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Lab Manual : Do-it-Yourself Solar LED Lantern Kit

Welcome, and thank you for purchasing this educational do-it-yourself kit! Not only will you learn about

solar energy, circuits, electromagnetism, batteries, and alternative energy, but you will also make your

own customized solar garden lantern! You can also feel good about your purchase of this product: your

‘upcycled’ jar or housing is being kept out of the waste stream (and the materials such as plastic or glass

takes hundreds to millions of years to decompose), you are creating and utilizing ‘clean’ energy, and a

portion of the proceeds from this purchase help to support projects for building solar-powered lanterns

for those in lesser developed nations who live without electricity (more on that later).

Solar cells, also called photovoltaic cells, convert sunlight into electricity. That electricity can be used to

power many different electrical devices, especially for outdoor use. Recently, there has been a surge of

solar-powered landscape lights on the market, and understandably so; no power cords to bury, no timer

to program, no access to a power supply, and zero operating cost. In this kit, you can design your OWN

customized solar-powered LED light, where the variations in style and décor are only limited by your

imagination!

The glass jar (or other opaque lantern cover) is NOT included in this kit. This is not just because the glass

is fragile and harder to ship. Rather, this kit was designed with ‘upcycling’ in mind. Glass jars with lids

are virtually everywhere, and creativity and individuality is fostered in your choice of other types of

repurposed materials to use.

The circuitry of designing a light that recharges itself in the sunshine seems fairly simple, at first thought.

The solar panel charges the battery by day, and at night, the battery discharges its stored electrical

energy back out through the LED light. The next day, the process repeats itself. As you’ll read further in

the Lab Manual, there’s a good deal of history, math, and science involved in what seems to be a

‘simple’ solar-powered lantern.

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Table of Contents

How it Works: Solar Cells……………………………………………………………………………….. page 3

How it Works: LEDs………………………………………………………………………………………… page 5

How it Works: Rechargeable Batteries………………………………………………….………. page 6

Putting it All Together…………………………………………………………………………………. page 7

Making sense of the schematic……………………………………………………………………… page 8

Assembly instructions……………………………………………………………………………………. page 8

Assembly pictures………………………………………………………………………………………….. page10

How Does the Dusk to Dawn Switch Work? …………………………………..…………….. page 11

Measuring and Calculating Voltage, Current, Power, and Resistance………….... page 11

Series vs. Parallel Circuits…………………………………………………………………………….. page 15

Calculating Recharge Times…………………………………………………………………………… page 16

Solar Insolation Maps…………………………………………………………..…………………….. page 17

The Future of Solar…………………………………………………………………………………….. page 19

STEM Application: Developing Solar Lanterns for South Africans……………… page 21

Solar Jar Light Kit Materials List

1 - 3V 70mA polycrystalline solar panels

1 - Circuit board (pcb) with dusk/dawn operation for 2 LED’s

2 - LED’s

1 – Battery holder for 2 AAA batteries

2 – AAA NiMH rechargeable batteries

1 - Red and black solar wires

1 - Shrink tubing (optional, for extending wires)

1 – Multimeter (for measuring voltage and current)

Not included: Soldering iron, solder and flux, Mason jars or other “containers” for your solar lantern, electric drill and drill bits, silicon adhesive and/or hot glue gun, wire cutters/strippers, extra External switch (optional).

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How it Works: Solar Cells

Solar cells work by tapping into the photovoltaic effect, which converts light into electricity at the

atomic level. More specifically, photons (a “packet” of solar energy of a particular wavelength) are

converted into released electrons.

First, a Brief History Lesson…

A.E. Becquerel (1839) first discovered the photoelectric effect at the age of only 19 when he exposed

platinum electrodes coated with silver compounds to light inside of an acidic solution which acted as an

electrolyte. The combination created both voltage and current, which creates power! Later in 1905,

Albert Einstein described the basic nature of light in which photovoltaic technology is based. In 1954,

Bell Laboratories created the first conventional photovoltaic system module, which was then called a

“solar battery.” By the 1960s, photovoltaic systems were further developed by the space industry, and

the 1970s energy crisis further increased the demand for residential and commercial use. By the 1980s,

photovoltaic cells were most popular to power small electronics, such as calculators, watches, and

radios.

