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“How Things Work” Gen Ed Engineering
Mission College / Hope College
Name:_______________
Lab #11
Solar Photovoltaic Battery Recharger
In this lab you will construct photovoltaic battery recharger. Photovoltaic cells are used to
generate current that is used to recharge two AA batteries. The photovoltaic cells, also called
solar cells, produce an electric current when illuminated with light. They are energy conversion
devices that convert light energy into energy in the form of electric current.
The batteries used are Nickel-Metal-Hydride (NiMH) rechargeable batteries. These are common
rechargeable batteries such as those made by Energizer or Duracell. A diode is also used. Diodes
allow current to flow in only one direction. The diode prevents the current from flowing
backwards and draining the batteries in low light conditions.
Materials List:
2 – Photovoltaic Cells with wire leads (PowerFilm® MP3-37, rated 3V @ 37 mA )
2 – AA Rechargeable Batteries (NiMH)
2 – AA Battery Holders
1 – Diode (1N5817 - low forward on voltage 0.45 V)
1 – Foamcore Base (5 inch x 5 inch)
1 - Insulated 22 gauge wire (12 inch)
Tools and Supplies:
Light Source (Halogen or Fluorescent)*
Hot Glue Gun
Scissors
Wire cutters/strippers
Pliers
DMM to measure Voltage and Current
Scotch Tape
Electrical Tape
Large Nail
Alligator Clip leads (4 per person)
PasPort Current & Voltage Sensor
Lamp with 60 watt Halogen flood bulb *
Decade Load Box
Light Meter
Diffraction Grating
Note: 60W Halogen bulb will get hot. –
95% of full spectrum
* Can also use High Intensity
Fluorescent Bulb
Resistors (10-30 ohms, 2 per person)
Tape Measure GLX Xplorer
DataStudio and PC
___________________________________________________________
In assembling the recharger, the photovoltaic cells will be connected in parallel. The batteries and
the diode will be connected in series. The combined currents from photovoltaic cell will flow
through the two batteries and the diode. The diagram below is a schematic of the circuit.
1
“How Things Work” Gen Ed Engineering
Mission College / Hope College
Name:_______________
Diode
Photovoltaic 2
Photovoltaic 1
Battery 1
Battery 2
2
Photovoltaic Battery Recharger
Part A: Draining the Batteries
The batteries must be completely drained or discharged. This is necessary to be able to prove that
the photovoltaic charger is able to recharge the batteries. The batteries will be drained, or
discharged, by connecting them to a resistor. Current will then flow through the resistor from the
battery until the battery is discharged.
(Note: In general the batteries do NOT have to be fully drained before being recharged.)
1. Put each battery in a holder and connect each end of the battery holder to a 27 ohm
resistor as shown. The resistor can be connected in either direction. The resistor allows
the batteries to discharge rapidly but not so quickly that they are damaged by
overheating.
Alligator Clip
Battery
Resistor
Resistor
Alligator Clip
2. Discharging may take 30 minutes or longer. Allow the battery to discharge while
working on other parts of the laboratory.
3. Test in flashlight (should NOT work).
If necessary continue discharging.
4. Measure the Voltage of each discharged battery. The voltage should be less than 1.2 V
for the discharged batteries. Record the measured voltages.
Battery 1 Voltage:______________
Battery 2 Voltage:______________
3
Part B: Assembly of photovoltaic charger
1. Attach photovoltaic panels (PVs) to foamcore base
Locate the two photovoltaic panels. HANDLE WITH CARE. Do not tug on the red and
black wires. The attachment between the red and black wires and the PV is easily broken.
Position the panels on the foamcore base. Wires should point to the middle. The red
(positive) should be at the top of each.
Tape photovoltaic panels on front of the foamcore. Do not put tape over the gray active
part of the PV. Tape will absorb light and reduce the amount of electric current produced
by the PV. Tape only on the metal edge.
Red Wires (+) at top
Tape on
edges
No Tape on
Active Part
No Tape on
Active Part
Tape on
edges
2. Send the PV wires from front to back of the foamcore
Punch a hole with nail through the foamcore at the top and bottom in the space between
the PVs. Put the wires through to back through the holes.
