Solar Powered, Multi-seated,
Internetted Computer System
Final Report
December 3rd, 2008
Sponsored By:
In Cooperation With:
Michigan State University
University of Dar es Salaam
ECE 480 - Design Team #2 - Fall 2008
Management
Jakub Mazur
Web
Josh Wong
Document
Ben Kershner
Presentation/Lab Eric Tarkleson
Executive Summary
With the increasing proliferation of affordable, reliable personal computers
into the marketplace, there is a great demand to develop affordable personal
computers for remote and underdeveloped areas. Before deploying a computer
system into such harsh conditions several obstacles must be overcome, including
source of electricity, telecommunications, and the savannah climate. The Lenovo
Corporation has tasked this team to develop a solar-powered computer workstation
that can accommodate up to eight users. The solution must be robust enough to
withstand the harsh environment with as little technical maintenance as possible,
yet still be affordable for rural schools.
A bank of solar panels charges a small battery bank using a commercially
available charge controller. A custom engineered battery management system
monitors voltages and currents from the solar panels and the battery to calculate
system data. The system also monitors case temperatures to insure everything is
operating within a safe range. Should the battery become discharged or
temperatures rise above the safe limits the system will automatically shut itself
down. All power data is logged to the computer and stored for system optimization.
A small LCD panel displays pertinent system information
A single computer can connect up to 8 monitors, keyboards and mice. Each
terminal allows independent simultaneous logons. The operating system is Linux
based and uses only free and open source software (hereinafter referred to as
FOSS).
Since this project has three vastly different, but equally important main tasks
that needed to be researched, tested, and built this report is split into three main
sections. First is solar power system design, second is power management, and third
is Multi seat hardware and software. Each section will address the research, design
issues, testing, and conclusions that were found. The last section will deal with
system integration and project findings and conclusions. Since we did not deal with
the telecommunications issues these will not be addressed in this report.
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Acknowledgement
We would like to thank our sponsor, Lenovo, for their help in making this
project a success. We also would like to thank our team members in the
Telecommunications department as well as the faculty and students working with
us at the University of Dar Es Salaam, Tanzania.
Also we would like to thank LEM and Man vs. Machine for their donations
and samples. Their engineers and salespeople were very helpful in guiding us in the
right direction.
This project would not have been possible without the help of our facilitator
Dr. Jian Ren. Also we would like to thank Dr. Goodman for helping keep us on track
and helping us plan our deployment of this project in Tanzania.
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Figure 1 - Power system block diagram.
4
Table of Contents
Solar Powered, Multi-seated, Internetted Computer System
1
Executive Summary .................................................................................................................................................2
Acknowledgement....................................................................................................................................................3
Section 1 - Photovoltaic System Design...........................................................................................................7
Determining Size of Photovoltaic Panel Array: ............................................................................................. 7
Determining Load Power Consumption: ........................................................................................................... 7
Determining Solar Insolation Levels: ................................................................................................................. 9
Sizing Battery Array .................................................................................................................................................11
Wire Sizing and Connections:...............................................................................................................................12
Maximum Solar Power Output: ...........................................................................................................................13
Inverter to Battery Wiring.....................................................................................................................................14
DC-AC Inverter.............................................................................................................................................................15
Charge Controller ......................................................................................................................................................16
Conclusion .....................................................................................................................................................................17
Section 2 - Power Management ....................................................................................................................... 19
Research .........................................................................................................................................................................19
Voltage Measurements ............................................................................................................................................20
Control Board ..............................................................................................................................................................22
Power Management Board....................................................................................................................................22
Conclusion .....................................................................................................................................................................23
Section 3 - Multi Seat System Hardware and Software .......................................................................... 25
Thin Clients ...................................................................................................................................................................28
Multiple-Motherboards ....................................................................................................................................... 29
Multi-User ......................................................................................................................................................................30
Testing/conclusion ....................................................................................................................................................31
Section 4 - System Integration and Conclusion ......................................................................................... 32
