Mobile Solar Charging Station

Mobile Solar Charging Station

Mid-Project Report

3/18/2014

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Mobile Solar Charging Station Contents:

Team Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Mechanical System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 System Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 General User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Sensor Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 DC/DC Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Updated DC/DC Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 PCB Layouts for DC/DC Converters and Switching Circuits . . . . . . . . . . 30 Redesign of Power Switching Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Enclosure Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Tipping Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Initialize Motor Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Encoders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Manual Bump Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Manual Override . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Emergency Switches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 The Purpose of Different Wiring Used. . . . . . . . . . . . . . . . . . . . . . . . . . . .47 The Types of batteries used for our System and Why . . . . . . . . . . . . . . .48 Wiring Harness for the Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48 Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 GUI Add-ons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

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Mobile Solar Charging Station Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

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Mobile Solar Charging Station Team Members:

Rudy Sanchez Undergraduate Electrical Engineer I am a 5th year Electrical Engineering student with a focus in Power Engineering and Materials. My engineering interests have to do with power transmission and material science. Over the past summer I interned at NASA Ames Research Center where I grew Nano Wires, and measured the power response of them. At the University of California Santa Cruz I have spent the past two years studying power electronics and software implementation into hardware through the courses of EE 176, CMPE13, CMPE 121, and CE 167. My main responsibility in this project is to design and program the control system through the use of a microcontroller. It is also my responsibility to create the general user interface in which the user of the project will interact with the system. James Ridgers Undergraduate Electrical Engineer I am a 5th year Electrical Engineering student with a primary focus on electrical power and energy production, storage, and transfer. I have some work experience building electrical circuits from building a dual-phase integrator to be used as an analog to digital filter for Professor Joel Kubby at UCSC. After graduating in the spring of 2014, I plan on either going to graduate school to get my masters, or find work as an electricians apprentice in southern California. My primary responsibility for this project is to create DC/DC converters that will step-down voltage from the panels to 12-V for the batteries, as well as helping with designing and building the MOSFET switches that control the flow of power through the system. Louie Vergara Undergraduate Electrical Engineer I am a fifth-year Electrical Engineering student at the University of California Santa Cruz with a primary focus in power engineering. My chief focus at Santa Cruz has been Power Electronics and Generation. I will be graduating this spring quarter with a Bachelor of Science in Electrical Engineering. After graduation, I plan to attend graduate school and study smart-grid design and power-system operations to enable renewable-resource integration into the smart-grid infrastructure. For the past couple of years I’ve interned at NASA Ames Research Center, Los Alamos National Laboratory, and Shanghai University of Electric Power, and researched the types of energy storage that maximize power output which is economically cost-beneficial, according to the location’s climate behaviors. My main responsibility for this project is to design a mechanical structure that will give the most reliable and efficient energy generation output for charging battery powered equipment. This research background will allow me to assist with

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Mobile Solar Charging Station determining which possible solar-tracking solution is best for the climate in Santa Cruz. I will also be laying out PCB designs for any circuitry components that must be done for the project.

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Mobile Solar Charging Station Abstract: As society moves into its future, there is a strong push from energy agencies to design and deploy various forms of energy-generation technologies that will reduce fossil-fuel emissions. There is great present and future demand for the provision of electrical energy within a locale’s environmental constraints. Under such constraints, the potential for reliable and efficient energy generation that integrates renewable resources assumes enhanced significance. This report presents a transitioning solution from fossil-fuel equipment used by the University of California Santa Cruz Groundskeeping Services to a mobile solar charging station that consists of a solar tracking system. The report assesses the functionality of a mobile charging station that will be used on campus to recharge groundskeeping tools. It also provides a comparison of different solar tracking systems, and explains which solution is best for the location’s climate. In addition, it provides a technological roadmap to how the system is designed and implemented, and the future work that remains to be done to demonstrate a reliable and independent renewable energy system.

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Mobile Solar Charging Station Acknowledgements: The Mobile Solar Charging Station is funded by the Sustainability Office, Carbon Fund, Grounds Services, California Alliance for Minority Participation (CAMP), and Michael Isaacson through the University of California Santa Cruz (UCSC). The members of this group would like to thank Chrissy Thomure from the Sustainability Office and Carbon Fund, Roger Edberg from Grounds Services, and Malika Bell and Yuli Ortega from CAMP for their financial support with this project. The Project support provided by Dr. John Vesecky, Stephen Petersen, and Paul Naud is gratefully acknowledged. In addition, Dave Thayer and Joe Cox from UCSC Division of Physical & Biological Sciences Machine Shop for their dedication to the project and consulting input on the structural drafts. This Project benefitted greatly from its Senior Design Advisory Committee, which offered crucial guidance and input during in-person weekly meetings. The members are grateful for the committee members’ dedicated support and insightful comments.

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Mobile Solar Charging Station Motivation:

Recently, there has been an increasing push from environmental and budget forces to pursue research in power systems, to reduce emissions from fossil fuels, and to accelerate the implementation of more renewable energy [1]. The need for these systems to integrate into dynamic environments will be the impetus for restructuring existing power technologies, as well as formulating new ones.

Groundskeepers of UCSC are currently transitioning over from gasoline powered tools to electric tools, but have run into the issue that they must travel to the one place on campus where they are able to recharge their tools. Due to this energy intensive protocol, the transition to electric tools has been dramatically hindered. Which is why, groundskeeping has requested IDEASS(Impact Designs: Engineering and Sustainability through Student Service) for a mobile charging station. Executive Summary: The purpose of this project is to create a mobile charging station that generates and stores the output energy through the use of solar panels to charge an electric lawn mower via an AC wall outlet. The system will be controlled by an on-board microcontroller, able to measure the solar irradiance, energy generation from the solar panels, the lawn mower’s current and voltage ratings, and switching control, and record the measurements through an on-board SD card. The microcontroller will also control DC motors to position the PV panels referenced to the sun for maximum power output. There will also be an accessible general user interface to monitor the power ratings. A major goal for this system is a robustness enabling it to be used on such other platforms as truck beds and trailers. Approach: Design Requirements: The requirements of this project from UCSC Groundskeeping Services is to build a solar charging station on a trailer, which will be able to output a high enough power to charge their electric lawn mower. This lawn mower charges up at a maximum of 864 W; therefore the system must be able to supply this power to charge the lawn mower regardless of the weather conditions. For this to happen, we are using three Photovoltaic (PV) 270 W SUNIVA Monocrystalline Solar Panels to generate enough solar power to charge the lawn mower. In order to generate and use PV power efficiently, the Mobile Solar Charging Station (MSCS) must be an autonomous solar tracking system consisting of a mechanical structure, a controlled system, an energy storage system, and all the circuitry. The complete MSCS is composed of these components, installed on a trailer that is 100 in x 65½ in x 6 ft, to have a complete MSCS. At the same time, all these components must be light enough for the trailer to support, since the groundskeepers will be driving the trailer to the various working locations around campus.