How Solar Cells Typically Operate Today…

Currently, the most common type of solar cell (and the type of solar cell used in this kit) is a ‘single-

junction’ silicon based photovoltaic cell. Silicon is a semiconductor, which will transfer electrons better

than an insulator but not as well as a conductor such as copper or most other metals. Silicon makes up

the bulk of the solar cell, but by itself it’s a poor conductor. It needs to be ‘doped up’ with other

elements that will lose and gain electrons more easily. Silicon that has phosphorus mixed in with it will

be electrically more negative, and silicon that has boron mixed in with it will be electrically more

positive. This separation of charge creates a voltage differential which causes electrons to flow from the

more negative silicon layer on the top to the more positive silicon layer on the bottom when connected

to a circuit, thereby creating electricity. Of course, this is the more ‘simplistic’ explanation, and an

educational kit such as this is obliged to explain how solar cells function on a more elaborate atomic

level .

Silicon has 14 electrons: 2 in its first shell, 8 in its second shell, and 4 in its third shell. You may recall in

a high school science class that atoms need a total of 8 electrons in its third shell to be ‘happy.’ Silicon

compensates for this by making atomic bonds (I call it ‘holding hands’) with four other silicon atoms.

This makes pure silicon strong and stable and give silicon the ability to form a predictable crystalline

structure with a latticework that is heart of the electronics industry. Phosphorus, however, has 15

electrons, therefore having 5 electrons in its outer shell, leaving it with one extra electron that “cannot

hold hands” with a silicon. Phosphorus’ extra electron is more likely to break free because it is not tied

up with a bond with other neighboring atoms, and makes the material more negatively charged. This

type of phosphorus-doped silicon is called n-type silicon.

Conversely, silicon that is doped with the element boron is called a p-type silicon. That’s because boron

has a total of 13 electrons, with 3 electrons in its outermost shell. Instead of free electrons, boron has

free openings (loosely called “holes”) and thereby carries a more positive charge.

The actual inner workings of the solar cell rely on the setup of its three layers: the negative layer, the P-

N junction (the gap between the two layers) and the positive layer. The “extra” electrons in the n-type

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layer on top will naturally move to the P-type layer on the bottom once they are placed together, but

only when the electrons are very close to the P-N junction. The P-N junction is actually the most

important part of all: it creates an electric field, and without it, the solar cell wouldn’t work. The P-N

junction also acts as a diode, which only allows the electrons to move in one direction. As more

electrons move from the top N-type layer to the bottom P-type layer, the electric field gets stronger, but

it acts against any more electrons moving to the bottom layer. Eventually, the movement of electrons

from the top layer to the bottom layer stops, but the electrons that have already moved from the top

layer to the bottom layer are kept in place.

Light is used in the final step of the operation of a solar cell. A photon of light travels into the solar cell material, and strikes an atom. The energy of the photon is absorbed by an electron in the atom, causing the electron in the atom to leave and become a free electron. This is the photoelectric effect! The free electron can move from the bottom layer to the top layer, opposite of the direction of the electric field. But once the electrons are ‘thrown’ back up to the top layer, they cannot cycle back down to the bottom layer again, due to the electric field. Therefore, electrons that are freed by photons ‘build up’ on the top n-type layer and just stay there. When many photons are absorbed, many electrons build up at the top of the solar cell. At the same time, positive atoms accumulate in the bottom layer. The separation of free (-) electrons in the top layer and fixed positive (+) atoms in the bottom layer creates a voltage between the two layers. If you connect a wire to the top layer, it becomes a negative terminal! Then, connect a wire to the bottom layer and it becomes a positive terminal! In sum, the electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

SIMPLY PUT…When light falls on a solar cell, electrons are freed by light. Because of the electric field

purposefully set up in the solar cell, freed electrons build up on the top layer. This is what creates voltage

between the (+) and the (-) terminals.