Integration ....................................................................................................................................................................32
Safety ...............................................................................................................................................................................32
Future Improvements: .............................................................................................................................................34
Intelligent Monitoring .............................................................................................................................................35
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Conclusion: ....................................................................................................................................................................37
Appendix 1 – Technical Roles and Responsibilities ................................................................................ 39
Eric Tarkleson .............................................................................................................................................................39
Joshua Wong ................................................................................................................................................................40
Jakub Mazur .................................................................................................................................................................41
Ben Kershner ................................................................................................................................................................42
Appendix II - Schematics .................................................................................................................................... 43
Appendix III - Nomenclature ............................................................................................................................ 46
Appendix IV - References and Recommended Reading ......................................................................... 47
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Section 1 - Photovoltaic System Design
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 solar panels and batteries act as a backup for the grid.
In cases where there is no access to grid power an “off-grid” system is used, in which
the battery bank stores and provides all the energy for the system without a backup.
Since this is generally the case in under-developed areas this will be the system
discussed here. There are also systems with generators as backups, they are
comparable to “grid-tie” systems and will also be omitted from discussion here.
Determining Size of Photovoltaic Panel Array:
There are several steps involved in sizing the PV array, determining load
power consumption, accounting for losses and dividing by solar insolation levels for
deployment region.
Determining Load Power Consumption:
The first step is to determine how much power the total system load will
draw. Power is measured in Watts:
P = V ∗ I (Joule’s Law)
However, the power rating is more useful when looked at in terms of time,
this is indeed how electric companies charge consumers. For example a 200Watt
light bulb running for 24 hours uses 4.8 KWh.
200Watts ∗ 24hrs = 4800 Watt-Hours or 4.8 KWh
A list of all devices connected to the system should be made with their
appropriate power draw available from specifications sheets or better yet, actual
measurements.
7
Component
Power (Watts)
Lenovo S10 (Idle)
91
Lenovo S10 (Full Processor + Hard
116.76
Drive)
Lenovo S10 (30% Duty Cycle)
98.73
Satellite Router (Idle)
53.8
Satellite Router (Busy)
72.5
Est. Typ. Satellite (30% Duty Cycle)
59.41
17” LCD Screen x 4
(20*4) = 80
Total
238.14 Watts
Figure 2 - Power measurements on 11/11/2008. Note: Headsets, Keyboards & Mice are
currently not included in calculations because the team is not in possession of them and their
power consumption should be minimal.
Since these devices are designed to plug into AC power, a DC-AC power
inverter is needed. The power inverter ideally operates at 90% efficiency. Therefore
the maximum inverter draw from batteries is:
238 Watts / 0.90 = 264.60 Watts
This system power draw is then multiplied by the amount of hours per day
that it will operate.
264.60 Watts ∗ 8hrs/day = 2116.80 Watt Hrs/day
To compensate for system losses during battery charge/discharge cycles the
total system power consumption is multiplied by a 20% compensation factor
(Sunwize).
2115.52 Watt Hrs/day ∗ 1.2 = 2540.16 Watt Hrs/day
8
Determining Solar Insolation Levels:
In order to determine a good approximation of how much power the PV
panels will output, solar insolation levels need to be considered. Solar insolation is
the amount of incoming solar radiation incident on a surface, for PV applications the
surface of interest is the earth’s surface. The values of solar insolation are commonly
expressed in kWh/m2/day, which is the amount of solar energy that strikes a
square meter of the earth's surface in a single day. This is commonly referred to as a
“Sun-Hour-Day”. The amount of insolation received at the surface of the Earth is
controlled by the angle of the sun, the state of the atmosphere, altitude, and
geographic location.
Figure 3 - World insulation levels.
This map divides the world into six solar performance regions based on
winter peak sun hours.
It is important to keep this in mind when designing the system because as
seen below in Figure 4, during the winter you have a much smaller ‘Solar Window’.
Worst case scenarios should be calculated as it is better to have extra energy in the
summer than not enough in the winter. Therefore the “Sun-Hour-Day” values for
December (since December days are shortest) are generally used.
9
Figure 4 - Sun path chart.
Solar Insolation Levels for Arusha, the prototype deployment area, are seen
below in Figure 5.
Figure 5 - Hankins.
The compensated total power consumption per day value calculated above is
then divided by the solar insolation values for given deployment region to
determine minimum PV panel array power output requirement:
2540.16 Watt Hrs/day / 5.5 = 461.84 Watts
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Sizing Battery Array
Nearly all large rechargeable batteries in common use are Lead-Acid type,
although there are three variations, flooded, gelled electrolyte (“Gell Cells”) and
absorbed glass matt (“AGM”). Flooded is the oldest and cheapest technology used
but can be dangerous, in case of a malfunction acid can spill. Gell Cells contain acid
that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass,
therefore even if the battery where cracked open, no acid would spill. Gell batteries
need to be charged at a slower rate (capacity / 20) but this is not a concern in the PV
setup as the panels will not be outputting nearly this much current. AGM batteries
are the newest technology and have all the advantages of Gell Cells without the
charging limitations.
All deep cycle batteries are rated in amp-hours. An amp-hour (Amps x Hours)
is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. The accepted
AH rating time period for batteries used in solar electric and backup power systems