Red Wires
through hole at
top
Make hole
with nail
4
Black wires through hole
at bottom
3. Attach battery holders to the back of the foamcore
Glue battery holders to the back side of the foam core. Positive toward the top. Attach
near edges toward the bottom as shown. Use sufficient amount of glue so the battery
holders are attached firmly. Mark positive and negative on the foamcore
Hot Glue
Hot Glue
4. Install the diode on the right-side battery holder positive terminal.
Install diode on right side positive. White band goes toward positive battery terminal.
Very important. Loop wire around itself. The diode must be secure and not able to
detach.
The diode only allows current to flow in one direction. It prevents the batteries from
discharging by preventing current from flowing out of the PVs and into the battery in low
light conditions. The diode ensures that current only flows out of the solar cells and into
the two batteries, and not in the reverse direction.
Diode connected here
IMPORTNANT Diode
white stripe toward +
side of battery holder
I
Diode wire through
tab and twisted
around itself for
secure connection
5
5. Prepare Red and Black Wires
Remove the plastic insulation from each red and black wire for the PVs. Remove about 1
inch (25 mm). Use wire strippers as shown.
About 1 inch (25mm) of
plastic insulation
removed
6. Attach PV wires red to red and black to black.
Attach red to red and black to black (positive to positive and negative to negative) Twist
ends of the wire as shown. This connects the two PV in parallel.
Twist wires
together
Twist wires
together
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7. Connect PV red wires (positive) to the diode
Attach the two red wires to the diode. Twist the wires around the diode. Bend or fold
over the diode and red wires so red wire will not slip off. It is very important that the red
wires cannot slip off the diode wire. Secure with electrical tape.
Twist red
wires with
diode
Bend to
secure
Tape
8. Connect PV black wires (negative) to the negative terminal of the battery holder.
Wrap the ends of the black (negative) PV wire around negative terminal on the left side
of the foamcore. Twist the wire around itself so it cannot be pulled off the battery holder.
Should look like picture.
Attach black
wires to (-)
side of
battery
holder on left
side
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9. Attach white wires to the battery holders.
Locate the white wire. Cut the wire in half to create two pieces. Remove the white plastic
insulation from both ends of each wire, about 1 inch (25 mm) as shown.
Attach one white wire to each of the battery holders. One to the positive, one to the
negative. Wrap wire around itself so it cannot be pulled off the battery holders.
DO NOT CONNECT THE WHITE WIRES TO EACH OTHER. LEAVE ONE END OF
EACH WHITE WIRE UNCONNECTED.
White
Wire
Remove 1 inch (25 mm)
of Insulation
White Wire
10. Install the batteries.
Be sure that the batteries are completely drained and discharged. Install batteries with
positive toward top of holder. It is very important that the batteries are installed in the
correct way. The charger is now ready to test.
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11. TEST. Connect meter between white wires to measure CURRENT.
Attach meter to two white wires. Set to measure CURRENT. It is very important to
establish that CURRENT is flowing through the batteries when the PVs are illuminated.
Set the meter on a LOW CURRENT scale of approximately 2 mA.
A
Connect CURRNENT
Probes to White wires
12. Turn PV charger over to face the light.
With the meter connected, turn the charger over so the PV panels are facing light. The
meter should show some small amount of CURRENT even with just the normal room
lights. Cover the photovoltaics with your hand (or a book) and the current should
decrease. This establishes that the PVs are working. Record your results.
CURRENT with PV in room light: _______________ (mA)
CURRENT with PV covered:____________________(mA)
NOT WORKING?
--Check the wires is every thing connected?
--Is diode in the right way?
--Are the batteries in the proper orientation (positive up)?
--Are the positive and negative (red and black wires) in proper locations?
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Ammeter
Part C: Testing charger and recharging batteries.
1. Testing using artificial light (indoor).
In this test the charging current produced by the PVs will be tested using an artificial light
(light bulb).
A meter should be connected between the white wires and set to measure CURRENT.