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Mobile Solar Charging Station Mechanical System: PV, the technology that converts sunlight into electricity, is one of the fastest-growing technology sectors of the renewable energy industry. The overall objective is to convert solar irradiance into electricity for use in any desired practical application. PV panels are used in larger on-site solar applications. For this type of application, PV panels are either stationary, or have some form of mechanical solar tracking system that tracks the sun throughout the day, repositioning the panels appropriately to maximize energy output [12]. Usually, the mechanical system is a single- or double-axis system that follows the sun’s azimuth and elevation. If the solar tracking system is a single-axis system, then the mechanical system will pivot the PV panels in one orientation following the sun daily or annual [3]. Typically this type of system, with only one angle tilt of orientation, is very simple to implement. Single-axis tracking provides an energy increase in the order of 25% or so, depending on the location’s irradiance [4]. A double-axis, or dual axis, tracking system tracks the sun’s azimuth and elevation. With this tracker, the system always maintains the optimal alignment to the sun. However, this type of mechanical system is technically more complicated compared to the single-axis tracking system. Despite its complications, the dual-axis tracking system can achieve an increase energy yield of up to 35%, as opposed to the 25% of single axis, but the system involves higher costs to implement [3]. Most trackers require a mounting system that can withstand different weather conditions, such as wind loads and storms. The drive system could be done manually, but is typically designed with an electric motor. The National Renewable Energy Laboratory (NREL) in Colorado has formed a performance and financial model, designed to facilitate decision-making for people involved in the renewable energy industry, known as the System Advisor Model (SAM). SAM makes performance predictions and cost of energy estimates for power projects based on capital outlay and operation costs as input by the user. SAM includes several libraries in its database of performance data and coefficients that describe the characteristics of PV modules under local climate behaviors for various regions of the United States. We used it to assess advantages and disadvantages of types of solar trackers for our application. NREL has a Solar Prospector database that shows the solar resource data and ambient weather conditions throughout the United States; this can easily be used with SAM. With this database, San Francisco’s data was downloaded. (San Francisco is the closest city to Santa Cruz that’s represented.) This data was used to determine the best solar tracker for the University of California Santa Cruz (UCSC). Figure 1 shows San Francisco’s 2011 annual data with the design parameters for PV panels average energy output for the three different solar tracking systems. Each modeled graph produces the actual power from the three PV panels, with their perspective tilts. PV panels won’t reach optimal output, but on average these produce about 240 W with a tilt of 30°.

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Mobile Solar Charging Station

Figure 1a: Stationary-Axis Solar Tracking System: Total Energy Output of 720 W with a tilt of

30°.

Figure 1b: Single-Axis Solar Tracking System: Total Energy Output of 720 W with a tilt of 30°.

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Mobile Solar Charging Station

Figure 1c: Dual-Axis Solar Tracking System: Total Energy Output of 720 W with a tilt of 30°.

Clearly, the dual-axis system produces the most energy gain, but, as stated earlier, is more difficult to implement. Figure 1a is modeled to stationary panels. The energy output has a maximum peak of 100-kWh for the month of July, whereas Figure 1b has one-axis of rotation with the 30° tilt and produces 120-kWh in July, for nearly a 20% increase in energy generated. Comparing the single-axis versus the double-axis system, then there is about a 6% increase with the double-axis system; this is why the team decided to build and implement a dual-axis system for this MSCS. The group considered this to be the best option for the project because all of the circuitry components, such as the microcontroller, raspberry PI, and LCD screen, could be powered by the additional energy produced, making the 26% energy gain worthwhile to implement for the circuitry components. The remaining increase in power goes toward the stored battery banks that supply the lawn mower. Having decided what type tracker will be used for the MSCS, our next step was to design the frame to hold the system together. The mechanical structure needed to be lightweight enough for the trailer to support, to be within the dimensions of the trailer, and to have a 6-foot clearance (the lawn mower has sunshades). Figure 2 shows a sketch-up of the structure with the entire system implemented. The structure will have two electric motors installed at the top of the frame where there will be a central pivot point that repositions the panels accordingly to the sun. In front of the trailer will be a triangle-shaped cabinet to hold the two Deep Cycle Batteries rated at 12 VDC 104 Ah, an inverter, and all of the circuitry components that connect the entire system. The electric motors will be positioned in such a way that they rotate the PV panels from east to west 30° on the tilted axis and 90° on the vertical axis. Figure 3 shows the proof of concept on how the panels will be rotated [10]. In Figure 3a, one electric motor centered underneath the panels on a central pivot point positioned in the horizontal axis to rotate 30° on each side, that

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Mobile Solar Charging Station is 30° on the west and 30° on the east. A second motor will be positioned underneath the central pivot to rotate the vertical axis 45° clockwise and 45° counterclockwise referenced from the center. The motors that will be used are two 24-V 40-lbs/in DC motors. To complete this dual-axis solar tracking system, the cost estimate is $6500, labor and parts inclusive. (The estimate is based on using 1-in x 1-in x 1/8-in stainless tube for the structural framework.)

Figure 2: Sketch up of the mechanical structure mounted onto the trailer. The cabinet is in front of the trailer with a 6-foot clearance that holds the PV panels in place. Wires will be connected

from the PV panels onto the cabinet where the whole system is integrated.

(a) (b)

Figure 3: Figure 3a shows how the PV panels will be rotated along the horizontal axis – 30° on each side, east and west. Figure 3b shows the rotation along the vertical axis. The rotation

along the vertical axis will be 45° from the center, a total of 90° in both directions [10].

System Control: The most important part of the MSCS is the system control, since this system must control motors and switches, track the sun’s position, and keep the user updated with a general user

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Mobile Solar Charging Station interface. A microcontroller was the obvious answer to control the system. The reason for this choice: the microcontroller can handle multiple input and output signals with its general-purpose input/output (GPIO) pins, and pulse-width modulation (PWM) control, all controlled with C-programming. The choice of which microcontroller to be used depends on what the system actually needs. For this system what is needed are 4 PWM signals that control the DC motors from the solar sensor readings, one universal asynchronous receive-and-transmit (UART) line, 8 analog inputs, and 10+ digital outputs. Three microcontrollers were considered: the PIC32, the PsoC5, and the MSP430. The PsoC5 and PIC32 were seen as viable options for the number of GPIO pins each has - 4 PWM signal outputs. The MSP430 ended up being rejected, since the microcontroller does not have enough PWM signals or as many GPIO pins. An alternative choice was to use the chipKIT Uno32 board. This board implements the PIC32MX320F128H and has external ports for GPIO and power regulation. The group ultimately decided on the chipKIT Uno32. The reason for this was the level of programming needed, and the power draw. Since the MSCS generates and stores power, this device operates at low powers. The PIC32 was chosen also because of the extensive libraries provided by Professor Elkeim (UCSC Computer Engineering Professor), who also provided a bootloader needed to gain the ability to program the PIC32 in C code. The overall system has two batteries that will be charged and discharged. For example, if Battery A has finished charging, then Battery A will change the state and begin to discharge, while Battery B does the opposite. One of the main tasks of the PIC is to control the switching circuit, which controls when each battery is being charged or discharged. Figure 4, shows the circuitry for the microcontroller, the pins of the schematic represent digital pins of the PIC32 that enable each switch. The PIC32 creates a voltage rail of 3.3-V to VGS of the each MOSFET to turn on, acting as a short. This allows 12-V 68-ADC that is generated from the PV panels to flow from the drain to the source of the MOSFET. When a logic 0 or 0-V is applied, the MOSFET is turned off, acting as an open circuit where no current flows through. The drain current to the battery input comes from the DC/DC converter (discussed later in the report).

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Mobile Solar Charging Station

Figure 4: Power Switching Circuit that is controlled by the PIC32. Each MOSFET is rated for 12-V 72-A. With the VGS set to 3.3-V, only 65-A is allowed to pass through since the PV panels MSCS

will never be at theoretical maximum. To dissipate the heat generated by each MOSFET, heat-sinks were purchased for each switching MOSFET. For this, thermal dissipation needed to be understood in order to integrate heat-sinks for integrated devices to ensure that any given device is operated within its defined temperature limits [2]. But first, we needed to calculate how much power the MOSFET dissipates. Given that the load generated is 65-A and the MOSFET’s maximum internal resistance is 13-mΩ, the power dissipation is:

This power loss determined which type of heat-sink should purchased by another set of calculation. The maximum allowable junction temperature (TMAX) is one of the key factors that limit the power dissipation capability of a device [4]. TJMAX is defined by the manufacturer and usually depends on the reliability of the die used in the manufacturing process. As a result, the typical equation used for calculation of the dissipation is:

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Mobile Solar Charging Station where θJA is the thermal resistance, TJ is the junction temperature, TA is the ambient temperature, and PD is the power dissipation. Therefore, using the NTD70N03R datasheet and solving for the thermal resistance, each MOSFET ended up having a thermal resistance of θJA=3.1 °C/W.