is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20
hour period while the total amp-hours it supplies is measured (Windsun).
The compensated total power consumption per day value is used again to
calculate minimum battery array size.
2540.16 Watt Hrs/12 Volts = 211.68 AmpHrs/day
Number of days of autonomy to support: 1 (8hrs)
211.68 ∗ 1 = 211.68 AmpHrs
“Battery life [deep cycle] is directly related to how deep the battery is cycled
each time. If a battery is discharged to 50% every day, it will last about twice as long
as if it is cycled to 80% DOD [depth of discharge]. If cycled only 10% DOD, it will last
about 5 times as long as one cycled to 50%. Obviously, there are some practical
limitations on this - you don't usually want to have a 5 ton pile of batteries sitting
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there just to reduce the DOD. The most practical number to use is 50% DOD on a
regular basis (Windsun).”
Depth of discharge for battery: 0.5
211.68 / 0.5 = 423.6 AmpHrs
This means that after 8 hrs of use without sun the battery will be discharged to 50%
8 Hrs of autonomy and battery depth of discharge at 0.80 (Half the life-span of 0.50):
264.60 Amp Hrs
6 Hrs of autonomy and battery depth of discharge at 0.50:
264.6 Watts ∗ 6 Hrs = 1587.6 Watt Hrs / day ∗ 1.2 = 1905.12 Watt Hrs / day
1905.12 Watt Hrs/12 Volts = 158.76 AmpHrs
142.8 AmpHrs / 0.5 = 317.52 Amp Hrs
4 Hrs of autonomy and depth of discharge at 0.50:
238 Watts ∗ 4 Hrs = 1058.4 Watt Hrs / day ∗ 1.2 = 1270.08 Watt Hrs / day
1270.08 Watt Hrs / 12 Volts = 105.84 Amp Hrs
105.84 Amp Hrs / 0.5 = 211.68 Amp Hrs
Wire Sizing and Connections:
Another important consideration for the system is the electrical wiring. All
wiring needs to safely accommodate the amount of current draw of the system with
an acceptable amount of losses. In a DC system losses quickly become an issue. This
is especially a concern PV systems as they can only handle a small voltage drop as
there must be enough potential to charge the battery array, and of course it is good
practice to keep energy loss sourced from the sun to a minimum. Generally a 3%
drop between PV array and batteries is acceptable. Also, “any type of connection
bigger than AWG 10 should have a proper compression connector, with appropriate
joint compound and preparation. This does require special tools and dies. Otherwise
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you are running the risk of burning up your connections if you get any kind of heavy
current flowing. (SolarForum)”
Losses associated with transmission of DC power:
CM = (22.2 ∗ A ∗ D)/VD
CM = Circular Mills In Copper
A = current in amps
D = one-way cable distance in feet
VD = Voltage Drop
22.2 = Constant for Copper
For wiring from the PV panels to charge controller the maximum PV short
circuit current specification (from PV data sheet) is used.
Maximum Solar Power Output:
24 Volt Systems:
Configuration
Max Current Out (Amps)
6 x PW080
3 x (5.14A-ISC) = 15.42
3 x ST-165
20.63
4 x KY125
20.83
Figure 6
12 Volt Systems:
Configuration
Max Current Out (Amps)
6 x 80 Watt
6*(5.14A-ISC) = 30.84
3 x PW165
41.25
Figure 7
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Using the loss equation above the following result was obtained for the
selected system:
Distance: 50ft
Voltage Drop: 0.72
Current: 15.42 Amps
Circular Mills: 23772.5
AWG: 6
Inverter to Battery Wiring
For current level estimates from the battery to inverter maximum power
draw levels are used although this distance is generally short and maximum
available wire gauge is recommended. This is also due to the fact that the system
will encounter surge currents as various components are ‘turned on’. Since the
system used as an example here is not continuously running and is to be turned off
every night and back on in the morning this was a serious issue that needed to be
tested. (Refer to Figure 9).
Maximum Power Draw:
Component
Power (Watts)
Lenovo S10 (Full Processor + Hard Drive)
116.76
Satellite Router (Busty)
72.5
17” LCD Screen x 4
(20 ∗ 4) = 80
Total
270 Watts
Figure 8
Assuming the inverter that will be sourced in deployment area is operating at
90% efficiency:
270 Watts = 300 Watts x 90%
Maximum current draw in 12 Volt system = 300 Watts / 12 Volts = 25 Amps
Maximum current draw in 24 Volt system = 300 Watts / 24 Volts = 12.5 Amps
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Power-Up DC Current Draw
35
30
25
Current (Amps)
20
15
10
5
0
1
1570 3139 4708 6277 7846 9415 10984 12553 14122 15691 17260 18829 20398 21967 23536 25105 26674 28243 29812 31381
-5
Samples
Figure 9
Figure 9 shows DC current draw as measured during power-up of Lenovo
S10 Workstation (custom configuration) and L193p Monitor. Although the system is
only drawing 5 amps while running the surge current spikes are clearly visible. This
is indeed one of the reasons why proper electrical connections are crucial.
DC-AC Inverter
Since the computer and monitors are designed to plug into AC power and
accessory plugs for phone charging are a project specification an inverter is
necessary. There are two types of inverters, pure sine wave and modified sine. Most
devices will work from modified sine, this is what common uninterrupted power
supplies provide and what was selected for this system. It is important to make sure
that the inverter is rated to provide enough power for everything running off of it.
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Charge Controller
The charge controller chosen for this system is the Outback Power FlexMax
60. This decision was based on versatility, efficiency, robustness, and availability in
deployment area. The Outback can accept a wide range of voltage inputs as well as
various battery arrays, this was important for this specific system as ultimately
whatever solar panels are in stock at the time of deployment in the region will be
used.
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 accept
85 and 100V.
Figure 10 - Outback charge controller.
The highlighted area on the graph represents the highest efficiency while
charging 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 Voltage
at Peak Power is around 17 Volts.
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Figure 11 - Outback charge controller.
The highlighted area in this graph represents the optimum efficiency if the
system where charging a 24V battery array. The charge controller is operating at
about 98% efficiency with an input Voltage around 34V. Two 12V panels in series
will typically have 34 Volt equivalent Voltage at peak power.
In an ideal setup the FlexMax 60 would operate at 98.1% efficiency with an
input of 68V while charging a battery array at 48V. This would be the case with the
optimum PV panel chosen in section 1, the Kaneka G-EA060 as the VPM is 67Volts.
Conclusion
Designing an off grid photovoltaic system involves many steps and although
the math is simple all calculations should be double checked. If the calculations for
one component are off chances are the whole system will not work, every stage
relies on the previous one. Designing the system for worst case scenarios is good
practice, it is better to have extra energy than not enough. All safety precautions
17
should be followed especially on electrical connections that have a possibility of
carrying a lot of current. Breaker boxes before and after battery connections for
