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									Proposal for Efficient Solar Power Converter Project

              ECE 345 – Senior Project
       Ting-Hau Ho, David Kline, Jet-Sun Lin
                 18 September 2002
Title: Efficient Portable Solar Power Converter
Objectives: This project aims to create a completely self-contained unit capable of converting light
         energy from the sun into electrical energy, storing that energy chemically, for later use, in the
         form of a battery, and then converting the chemical potential energy back into electrical energy
         capable of driving a 120 V AC appliance. The initial goal is to build a device that is easily
         carried for the purposes of demonstrating the viability of solar power to various audiences.
         However, the converter should be easily adaptable and scalable for use in a variety of
         applications, including as a portable, renewable power source for field scientists, busy travelers,
         and even recreational uses such as camping or tailgating. Because it derives energy from the sun,
         it will also save the user money on energy costs.


         The Converter will have four major components, outlined as blocks in Diagram 1. The functions
of each of these stages will be described in the section below.
           Solar Energy

           Stage 1.             Stage 2.          Stage 3.        Stage 4.
           Solar Panel          Solar/Battery     Battery         DC/AC
           Array                Coupling                          Converter             Output

Diagram 1. Block Diagram of Main Circuit Stages

Stage 1: Solar Panel Array

         The Solar Panel Array will convert light energy from the sun into an electric current by means of
photovoltaic cells. This array will consist of 1 or 2 BP Solar bp sx 30 solar panels mounted on a folding
platform which can be easily adjusted to maximize the incident light on the array as well as easily
disassembled for storage and transport. The bp sx 30 specifies a maximum output of 30 W. At Pmax, the
voltage and current specified are 16.8 V(dc) at 1.78 A.
         16.8V 1.78A 1h  29.90Wh (Energy Output for one hour from one Solar Panel)
        120V  1A  h  20Wh (Energy Requirement to meet goal)
       The expected power output from one solar panel should be sufficient to achieve our goal of
operating a 120 W device for at least ten minutes on a 60 minute charge, including losses within the
system. With more than one of these solar panels in a parallel configuration, charging times would
decrease and more energy would be available for the output. The system should be easily scalable for
larger applications.
       The bp sx 30 is 19.7”W x 23.1”L x 0.89”D which meets the portability requirement.
       The interface between the Solar Panel Array and the Solar/Battery coupling section will be a
straightforward direct connection.

Stage 2: Solar/Battery Coupling

       The Solar/Battery Coupling will provide protection of the Solar Panel Array against reverse
charging, as well as matching the output voltage from the Solar Panel Array with the input voltage of the
Battery. As long as there is sufficient incident light on the solar panels, there should be a positive
current flowing out of the Solar Panel Array and into the Battery. However, if there is no light or
insufficient light on the solar panels, the direction of the current cannot be guaranteed without an
intervening circuit.
       The output voltage from the solar panels (16.8 V(dc) will not match the float charge input
voltage requirements of the battery (13.5-13.8 V(dc) at 77°F). Therefore, the Solar/Battery Coupling
will also include a DC/DC voltage transformer circuit to compensate for the mismatch.

Stage 3: Battery

       The battery will serve as an energy storage device. Due to the fact that current solar cell
technology does not make it practical to provide the power necessary to drive the 120 W load in real-
time from a solar panel array small enough to be considered portable, the battery stage becomes
necessary in order to build up enough energy to drive the load for a meaningful amount of time.
       A Johnson Controls U1-31 lead-acid battery will be used. This battery has a float charge voltage
range of 13.5 to 13.8 V(dc) at 77°F, which will be provided by the Solar Panel Array via the
Solar/Battery Coupling stages described above.      The manufacturer’s specifications indicate a rated
output of 150 A at 12 V(dc), which is more than enough to drive the converter in the next stage.
Capacity is 31 Ah at 20 hours; since the expected charging cycle will be considerably shorter than this
the capacity for these shorter charging cycles will have to be determined.
        The interface from the Battery to the DC/AC Converter will be controlled by a manual switch.
This is to allow the user to actively switch from a charging state into a usage state, and to isolate the
charging circuit (Stages 1 & 2) from the converter and discharge portion of the device (Stage 4).

Stage 4: DC/AC Converter

        The DC/AC Converter stage will transform the DC signal from the Battery into a 120 V(ac)
waveform capable of driving a small appliance. Because the efficiency of this section is pivotal to the
success of the project, special emphasis will be placed on the design and selection of components.
        The first stage of the converter will step up the 12 V(dc) output from the battery to a higher
voltage signal (for example 144 V(dc)) so that the resulting AC signal from the inverter has voltage of
120 V(ac).
        The inverter stage will consist of either a full-bridge inverter or a pulse-width modulation
        The DC/AC inversion stage will consist of either a full-bridge inverter or an inverter with the
switching controlled by a pulse-width modulator.
        The full bridge inverter consists of four transistors connected in parallel with a diode. Each of the
transistor/diode pairs works to conduct current in such a way that approximates an AC signal. The full
bridge inverter also blocks all of the original DC signal, allowing only the AC signal to come through.
Additionally, the full bridge inverter has the ability to be tuned so that a voltage amplitude at the output
that is up to 27% greater than the input voltage can be attained. An L-C filter may be applied to the
output signal to reduce any unwanted harmonics produced by the inverter.
        If test reveals the 120 V(ac) 60 Hz waveform produced by the full bridge inverter is not within
acceptable range, we will use a pulse-width modulated inverter. In this inverter, a PWM times the action
of four switches. This allows for a smoother waveform to be produced at the output. The disadvantages
of this approach are that the maximum voltage amplitude at the output is equal to the DC input voltage.
Additionally, he PWM switches each switch at a much higher rate than the full-bridge inverter, and each
switching event causes a slight energy loss, so more energy is lost using the PWM inverter than the full-
bridge inverter.
        The output from this stage will be a standard 3-prong wall outlet.

