possible to increase efficiency and to avoid missing the maximum power point before it moves causing lots of oscillations. The charge controller will use the perturb and disturb algorithm of finding the maximum power point. This method is not the fastest algorithm but it is the

4 simplest to utilize and will find the maximum power point fast enough if the code used is very slim. The system will have a DC-DC buck boost that provides an accurate reading of the battery s stage, estimating a certain level, thus the charge controller can perform its task and determine how many pulses it should produce to keep charging the batteries. The charge controller will be designed and manufactured on a custom printed circuit board. The data PCB will house the microcontroller, sensors and display peripherals. This being the data PCB will be then attach to the high power circuit board which consists of a full bridge, a DC-DC buck boost circuitry, input and output terminal blocks, and several protection switches. The charge controller will measure voltage and current of both the solar panel and battery bank. This specific integrated circuit performs MPPT to achieve the most efficient value to charge the battery bank. The following Buck Mode and Boost Mode equations were used to calculate the value of the inductor used for the two different modes of the charge controller Equation 1 Buck Mode Equation 2 Boost Mode Figure 4 Power Printed Circuit Board Figure 5 Data Printed Circuit Board

5 Due that the charge controller can communicate with the microcontroller through IIC, all the digital sensors, integrated circuits, and LCD were design to use IIC as well to minimize the trouble of using different communication modules. Therefore to make the overall system more user friendly, an LCD screen is include to display various the necessary data, from power in/power out to temperature. This LCD is a 16x2 character display in order that two lines of values can be displayed simultaneously. The LCD also features an adjustable backlight. This style of LCD will provide enough detail for the system while consuming as little power as possible to maintain the best efficiency, drawing only 50 ma during normal operation. A. Microcontroller The microcontroller chosen was the MSP430G2553, this microcontroller provides all hardware functionality required in the charge controller; all the analog and digital pins required by the sensors and other peripherals are satisfied by this microcontroller. In addition, the 16 MHz clock speed, I2C communication, and robust programming language were also deciding factors in this decision. Protecting the inside from excess dust and water, it adds thermal complications, in great need to be well ventilated and cooled by fans to prevent any overheating. The enclosure made out of wood and covered with Plexiglas has three different compartments where the components are placed securely inside. The batteries are housed in a separate compartment with its own ventilation, due that are capable of leaking fumes and spilling. The outside user interaction The front panel is design to include: LCD display, power outlet sockets, American and European standard, USB socket, fans and a switch for emergency shut-down the system. All electrical components are electrically isolated from the enclosure, in order to prevent any problems with static electricity and accidental electrocution with human interaction The speed of the microcontroller needs to be sufficient to run the code without any harmful delays, however is not of main concern. Temperatures will not rise and shift in the orders of microseconds and the peripheral devices will not need to communicate with the main controller constantly, this allows the processing speed to be in the medium range of around 25MHz. At this speed the charge controller can perform its entire task without noticeable changes. Base on the initial design, one goal was to choose a low power microcontroller. This will be essential in the case of no energy collected from the solar or wind source; the batteries will then supply power to the microcontroller to keep its normal operation. RISC architecture over CISC architecture will be preferred. The main controller will not perform any complex instructions or calculations. The more RISC the microcontroller s architecture, the faster the number of clock cycles per instruction. This will allow the microcontroller to quickly move from instruction to instruction and taking care of the system. The MSP430 by Texas Instruments is a processor that falls into all of the criteria above. It has RISC architecture, is low power, and has a moderate processing speed. The microcontroller can then be taken from the Launchpad and place into a designed PCB board after all the code is completed. B Sensors Standard voltage divider techniques will be implemented in order to sense the voltage on both sides of the charge controller as shown in Figure 6 which illustrates the use of sensors, analog to digital converter to read all data transmitter on the charge controller. The battery voltage needs to be known in order to determine the charging state and continue with the charging algorithm. Figure 6 MPPT Charge Controller Data There will be several resistor chosen with designated values to pins that will fed the microcontroller to a nominal operation of 3.3 V, thus the microcontroller can interpret and The resistor values are chosen so that the designated pin is fed an analog voltage between 0 and 5 V that the microcontroller can interpret and induce the actual voltage value drawn from the system. Similarly, the current sensors send an analog voltage value to the microcontroller, which will determine the nominal value of the current. The current sensors, which consist of shunt