easy power disconnect should be implemented. These breakers should be rated for
DC voltages.
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Section 2 - Power Management
The core of the power management system is the data-gathering module. The
module uses a PIC microcontroller to measure voltages, currents and temperatures
and send the results to the computer via USB.
The end result of this system is similar to the power management utility in
your laptop. Just as with a laptop this is a self contained system that will not have
access to external power some of the time (when there is no sunlight). We feel that
because of the complexity of the multi user computer as well as the photovoltaic
system we needed to design a system that not only watches battery voltage but also
stores and interprets data for intelligent power decisions.
Research
The current sensors used are LEM FHS series
sensors. They are Hall effect sensors which operate by
measuring the electric field produced by the current
carrying wire. The field is proportional to the current.
These current sensors are isolated from the actual power
path and can be turned off to conserve power. Different
models of the sensors are capable of measuring anything from milliamps to over 50
amps. The biggest advantage of these sensors is scalability. A single chip can handle
any range of current by clever PCB board design. We acquired several sample units
from LEM and put them into small project boxes that can be used anywhere. They
attach to the power management system via d-sub cables.
Temperature measurements use a small probe, which outputs a voltage
proportional to the ambient temperature. We chose the national instruments
LM335z. These are Zener diodes with a linear internal voltage drop proportional to
the temperature of the device. Our design allows for up to 4 sensors.
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Because the PIC uses a very small amount of power, it is continuously
operating to ensure system safety. The PIC is connected to a small LCD screen which
displays pertinent information about the condition of the battery and solar panels.
Two buttons mounted beside the screen allow a user to scroll through the
information showing them battery voltage, current, and amp hours remaining. Also
there will be displays for voltage and current from the solar panel. This information
will 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, battery
good, or battery low.
The PIC communicates to the computer via USB. This is done using the FTDI
UM232R. This device automatically converts the USB signal to a standard serial
signal. It also registers with the computer as being a serial port. With the
connection, the computer can query the PIC about the status of the battery and can
shut the system down safely when the battery is low. The computer can also query
the PIC to get real time data on the voltages, currents and temperatures. The
information is stored in an XML file and can be easily reprocessed into any file
format for analysis of the data.
Voltage Measurements
Voltage measurements require the addition of a voltage divider into the
circuit because the PIC has a maximum dynamic range of 5 volts. An advantage of
using the PIC to measure voltage is that the value of the resistors in the voltage
divider can be accurately measured and the program can use that information to
improve the accuracy of the voltage measurement.
The built-in 10-bit ADC of the PIC 18F4520 microcontroller is a cheap way to
measure 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 IO
pin as a current sink, we can efficiently and accurately measure much larger
voltages.
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Figure 12 - Voltage divider with selectable current sink.
The 10-bit ADC on the PIC18F4520 is implemented as an RC circuit inside of
the PIC. Because of this, there are certain parameters to be aware of, all of which
have been taken from the PIC18F4520 Datasheet.
By adding a voltage divider to the input pin of the PIC, we are now able to
scale the input voltage. All that is left is the selection of resistor values. Starting with
known values, we can select vout to be 5V, as that is vref+ on the PIC. Also, recalling
from the first section, the nominal input impedance to the PIC is 2.5kΩ, and as such
will be selected for R2. The node labeled vin is the high voltage input to the voltage
divider, and should be selected as the highest voltage desired to be measured. For
this 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 / vout
R1 = R2 ∗ ((vin / vout) – 1)
R1 = 2.5kΩ ∗ ((50V / 5V) – 1)
∴ R1 = 22.5kΩ
We can now build our new voltage divider and accurately measure
voltages up to 50V! To convert the ADC result to actual voltage, the following
equation is now used:
van0 = ((VDD ∗ ADC_output) / 1023) ∗ (R2 / (R1 + R2))
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Control Board
This is the schematic and for the control board. It is mounted behind the
control panel and connects directly to the LCD header. There are headers for
connection to the status LEDs as well as the control buttons. A ribbon cable connects
this board to the rest of the circuit
Figure 13 – LCD control board schematic.
Power Management Board
This circuit allows connection of up to 4 current sensors, 4 temperature
probes, and 4 independent voltages. The voltage inputs have a voltage divider
circuit that can be configured to accept voltages for systems running at 12, 24, or 48
volts. A USB connection allows data sharing with the computer.
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Figure 14 - Power system monitor schematic.
Conclusion
Our power management system performs as it was designed to. Each
channel accurately reads a voltage from the sensor and converts the voltage to a
value. The PIC then uses this information and displays this information clearly on
the LCD panel. Both boards are mounted in an enclosure with connectors for each
sensor. The board will be mounted in the main enclosure and will allow a user to
turn of the entire system with the touch of a button. The sturdy PCB design and
construction will stand the test of time. The PCB board also allows for on site
reprogramming if it is deemed necessary.
23
Figure 15 - Power system monitor and faceplate.
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Section 3 - Multi Seat System Hardware and Software
The primary goal of this project is to help promote education in developing
countries by providing grade schools with electronic resources. There are a variety
of other groups that have already initiated solutions to this problem. The most
prominent group is the One Laptop Per Child Association (hereinafter referred to as
OLPC), which has created a cheap, durable laptop known as the XO-1. Other groups
such as the Center for Scientific Computing and Free Software (hereinafter referred
to 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 to
producing low cost laptops and distributing them to low-income areas. There exist
several problems with the program, including the per-deployment cost and
deployment into rural areas. The original intent was to deliver a laptop to every
child for a cost of $100 per device. The program, however, is unable to deliver the
laptop 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 a