Place the PV close to the light bulb about 1 inch (25 mm) away. Record CURRENT in
milliamps (mA)
Repeat for distances of 5 cm, 10 cm, 25 cm, 50 cm, 100 cm. Record the results
Artificial Light Test Results. CURRENT in milliamps (mA)
1 inch (2.5 cm) ___________
5 cm: _______________
10 cm: _______________
25 cm: _______________
50 cm: _______________
100 cm: _______________
QUESTION: From the test results what distance from the light should the PV charger be
located to recharge the batteries the fastest?
________________________________________________________________________
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2. Testing using sunlight (outdoor test).
In this test the charging current produced by the PVs will be tested using sunlight.
A meter should be connected between the white wires and set to measure CURRENT as
before.
Point the PV directly toward the sun. Record CURRENT in milliamps (mA)
Try holding the PV at a tilt or angle to the sun. Try several angles. Record the results
Sun Light Test Results. CURRENT in milliamps (mA)
Direct Sun ___________ (Perpendicular to sun’s rays)
Angle 1 _______________
Angle 2 _______________
Angle 3 _______________
Angle 4 _______________
QUESTION: From the test results what is the best angle or orientation toward the sun to
recharge the batteries fastest?
________________________________________________________________________
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3. Start Battery Recharge
Locate your PV charger either indoors close to the lamp or outside in the sunlight. Record
the CURRENT.
Recharging CURRENT:_______________________ (mA)
DISCONNECT METER AND ATTACH WHITE WIRES TOGETHER
Disconnect the meter from the PV charger. The white wires must be connected together
to have a complete circuit. Twist the ends of the white wires together and cover with
electrical tape.
Twist
together
ends of
white wires
Tape
4. Recharge Batteries for 30 minutes.
Leave the PV charger in the sun or by the lamp for at least 30 minutes.
Conduct other laboratory activities while the batteries are being recharged.
5. Test Recharged Batteries.
After charging, battery voltage should measure 1.2 V or higher with the meter. If the
voltage is less than 1.2 V continue charging.
Record Battery VOLTAGE after recharging.
Battery 1 Voltage:______________
Battery 2 Voltage:______________
6. Test the recharged batteries in the flashlight.
7. Customize and/or decorate your PV charger.
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Explanation of charger operation.
Combined Current PV 1 + PV 2
Current
from PV 1
(37+ 37 = 74 mA, 3V)
Current
from PV 2
(37 mA, 3V)
(0.45 V, 74 mA)
Diode
(37 mA, 3V)
Photovoltaic 1
Photovoltaic 2
Battery 1
Battery 2
(1.2 V, 74 mA)
(1.2 V, 74 mV)
3.0 V > 0.45 + 1.2 + 1.2 V
Current flows from PV to Batteries
37 mA per cell is a typical average value. PV current depends
on the amount of light available and the other circuit
components, individual results will vary.
The photovoltaic cells are connected in parallel. Each cell produces 37 mA at 3V in full sun. The
37 mA is an average value that varies with the amount of light present and the type of
components connected to the cell so individual results will vary. The combined currents from the
photovoltaic cells will flow through the two batteries and the diode. This current input recharges
the batteries. Diodes allow current to flow in only one direction. The diode prevents the current
from flowing backwards and draining the batteries in low light conditions. The combined
voltages of the diode and the batteries must be less than or equal to the 3V produced by the
photovoltaic cells. This insures that current will flow from the cells (higher voltage) to the
batteries (lower voltage).
Batteries
The energy content of batteries is specified in units of mAh (milli-amp-hours). For example a
battery rated at 1700 mAh can supply a current of 1700 mA for 1 hour of total operation or it can
supply half as much current for twice as long – 850 mA for 2 hours of total operation. If a
flashlight requires 60 mA of current, then a 1700 mAh battery will run the flashlight for 1700
mAh / 60 mA = 28.3 hours.
When a rechargeable battery is recharged, current is put into the battery, reversing the chemical
reaction in the battery and restoring the ability of the battery to provide current. In recharging, the
energy input is comparable to the energy removed. A 1700 mAh battery that is completely
drained, will require at least 1700 mAh of energy to recharge. If the recharging current is 200mA
then the time needed to recharge would be at least 1700 mAh / 200 mA = 8.5 hours.
