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you are running the risk of burning up your connections if you get any kind of heavycurrent flowing. (SolarForum)”Losses associated with transmission of DC power:CM (22.2 A D)/VDCM Circular Mills In CopperA current in ampsD one way cable distance in feetVD Voltage Drop22.2 Constant for CopperFor wiring from the PV panels to charge controller the maximum PV shortcircuit current specification (from PV data sheet) is used.Maximum Solar Power Output:24 Volt Systems:ConfigurationMax Current Out (Amps)6 x PW0803 x (5.14A‐ISC) 15.423 x ST‐16520.634 x KY12520.83Figure 612 Volt Systems:ConfigurationMax Current Out (Amps)6 x 80 Watt6*(5.14A‐ISC) 30.843 x PW16541.25Figure 713

Using the loss equation above the following result was obtained for theselected system:Distance: 50ftVoltage Drop: 0.72Current: 15.42 AmpsCircular Mills: 23772.5AWG: 6Inverter to Battery WiringFor current level estimates from the battery to inverter maximum powerdraw levels are used although this distance is generally short and maximumavailable wire gauge is recommended. This is also due to the fact that the systemwill encounter surge currents as various components are ‘turned on’. Since thesystem used as an example here is not continuously running and is to be turned offevery night and back on in the morning this was a serious issue that needed to betested. (Refer to Figure 9).Maximum Power Draw:ComponentPower (Watts)Lenovo S10 (Full Processor Hard Drive)116.76Satellite Router (Busty)72.517” LCD Screen x 4(20 4) 80Total270 WattsFigure 8Assuming the inverter that will be sourced in deployment area is operating at90% efficiency:270 Watts 300 Watts x 90%Maximum current draw in 12 Volt system 300 Watts / 12 Volts 25 AmpsMaximum current draw in 24 Volt system 300 Watts / 24 Volts 12.5 Amps14

Figure 9Figure 9 shows DC current draw as measured during power‐up of LenovoS10 Workstation (custom configuration) and L193p Monitor. Although the system isonly drawing 5 amps while running the surge current spikes are clearly visible. Thisis indeed one of the reasons why proper electrical connections are crucial.DC‐AC InverterSince the computer and monitors are designed to plug into AC power andaccessory plugs for phone charging are a project specification an inverter isnecessary. There are two types of inverters, pure sine wave and modified sine. Mostdevices will work from modified sine, this is what common uninterrupted powersupplies provide and what was selected for this system. It is important to make surethat the inverter is rated to provide enough power for everything running off of it.15

Charge ControllerThe charge controller chosen for this system is the Outback Power FlexMax60. This decision was based on versatility, efficiency, robustness, and availability indeployment area. The Outback can accept a wide range of voltage inputs as well asvarious battery arrays, this was important for this specific system as ultimatelywhatever solar panels are in stock at the time of deployment in the region will beused.Note, the efficiency curves (Figure 10 and Figure 11) are for the Flexmax80,they are identical to the Flexmax60 other than the fact that the FX60 does not accept85 and 100V.Figure 10 Outback charge controller.The highlighted area on the graph represents the highest efficiency whilecharging a 12V battery array. The charge controller is operating at about 95.5%efficiency with an input Voltage between 17‐34V. Typically a 12V PV panel's Voltageat Peak Power is around 17 Volts.16

Figure 11 Outback charge controller.The highlighted area in this graph represents the optimum efficiency if thesystem where charging a 24V battery array. The charge controller is operating atabout 98% efficiency with an input Voltage around 34V. Two 12V panels in serieswill typically have 34 Volt equivalent Voltage at peak power.In an ideal setup the FlexMax 60 would operate at 98.1% efficiency with aninput of 68V while charging a battery array at 48V. This would be the case with theoptimum PV panel chosen in section 1, the Kaneka G‐EA060 as the VPM is 67Volts.ConclusionDesigning an off grid photovoltaic system involves many steps and althoughthe math is simple all calculations should be double checked. If the calculations forone component are off chances are the whole system will not work, every stagerelies on the previous one. Designing the system for worst case scenarios is good17

practice, it is better to have extra energy than not enough. All safety precautionsshould be followed especially on electrical connections that have a possibility ofcarrying a lot of current. Breaker boxes before and after battery connections foreasy power disconnect should be implemented. These breakers should be rated forDC voltages.18

Section 2 ‐ Power ManagementThe core of the power management system is the data‐gathering module. Themodule uses a PIC microcontroller to measure voltages, currents and temperaturesand send the results to the computer via USB.The end result of this system is similar to the power management utility inyour laptop. Just as with a laptop this is a self contained system that will not haveaccess to external power some of the time (when there is no sunlight). We feel thatbecause of the complexity of the multi user computer as well as the photovoltaicsystem we needed to design a system that not only watches battery voltage but alsostores and interprets data for intelligent power decisions.ResearchThe current sensors used are LEM FHS seriessensors. They are Hall effect sensors which operate bymeasuring the electric field produced by the currentcarrying wire. The field is proportional to the current.These current sensors are isolated from the actual powerpath and can be turned off to conserve power. Differentmodels of the sensors are capable of measuring anything from milliamps to over 50amps. The biggest advantage of these sensors is scalability. A single chip can handleany range of current by clever PCB board design. We acquired several sample unitsfrom LEM and put them into small project boxes that can be used anywhere. Theyattach to the power management system via d‐sub cables.Temperature measurements use a small probe, which outputs a voltageproportional to the ambient temperature. We chose the national instrumentsLM335z. These are Zener diodes with a linear internal voltage drop proportional tothe temperature of the device. Our design allows for up to 4 sensors.19