Limitations of Solar Cells

Besides the well-known general issues concerning solar energy’s intermittency, unpredictability, and

expense (price is dropping fast!), the solar cells we are using have their own specific setbacks which limit

their efficiency in energy production. Solar panels currently on the market have efficiency values

between 12 and 20 percent. So why is it so challenging to make the most of a sunny day?

The primary issue involves a limited wavelength of energy that actually has the ability to separate the

electrons from the atoms. Light energy, of course, comes in a range of wavelengths with different

energy levels. But only a certain wavelength range (with an optimum on the red end of the visible light

spectrum) has the right amount of energy to alter an electron-hole pair. The band gap energy is the

minimum amount of energy needed to free an electron from its bond, and this energy differs among

semiconductor materials. For crystalline silicon, the band gap energy required to knock an electron

loose has been measured to be 1.1 electron volts (eV). The rest of the sun’s energy is reflected, gets

converted into heat or re-emitted as light, or simply passes right through the solar cell causing no effect.

This accounts for the loss of an estimated 60-70 percent of the total radiation energy potential of

sunlight.

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Other shortcomings in solar cell physics, materials, or design affect the cells’ overall efficiency. Issues

such as the lack of a possible ‘ideal’ material with the capability to absorb the entire solar spectra, the

random scattering of released electrons (which are of no use unless they reach the p-n junction),

inadvertent ‘electron-hole’ recombinations (which limit electrons from reaching the p-n junction), the

overheating of the solar cell (which decreases the cell’s efficiency), the reflective properties of silicon

itself, and other resistances to electron flow impact the solar cell’s power output. Additionally, a dense

grid of top-surface electrical contacts is required to maximize the amount of electrons collected, but

come at the consequence of loss of power due to shading effects.

How it Works: LEDs

Pairing photovoltaic cells and LEDs is like “coming around full circle”. Photovoltaic cells produce energy

in the form of direct current (DC). This direct current can be run through an LED in order to convert it

back to (you guessed it) light! Basically, a photovoltaic cell and an LED run on the same basic concept,

but their inputs and outputs are reversed!

Light Emitting Diodes, commonly called LEDs, do dozens of different jobs and are found in all kinds of

electronic devices. They are the basic functional unit in a remote control, form the numbers on digital

clocks, light up watches, tell you when your appliances are turned on, illuminate traffic lights, and are

the base of the thinnest flat screen televisions and computer monitors on the market today. Because

they can be made so small, LEDs can embellish false eyelashes or be infused into contact lenses!

As the name implies, an LED acts as a diode, which only allows electrons to move in one direction. The science behind the way an LED works is parallel to how a photovoltaic cell works. First, semiconductors are again doped with impurities; in LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure AlGaAs, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively charged particles) to conduct electric current. In doped material, additional atoms change the balance, either adding ‘free electrons’ or creating ‘holes’ where electrons can go. The AlGaAs with extra electrons is called an N-type material, and the AlGaAs with the ‘holes’ is called a P-type material. The N-type material and the P-type material are sandwiched together, and the free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone. To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you put in electrical energy by connecting the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. These free electrons moving across a diode can fall into empty holes from the P-type layer. When the ‘free’ electrons drop into a ‘hole’ in the atom, it drops into to a lower electron orbital. When the electrons drop to a lower orbital, they release energy in the form of photons! The frequency (or color) of the photon depends upon the ‘band width’ or the gap in the conduction band in the LED. LEDs can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between, depending on the types of materials used and their setup.

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Advantages of LEDs

Unlike ordinary incandescent bulbs, LEDs don't have a filament that will burn out, they don't get

especially hot, and they are much more energy efficient. Incandescent bulbs must generate a lot of heat

in order to create light: so much heat that you can bake a small cake with them, as EasyBake ovens have

utilized ordinary 100 watt light bulbs for decades! LEDs are also more efficient, more compact, last

longer, and are less toxic than fluorescent light bulbs. LEDs cost much more up front, but in the long run

they save you more money in replacement bulbs and operating costs than incandescent bulbs or even

fluorescent bulbs. For the purpose of constructing a solar garden lantern for this kit, only an LED light

would be small enough and efficient enough in order to work.