Recharge
60 mA
1700
mAh
200 mA
1700 mAh / 200 mA = 8.5 hours
1700 mAh / 60 mA = 28.3 hours
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1700
mAh
Solar Photovoltaic Battery Recharger Questions
1.) Comparing cost of solar recharged batteries to non-rechargeable batteries.
The cost of the components for the photovoltaic battery recharger is $14.13. This
includes the cost of the rechargeable batteries ($2.37 each). The NiMH rechargeable
batteries can be recharged more than 100 times.
a) Assuming that the batteries are completely discharged each time they are used,
and they are recharged 100 times using the solar charger. What is the cost per
use?
b) A package of 8 non-rechargeable AA batteries of comparable capacity costs
$3.99. What is the cost per battery?
c) Are the solar recharged batteries less expensive than non-rechargeable batteries?
Explain why or why not.
2.) Determination of recharging time.
One person’s solar photovoltaic charger is found to produce 54 mA of current to the AA
batteries in sunny conditions. The batteries used have an energy capacity of 2000 mAh.
The recharging process is 66% (2/3) efficient. That means that only 66% of the input
current is converted into stored energy in the battery. How much time is needed in sunny
conditions to fully recharge the battery?
3.) Decreasing the time needed to recharge the batteries.
It is desired to be able to fully recharge a AA battery with 2000 mAh capacity in 8 hours
of sunny conditions. Assuming 66% efficiency how much current must be input to the
battery to accomplish this?
4.) Design of 8 hour charger.
Assume that a typical photovoltaic charger of the type built in this lab can supply 50 mA
of current from 2 cells in parallel in sunny conditions:
a) How many cells would be needed to fully recharge a 2000 mAh AA battery in 8
hours (as described in question 3)?
b) About how big would the solar charger have to be? What would be approximate
dimensions? Make a drawing showing your design.
c) The materials for a charger with two cells cost $14.13 including the cells. The
cells cost $3 each. What would be the minimum cost for a charger that could
recharge the batteries in 8 hours?
d) How many times would this charger have to be used to equal the cost of buying
non-rechargeable batteries? Use the data given in question 1.
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Characterizing the Solar Cell for Designing Power Systems
1. Introduction
In this portion of the lab you will use the sunlight or a high intensity lamp to plot
voltage and current produced by the solar cell. The voltage and current will
depend on the resistance of the load resistance.
Power = Voltage * Current
(watts = volts * amps)
(milliwatts = volts * milliamps)
 units
 units
Power * Time = ENERGY
Kilowatt * hour = KWH
(1 KWH costs about 15 cents)
 units
At some load resistance the power (V*I product) produced by the solar cell is
maximized.
Once you know the optimal load resistance, then it is possible to optimize the
performance of the solar cells by maintaining a load. From this data you can also
compute the time it take to charge a battery.
Placing multiple cells in an array pattern can boost both voltage and/or current.
This allows cells to be designed to power an appliance with specific voltage and
current requirements.
The initial investment of solar cells is offset by the incremental monthly savings
in power that is not needed from the utility company.
Based on the power produced and the cost of a KWH in your community, you can
plot how long it will take you to repay the initial investment in solar cells.
For the following tests, in order to maintain consistency make sure the
distance from the light source and the panel(s) remains the same. 4 inches is
a good target.
At any time if the solar cells appears to be getting to hot (signs of this will be
plastics curling at the edges), increase the distance from the light source.
Measure the distance now from the light bulb to the panel.
Distance = _____________________
Turn off light when not in use. This saves energy as well as limits the heat.
15
2. Use DataStudio to determine at what load power is maximized and find the
maximum power output for one solar cell:
Connect the Pasco Voltage-Current sensor to your solar cell as shown in figure 1.
Red
Black
I
Decade Box
RL
I
I Meter
V Meter
Figure 1
Use a decade resistance box to vary the load from 0 to 5000. Make sure you
go all the way to 0 .
Use the following load resistance when measuring current and voltage:
5000
3000
1000
500
300
100
80
50
40
20
10
5
0
16
Using DataStudio:
1. Turn on the GLX. Make sure it is connected to the USB connector from the
PC.
2. DataStudio should automatically start-up. Click on:
3. You should now double click graph to create a new graph for your data.
4.