very significant financial burden, though once deployed, it is difficult to integrate
multiple PCs into a cohesive learning environment, and this takes away from
educating the students. However, the XO-1’s are extremely rugged PCs and do not
depend on any external power sources except to charge the batteries.
C3SL’s solution integrates into school systems better, and was widely
deployed in the Paraná Digital project. This project involved having multiple
terminals running off of a single computer in multiple schools. This program has
been very successful and shows great promise, but there is a critical flaw. The
program is entirely software, and this software was intended to run in a classroom
equipped with basic utilities, such as power and internet connectivity.
Our solution was to integrate the OLPC's ruggedness and the C3SL's novel
software solution into one robust package. The design team preceding ours built a
25
solar powered computer system that can be deployed in a relatively durable
building. They assembled a solar panel, battery, and a charge controller into a self
contained solution, such that deployment in a wide variety of climates and locales is
possible, but they were unable to decide on the computer system. Our primary goal
for this project is integrating the solar charging and battery system with a computer
system that is suitable for educating youth, regardless of regional or socio-economic
boundaries.
The requirements for the computer system were fairly simple; four seats, low
power. This could be established very easily with four laptop computers, but there
were design issues that had to be considered. First, the system will be deployed in a
relatively extreme environment. It also will not be running off of mains power, and
will therefore have to be low power. It should also be as cost efficient as possible, i.e.
lowest cost per seat. Four architectures were discussed for the computer system:
laptops, thin clients, multiple-motherboard, and multi-seat.
Cost Outline:
Total Cost:
Pros:
Laptops
Baseline:
- Router with
advanced
features ($200)
Per Seat:
- Lenovo
IdeaPad S10
Latop ($439)
- Mount ($50)
Thin Client
Baseline:
- Server ($500)
Multi-User
Baseline:
- Powerful
Server ($800)
Per Seat:
- Diskless
Workstation
LTSP 1220PXE
Thin Client
($285)
- Lenovo L197
Monitor ($239)
- Keyboard/
Mouse ($30)
Per Seat:
- Lenovo
L197 Monitor
($239)
- Keyboard/
Mouse ($30)
- Video card
($30)
- Optional
Software
($100)
Base: $200
Per Seat: $489
Easy, Reliable,
Serverless, Redundant,
Low
Base: $500
Per Seat: $554
Easy, Reliable,
Stable, Low
Power, COTS
26
Blade Client
Baseline:
- Server
($500)
Per Seat:
- Small
Motherboard
with RAM &
CPU ($100)
- DC-DC
Power Supply
($50)
- Keyboard/
Mouse ($30)
- Lenovo
L197 Monitor
($239)
Base: $800
Base: $500
Per Seat: $399 Per Seat: $419
Cheap,
Possibly
Lowest Power Cheaper than
Consumption, Thin
Single Point
Client, 100%
Power
Consumption
Small Screens,
Defeats
Purpose of
Designing a
New system as
Opposed to
Donating
Laptops,
Security
Concerns
Cons:
Relatively
Expensive,
Lenovo
Does not Make
a Thin Client
of
Maintenance,
100% Lenovo
Hardware
Lenovo
Hardware
COTS
Software is
Expensive and
Open-Source
is Immature,
Reliability is
Main Concern
Lots of
Enclosure
Work,
Reliability
Figure 16 - System architecture prototypes.
Laptops
Figure 17 - Laptop-type system architecture mockup.
The first prototype was a simple laptop-server setup. Each workstation
would consist of small laptop (a 10” form factor, such as the Lenovo S10). The
laptops would be connected to the Internet either by an Ethernet cable, or even WiFi. Laptops would be run without being directly connected to AC power; a charging
station would be setup next to the server.
27
This style of architecture would be very simple to configure. The server and
the laptops would all be off-the-shelf Lenovo products. The workstations would
have low power consumption, given the fact that the monitor, computer, keyboard,
and mouse are all combined into one device. Should a laptop be damaged, it would
also be very easy to replace, requiring little re-configuration, and no custom
engineering. The laptops, however, would be most susceptible to accidental damage
given that they would be out in the environment, rather than safe inside the case.
Thin Clients
Figure 18 - Thin client-type system architecture mockup.
The thin client architecture is much closer to a traditional desktop user
experience. A thin client is little more than a small form factor motherboard,
processor, and RAM (no HDD necessary) loaded with a special BIOS. The server
does most of the processing and the think client simply helps to shuttle data back
and forth. With this setup, four thin clients are each connected to their own monitor,
keyboard, and mouse. All of the applications and processing tasks are handled on a
central server.
This is another easy solution that uses all commercial off-the-shelf (COTS)
hardware and software. It also has much lower power consumption when compared
28
to installing four individual desktops, but the power consumption is not as low as
other methods. The key drawback is price. Since Lenovo does not produce a thin
client, they would have to be purchased at full market price. This brings the total per
seat cost up to around $700.
Multiple-Motherboards
Figure 19 - Blade client-type system architecture mockup.
One method of avoiding this high per seat cost would be to build custom thin
clients, very similar to the blades of a blade server. The team estimates that even
paying market prices, a hand-built thin client could be built for around $100,
compared to the nearly $300 for third-party thin client.
The pros are the same as that of the thin client, with the added benefit of
being cheaper and being made totally of Lenovo hardware. The key drawback
though is the packaging. Since these “thin clients” are just a motherboard, they
would need to be put into a proper case, first. This would add to the cost and could
cause lifecycle issues (a poor case would lead to high failure rates of the
motherboard).
29
Multi-User
Figure 20 - Multi-user desktop-type system architecture mockup.
The most attractive architecture is the Linux based multi-seat system. It is
implemented by building a central PC with multiple video cards (2-4), multiple
keyboards, and multiple mice. Each workstation consists of a monitor, keyboard,
and mouse plugged directly into the PC (acting as an X window server).
The cost for such a setup is low. Since no thin clients are required, the perseat cost consists of only a monitor, keyboard, and mouse. The power requirements
are also lower, given that the CPU and all of its resources are be shared by all of the
users. The system would also respond much quicker than a thin client, without the
LAN bandwidth and latency issues. With significant customization of the operating
system (UNIX based), this architecture was completed using only free, open-source
software.
This is the architecture that has been chosen by the design team. The low
cost and low power requirements make it an ideal solution. Also the server can be
safely mounted inside a case and only the monitors, keyboards, and mice need to be
outside of the case.
30
Testing/Conclusion
To test the computer terminals, four accounts were created and each account
attempted to do the following tasks:

Write a document using a word processor

Watch an offline video

Play an educational game

Browse a simple webpage

Browse a rich webpage

Watch an online video.
These tasks represent what a typical student might be doing on a school
computer. Support for advanced programs, such as MatLab, were not tested. Each
station was able to perform the tasks listed, and therefore the computer terminals
are ready to be deployed.
CPU
usage
Open Office
Firefox basic
Firefox rich
Totem
Gcompris
Video
Audio
Comment
Basic document preperation. Simulates multiple students
low
good n/a
writing a paper
Basic web surfing. Simulates multiple students reading
low
good n/a
static webpages, ie wikipedia
Advance web surfing. Simulates multiple students
high
good good
browsing flash enabled websites, ie Youtube
Video/audio player. Simulates multiple students watching
mid
good good
an offline video
Education games. Simulates multiple students playing non
low
good good
accelerated games
Figure 21 - Programs used to test multi-seat system.
31
Section 4 - System Integration and Conclusion
Integration
Our final project consists of a new more robust charge controller and a
custom engineered battery management system. New solar panels have been
chosen for our deployment area but due to their similarity to our current
components they were not ordered here. We decided to change our solar panel
array and battery bank from a 12V to a 24V system. This lowers our current
throughout the system which leads to lower losses and smaller wire gauges. We
purchased a 24V inverter to replace our 12V inverter that was left to us from the
previous team.
The final architecture that we chose was the single workstation multi-seat
system. This allows for relatively low power consumption and the lowest cost per
seat of the other architectures we researched. We were successful in having up to 4
independent simultaneous logons from a single desktop computer with multiple
graphics cards. With our current hardware we can easily support up to 8 seats with
only the addition of more graphics cards and accessories.
We were successful in combining our objectives into a complete and
functioning prototype that can and will be deployed in a rural village in Tanzania for
testing. Our finished prototype has 4 user terminals connected to a large case that
will hold the communications equipment, workstation, batteries, charge controller,
and battery management system. This system will be hooked up to a solar panel
array that should provide enough power for the system to function throughout a
normal school day.
Safety
Since we are designing a system that will be deployed in a school with no
preexisting power, and around people without much experience with electrical
equipment we need to take some extra precautions to make sure that the entire
32
system is as safe as possible. There will be solar panels that will either be mounted
on poles outside of the building or on the roof so we will have to make sure to
protect these cables because they will carry high currents. The battery bank also
needs extra safety measures because it will have the potential to unleash huge
amounts of current should a short, or other low resistance connection (ie. body)
cross the terminals.
The solar panels will be mounted outside. If we mount them on the roof then
we need to make sure that the cable runs go into the structure through a watertight
seal. Also there will need to be an earth ground installed in case lightning hits the
building. The building itself must be structurally sound enough not only for the solar
panels but also for several people so that installation and maintenance is safe.
If the building is orientated in such a way that the panels can not face the sun
while they are on the roof then they will have to be mounted on a pole outside of the
school building. In this case extra precaution must be used to insure that a person is
unable to access any wire leads because the current can easily kill a person. Also the
cabling will have to be run underground so the wires will have to be enclosed in the
proper type of duct to that they cannot be cut by someone digging in the area. They
also must be completely weatherproof to avoid the wire insulation breaking down
from the elements. One advantage to this setup is that the structure that the panels
hang on can be used as an earth ground for the entire system.
Once inside the building all wiring outside of the case should be in ductwork.
This prevents a person from easily cutting the wires and also prevents animals from
chewing through the insulation. The system case must have a hole that is not sharp
so that the wires cannot fray over time. Also cabling should be secured to the case so
that it cannot easily be pulled out which could cause short circuits or broken
hardware. Any short unintentional short circuits have a large probability of
destroying the expensive equipment nearby or causing a fire.
The battery bank must be carefully installed so as to minimize any possibility
of short circuits. Each battery terminal should have a cover installed so that it is
difficult to touch the electrodes accidently. All batteries should also be secured to
the casing so that they cannot move if the whole case is tipped or moved.
33
There must be circuit breakers at several key points in the system to insure
that if there is a problem the system will cut off power to itself and hopefully save
the equipment from being ruined. There will be a circuit breaker between the solar
panel array and the Charge Controller and also between the Battery Bank and the
Inverter. These will trip if current exceeds expected ratings and will also allow the
system to be manually shut down if the need arises.
Following these simple precautions should allow the system to be safe from
prying fingers, weather, animals and equipment failure. Also in the case of a failure
troubleshooting should be easy because if our battery management system is
running it will provide feedback about which components are not working. If a
breaker is reset then it will also provide clues as to which part of the system failed.
Future Improvements:
A major improvement to the efficiency of the system could be accomplished
by using a computer system that is optimized for minimal power consumption yet
within requirements for the ‘Multi-Seat’ setup. The computer used in the prototype
is a Lenovo S10 workstation. This computer is designed to be a state-of-the-art
processing power house, efficiency was near the bottom of requirements during its
design. However, it is the only hardware that Lenovo currently offers that would
support the ‘Multi-Seat’ architecture; therefore it was used in this proof of concept
prototype. After returning from Tanzania the team is traveling to North Carolina to
give a presentation to Lenovo Corporation about requirements and suggestions for
developing a more efficient computer system for use in PV systems. Amongst
specific suggestions of low-power components include using the Intel Atom
processor and lower power hard drives, this will also include a design modification
to their power supply. The power supply is designed to transform and rectify AC
power to DC which is used by the computer. Since power sourced from PV systems
is already DC unnecessary losses can be avoided by using a DC to DC power supply.
Most modern power inverters optimally operate at 90% efficiency; hence
eliminating these conversions from the system cuts losses significantly.
34
Although the team used the most efficient PV panels on the market at the
time of deploying the prototype, a close eye should be kept on advancements in PV
technology. Current panels are still very inefficient and expensive. This is an area
that is heavily researched and as advancements develop in this field they will
greatly benefit the system.
Intelligent Monitoring
One of the goals of developing the power monitoring unit is to collect usage
data. This includes computer and telecommunication equipment duty cycles as well
as specific component power consumptions and PV array and battery efficiencies at
measured operating temperatures. Before deployment many factors are estimated,
once experimental data is available the system can be designed within tighter
tolerances. As part of the product life cycle management minor changes can be made