The battery must not drop below 50% of full charge, and not be overcharged to increase the lifetime of each battery. Due to these limitations, code has been implemented to control the logic of the MOSFETS and batteries. Figure 5 shows the state machine of the switching control. The switches are controlled by the PIC32 based off the voltage readings from the voltage divider from each battery, which are used to calculate the current charge of each battery. The state machine works by first, of course, having the system control on. Then we check which of the two batteries need to be controlled first. For instance, if Battery A is chosen first the code checks for two conditions: If Battery B is not charged, then charge Battery B; if Battery B is charged, then do not charge Battery B. After this condition, the code goes to the next stage and discharges Battery A and not B. Finally, if Batteries A and B are depleted, then stop discharging Battery A and jump to check the status of Battery B. If Battery A is depleted and not B, then start discharging Battery A. When this condition is met, check that the system is off. Once this condition has been met, then the code will check if Battery A is charged and greater than Battery B. If Battery A is greater, then restart the state machine at Battery A. The same conditions apply to Battery B (with conditions reversed).

Figure 5: State machine for checking the status of each Battery to charge and discharge onto the load.

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Mobile Solar Charging Station Another important function of the PIC32 is reading the values from the solar tracker. A solar sensor is used to calculate the azimuth and elevation of the sun to maintain optimum alignment to the sun that maximizes the power generated. This sensor calculates the sun’s positioning using real-time light intensity readings by monitoring the voltage, and the current produced. This solar sensor is composed of four LDR photoresistors with a rating of 80-kΩ to 240-kΩ (With more surface area, the photoresistors are more sensitive to light), located at the east, north, south, and west, to detect the light source intensity in the four orientations by forming 45° angle with the light source. At the photoresistors' positions, MDF shades isolate the light from other orientations to quickly determine the sun’s position [6]. The four photoresistors are divided into two groups, east/west and north/south. In the east/west group, both directions compare LDR intensities of received light. If the light source intensity (greater voltage) received by the sensors are different, the system obtains signals voltage outputs from the photoresistors in the two orientations. The PIC32 will then determine which sensor received the more intensive light based on the sensor output, then read the second greatest voltage output from the other orientation. If the sensor voltages are equal, then the sensor will determine the greatest voltage reading from the other two orientations and compare whether the sun is located between two directions or just one direction. A voltage type A/D converter interprets each voltage value from the solar sensor. Figure 6 shows the state diagram that tracks the sun’s position based on the solar sensor readings.

Figure 6: State Diagram for how the Solar Sensor determines the sun’s position.

Figure 7 shows how length of the shading structure was determined.

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Mobile Solar Charging Station

Figure 7: Sensor Precision

From the trigonometric equation, the precision of the sensor was derived [11]. For instance, the group wanted a precision of 3.2°, and place the photoresistor about 0.5 inch away from the center shade. Solving this trigonometric equation yields:

This is acceptable since the sun moves approximately 15° every hour (360°/25-hours = 15°/hour). An angle of 3.2° is chosen as the precision due to the height limits of the actual sensor (the opposite side of the sensor). If we wanted more precision, the angle would decrease giving a higher length (h). Figure 8 shows how the solar sensor separates each of the orientations. Each photoresistor uses a voltage divider to produce a voltage value for the PIC32 to read. When the irradiance increases on the photoresistor, Vout increases,

Figure 8: Photoresistor’s orientation on the solar sensor and voltage divider.

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Mobile Solar Charging Station General User Interface: Another component to the MSCS is an on-board user interface to check the statuses of how charged up the batteries are, and the performance of the entire system. Using the PIC32 is not a viable option for this purpose, because C-programming does not offer all the capabilities for general user interfaces and most microcontrollers are not equipped with HDMI or VGA interface. As a solution the team decided to use an on-board computer to due the job. However, the issue with this solution is that the computer needs to be small enough to not encumber the user, and draw low power since the point of the entire system is to charge other devices. Therefore, it was decided to use a Raspberry Pi. Raspberry Pi's are cheap devices that cost around $40 that can do the basic functions as most computers. They can easily be controlled using Python programming, and have the ability to interface with monitors and screens. Other options were considered compared to the Pi however, due to the extensive online documentation of the Raspberry Pi by other users, it was decided that the Pi would be the better option. The Model B Raspberry Pi comes with both an HDMI port and a VGA port, and 2 USB ports with 512MB that can store any program easily.

The reasons for using the Raspberry PI are to display the systems condition on a LCD screen to UCSC Groundskeeping crew through the General User Interface (GUI). The interfacing with this GUI will come from a physical light switch that will allow the user to turn the system on and off. Figure 9 shows what the GUI will display on the LCD screen.

Figure 9: LCD display using GUI interface.

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Mobile Solar Charging Station The input and output powers show how efficient the overall system is in real time, and will point where the kill switch is for any user to turn off the system. The bar graphs represent the battery charge of each battery. The GUI is updated by the sensor readings done by the PIC32 in unnoticeable sample rate. The library being used to create the GUI is TkInter, which is a standard tool in Python. Being that Python is open-source most of the documentation comes from online sources. By using commands as simple as "create_rectangle", a rectangle can be easily made and modified to fit the situation. There is one downfall to using TkInter as the user interface, and that is after it’s finishing creating the GUI, the file needs to run in an infinite loop in order to keep it on. This means that the program cannot do anything else while the GUI is on. This is a problem since the PIC is going to be sending data to the Raspberry Pi in order to have an updated interface. In order to fix this issue the program needed to be threaded. Threading is parallel programming that allows two main loops to happen at the same time. This is how GUI system was created. The program was threaded into two different threads, one for picking up the UART transmission from the PIC32, and the other for running the GUI. Communication: For communication between the PIC32 and the Raspberry Pi, it was decided to do this through a (UART). The reason for this choice stems from the fact that the team has already had experience using UART’s, and a wired connection will allow for optimum stability between the two devices. Sensor Readings: The PIC32 is used to interpret various sensor readings on the MSCS. Originally, this was going to be controlled though the Raspberry Pi since the readings are displayed on the LCD screen. However, the Raspberry Pi does not come with an attached ADC converter. This means that an external ADC converter would be needed in order for the Raspberry Pi interpret the sensor readings. Instead, the PIC32 does the sensor readings, and then sends the information to the Raspberry Pi through UART. The PIC32 can only sense 0 to 3.3-V into its pins since there are 12 A/D pins. If the voltage is higher, the PIC32 can get destroyed so it is mandatory that the voltage is not higher than 3.3-V. The 12 A/D pins convert analog signal into digital readings that a programmer can use. This conversion is dependent on the analog to digital converter built into the microcontroller. The A/D converter is built into the PIC32 that are 10 bits of resolution. In other words, the PIC32 can take a voltage between 0 to 3.3 and give that reading a value between 0-1024. By using the equation:

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Mobile Solar Charging Station

x= x1024

∗ 3.3

Obviously there is some resolution error, however this error is small enough that it does not affect our system in the long run. The specific sensor readings that we are doing is the amount of voltage left in the batteries, the solar sensor readings, and the current going through our system. The voltage sensor for the battery was made with a simple voltage divider:

Large values for resistance had to be used to keep the current under the current limit, 17-mA the PIC32 can handle. This voltage divider takes in 0-13.5-Vbattery. The reason for the battery voltage being 13.5-V instead of 12-V is to avoid the batteries becoming overcharged. For measuring current, a Hall effect sensor has been implemented after the DC/DC converter. This Hall effect sensor varies its output voltage in response to a magnetic field. When there is current running through a wire, a magnetic flux is produced that is measured into an output voltage from the Hall effect sensor. Characterizing the sensor first begins by supplying an initial voltage, 5-VDC, into the sensor. The sensor simply divides its supply voltage in half (2.482-V). For every amp that flows through the wire, the voltage output from the sensor increased about 0.018-V/A. Therefore; this formula shown below could be used to convert from Voltages to Amps as follows:

This voltage produced by the Hall effect sensor is read by the PIC32 and converted into current using a scaling factor that has been programmed in. This scaling factor depends on the load that is being applied to the system. As soon as the current has been scaled, the current measured with the output voltage of the DC/DC converter can determine how much power the MSCS is generating. DC/DC Converter: As mentioned earlier, the MSCS is using three SUNIVA Optimus Series Monocrystalline 270-W PV panels to generate energy. The voltage at max power point is 31.2-V and the current at max power point is 8.64-A. The open circuit voltage is 38.5-V and the short circuit current is 9.15-A. Therefore a DC/DC converter must be to handle an average voltage and current equal to that of the max power point, handle voltage up to the open circuit voltage, and current up to the short circuit current. The DC/DC converter also needs to be simple enough that it could be made

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Mobile Solar Charging Station within the design’s course time frame, and a low cost that fits within the teams budget (under a hundred dollars per DC/DC converter). Figure 10 shows the IV curve, and the nominal output for each PV panel.

The research for the DC/DC converter started out by reviewing other solar systems that had already been created at other universities. In the end the DC/DC converter for MSCS system was chosen based off San Luis Obispo SUPER project. A buck converter similar to the SUPER project was selected for this DC/DC converter [9]. Though the MSCS system has to be more efficient to design and assemble from the ground up, because of the time constraint, therefore it was decided to use a control ship to operate two MOSFETs of the buck converter. Some of the buck controllers that were considered included the LTC3835, LTC3703, LTC3840, and the LTC3845. After looking at multiple buck controls it was realized that it would be much easier and economical to create a buck converter for each individual panel. This approach to lows the maximum current that would go through the MOSFETs, and would be able to be within the maximum current sensed by the controller. In the end the LTC3845 was chosen for the buck converter because it suits all of the parameters needed. Multiple examples were found that outputs a constant 12-V.

(a)

(b)

Figure 10: (a) IV Curve for a SUNICA PV panel.

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Mobile Solar Charging Station

(b) Nominal Output for 255-270 Watt PV Panels

Before creating the physical DC/DC converter the circuit was designed using LTSpice. LTSpice was used because Linear Technology only has the LTC3845 part formatted in LTSpice. Figure 11 shows the DC/DC converter designed in LTSpice, and Figure 14 displays the voltage output for the LTSpice model. The output shown in Figure 12 is the voltage across Rload. Rload is 0.5-Ω to show that the current could be as high as 24-ADC, and would not be limited by the MSCS system. As shown the DC/DC converter is be able the handle the absolute maximum output current of 22.5-ADC.

Figure 11: LTSpice model of the DC/DC converter

Figure 12: DC/DC converter Output

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Mobile Solar Charging Station Figure 12 also shows the current output of the DC/DC converter model. Though the capacitors, inductor, and resistors in the DC/DC converter are ideal in this model, except for the MOSFETs. The simulation model uses Si7370DP N-Channel 60-volt MOSFETs. These MOSFETs are not the correct ones used for the physical DC/DC converter, but VGS is the same, 2-V to 4-V [10]. The majority of the time spent designing the DC/DC converter was spent on calculating and selecting the proper elements for the DC/DC converter. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold of 100-mV/RSENSE, and the input common mode range of SGND is 10-V that allows a margin for variations in the IC and external components values yielding:

The maximum output current that the DC/DC converter can output is 22.5-A. Therefore, RSENSE needs to be equal to or less than 3.56-mΩ. Choosing to be on the safe side RSENSE = 3-mΩ. For the physical DC/DC converter seven 20-mΩ resistors were used to find smaller resistors that could handle the maximum power across RSENSE.

Another condition that the DC/DC converter has to deal with is the operating frequency. The chosen frequency has tradeoffs between efficiency and component size. For lower frequencies MOSFETs have lower losses, but requires more inductance resulting in a larger inductive size and higher cost. If the frequency is larger, then bigger output capacitors are needed. Tables of RSET in Figure 13 show the corresponding frequencies given in the data sheet. From the different frequency ranges, 300-kHz was used to try and balance the inductive and capacitive cost. Nonetheless, the resistance was rounded to 50-kΩ in hope that the frequency did not change.

Figure 13: RSET versus Frequency

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Mobile Solar Charging Station Selecting the minimum inductive value is given by:

The typical value of ΔIout = (0.2 x Iout,max) = 4.5-A was chosen for the DC/DC convert, and the minimum inductive value is

but ended up rounding the inductor value equal to 3 x 10-6 H to make sure that the inductor is large enough. The MOSFETs were chosen to handle the maximum current and voltage across the source and gate, whilst still being controlled by 2-4 volts. These MOSFETs able to handle much more than the maximum voltage and current, but are also the same MOSFETs that we will be using as the switches and therefore it was decided that it would be bets to limit the number of different components across the system. The MOSFETs in the DC/DC converter are the HUF75645P3. The maximum voltage and current of these MOSFETs are 100-V and 65 A. The maximum voltage difference between the input and output is (40-V – 12-V = 28-V), and the maximum current we get from any of the DC/DC converters is 22.5 amps. As a result these MOSFETs meet the requirements. The input capacitance was selected based on the acceptable input voltage variance and minimum input voltage. The acceptable voltage variance was selected as 200-mV. The equation given in the data sheet to compute input capacitance is:

Deciding on the output capacitor selection the voltage is based on the design’s output voltage ripple, and the transient load requirements. The output voltage ripple is a function of the inductive current ripple and the output capacitance given by:

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Mobile Solar Charging Station

The maximum equivalent series resistance (ESR) required a specified ΔVout:

With this ESR(MAX) Cout was found:

After finding the inductor and capacitors, a resistive divider was used to find the DC output voltage:

This divider sets R1 = 16.2 [kΩ] and R2 = 142 [kΩ]. Using these resistors the output voltage equals 12.021 [V] which is close enough to the desired value.

After finalizing all of the elements we had to purchase the proper wiring that would handle the current. 12 American wire gauge (AWG) wire is used for the line of the DC/DC converter because it is one size larger than the minimum wire gauge that can handle the maximum voltage and current across the wire. The maximum current 14 AWG wiring can handle is 3-A, therefore the 12 AWG wiring is used to assure us that 55-A is handled properly without the wire heating up or deteriorate in case of the current becoming high [11]. Afterwards, we designed a prototype of the DC/DC converter onto a perfboard by soldering all of the components of the DC/DC converter together, yet maintaining everything organized. First the main line was connected with heavy duty solder, but we were concerned that the

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Mobile Solar Charging Station solder would create shorts everywhere so it was decided to use 12 AWG wire onto the perfboard as the main line. When the DC/DC converter was tested we were able to get some readings, but ended up being unreliable. Consequently, troubleshooting became problematic for finding any damaged or broken parts, which ended up being many “shorts” within the DC/DC converter that varied RSENSE because of all the solder.

Updated DC/DC Converter Designs: A more efficient buck converter may have been able to be achieved if we had built the circuit from the bottom up, but because of the time constraint we decided to use a control chip, which would be used to control the two mosfets of the DC/Dc converter. Some of the control chips considered for the DC/DC converter were the LT3845, LTC3703, LTC3840, and the LTC3835. We choose the LTC 3845 because it fulfilled the needs for our DC/DC converter, and required the least amount of external parts to work properly. Due to how much current is being produced by the three solar panels, instead of creating one DC/DC converter to handle the input from all three solar panels we decided to create a DC/DC converter for each individual panel. Figure 14 shows the engineering schematic for the DC/DC converter. The outputs of all three DC/DC converters are in parallel; unfortunately this does not lower the complicity of the design for the DC/DC converter. However, this reduces the amount of power loss due to heat dissipation.