How it Works: Rechargeable Batteries

Of course, the photovoltaic energy produced must be stored and later released through the LED in order

to create a functional garden lantern. In effect, you are saving the sun’s photons in the form of electrical

current, storing the current in a battery, and converting it back into photons at night!

Non-rechargeable batteries, or ‘primary’ cells, and rechargeable batteries, or ‘secondary’ cells, produce

current exactly the same way: through an electrochemical reaction involving an anode (the negative

terminal), cathode (the positive terminal) and electrolyte (a conducting fluid or paste that allows the

transfer of electrons). In general, a battery creates a voltage by introducing two chemical reactions

called “redox reactions” which produce separate negative and positive terminals. The reaction in the

anode releases electrons (oxidation), and the reaction in the cathode needs them (reduction, hence

redox). Solid conductors (electrodes) are located on the anode and cathode and a conducting liquid

solution or paste (electrolyte) is found in between. By simply connecting the supply and demand side

electrodes with a conducting wire, you allow these electrons to create a current, thereby supplying

power. In a non-rechargeable primary cell, the chemical reaction at the anode will eventually reach a

limit where it can no longer oxidize and produce electrons, the battery is dead. In a rechargeable

battery, however, the reaction is reversible (but power input is needed to ‘recharge’ the battery). When

electrical energy from an outside source is applied to a secondary cell, the negative-to-positive electron

flow that occurs during discharge is reversed, and the cell's charge is restored.

For our solar lantern, a nickel-metal hydride (NiMH) battery will be used. It’s the most common type of

rechargeable battery available at a small size (in this case, we are using two AAA batteries). The positive

electrodes of a NiMH battery consists of nickel oxyhydroxide (NiOOH), and the negative electrode is

made of a metal alloy (typically a metal hydride). The electrolyte is typically a potassium hydroxide

paste.

When the NiMH battery is discharged, the metal alloy at the negative electrode releases a hydrogen, which combines with a hydroxyl ion in the electrolyte to form water while also contributing an electron to the circuit, according to the following reaction:

Alloy (H) + OH`‹----› Alloy + H2O + e` Meanwhile, on the positive electrode, nickel oxyhydroxide is reduced to its lower valence state, nickel hydroxide.

NiOOH + H2O + e`‹----› Ni(OH)2 + OH`

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When the NiMH battery is recharged, the current placed back into the battery breaks down the water in the electrolyte paste into hydrogen atoms, which are absorbed into the metal alloy, and hydroxyl atoms are released, according to the following reaction:

Alloy + H2O + e`‹----› Alloy (H) + OH` Meanwhile, on the positive electrode, the hydroxyl atoms are used to oxidize the nickel hydroxide, creating nickel oxyhydroxide. The electrical potential energy is stored in the nickel oxyhydroxide, which is of a higher valence state. The nickel oxyhydroxide will eventually go on to release water and an electron as in a ‘charged’ battery according to the following reaction:

Ni(OH)2 + OH`‹-----› NiOOH + H2O + e`

Most NiMH batteries can be recharged several hundred times over the course of its life. They also do

not lose much of their potential power output after they are recharged. NiMH batteries normally

operate at 1.2 V per cell, somewhat lower than conventional (non-recharageable) 1.5 V cells, but will

operate most devices designed for that voltage.

Limitations of NiMH Rechargeable Batteries Although NiMH batteries can be recharged hundreds to times, they still have a life expectancy limited

only to 2 to 5 years. NiMH batteries also have an operable temperature range of -20 and 60 degrees

Celsius. It is well known that higher temperatures increase the collision rates of molecules, effecting

deterioration and self-discharge. Conversely, lower temperatures reduce the rate of reaction, creating

less power. This is the reason why cars containing rechargeable batteries (such as the Toyota Prius)

have their own thermal management systems.