6 resistors, will function alongside the voltage sensors to sense current coming into and out of the charge controller. The 50Amperes-50mV shunt resistor will be used to read the input current coming into the system. Other shunt resistors are the MP2060 power resistor for maximum amperage of 60A and.01ohm resistance. This specific model is an accurate and low power circuit, which is advantageous when designing for efficiency and reliability on the system. TMP175 temperature sensor is a two-wire, serial output interface which functions with the IIC bus on the microcontroller and, requiring no external components and is capable of reading temperatures with a resolution of C. The DS1624 is a low power, precise digital temperature sensor that supports a wide range of temperatures from - 40 C to +125 C not limiting its performance. This sensor has the capacity to be assigned a digital address, which allows up to eight temperature sensors to be used in the design and they can all be accessed from the microcontroller through the same I2C bus line. The TMP175 temperature sensor was selected to monitor the temperature of the charge controller. These values will not affect the operation of the project but provide additional insight into the performance of the system under varying conditions; also it will help prevent damages on overheating the circuit board. TMP75 temperature sensors will be program using the microcontroller to successfully read in the values and perform safety operations concerning if the temperature falls outside the nominal values. Besides the digital temperature sensor, the system will have several analog thermocouples type K, which are going to be place at the PV panel and the battery bank. The temperature on the panel will be recorded and can be correlated against the output power of the panel to examine its performance is desired. The temperature on the battery will be useful for safety considerations to make sure it does not overheat. These thermocouples will be connected with a wire to an analog to digital converter to transfer the value in a digital manner. The ADS7830 is a single-supply, low-power, 8-bit data acquisition device that features a serial I2C. With the advantage of an 8 channel analog input module, it can intake several analog devices and values. Also, its nominal voltage supply matches the MSP430 voltage supply of 3.3V, making this device low power consumption. A. DC-DC Regulators In the design of this project a Buck Boost DC to DC converter is going to be used. Figure 7 below better illustrates the circuitry of this converter. Main reason for this topology is because a Buck-Boost DC to DC converter allows the user to control the input and desired output voltage. So for example if the input voltage is higher than the desired battery terminal voltage in this case also called output voltage, Buck-mode will be implemented to lower the voltage. If the input voltage is lower than the desired battery terminal voltage, then Boost-mode will be implemented. This is possible due to high speed switching transistors and active elements in the circuit. The semiconductor device used for switching in this circuit is the MOSFET (N-channel metal oxide). Figure 7 Buck Boost DC to DC Converter Buck-Boost DC to DC converters is the ideal topology for battery charging controllers, since the input can be lower or higher than the desire Output voltage. Another way to make it more efficient is to substitute the rectifier components with actual switches; Bipolar junction transistor (BJT), Metal-oxide semiconducting field effect transistor (MOSFET), Insulated gate bipolar transistor (IGBT), Silicon controlled rectifier (SCR), also known as the Thyristors, or Gate turn off Thyristors (GTO). MOSFETS always seem more practical for robust designs and for cost considerations. 0 Figure 8 Buck Boost Converter Output to Input Ratio vs Duty Ratio

7 V. SOFTWARE All the integrated circuit on this system utilizes the IIC bus to communicate with the main controller. As specify on the IIC bus communication protocol, the two main functions utilized are the read and write through the data bus. The main program will read in all the values from the sensors and integrated circuit in order to determine the charging stage of the battery and implement MPPT charging algorithms. This will not only improve the charging of the battery, but it will also protect the system s components. It is critical to control all the switching devices due to the fact that this system will undergo two major energy sources, wind and solar. At the startup of the system the microcontroller communicates to the slave devices to read the input current and voltage drawing from the PV panels and wind turbine. In order to integrate both systems, it is essential to maintain a constant input voltage that will go through the MPPT IC, which chooses the maximum power deliver to the battery by applying a buck and boost converter. A decision three is utilized in the code to compare all the values to the nominal, in order to protect and optimized the system. The microcontroller attains that data of the system through the implementation of the following components: shunt resistors, thermocouples and analog to digital converters. It will perform a voltage divider to calculate the voltage draw and supply. With such information and decision three codification, the microcontroller open or closes the MOSFETs and relays at essential part of the circuit board. The data converted as power is display to the LCD screen. The MSP430G2553 microcontroller will control the components by following the nominal values. The input values will vary depending on the conditions of the ambient, the PV panel and wind turbine. Thus having a nominal value will ease the input values to the charge controller and protect the system overall form over current and voltages. VI. I/O There are multiple input and output components in the overall system. The input connectors which capture the energy from initial sources, PV panels and wind turbine, are inserted into our system using regular plug connectors. Transmitting the current and voltage to the charge controller, this will charge the batteries, and output the energy through the inverter and transformer to the end user. The system will include a dummy load for the excess of current drawn from the wind turbine. There is an enclosure that will house all the components protecting them from the environment and inexperienced individuals from tampering with the equipment. VII. PRINTED CIRCUIT BOARD This custom printed circuit board has been designed using CadSoft s Eagle PCB Design software. The final design includes two separate boards one for high power and one for the data transmission with the following dimensions 10 x 6 respectively. The largest trace widths are.5 inch allowing them to handle a maximum of 20A at a rise temperature of 10 C. The bottom layer of the twolayer board will be a ground plain which will help reduce external noise to the electronics and help keep all grounds well connected. In the power electronics circuitry, top copper plains where added in place of traces to assure high current capacities could be handled without overheating. VII. TESTING A testing plan was developed to ensure that all of the features of the system work as expected. First, during initial prototyping, each part was tested at the component level to confirm that it works individually before integrating it into the system. As mentioned above, each integrated circuit is interface with the microprocessor using I2C bus, thus were connected to the MSP430 development board and check for accurate communication before actual data was transmitted. The power and data circuitry was replicated on a solder less breadboard to demonstrate its functionality before its layout was designed into the PCB. The power components were also tested individually. The PV module has been tested by the mechanical engineers group. The battery was connected to a voltmeter and allowed to discharge when a load was attached. The dummy loads were connected to a power supply of approximately 9A. Finally the inverter was connected to the battery and was able to power several electronic devices as intended. The system testing will be performed when the major components are connected together. It will have a check list to verify the correct operation of all components including solar panel and wind turbine outputting power, LCD display, battery being charged/discharged, inverter supplying power, and transformer switching output voltage and frequency.