Figure 14: LTSpice model of the DC/DC Converter

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Mobile Solar Charging Station After creating a prototype using a breadboard, we created a PCB for the DC/DC converter. Below is an image of the DC/DC converter that we printed after all of the components have been soldered.

Figure 15: First Printed DC/DC Converter

After soldering all of the components and tested this circuit we found that this DC/DC converter would only output maximum of 6 [V]. We consulted with an electronics professor here at UCSC, and came to realize that the reason for this inadequate voltage is because we did not correctly separated the signal and power grounds of the DC/DC converter.

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Mobile Solar Charging Station We recreated how the wiring of the DC/DC converter was designed to separate the signal and power ground, as well as low the amount of noise of the signal ground as much as possible. We decided due to cost to create the DC/DC converters using sheets of copper (1 [ 𝑜𝑜𝑜𝑜

𝑓𝑓𝑓𝑓2]) and routing

our own circuit boards. Before we printed the first of the DC/DC converters on copper we dead-bugged the DC/DC converter, shown in Figure 16 to make sure that our design was sound and we would get the correct 12 [V]. The dead-bug ended up looking rather messy, but were able to find that by raising the output capacitance (bypass capacitor between the input and signal ground, power ground and VCC, and placing a ferrite bead along the signal ground line) we were able to get a much cleaner 12 [V] output shown below in Figure 17. Appendix B lists all of the parts used for the DC/DC converter.

Figure 16: Deadbug of the DC/DC Converter

Figure 17: 12 [V] output after dead-bugging the DC/DC converter. The voltage spike is

due to MOSFET switches and inductor.

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Once we had finished creating our dead-bugging for the DC/DC converter we then routed the DC/DC converter on a copper plate and soldered on all the pieces, shown in Figure 18 and 19. Though this significantly dropped the cost of each DC/DC converter, the time to connect the top and bottom layers of the copper plate where needed, which makes construction of the DC/DC converter extremely time consuming.

Figure 18: Final DC/DC Converter soldered onto a Copper Plate

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Figure 19: 12 [V] output from the final DC/DC converter soldered onto a copper sheet.

After creating and testing all of the DC/DC converters to make sure they output is a constant 12 [V], we realized that there would not be enough space for all three DC/DC converters in the enclosure, and would therefore need to create a small shelf to hold all three DC/DC converters. We decided to design the shelf using MDF in SolidWorks. We then printed out the shelves using a laser cutter and assembled and placed in the enclosure.

After installing all of our components and wire harness we realized that the shelves needed to be improved, or have the DC/DC converters PCB smaller. Because it is extremely difficult to install the wiring into the DC/DC converters without pulling out, bending the terminal blocks, or having the wires come out of the terminal blocks. PCB Layouts for DC/DC Converters and Switching Circuits: Trace maximum current is a requirement that designers specify to ensure that a trace in a circuit can handle the required current capacity. A set of equations calculates the maximum trace current based upon the following specifications: trace width, trace thickness, and maximum desired temperature rise. This is based on IPC (Association Connecting Electronics Industry): 221 Generic Standard On Printed Board Design. The trace width is calculated as follows:

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Mobile Solar Charging Station First, the area is calculated:

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 [𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚2] = �𝐼𝐼 [𝐴𝐴]

𝑘𝑘 ∗ (𝑇𝑇𝐴𝐴𝑚𝑚𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴𝑇𝑇𝑇𝑇𝐴𝐴𝐴𝐴_𝑅𝑅𝑚𝑚𝑚𝑚𝐴𝐴[°𝐶𝐶])𝑏𝑏�1𝐶𝐶

Then, the width is calculated:

𝑊𝑊𝑚𝑚𝑊𝑊𝑇𝑇ℎ [𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚] = 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 [𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚2]

𝑇𝑇ℎ𝑚𝑚𝑖𝑖𝑘𝑘𝑖𝑖𝐴𝐴𝑚𝑚𝑚𝑚 [𝑜𝑜𝑜𝑜] ∗ 1.378 �𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑜𝑜𝑜𝑜 �

For IPC-2221 external layers in air: k = 0.048, b = 0.048, c = 0.725 where k, b, and c are constants resulting from curve fitting to the IPC-2221 curves [11]. The geometry diagrams are shown below in Figure 20:

(a)

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Mobile Solar Charging Station

(b)

Figure 20: Geometry Diagrams for PCB Trace Widths – These diagrams determine the area of the trace based from the IPC-2221.

Using these equations, I determined the trace width needed for our DC/DC converters and switching circuits given that the DC/DC converters needed 10 [A] through the device, and the

switching circuits are rated for 70 [A]. Given these current values, we used 2 � 𝑜𝑜𝑜𝑜𝑓𝑓𝑓𝑓2� to handle the

current and any temperature rise. Similarly, this copper thickness allowed us to have shorter trace widths since this copper weight can handle any temperature increase. The only limitation

of using this type of copper thickness is having a higher power loss of 0.5 �ozft2�. Figure 21 shows

the PCB layouts for the DC/DC converter and switching circuits.

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Mobile Solar Charging Station

(a)

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Mobile Solar Charging Station

(b)

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Mobile Solar Charging Station

(c)

Figure 21: (a) PCB layout for the three DC/DC converters used. The main traces are the input that carries the negative and positive signals. They’re rated to carry up to 10 [A]. (b) and (c) This PCB shows the input switching circuit and output switching circuit respectively. The input switching circuit is connected between the DC/DC converter output and the batteries. Each trace must be able to handle up to 70 [A]. The output switching circuit is what connects the batteries onto the inverter load. Again, each trace must be able to carry a maximum of 70 [A]. The area for each trace isn’t the exact calculation due to the fact that there had to be spacing for the components. For example, the heat sinks used are large that require its own space as well as each terminal block and components.

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Redesign of Power Switching Circuit:

Earlier in this report, we had started off using NMOS transistors as our switching components. However, when we tested the circuits, there was a limitation with the design. When the output of the DC/DC converter is connected onto the input switching circuit and the output is the battery, the input voltage has to be greater than the battery supply voltage; otherwise there will always be feedback current, ultimately shorting the NMOS, which literally happened to our circuit. After troubleshooting analytically what had happened, we realized that the VGS of the NMOS transistors became a function of our load, which needed to be biased at a higher voltage than the battery voltage (12 [V]). As a solution, PMOS transistors were our second option. In this instance, the MOSFET switch is connected between the load and the positive supply rail as in PNP transistors.

In this device, the conventional flow of drain current is in the negative direction so a negative gate-source voltage (VGS) is applied to switch the transistor “ON” as shown in Figure 23. This is achieved because the PMOS is “upside down” with its source terminal tied to the positive supply +VDD. Then when the switch goes LOW, the MOSFET turns “ON”, and when the switch goes HIGH the MOSFET turns “off”.

A LM324 OP AMP is used as a non-inverting OP AMP to produce the necessary voltage to control the MOSFET. The OP AMP is controlled by the PIC32 that gives a 3.3 [V] to turn on and off the amplifier, which is powered by the battery voltage rail. So when the PFET goes low, a 0 [V] is being applied onto the VGS of the PFET, and when the PFET is high there is a -12 [V] being applied to VGS to turn on the PFET. In other words, the resistors shown below in Figure 22 are a 3 to 1 ratio in order to produce -12 [V] to turn on the PFET.

Figure 22: Non-inverting Amplifier to produce 12 [V] that power the PMOS to turn “ON”. The resistors must be a 3 to 1 ratio.

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Mobile Solar Charging Station

(a)

(b)

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Mobile Solar Charging Station Figure 23: (a) and (b) shows the input and output switching circuit respectively. A non-inverting

op amp controls each PMOS.