You will now select data to display on the graph. Choose current.
5. In order to display both current and voltage on one graph, click and drag
voltage onto the graph.
17
6. Select Overlay:
7. You should now see an empty graph with both current and voltage overlaid on
the same graph:
8. Now we need to change the horizontal axis from time to load resistance.
Select:
.
18
9. You should now see:
10. First select Power below Measurements so you can view Power readings as
load changes. Next select Sampling Options.
11. Enter your sampling options as shown below and then click OK:
19
12. Now close the Experiment Set-Up window.
13. Change the x-axis on the graph to be load resistance. Left-click on Time and
select Load Resistance. You may have to do this for both current and voltage
graphs.
14. You are now ready to collect your data. Set the decade resistance box (DRB)
to 5000 Ohms. Note that the multiplier K = 1000. Put the x1K dial on 5. All
other dials should be on zero. The dials all add together to give the desired
resistance.
15. Turn the lamp on.
16. Now press the Start button:
17. If you are set to 5000 on the DRB, then click Keep:
18. DataStudio will then ask you for the load resistance. Enter your load and then
click OK:
19. Now change the load to 3000, press Keep again and enter the load. Do this
repeatedly until you get to 0 ohms using the resistor values given on pg 16.
20. Once you have collected all your data, to stop the run press red square:
20
21. After you’ve collected all the V and I measurements, your graphs might look
something like:
22. To adjust either horizontal or vertical scale, place the cursor over one of the
axis numbers. When you see the curly line
drag the mouse left or right to change the scale.
then left-click and
23. Now click and drag Power from the Data column over to the graph. This will
allow you to see at what load resistance power is maximized. You should see
something like:
21
24. At what load resistance was power at a maximum for YOUR solar cell?
_________
This is called the maximum power point.
25. Using the  tool, select Maximum. What is the maximum power produced by
YOUR solar cell? ___________
26. You can now see the maximum values for all three measurements:
22
27. Using the light meter, measure the light intensity of your light source.
______________ Lux
Be sure to place the meter in the same location as the solar cell so you record
the light intensity that the panel receives.
Using a ruler measure approximate length and width of the solar cell. Try not
to measure the plastic housing.
l = _______, w = ______
Using the internet, find a definition for the unit LUX.
Write the definition below:
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Examining Voltage vs. RLoad, you will see that voltage reaches a maximum with a
large load and gradually drops to zero as the load drops.
What is the maximum voltage you recorded with YOUR cell?
What is the voltage when power is at a maximum?
___________
___________
Examining Current vs. RLoad, you will see that current reaches a maximum with a
zero ohm load and gradually the current drops to zero as the load increases.
What is the maximum current you recorded with YOUR cell?
What is the current when power is at a maximum?
24
___________
___________
Series Configuration
3. What happens to data when two solar cells are in series?
Using a load resistance that produced maximum power, find the voltage and
current produced from two cells in series.
What is the maximum series voltage you recorded? ____________
What is the maximum series current you recorded? ____________
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Parallel Configuration
4. What happens to the data when two solar cells are in parallel?
Using a load resistance that produced maximum power, find the voltage and
current produced from two cells in parallel.
What is the maximum parallel voltage you recorded? ____________
What is the maximum parallel current you recorded? ____________
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5. For the next questions, set the load to the load that produced maximum power.
This should be somewhere between 50 and 100 .
What happens when a light absorbing object (your hand, a piece of paper, sheer
fabric, etc.) is placed between the light source and your solar cell?
Using a diffraction grating, determine which light colors are present from the
lamp’s light.
Colors Present: _____________________________________________
For the following tests, put a fluorescent light source at the SAME distance from
the panel as the first light source.
Using this other lamp source in the lab, see what effect a different light source has
on the voltage and current. Use the load that produced maximum power.
Rload Voltage (V)
Current (mA)
P = V*I (mW)
Measure the intensity of this fluorescent light source with the light meter.
_______________________(Lux)
Using a diffraction grating, determine which light colors are present from the other
lamp’s light (fluorescent source).
Colors Present: _____________________________________________
Do you think the solar cell’s performance depends on the frequency of the light that
reaches the cell (color of the light)?
