Tape photovoltaic panels on front



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)

Resistors (10-30 ohms, 2 per person)

Tape Measure GLX Xplorer

DataStudio and PC

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

___________________________________________________________

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.

[pic]

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.

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:______________

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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?

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

[pic]

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?

________________________________________________________________________

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

[pic]

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?

________________________________________________________________________

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.

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.

Explanation of charger operation.

[pic]

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.

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.

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) ( units

(milliwatts = volts * milliamps) ( units

Power * Time = ENERGY

Kilowatt * hour = KWH ( units

(1 KWH costs about 15 cents)

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.

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.

[pic]

Figure 1

Use a decade resistance box to vary the load from 0 to 5000Ω. Make sure you go all the way to 0 Ω.

Use the following load resistance when measuring current and voltage:

|5000 |

|3000 |

|1000 |

|500 |

|300 |

|100 |

|80 |

|50 |

|40 |

|20 |

|10 |

|5 |

|0 |

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:

[pic]

3. You should now double click graph to create a new graph for your data.

[pic]

4. You will now select data to display on the graph. Choose current.

[pic]

5. In order to display both current and voltage on one graph, click and drag voltage onto the graph.

[pic]

6. Select Overlay:

[pic]

7. You should now see an empty graph with both current and voltage overlaid on the same graph:

[pic]

8. Now we need to change the horizontal axis from time to load resistance. Select:

[pic].

9. You should now see:

[pic]

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:

[pic]

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.

[pic]

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:

[pic]

17. If you are set to 5000 on the DRB, then click Keep:

[pic]

18. DataStudio will then ask you for the load resistance. Enter your load and then click OK:

[pic]

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:

[pic]

21. After you’ve collected all the V and I measurements, your graphs might look something like:

[pic]

22. To adjust either horizontal or vertical scale, place the cursor over one of the axis numbers. When you see the curly line [pic]then left-click and drag the mouse left or right to change the scale.

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:

[pic]

24. At what load resistance was power at a maximum for YOUR solar cell? _________

This is called the maximum power point.

25. Using the Σ tool, select Maximum. What is the maximum power produced by YOUR solar cell? ___________

[pic]

26. You can now see the maximum values for all three measurements:

[pic]

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:

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.

[pic]

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.

[pic]

What is the maximum current you recorded with YOUR cell? ___________

What is the current when power is at a maximum? ___________

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? ____________

Parallel Configuration

4. What happens to the data when two solar cells are in parallel?

[pic]

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? ____________

5. For the next questions, set the load to the load that produced maximum power.

This should be somewhere between 50 and 100 Ω.

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 (Ω) |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)?

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):

[pic]

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: [pic]

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.

[pic]

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:

[pic]

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:



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

η ’ Power-out/Power-in

η = (Your max. power density) / (The power the cell receives from light)

η = 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: __________

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:



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

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.



Do you believe this claim?

11. Discuss solar energy for other uses besides generating electricity.

Other Information Sources:

-----------------------

Alligator Clip

Alligator Clip

Resistor

Battery

No Tape on Active Part

Tape on edges

No Tape on Active Part

Tape on edges

Red Wires (+) at top

Make hole with nail

Red Wires through hole at top

Black wires through hole at bottom

Hot Glue

Diode connected here

Hot Glue

IMPORTNANT Diode white stripe toward + side of battery holder

Diode wire through tab and twisted around itself for secure connection

About 1 inch (25mm) of plastic insulation removed

Twist wires together

Twist wires together

Twist red wires with diode

Bend to secure

Tape

Attach black wires to (-) side of battery holder on left side

Remove 1 inch (25 mm) of Insulation

White Wire

White Wire

Connect CURRNENT Probes to White wires

Twist together ends of white wires

Tape

Current from PV 1

(37 mA, 3V)

Combined Current PV 1 + PV 2

(37+ 37 = 74 mA, 3V)

Photovoltaic 1

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.

(37 mA, 3V)

(1.2 V, 74 mV)

(1.2 V, 74 mA)

(0.45 V, 74 mA)

Current from PV 2

Photovoltaic 2

Battery 2

Battery 1

Diode

Photovoltaic 1

Diode

Battery 1

Battery 2

Photovoltaic 2

1700 mAh

60 mA

1700 mAh / 60 mA = 28.3 hours

1700 mAh

200 mA

1700 mAh / 200 mA = 8.5 hours

Recharge

A

Ammeter

Resistor

I

Black

Red

V Meter

I

I

I Meter

Decade Box

RL

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