Putting it All Together

So, all we have to do is hook up the solar panel to the NiMH battery and the LED lights, and we’ve got

the makings of an operable solar lantern, right? Of course, it’s not that easy!

Electricity flows in circuits: closed paths through which electrons flow. Within these circuits or “loops”

for the electrons to flow, one needs to incorporate a voltage source, switches, and loads (in this case, it

is the LEDs or the NIMH batteries). BUT…the LEDs cannot get too much power, the electricity has to be

blocked from leaving the batteries when they are charging, and an automatic switch needs to be

integrated into the circuit so that the LEDs turn OFF when there is light available but turn ON when the

degree of light decreases.

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Making Sense of the Schematic of the PCB (printed circuit board)

Working from left to right, you can see that there is an outer and an inner circuit leading from the solar

cell to the blocking diodes (1N5819) before it gets to the batteries, in order to keep energy in batteries.

The outer circuit is for the optional external on/off switch (Switch1), so that the lantern can be manually

turned on or off regardless of time of day or battery charge level. In the inner circuit, there is a resistor

which keeps the LEDs from getting too much power. The ‘dusk to dawn’ switch is actually a PNP bipolar

junction transistor, which will be further described below. Inside the jar, there are two switches

(Switch2 and Switch3) which can manually turn on or turn off the LEDs from within in order to set the

brightness desired or conserve energy if sun hours are low.

Assembly Instructions

1. Identify all the parts as listed in the materials section.

2. Using a little flux on the small copper circles on the back of the solar panel, solder on the red +

and blank – wires on the solar panel.

3. Look at the pcb (printed circuit board) closely and identify the soldering contacts for the solar

panel, batteries, and LEDs.

4. Depending on what type of container you will be using, you may want to drill holes in the lid so

that you can place the solar panel on top, place the optional external switch on top (not

included), and cut the solar wires to the desired length.

5. Soldering the LEDs: On each LED there is a longer wire (+) and a shorter wire (-). It is easiest to

solder on the underside of the pcb. Feed the LEDs from through the top. Solder the + end and

the – of the LED to the + and – contacts on the pcb for LED1. You may also solder wires to these

contacts if you want to locate the LEDs in a different location. Repeat for LED2. Trim the excess

LED wire with cutters.

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6. Find the 2 AAA battery holder with wires. If you are using a Mason jar type container, you can

locate the battery holder under the lid inside the jar. You can attach the battery holder to the

lid with a hot glue gun or silicon adhesive as well as the solar panel to the top of the lid (with

holes or wires).

7. Cut the + and – wires on the battery holder to the desired length and solder them to the + and –

battery contacts on the pcb.

8. Solder the + and – wires from the solar panel to the + and – solar contacts on the pcb.

9. The pcb can be attached to the cover of the battery holder using a small amount of hot glue or

silicon adhesive on the corners or outer edges. DO NOT USE ADHESIVE NEAR THE

COMPONENTS.

10. Install the AAA rechargeable batteries, and your solar LED light is complete!

OPERATION…The solar garden light should get as much sun as possible. When the solar panel is

covered or when it is dark, turn on inner switches for the LEDs until they light. If you take the

lantern back out into the sun, the LEDs will go off. At night, the LEDs will turn back on

automatically. Have fun!

Note: Optional external switch is no longer part of the circuit. It is not necessary to solder these

contacts together.

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Assembly Pictures

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How Does the Dusk to Dawn Switch Work?

The key feature of our solar powered lantern is that it charges the battery during the day, but then

discharges the battery to light the LED at night. To do this, one could install a photosensor component

which separately measures light intensity to switch from one circuit to another. But why waste the

power and increase the complexity of the system when the photovoltaic cell itself serves as a light

sensor? For our solar garden lantern, a type of transistor called a PNP bipolar junction transistor will be

utilized. Transistors are devices that control the movement of electrons, and consequently, electricity.

They work something like a water faucet -- not only do they start and stop the flow of a current, but

they also control the amount of the current. With electricity, transistors can both switch or amplify

electronic signals, letting you control current moving through a circuit board with precision.