at identified weak links in order to maximize system efficiency.
35
36
Item
Cost
Quantity
Total Cost
Miniature Pushbutton
3.40
2
6.80
Metal Binding Posts
5.47
2
10.94
Liquid Tin
31.94
1
31.94
Project Box
6.99
1
6.99
Outback Flexmax60
545.22
1
545.22
PSE-24125A Inverter
350.00
1
350.00
Winford BCF9 Adapter
6.50
2
13.00
LEM
37.20
1
37.20
Calling Card
10.00
1
10.00
Mobile Computer Cabinet
1871.00
1
1871.00
VGA & USB extensions
54.35
1
54.35
i-rocks USB Hub
11.99
4
47.96
SYBA USB Audio Adapter
7.99
4
31.96
Lenovo S10 Workstation
1190.60
1
1190.60
Total:
4207.96
Figure 22 - Team budget.
Conclusion:
This project is special because the driving force behind it is not artificial. The
team members do not care about their grade. They do not care about the outcome of
the design day competition. This team is driven by passion, and the sole thing this
team cares about is the successful deployment of a system that will provide
hundreds of kids living in a world of great hardship with a priceless tool, a tool that
all of us grew up taking for granted. A tool that has the ability of transforming
people, a tool that transforms kids into students, a tool that transforms thoughts
into dreams… this tool is access to information.
We live in the information age. We live in a society where the term “Just
Google it” is used on a daily basis. Where we are two clicks away from the answer to
37
any question we have. So, what is it that separates us? What makes us different?
How do we have access to this tool when there are people living in this world that
are deprived of something so fundamental to us? We are no different. We are just
lucky to be born into this society.
This team is fortunate enough to have the ability to give our most prized tool
to hundreds of under-privileged children. Children that go to bed at night and
dream, and we want to let them dream big and turn those dreams into realities.
The team members participated in every aspect of engineering a product and
service from start to finish. The team not only accomplished everything outlined in
their initial project description but everything that was thrown at them along the
way, and what is yet to come.
This December, Michigan State University’s Design Team 2 will deliver the
most priceless present anyone can ever give, this is all that we care about and there
is nothing that can stand in our way.
38
Appendix 1 – Technical Roles and Responsibilities
Eric Tarkleson
Eric Tarkleson was this semester’s lab facilitator and
document preparer. He worked with Ben on the
current and voltage monitoring system. Specifically
he researched and acquired and built the current
sensing equipment used in the final prototype.
Eric also worked with the team to decide on casing
options. He chose the final case design that will be
used on design day. Eric worked with Jakub on
designing and manufacturing the faceplate and box
that will house the battery management system. The
end result is a sleek easy to use interface to the system that was custom designed for
monitoring a solar powered computer system.
Almost all of the PCB design was completed by Eric. The final project includes two
small PCB boards. The main board includes a microprocessor controlled data
acquisition system that monitors the various components power usage. It then
sends this data to the computer via USB for datalogging. The second PCB mounts to
the LCD and routes the appropriate signals to the status LEDs as well as the power,
reset and menu buttons.
Testing was performed by the whole team and Eric was no exception. Much testing
needed to be performed to insure that the correct solar panels, battery banks, and
charge controllers were purchased and ready in Africa. Much power testing was
done on both the Lenovo S10 PC and the monitors. This data will be useful in
determining how to streamline the system in the future.
39
Joshua Wong
Josh’s primary duties for the semester were to find
and implement a multi-terminal solution. The first
attempt to implement a multi-terminal solution was
installing and testing an LTSP server. LTSP allows
Linux based system to run thin clients which support
PXE booting and uses the x86 instruction set. This
was rejected due to the use of non-lenovo hardware.
The second attempt was to implement a multiseat
system by modifying the login manager and the X
windowing system. During the course of searching
for up to date documentation on Xephyr and gdm a new program called MDM was
discovered. MDM provided scripts to do most of the work referred to by the older
documentation. However, the software was immature and several patches were
needed to ensure it functions correctly. Specifically, there were several bugs which
the developers never tested for, including a bug where the program terminates
prematurely.
Josh was able to analyze the codes and fix most of the bugs. The software
required major retooling to be compatible to with the Nvidia drivers. Fortunately
one of the developers uses an Nvidia system and was able to provide valuable
assistance in adapting the existing code to use the Nvidia drivers. Another major
development was discovering how to enable audio devices for the system. Enabling
audio devices system wide required solving a problem where certain programs
would work properly while others failed. This was eventually traced to an issue
where a file was not getting processed correctly. Other tasks included locating a USB
serial interface and helping my teammates develop software for the USB interface.
Overall, most of my tasks involved dealing with software, or interfacing hardware
with software.
40
Jakub Mazur
Jakub Mazur was the Project Manager of the team. He
was heavily involved in photovoltaic component
selection including: Determining the size of the PV
array, sizing the battery array, wiring gauge
selection, and the DC-AC inverter selection.
Jakub acted as supply chain manager since there
needed to be a serious amount of logistics to be
taken care of to source parts in Africa and
communicate and assign tasks to the Tanzanian
students. This included plenty of research, keeping
up with emails on a daily basis and making phone calls at 2am to motivate third
parties to do their job. This task turned out to be extremely time consuming and
quite frustrating. The time-difference also played a role in this as often only one
communication per day is possible.
Jakub was also heavily involved in testing stages, he performed load current
measurements using data acquisition hardware and programming in LabView. This
provided critical power consumption data as well as capturing surge current data.
His technical roles also included helping all the other team members with any issues
they where having. He worked with Ben and Eric on the microprocessor power
monitoring circuitry. He also worked with Eric to design and physically build the
faceplate of the power monitoring equipment.
41
Ben Kershner
Ben Kershner, the document prep, handled the
collaboration, formatting, and finalization of the team
documentation. He also spent several days researching
the types and prices of solar panels and batteries, and
building a spreadsheet that helped to organize and
analyze this data, so that the team could choose the
most cost efficient parts of the power system.
The majority of Ben’s time was spent designing
and programming the circuitry for the power system
monitor. He built the prototype on a breadboard and
wrote a series of proof-of-concept coding examples to test the various digital and
analog I/O features of the PIC, including a custom LCD driver library to interface the
PIC to a Motorola 44780-type chipset.
Ben’s application note in measuring high DC voltages with the built-in ADC of
the PIC in a power efficient manner was implemented in the final design of power
system monitor.
Ben also wrote the server side code to accompany the power system
monitor. Written in C++, and taking advantage of UNIX system calls, it enables the
server to pull metric data off of the PIC. It then writes this to an XML file, which is
uploaded to an offsite server on a daily basis.
42
Appendix II - Schematics
Figure 23 - LCD control board schematic.
Figure 24 - LCD control board layout.
43
Figure 25 - Power system monitor schematic.
Figure 26 - Power system monitor layout.
44
Appendix III – Gantt Chart
45
Appendix IV - Nomenclature