8 VII. CONCLUSION The purpose of selecting this project was not only to design a system that will show all the knowledge acquired during the college career as engineers, but to come together as a group of electrical engineers, and solve an immense problem for an impoverished community located in Johannesburg, South Africa, where it seem impossible to supply source of power, due to high cost and lack of knowledge in the power system and power electronics field. Amandla Aluhlaza consist of a design that transfers the energy acquired from renewable sources solar and wind to a storage unit, which the people in Johannesburg will have access to for a couple of hours every day so that they can charge their cell phones and other personal electrical devices. In addition it will capable of delivering enough energy to power up a computer, a projector screen, and a stereo system that will be used with the main goal of educating and entertaining the people who have never had access to this type of technology. Overall this sophisticated design which is described and researched in this report was based on organized procedures, data analysis and meticulous calculations that lead to the right and most efficient solutions, by which the group Amandla Aluhlaza worked together to find the most practical and robust design for the specified conditions. In this project the group worked and will keep working with physical concepts that have been demonstrated and will be demonstrated in the next phase of senior design; Research, hypothesis, design, and testing. VII. ACKNOWLEDGEMENTS The authors wish to acknowledge the support provided by their faculty committee members Michael G. Haralambous, Wasfy B. Mikhael, Zack Abichair; This project would not have been possible without the support of Martin Rodriguez, he have not only contribute to the team with exemplary insight, but he has provided his valuable time in assisting during the design and testing of the project for successful completeness. In addition, the authors would like to acknowledge their sponsor, Progress Energy, Dr. and Alvin Wang, and to Dr. Richie for facilitating this grant money. VII. REFERENCES [1] Mukund, Patel. Storage Systems. New York: CRC Press, Print [2] Mukund, Patel. DC to DC Converters. New York: CRC Press, Print BIOGRAPHIES Andrea Patricia Solano is a senior in electrical engineering with an interest in power electronics and communication systems. She has been an active member and current Vice-President of the Society of Hispanic Professional Engineers (SHPE). Also, she enjoys cooking and traveling. Andrea will working at Chevron as a facilities engineer in Bakersfield, CA upon graduation. Juan Jose Valera is a senior in electrical engineering with expertise in power electronics and control systems. He is an international student who enjoys being exposed to new cultures and languages. Currently Juan is the Volunteer Coordinator for the Society of Hispanic Professional Engineers (SHPE). Juan future aspirations is to work developing reliable and efficient eco-friendly energy harvesting solutions that could potentially impact the way we live. Manuel Edward Keesee is a senior in electrical engineering with strong interest in physics and power systems. He is currently pursuing a career in the energy field. He is currently the Outreach Coordinator for the Society of Hispanic Professional Engineers (SHPE). In addition, he enjoys extreme sports and learning other languages. Art is something that attracts him, and he feels that traveling is a great way to learn from other unique cultures and traditions around the world. Randall Lay is a senior in electrical engineering with a minor in mathematics. His professional interests are control systems and power systems. He is employed by Power Systems Mfg. as a control system engineer. He will specifically be working on gas turbines for power generation. His hobbies mainly include rock climbing (seen in picture) and various other outdoor activities