Frame Structure:

The Solar Charging Station has to be built and mounted onto a trailer to charge lawn mowers that charge up at a maximum of 864 [W], and must withstand all types of weather conditions. The structure must be light enough to be supported by the trailer, yet sturdy for use. Figure 24 shows a picture and dimensions of the trailer where the systems will be installed. The dimensions of the trailer are 97’’ x 60 ½’’ x 6’. (The 6-foot clearance is necessary since the lawn mower has sunshades that are that high.)

Figure 25 shows the actual frame built from the machine shop. This frame consists of two motors for a dual-axis rotation, two screw jacks (4-to-1 gear ratio) and a driveshaft that directs the motor’s output to move the panels in the elevation direction, with four gears -- two for each motor.

Figure 24: Picture and dimensions of the trailer where the system will be installed.

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Mobile Solar Charging Station

(a)

(b)

Figure 25: (a) Motor 1 turns the panels in the elevation direction (up to 30°). You can see that there is a drive shaft, turned by two gears. As the driveshaft turns, the screw joints turn

clockwise, increasing the panel’s angle. (b) Motor 2 rotates the panels in the azimuth direction of a total of 360° - 180° clockwise and another 180° counterclockwise.

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Mobile Solar Charging Station Enclosure Design: For the enclosure used to house all of our electronics, our first prototype, was constructed out of ¾’’ thick plywood. The enclosure needs to be large enough to fit all of our electronics, yet small enough to fit in front on the trailer and not take up any of the space that groundskeeping uses. To keep the enclosure out of the way of groundskeeping, a first prototype was built to fit at in front of the trailer. As you can see in Figure 26 and 27, the designs of the first prototyped are shown.

Figure 26 Dimensions of the Enclosure

. Figure 27: Enclosure Primed for the Outdoors

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Mobile Solar Charging Station Tipping Calculation: As soon as we had a frame design, there was concern about this structure, and that was: whether it would be top-heavy. In the case of the structure being top-heavy, this could cause the trailer to tip over on steep parking grades. In order to see what angle the trailer would have to be in order for it to tip, some calculations were made. First, the weight of the trailer without the lawn mower is taken into account, which is approximately 450 [lbs.]. Second, the approximate weight of the frame on top of the structure (150 [lbs.]) also needed to be taken into account. The height of the top of the trailer was estimated to be about 9 [ft.] from the ground.

With this data we use the equation 9tan-1() = x. 9 is the length from the top of the trailer to the ground. X is the length from the middle of the trailer/structure to the outside edge of the trailers tires. Taking this length x and making it proportional to the base weight and the top weight, we were able to find the center mass of the trailer which is where the tires sit on the ground. Looking at the data, it was found that the trailer needs to be at a 40° tilt in order for the trailer to fall over its own weight. This degree was found to be reasonable so the project continued. If you’re curious about these numbers, please see the Appendix A. Manual Override: When the PIC32 starts up, an implemented algorithm will begin to start tracking the sun. However, in order for the PV panels move to the correct position to track the sun, the PV panels has to start at some default location. To do this, the PIC32 will use the bump sensors as well as the encoders that are on the motors to determine the default position. The solar panels will first move the 30° tilt which is monitored by a mechanical bump sensor. When the panels have reached maximum tilt, the mechanical sensor will trigger an electric pulse onto the PIC32 informing that the panels have reached maximum.

Then the motors will turn the panels in the azimuth duration that, again, is being monitored by a bump sensor. While the PV panels are moving, the PIC32 will track how far the motors have moved through the use of encoders. Afterwards, the motors will move in the opposite direction, a total of 360° (180° from the zero position) in the opposite direction until it hits the bump sensor for that direction. Also again, the PIC32 will keep track of how far the panels have moved with the encoders. The way the encoders work is by first subtracting its first encoder value from the second giving an absolute value. This is the number of rotations the motor has to turn on the encoder for it to reach its initial starting position.

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Mobile Solar Charging Station Encoders: For added safety and precision of the solar tracking and motor movement, encoders were bought and installed onto the motors of the design. The encoders that were used were the AMT 103 capacitive rotary encoder. Typically, encoders are optical, so when the shaft of the motor makes a rotation, a beam of light is sent from the device and a signal is sent through the encoder. However, a capacitive encoder uses capacitive properties to detect a turn. This renders the encoder more reliable, for there is no fear of dust or debris interfering with the optical sensor.

These encoders have 5 pins -- a ground rail, a 5 [V] rail, 2 phase signals, and one absolute signal. The phase signals use a code call grey coding. With this, the encoder gives off precise angles of how far it has gone for 1 shaft rotation. This was deemed unnecessary for the project. This level of precision was not needed since the encoders are being used to make sure the panels have not moved too far in any direction.

Instead, what is being used is the absolute position. The encoder gives a pulse every time the shaft has made one full rotation. The motors that we are using have a 240:1 gear ration. This means that every time we receive a pulse on the encoder, the main shaft of the motor has moved 1.5 degrees. This, along with the 4:1 and 5:1 gear ration that are implemented on the motors, was deemed precise enough to ensure that motors are moving safely.

Manual Bump Switches: After installing the encoders onto the motors and we realized there has to a back up to make sure that the panels did not rotate, or tilt too far in any direction because this would cause damage to the structure. For this we created a simple circuit implementing bump switches. For this circuit, we voltage divider is used attached to one of the batteries that would convert 12 [V] down to approximately 2.7 [V]. This line was then led to the bump switches and attached such that they would output 2.7 [V] when triggered. The output was fed to the PIC32 and when a non-zero is read, the PIC32 would know that one of the switches had been triggered and would automatically stop all of the motors so that no damage would be done to the structure. Metal plates are installed to trip and place the bump switches such that the structure would only ever rotate 180° clockwise and counter clockwise, the panels would tilt to only 30°, and a switch will be tripped when the panels are in the default position - 0°.

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Mobile Solar Charging Station

Figure 28: Bump Switch Schematic (Note: Schematic is the same for all 4 Bump Switches)

Figure 29: Bump Switch

Figure 30: Bump Switch Circuit

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Mobile Solar Charging Station

Figure 31: Rotating Bump Switch (Top View)

Figure 32: Rotating Bump Switch (Side View)

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Mobile Solar Charging Station

Figure 33: Tilt Bump Switch (Side View)

To protect the bump switches from the weather we wrapped all of the bump switches with plastic and duct tape. Though it wasn’t the prettiest solution this did work in protecting the bump switches while also allowing the nuts through used to hold the bump switches onto the braces as shown in Figure 34 and 35.

Figure 34: Rotating Bump Switch Components on Structure

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Mobile Solar Charging Station

Figure 35: Tilt Bump Switches on the Structure

Manual Override:

In the case of electrical components being damaged or broken, there is a series of manual override switches placed on the enclosure so that the system will still work -- just not autonomously.

Emergency Switches: For this system, considering that large currents are running through each circuitry component, safety is its first priority. For safety reasons, two emergency switches have been placed on the system to allow the user to turn off key aspects of the project. The first emergency switch is a 3-pole circuit breaker.

Other options were considered when deciding on this breaker. Other physical switches could have been put in place of the breaker, such as large pushbutton switches or other levers. However, we decided to go with the breaker for its simplicity. The maximum amount of current that will go through our system is 70 [A], and given that there are existing breakers rated for this current, which is why this breaker is an easy solution.

Another reason we decided for the breaker was for its automatic switching. If the current in our system were to somehow start going over 70 [A], then the breaker will automatically flip itself on, disconnecting the lines.

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Mobile Solar Charging Station This circuit breaker is in place for two reasons. The first reason is for the automatic safety that it provides, and the second is that of a manual switch. Since the circuit breaker has 3 poles, we are able to disconnect three lines at once with this breaker. The first line turns off the line between the inverter and batteries. The second line interrupts the DC/DC converter to the input switch. Lastly, the final one disconnects all of the lines connecting the input switches to the batteries. By turning off these key locations, all of the power generation is detached and all the circuitry becomes open circuits. This guarantees that the batteries are completely cut off from the system, so that nobody is in danger of getting seriously injured in the case of a failure.