To work properly, transistors require pure semiconductor materials: materials that have properties in-

between insulators and conductors, allowing electrical conductivity in varying degrees. The type of

technology applied in a transistor is (you guessed it) another utilization of N-doped and P-doped silicon!

When the N-type and P-type adjacent to each other, a P-N diode is created. This diode allows an

electrical current to flow, but in only one direction, a useful property in the construction of electronic

circuits. Full-fledged transistors were the next step. To create transistors, doped silicon is engineered to

make two layers back to back, in a configuration of either P-N-P or N-P-N. The point of contact was

called a junction, thus the name junction transistor. With an electrical current applied to the center

layer (called the base), electrons will move from the N-type side to the P-type side. The initial small

trickle acts as a switch that allows much larger current to flow. In an electric circuit, this means that

transistors are acting as both a switch and an amplifier.

In our setup, the PNP bipolar junction transistor is connected to the solar panel. The other 2 contacts on

the transistor are BATTERY IN (from + battery) and TO LED’s (to + on LED). During the daytime, the

power from the sun will cause an electrical flow that will “switch” the transistor to the OFF position, not

allowing any power to the LED(s). Instead, the transistor allows all the solar energy from the sun to be

stored in the 2 AAA batteries to be used later. When the transistor does not get any solar power at

night, the “switch” in the transistor diverts the energy stored in the battery by turning ON the LED(s).

The simple switch operation of transistors is what enables your computer to complete massively

complex tasks. In a computer chip, transistors switch between two binary states -- 0 and 1. This is the

language of computers. One computer chip can have millions of transistors continually switching,

helping complete complex calculations.

Measuring and Calculating Voltage, Current, Power, and Resistance (optional experiments)

It is useful to envision electricity running through a circuit as being similar to water flowing through a

pipe. Just as water flows downhill, electrons flow from places of higher potential to places of lower

potential. The difference in the electrical potential provides the force that pushes a charge through a

circuit. The unit of measure of potential difference is called the volt (V). Electrons will flow as long as

there is a potential difference, or voltage, between the two parts of a circuit. Voltage supplies the

“push” to the electrons that are flowing. The amount of electrons flowing through a material is called

the current. The standard unit for measuring an electric current is the ampere (amp), but is usually

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abbreviated with an I (think of “I” as describing amps by using the word “Intensity”). Using the water

analogy, current would be similar to the “water pressure”, determined by the volume of water flowing

in a defined area. The third factor which must be considered is resistance, a property which impedes

the electron’s ability to flow. The standard unit for measuring resistance is the Ohm (Ω). Using the

analogy such as water running through a pipe, certain factors such as the length of the pipe, the

diameter of the pipe, and pipe blockage may slow down the flow of water. The greater the resistance,

the less current there is for a given voltage.

Measuring Voltage

To measure voltage of a solar cell with a multimeter, the black probe should be in the black multimeter

port (usually labeled COM) and the red probe should be in the red multimeter port labeled “V”. For

some multimeters, this may be the same port used to measure current. It may be labeled “VMAW”, or

something like that, instead of just “V”.

Connect the solar panel with no load attached, by attaching the probes directly to the wires of the solar

panel. Make sure the meter is set to DC volts, not AC volts. Again, if you get a negative number in the

display just reverse the probes.

Your DC voltage should be close to 3.0 V in direct light/full sun, due to the specifications of the solar

panel.

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Measuring Current

To measure current of a solar cell with a multimeter, the meter must be wired into the circuit in series,

and the circuit must be closed (all connected) for current to flow. Most multimeters will measure up to

200 mA (milliamps), and some have an additional port to measure up to 10A (amps, 1 amp = 1000 mA).

In either case, the black multimeter cord will plug into the black (negative) port on your meter. For low

currents (200 mA or less), you will usually plug the red multimeter cord into the red port labeled “mA”.