Performance Requirement

        The goal of the prototype is to be able to drive a 120 V(ac) appliance which draws 1 A for a
period of 10 minutes on a 60 minute charging cycle. The total efficiency of the device (taken as power
delivered at the final output (3-prong wall socket) over the power input from the Solar Panel Array),
given the manufacturer’s specifications, should be at minimum 67%.            It is hoped that the actual
efficiency of the system will far exceed this minimum quantity. Because the performance of the device
depends on the incident light available, it will be necessary to conduct low-light tests to determine
expected performance requirements for less-than-ideal conditions. The manufacturer data on the solar
panels’ efficiency at lower light levels will be utilized to determine the performance expectations prior to


           The practical test of the success of the project will be plugging in an actual AC appliance and
running it for at least ten minutes after charging the system for 60 minutes. However, on the way to that
goal, certain measurable parameters can be verified. This section details these parameters and the
methods which will be used to measure them.

Testing Procedures

           The Johnson Controls U1-31 battery requires a charging input between 13.5 and 13.8 V(dc).
Because of these requirements, it will be necessary to closely regulate the output from the Solar/Battery
Coupling. The DC output from this stage will be monitored for various amounts of incident light by use
of a multimeter, and the AC ripple will also be measured to ensure a clean signal into the battery. The
cleanliness of the signal as well as how close the signal is to the ideal float charge voltage the more
efficient the battery charging will be.
           Similarly, the DC output from the battery will be subjected to the same measurements, and again
after the DC step-up the same measurements will be taken.
           Output from the DC/AC Converter should be 120 V(ac, rms) at 60 Hz with no DC bias. This
value will be measured by a multimeter. Furthermore, the waveform of the output should closely
resemble the sinusoidal output from a typical wall socket. Therefore, the output will be observed on an
oscilloscope and compared with the oscilloscope tracing of the wall socket output.

Tolerance Analysis

           In order to achieve a clean, efficient DC/AC conversion with a sinusoidal output, it will be
extremely important to select closely matched BJTs in the converter stage. Because BJT performance is
highly dependent upon temperature, and input voltage cleanliness, among other factors, and because the
cleanliness of the output as well as overall efficiency of the system are so greatly affected by this
element, a thorough tolerance analysis will be performed on the BJTs.

Cost and Schedule

         The major parts required for the project are listed below, along with quantity and approximate
Description                                     Quantity     Cost/Unit                     Total Cost
bp sx 30 solar panel                            1            $219.95                       $219.95
Johnson Controls Dynasty U1-31 Battery          1            $35.00                        $35.00
Solar/Battery coupling ckt components           1            $10                           $10
DC/AC converter ckt components                  1            $30                           $30
Carrying case                                   1            $20                           $20

         The total parts estimate is $324.95.
         There will be three engineers working on this project for ten hours per week. The labor cost for
these three engineers is $25/hour. Therefore, the total labor cost is
              $25                10 hours
                          2.5            10 weeks  3engineers  $18,750
         engineer  hour          week
         The project timeline spans ten weeks. Table 1 gives an outline of the major tasks as well as the
completion date goal and engineer with primary responsibility for that task.

                              Jet                            Ting-Hau                      Dave
Week of 9/16                  Order solar panel              Obtain battery                Begin design of DC/AC
                              Begin design of                Test battery parameters       converter
                              Solar/Battery coupling
9/23                          Test solar panel parameters    Simulate DC/AC converter      Simulate DC/AC converter
                              outdoors in various lighting   Tweak converter design        Tweak converter design
                              conditions                     based on simulation results   based on simulation results
                              Tweak coupling circuit
9/30                          Finalize coupling circuit      Finalize DC/AC converter      Finalize DC/AC converter
                              design                         circuit design                circuit design
                              Prepare for design review      Prepare for design review     Prepare for design review
10/7                          Determine output               Obtain circuit elements for   Obtain circuit elements for
                              expectations for various       DC/AC Converter               DC/AC Converter
                              lighting levels                Build DC/AC Converter         Build DC/AC Converter
                              Obtain circuit elements for    Test DC/AC Converter off      Test DC/AC Converter off
                              Solar/Battery coupling         lab power supply (to make     lab power supply (to make
                              circuit                        tolerance analysis)           tolerance analysis)
                              Build coupling circuit         Tweak DC/AC Converter         Tweak DC/AC Converter
                              Test coupling circuit
10/14               Test battery charging with      Test DC/AC converter off        Test DC/AC converter off
                    solar panel                     battery                         battery
                    Tweak coupling circuit if       Tweak DC/AC converter           Tweak DC/AC converter
                    necessary                       design if necessary             design if necessary
10/21               Integrate all stages            Integrate all stages            Integrate all stages
10/28               Test and verify output          Test and verify output          Test and verify output
                    expectations of the             expectations of the             expectations of the
                    integrated system               integrated system               integrated system
11/4                Prepare for mock-up demo        Prepare for mock-up demo        Prepare for mock-up demo
11/11               Make final adjustments to       Make final adjustments to       Make final adjustments to
                    improve efficiency and          improve efficiency and          improve efficiency and
                    polish the design               polish the design               polish the design
11/18               Prepare for presentations and   Prepare for presentations and   Prepare for presentations and
                    demos                           demos                           demos
                    Write final report              Write final report              Write final report
Table 1. Timeline

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