The circuit breaker is rated for 100 [A]. This breaker is rated this high because of the high current in our system. If the current was to become larger than 100 [A], the breaker will automatically flip and turn off the system, known as automatic switching.

Since this circuit breaker has a physical switch, it can also be used to disconnect those same three lines mentioned earlier manually. In other words, if the system is unsafe, the user can flip the breaker and stop all power from entering or leaving the batteries.

The other safety switch that is being used is a large mushroom button switch. This switch is brightly colored and is a double pole-double throw. This switch is placed right below the center of the rotating axis of the structure. This line has the power for the motors going through it. When it is pressed, the line will be cut and the motors will not move. This is in place so that if the user is near the panels when they are in rotation and suddenly a body part comes in danger of becoming injured, the user will be able to push this switch and kill the power to the motors.

The Purpose of Different Wiring Used: When embarking on a project having this much current running through the system, choosing the proper gauge of wire is critical. Using an improper gauge of wire can damage sensitive electrical components. Under-gauge wires, for example, can overheat and potentially catch on fire. Safety is an important consideration when determining the proper gauge of wire for any electrical project.

For our system, there is 10 [A] and 70 [A] of current running through the system as well as smaller currents that can be used with regular 22-AWG wire. For these higher currents a 12-AWG wire is used that connects the PV panels onto each DC/DC converter. 4-AWG wires are used to connect all three DC/DC converters onto the input switching circuit. From the switching circuit 4-AWG is still used that connects onto the batteries, output-switching circuit, and onto the inverter.

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Mobile Solar Charging Station The connections coming from outside of the enclosure, such as the power from the PV panels, lines to power the motors, and reading the solar sensor, are all connected to the enclosure through standard connectors. Molex connections are used to send power to the motors, read the encoder values, receive readings from the solar sensor, and read the bump switches. The input power from the solar panels cannot use Molex connectors, because a larger gauge wire is being used. Instead, MC4 connectors are being used; they’re commonly used for connecting PV panels.

Inside of the enclosure 12 AWG wire run from the MC4 connections to the DC/DC converters. From the DC/DC converter they combine into 4 AWG wire in order to handle the amount of current going through the system, as stated previously.

The connections that connect the PIC32 onto the motors, Molex connectors are used that are installed with 22 AWG wire. The amounts of current passing through these connections are a maximum of 1 [A], so this wire can easily handle this current density. The Types of Batteries Used for Our System and Why:

The Sun Xtender 12 [V] 104 [Ah] batteries are deep cycle unique high-density plate technology that provides superior reliability, power, and extended life. The U.S. military and other worldwide airframe manufacturers adopt these batteries. These types of batteries are referred as solar batteries (Deep Cycle Batteries), which are a key component in a stand-alone renewable energy system. Solar batteries provide energy storage for solar, wind, and other renewable energy systems. Different from a bar battery, deep cycle battery is capable of surviving prolonged, repeated, and deep discharges which are typical in renewable energy systems that are “off grid”.

In order to maintain healthy batteries and prolong battery life, these batteries should not be discharged below 50% depth of discharge (DOD). Often an inverter will have a low voltage disconnect feature that will disconnect loads at a given set point. This is why we monitor both batteries to help better maintain deep cycle battery health and to provide statistics about the overall health of the system.

Wiring Harness for the Structure:

For the wire harness of the structure, we are using ¾’’ split duct conduit that will insulate all of the wiring that connects the bump sensors, motors, and PV panels. Figure 36 shows the structure model that has been built by the machine shop.

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Figure 36: Structure Model for the machine shop to build.

The grey arrows show where the conduit is placed on the frame. The red and black arrows represent where the wires are distributed for each panel, motor, and bump sensors located on the structure. There are a total of 6 wires for the three PV panels, 10 wires for both motors, and 8 wires total for the bump sensors. All these wires in installed and directed through the grey arrows in the figure that represents the split duct conduit.

Inverter: Due to the time constraint on our project we knew from the start that there wouldn’t be enough time to design our own inverter, therefore an inverter was found that output 12 [V] onto the batteries. This inverter can also handle the maximum amount of current our system is required to handle. We decided on the Xantrex 806-1010 Pro Series Inverter - 1800 Watts. This inverter was designed to run off of 12 [V] car batteries, which fulfilled all of our requirements for our system. Figure 37 shows the datasheet of electrical performance of the inverter.

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Mobile Solar Charging Station

Figure 37: Electrical Performance of Inverter

GUI Add-ons: The bar graphs of the GUI are colored in order to give more information to the user. If the bar graph of a battery is blue, that means that that specific battery is neither charging nor discharging. If the bar is green then that battery is currently charging. Lastly, if the graph is red, then the battery is discharging. This is all provided on the legend that is at the bottom of the GUI. The screen for the GUI is a basic car LCD screen a .9’’ x 3.2’’ x 1.6’’. This size can easily fit inside of the enclosure that is being installed onto the trailer. The GUI loads upon startup, in other words when the user turns on the system from its off state the GUI will automatically load. However, this also has an unexpected implication that an additional switch must be flipped after the Pi has achieved full startup. When the Raspberry Pi starts, it uses the built in UART lines. However, as soon as the lines have finished starting up the UART lines are free to use. If the UART is still connected when the Raspberry Pi launches, the GUI will not be able to update. To overcome this issue, a light switch is placed on the design that will allow the communication to be broken up while the Pi boots up.

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Mobile Solar Charging Station This Raspberry Pi was configured to make the GUI load on startup by adding a folder in its memory called Autostart. With this folder created, there also needs to be a text file that includes: [Desktop Entry] Encoding = UTF-8 Type = Application Name = filename Comment = Exec = sudo python3 /home/pi/filename.pi StartupNotify = false Terminal = false Hidden = False When the Raspberry starts up, the folder called Autostart, it finds this folder and executes anything in that folder. This code shown above loads the file that is listed, in our case the GUI. System Overview: The figure below shows an overview of the systems main components which are solar panels, DC/DC converters, MOSFET switches, Raspberry PI and PIC32 controllers, batteries, user interface, motor for panel control, and AC output through the inverter.

Figure 38: Block Diagram of the Program Overview

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Mobile Solar Charging Station Future Work: I) Enclosure for the Batteries and the Electronics After completing our first enclosure, we came to realize that despite having space for all of our components we did not have the correct spacing, or correct wiring harness. This became a major issue because the wiring always became tangled, pop out of components, or pulled some of the components apart. Our first enclosure was designed to be mounted in front of the trailer. With this design we built our enclosure to have three levels to hold all of the components and the batteries. The enclosure had a vent placed over the batteries to circulate out any hydrogen gas produced by the batteries. The next enclosure should be built and placed along the side for the trailer to give more space for all of the components. With this layout we will no longer need to place the components and batteries on different levels, but rather keep them all on the same level. The new enclosure will also be made out of a different material instead of plywood. Though plywood is rather light and sturdy, we needed more caulk to insulate the enclosure as much as possible. A second prototype is needed for complete insulation. The major change needed for the enclosure is to have the wiring insulated with conduit from one component to another to keep the wires from becoming tangled, or pulling any of the components out. II) DC/DC Converter Though the DC/DC converter will output 12 [V], shorts along the DC/DC converter happened numerous times when placing the devices onto the enclosure. Terminal blocks used to hold the wiring continuously were ripped out of the copper plates. New DC/DC converters will need to be created with new connections to the input and output wires. It is also advised that the DC/DC converters be printed on a PCB board to make sure that there are no shorts made on the circuit. Conclusion: Ultimately we were not able to fully integrate all the components of our project and assemble a working prototype. The main component of this project, being the frame, was delivered 19 days later than the original estimate time. The team only had 9 days to try and finish a working prototype, and eventually led to an incomplete product. Originally we had 28 days scheduled for installing a testing, but the installment phase of the wire harness and enclosure with all of our electronics ran out our time (that is it took the 9 days after our frame was delivered.). However, there are plans for this project to be continued over the summer, and possibly be reconsidered in Senior Design Capstone Project for next year.