Do you think the solar cell’s performance depends on the intensity of the light
(Lux)?
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6. Plotting Voltage vs. Current for both series and parallel configurations will give
a plot that looks something like (you do not have to do this):
7. Calculating power density and efficiency:
Divide the power by the area of the panel, to get the power density. Do this for
the single solar cell. Use units of watts/meter2
To do this using DataStudio, click on:
Next define a new variable called ‘Power Density’ as shown below:
Divide Power by the area of your solar cell. Area = length X width. Convert
area to meters before calculating the solar cell’s area.
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Now click on Graph to create a new graph. Drag the Power Density data into the
graph. Change the x-axis to load resistance. Your graph should look something
like:
The noon-day sun puts out an approximate average of 1000W/m2 onto a
perpendicular surface at the earth’s surface on a clear day at sea level.
This number is called Solar Insolation. Solar Insolation varies based on location,
time of year, and atmospheric conditions.
Determine the power of the sun given our location and this time of year.
You may want to refer to:
http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi
29
Scroll down to the table with the heading:
Monthly Averaged Insolation Incident On A Horizontal Surface At Indicated
GMT Times (kW/m2)
Since we did this lab using lamps (not the sun), use the light meter readings to
compare power onto the surface vs. power out.
We will now calculate efficiency by dividing the approximate power received by
the solar cell by the power produced by the solar cell. Use your maximum power
density for this calculation:
High noon in Spring and Fall Equinox: +/- 80,000 LUX  0.60 kW/m2
(This is for Santa Clara County, CA. Use internet to find your local data.)
Converting our Light Data to power/area: 30,000 LUX  (0.6)*3/8 kW/m2
(Assuming student recorded about 30,000 lux from the light source.)
Insert your data as follows:
Using your LUX reading: (Your lux)/80,000) * 600 W/m2  power into
cell
Power-out/Power-in
 = (Your max. power density) / (The power the cell receives from
light)
 = 13.8 %
Note: You may not get 13.8% but your number should be between 10-20%.
8. Design a solar cell array to power one of your portable electronic devices (such
as an MP3 Player, CD Player, etc.) during hours of high solar insolation (i.e.
10 a.m. to 4 p.m.):
(Use the data taken in lab for one solar cell and assume that the load is set such
that power is maximized.)
Find the specifications for your device:
Voltage:
__________
Current:
__________  you may need to measure this with a
multimeter
Power: __________
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Voltage out of Cell: _____________ You can use data from using artificial
light or sunlight for this
Current out of Cell: _____________
Sketch below the solar cell module needed to power your device:
9. What if you wanted to power an appliance 24 hours per day? How would you
size the array of solar cells? What other equipment would be needed for 24
hour operation?
10. Estimate the cost recovery for a solar cell. Assume the installation price is
$4.35 per watt of power generated. In what month do you break even?
Indicate your costs per KWH or you can use the average cost is about 15 cents per
KWH.
$4.35/watt / [$0.15/kWh] = 29,000 hours or sunshine  14-16 years
Retail prices for PV panels according to Real Goods:
$9.72/watt installed price w/ permits but no rebates
$2.00/watt rebate from California Million Solar Roofs Program
(Does your state or region offer any programs that offset the cost of installing
solar panels?)
How do your current electricity rates (cost per KWH) effect the cost recovery
time for a solar system installed on your roof?
More information is at:
www.dsireusa.com
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Optional Exercise:
What factors influence the use of solar cells?
Consider: Rebates, costs, efficiency, local electricity rates, availability of power,
sunshine etc.
You can calculate cost recovery for residential PV panels with the online
calculator at Real Goods. Refer to http://www.realgoodssolar.com
Consider the following quote:
The United States Department of Energy indicates the amount of solar energy
that hits the surface of the earth every +/- hour is greater than the total
amount of energy that the entire human population requires in a year.
Another perspective is that roughly 100 square miles of solar panels placed in
the southwestern U.S. could power the country.
http://www.solar4power.com
Do you believe this claim?
11. Discuss solar energy for other uses besides generating electricity.
Other Information Sources:
http://www.apricus.com/html/solar_collector_insolation.htm
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