C3SL – Center for Scientific Computing and Free Software.
CPU – Central Processing Unit, refers to the main processor chip on a
computer motherboard, not the computer as a whole.
COTS – Commercial Off The Shelf, describes hardware or software that may
be purchased rather than designed and built.
FOSS - Free and Open Source Software.
MPPT – Maximum Power Point Tracker, a style of solar charge controller.
multi-seat – A type of system architecture in which many workstations are
built onto a single machine.
PIC – The company that produces the microcontroller used, may also refer to
the microcontroller itself.
PV – Photo-Voltaic, i.e. solar panel.
PXE – Pre-boot eXecution Environment, a manner of booting computers over
the network without a locally installed operating system.
OLPC – One Laptop Per Child.
OSS – Open Source Software.
system architecture – The term used within this document to describe how
the style in which the workstations are deployed.
thin client – A client computer that relies on a central server for a majority of
its processing tasks.
46
Appendix V - References and Recommended Reading

Hankins, Mark, and Francis Njeru. Solar Electric Systems for Africa : A Guide
for Planning and Installing Solar Electric Systems in Rural Africa. Ed. Timothy
Simalenga. Beverly: Commonwealth Secretariat, 1995. 7-8.

http://www.knowledgehound.com/topics/solar.htm

World Insolation Map: http://www.sunwize.com/info_center/insolmap.htm

Sun path chart: http://www.oksolar.com/images/solar_path_large.jpg

Outback Power Flexmax User Manual:
http://www.outbackpower.com/pdfs/manuals/flexmax.pdf

Wiring Safety Concerns:
http://www.usbr.gov/power/data/fist/fist3_3/vol3-3.pdf

Also,
http://www.solarpowerforum.net/forumVB/showthread.php?t=1890&page
=3

** Source: Sunwize PV installation guide as well as other installer personal
experiences

http://www.sunwize.com/catalog/images/design_sunwize_guide.pdf

http://www.windsun.com/Batteries/Battery_FAQ.htm

http://www.windsun.com/Batteries/Battery_FAQ.htm

http://www.nmsu.edu/~tdi/PV=NEC_HTML/pv-nec/pv-nec.html

http://www.nwes.com/Wire%20Loss%20Tables.htm

http://www.egr.msu.edu/classes/ece480/goodman/spring08/group03/

http://www.outbackpower.com/

http://ww1.microchip.com/downloads/en/DeviceDoc/39631E.pdf

http://ww1.microchip.com/downloads/en/devicedoc/C18_Lib_51297d.pdf
47