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Mobile Solar Charging Station Despite of these bureaucratic hurdles most of the electronics function correctly. All of the code for the GUI functionalities worked great, as well as the switching circuits and DC/DC converters. The only design flaw within the system is the enclosure since it’s a first prototype, and all of the internal wiring in the enclosure. Some wiring segments required big gauge wires making it difficult to bend into its proper locations without extracting any components out of the circuitry, or being disconnected. Hopefully with the future work recommended, someone or a team of electrical engineers can improve our first prototype design into a working prototype.

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Mobile Solar Charging Station Appendix A: Angle 9*tan(angle) 2*tan(angle) .61*col3 + .39*col2 of Tilt Top of Trailer Weight Proportioned

Structure

0 0 0 0

1 0.157015927 0.034892428 0.072750713

2 0.314127466 0.069806104 0.145545726

3 0.47143046 0.104762324 0.218429446

4 0.62902122 0.139782493 0.291446499

5 0.786996762 0.174888169 0.364641833

6 0.945455046 0.210101121 0.438060838

7 1.10449522 0.245443382 0.511749452

8 1.264217874 0.280937305 0.585754282

9 1.424725299 0.316605622 0.660122722

10 1.586121754 0.352471501 0.734903079

11 1.748513743 0.38855861 0.810144701

12 1.912010311 0.42489118 0.885898111

13 2.076723347 0.461494077 0.962215151

14 2.2427679 0.498392867 1.039149127

15 2.410262527 0.535613895 1.116754971

16 2.579329643 0.573184365 1.195089401

17 2.750095908 0.611132424 1.274211104

18 2.922692633 0.649487252 1.35418092

19 3.097256211 0.688279158 1.435062044

20 3.273928585 0.727539686 1.516920244

21 3.452857752 0.767301723 1.599824092

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Mobile Solar Charging Station

22 3.634198295 0.807599621 1.68384521

23 3.818111974 0.848469328 1.769058548

24 4.004768349 0.889948522 1.855542668

25 4.194345467 0.932076771 1.943380067

26 4.38703061 0.974895691 2.032657516

27 4.583021097 1.018449133 2.123466441

28 4.782525173 1.062783372 2.21590333

29 4.985762978 1.107947328 2.31007018

30 5.192967604 1.153992801 2.40607499

31 5.40438626 1.200974724 2.5040323

32 5.620281556 1.248951457 2.604063787

33 5.840932913 1.297985092 2.706298916

34 6.066638126 1.348141806 2.810875665

35 6.29771509 1.399492242 2.917941325

36 6.534503721 1.452111938 3.027653391

37 6.777368081 1.506081796 3.140180544

38 7.026698759 1.561488613 3.255703758

39 7.282915514 1.61842567 3.374417521

40 7.546470248 1.676993388 3.496531215

41 7.817850325 1.737300072 3.62227065

42 8.097582315 1.799462737 3.751879806

43 8.386236205 1.863608046 3.885622775

44 8.684430159 1.929873369 4.023785974

45 8.992835911 1.99840798 4.166680639

46 9.312184889 2.06937442 4.314645665

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Mobile Solar Charging Station

47 9.643275189 2.142950042 4.468050838

48 9.98697955 2.219328789 4.627300525

49 10.34425448 2.298723218 4.792837909

50 10.71615076 2.381366834 4.96514985

51 11.10382554 2.467516786 5.1447725

Appendix B:

. LTC3845

. Fixed Terminal Blocks 6.53MM 2 CKT 300V

. Diode- General Purpose, Power, SwitchingSW 75V 250MA HS

. HUF75645P3 - MOSFET75a 100V N-Ch UltraFET

. SMD1/8watt 49.9Kohms 1% 100ppm

. SMD1/8watt 82.5ohms 1% 100ppm

. EMI Filter Beads, Chips & ArraysFER BD HI CUR CHIP 0805 21OHMS 30%

. SMD1000uF 25V Capacitor

. Capacitors50V 180uF 20%

. IHLP5050CEER3R3M01 - Fixed Inductors3.3uH 20%

. SMD/SMT0805 2.2uF 25volts X7R 10% Capacitor

. Multilayer Ceramic Capacitors MLCC - SMD/SMT16volts 1.5uF X7R 10% Capacitor

. Multilayer Ceramic Capacitors MLCC - SMD/SMT1uF 50 Volts 10% X7R Capacitor

. Multilayer Ceramic Capacitors MLCC - SMD/SMT50volts 0.1uF X7R 10% Capacitor

. Thin Film Resistors - SMD0805 1/8W 1.0Mohms

. Thin Film Resistors - SMD0805 100Kohm 0.05% 10ppm

. Thin Film Resistors - SMD82.5K OHM .1% 25ppm

. Thin Film Resistors - SMD49.9Kohms .1% 25ppm

. Thin Film Resistors - SMD42.2Kohms .1% 25ppm

. Thin Film Resistors - SMD0.1W 16.2K ohm 0.1% 25ppm

. Thin Film Resistors - SMD.375watt 20Kohm .1% 25PPM

. Thin Film Resistors - SMD.625watt 20Kohm .1% 25PPM

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Mobile Solar Charging Station References:

[1] S. D. University of California, Berkeley School of Law; University of California, Los Angeles; University of California, “2020 STRATEGIC ANALYSIS OF ENERGY STORAGE IN CALFORNIA,” California Energy Commission 2011, 2011.

[2] Planning and Installing Photovoltaic Systems: A guide for Installers, Architects and Engineers, 2nd ed. London: Earthscan, 2008, p. 384.

[3] J. Simon and G. Mosey, “Feasibility Study of Economics and Performance of Solar Photovoltaics at the Kerr McGee Site in Columbus , Mississippi A Study Prepared in Partnership with the Environmental Protection Agency for the RE-Powering America ’ s Land Initiative : Contaminated ,” no. January, 2013.

[4] N. Seshasayee, “Understanding Thermal Dissipation and Design of a Heatsink,” no. May, pp. 1–4, 2011.

[5] P. Nortilien, Amos; Carvajal, David; Obeng, “Senior Design Project Portable Solar Power Supply Submitted to : Dr . Samuel Richie EEL 4914 Senior Design II Submitted by : Group V Amos Nortilien David Carvajal Peter Obeng,” 2012.

[6] P. Singh, E. Engineer, and M. Felton, “Dual ’ Axis ‘ Solar ’ Tracker,” pp. 1–40, 2011.

[7] Joseph Witts, “CalPoly SuPER System Photovoltaic Array Universal DC-DC Step Down Converter” M.S. thesis, EE, CalPoly, San Luis Obispo, CA, 2008.

[8] Linear Technology, “N-Channel 60-V (D-S) MMOSFET,” Si7370DP datasheet, Feb. 2009 [Revised Jan. 2008]

[9] Mark Lund, “Wire Gauge and Current Limits Including Skin Depth and Strength,” powerstream.com, Jan. 2014. [Online]. Available: http://www.powerstream.com/Wire_Size.htm

[10] http://blog.pepperl-fuchs.us/blog/bid/253098/5-Ways-to-Track-Your-Solar-Tracker

[11] Brad. "The CircuitCalculator.com Blog." » PCB Trace Width Calculator. N.p., 31 Jan. 2006. Web. 11 June 2014.

[12] "SEIA." Solar Industry Data. N.p., 2013. Web. 10 June 2014.

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