Because the PIC uses a very small amount of power, it is continuouslyoperating to ensure system safety. The PIC is connected to a small LCD screen whichdisplays pertinent information about the condition of the battery and solar panels.Two buttons mounted beside the screen allow a user to scroll through theinformation showing them battery voltage, current, and amp hours remaining. Alsothere will be displays for voltage and current from the solar panel. This informationwill be used to compute time and percent remaining before system power down.There are also three status LED's that inform the user of system power, batterygood, or battery low.The PIC communicates to the computer via USB. This is done using the FTDIUM232R. This device automatically converts the USB signal to a standard serialsignal. It also registers with the computer as being a serial port. With theconnection, the computer can query the PIC about the status of the battery and canshut the system down safely when the battery is low. The computer can also querythe PIC to get real time data on the voltages, currents and temperatures. Theinformation is stored in an XML file and can be easily reprocessed into any fileformat for analysis of the data.Voltage MeasurementsVoltage measurements require the addition of a voltage divider into thecircuit because the PIC has a maximum dynamic range of 5 volts. An advantage ofusing the PIC to measure voltage is that the value of the resistors in the voltagedivider can be accurately measured and the program can use that information toimprove the accuracy of the voltage measurement.The built‐in 10‐bit ADC of the PIC 18F4520 microcontroller is a cheap way tomeasure analog voltages. A drawback is that it is limited to measuring only 0‐5VDC.By adding a voltage divider, customizing the software slightly, and using a digital IOpin as a current sink, we can efficiently and accurately measure much largervoltages.20

Figure 12 Voltage divider with selectable current sink.The 10‐bit ADC on the PIC18F4520 is implemented as an RC circuit inside ofthe PIC. Because of this, there are certain parameters to be aware of, all of whichhave been taken from the PIC18F4520 Datasheet.By adding a voltage divider to the input pin of the PIC, we are now able toscale the input voltage. All that is left is the selection of resistor values. Starting withknown values, we can select vout to be 5V, as that is vref on the PIC. Also, recallingfrom the first section, the nominal input impedance to the PIC is 2.5kΩ, and as suchwill be selected for R2. The node labeled vin is the high voltage input to the voltagedivider, and should be selected as the highest voltage desired to be measured. Forthis example we will say 50V.As can easily found by inspection, the formula for a voltage divider is:vout vin (R2 / (R1 R2))This gives us a formula, three knowns, and one unknown. Solving for R1:R1 R2 vin R2 / voutR1 R2 ((vin / vout) – 1)R1 2.5kΩ ((50V / 5V) – 1) R1 22.5kΩWe can now build our new voltage divider and accurately measurevoltages up to 50V! To convert the ADC result to actual voltage, the followingequation is now used:van0 ((VDD ADC output) / 1023) (R2 / (R1 R2))21

Control BoardThis is the schematic and for the control board. It is mounted behind thecontrol panel and connects directly to the LCD header. There are headers forconnection to the status LEDs as well as the control buttons. A ribbon cable connectsthis board to the rest of the circuitFigure 13 – LCD control board schematic.Power Management BoardThis circuit allows connection of up to 4 current sensors, 4 temperatureprobes, and 4 independent voltages. The voltage inputs have a voltage dividercircuit that can be configured to accept voltages for systems running at 12, 24, or 48volts. A USB connection allows data sharing with the computer.22

Figure 14 Power system monitor schematic.ConclusionOur power management system performs as it was designed to. Eachchannel accurately reads a voltage from the sensor and converts the voltage to avalue. The PIC then uses this information and displays this information clearly onthe LCD panel. Both boards are mounted in an enclosure with connectors for eachsensor. The board will be mounted in the main enclosure and will allow a user toturn of the entire system with the touch of a button. The sturdy PCB design andconstruction will stand the test of time. The PCB board also allows for on sitereprogramming if it is deemed necessary.23

Figure 15 Power system monitor and faceplate.24

Section 3 ‐ Multi Seat System Hardware and SoftwareThe primary goal of this project is to help promote education in developingcountries by providing grade schools with electronic resources. There are a varietyof other groups that have already initiated solutions to this problem. The mostprominent group is the One Laptop Per Child Association (hereinafter referred to asOLPC), which has created a cheap, durable laptop known as the XO‐1. Other groupssuch as the Center for Scientific Computing and Free Software (hereinafter referredto as C3SL) have made significant strides in reusing older computers for schools;however, both of those programs have some significant drawbacks.The primary competitor identified is the OLPC. The OLPC Association is dedicated toproducing low cost laptops and distributing them to low‐income areas. There existseveral problems with the program, including the per‐deployment cost anddeployment into rural areas. The original intent was to deliver a laptop to everychild for a cost of 100 per device. The program, however, is unable to deliver thelaptop at the 100 target; in fact, the cost to donate a system is almost 200.Deployments also require a minimum commitment of 100 laptops. This represents avery significant financial burden, though once deployed, it is difficult to integratemultiple PCs into a cohesive learning environment, and this takes away fromeducating the students. However, the XO‐1’s are extremely rugged PCs and do notdepend on any external power sources except to charge the batteries.C3SL’s solution integrates into school systems better, and was widelydeployed in the Paraná Digital project. This project involved having multipleterminals running off of a single computer in multiple schools. This program hasbeen very successful and shows great promise, but there is a critical flaw. Theprogram is entirely software, and this software was intended to run in a classroomequipped with basic utilities, such as power and internet connectivity.Our solution was to integrate the OLPC's ruggedness and the C3SL's no

There are many types of solar systems but most can be categorized into a variation of the following: A “grid‐tie” system where there are no batteries and the power grid provides back‐up power. A hybrid “grid‐tie” system where the power grid provides back‐up for the