Attach the multimeter into your circuit as described above. Turn the multimeter dial to the DC mA

setting (not the AC setting. The DC setting has a symbol that is a solid line above a dashed line). If you

get a negative sign on the multimeter display, just reverse the black and red probes – you have them

hooked up backwards.

Your DC current should be close to 70 mA in direct light/full sun, due to the specifications of the solar

panel.

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Calculating Voltage, Current, Power, and Resistance

In the 1820s, German physicist Georg Ohm experimented with many electrical substances to determine

their resistance. Ohm found out that the resistance for most conductors does not depend on the

voltage across them. A conductor or any other device that has a constant resistance regardless of the

voltage is said to obey Ohm’s law, stated below:

Resistance (R or Ω) = Voltage (V) ÷ Current (Amp or I)

This formula can be rewritten in numerous ways: Ω = V/A, R = V/A, A = V/R, V = AR.

Another way of visualizing Ohm’s Law is with this ‘pie diagram’. Put your finger on the variable you want to solve for and the operation you need is revealed.

V = I (or Amp) x R (or Ω), I (or Amp) = V ÷ R (or Ω), R (or Ω) = V ÷ I (or Amp)

As stated before, power (P) is the product of volts (V) times current (Amp or I). The standard unit for

measuring power is the watt (W), which is equivalent to one joule of energy per second. Therefore,

P (Watts) = Voltage (V) Current (Amp or I)

Now substitute a version of Ohm’s Law (V = IR) into the equation for power, and you get:

P (Watts) = (AR) A

P = A2R

Now substitute another version of Ohm’s Law (I = V/R) into the equation for power, and you get:

P (Watts) = V (V/R)

P = V2/R

Therefore, it is possible to calculate any relationship between power, resistance, voltage, and

current, depending on what information is available and which formula is chosen.

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Series vs. Parallel Circuits

In order to get certain loads (such as one to several LEDs or a motor) to work, a certain voltage or amperage is required. If the current (amperage) is too high, a resistor is used to slow down the current. But what if the voltage or the current is too low? Aside from getting a larger capacity power source, the voltage or current could be increased by adding an additional solar panel and/or changing the type of circuit used. A series circuit has only one path for the current to take. A series connection of solar panels adds voltages of the panels but does not add current.

Example: Two 3V, 70 mA panels wired in series will produce 6V, 70 mA

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A parallel circuit has several paths for the current to take. A parallel connection of solar panels adds

current outputs of panels but does not add voltages.

Example: Two 3V, 70 mA panels wired in parallel will produce 3V, 140 mA

Calculating Recharge Times

For solar-charging batteries, it’s more important to know about battery capacity than it is to know about

resistance. Battery capacity is measured in a unit called mAh, standing for milliAmps per hour (the

prefix ‘milli’ means one thousandth). These batteries have a capacity of 1000 mAh when they are fully

charged. You can find this number printed on the battery. This tells you that the how much of a certain

current it can produce before the battery needs to be recharged. For example, if the load in your circuit

uses 400mA (or .4 A) then a single NIMH battery can power it for 2 ½ hours (400 mAh x 2.5 hours = 1000

mAh).

You can use the same calculations to figure out how long it will take to charge a battery with a certain

amount of current. For example, if a solar module is producing 100 mA of current, it would take 10

hours to recharge the battery (100mA x 10 hours = 1000mAh).

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So the calculations are pretty simple, but there is one complication: if you set up your solar module to

face the sun in the morning and then leave it to charge all day, you will find that the current produced

by the solar module changes as the sun moves through the sky! These simple calculations only work

when the current stays the same. Therefore, the next topic to discuss is how to calculate the charge

time of a battery no matter where you are on the Earth. To do this, you need to refer to a solar

insolation map.

Solar Insolation Maps

When sunlight reaches the earth’s surface, it has the potential to produce about 1000 watts per square

meter at noon on a cloudless day (that is, if the sun was shining at the most direct angle and assuming

that 100% of it was able to be converted into electricity). The standard 1000 watts/m2 is also called the

‘peak sun’ or ‘peak hours’. This potential energy varies, however, due to time of day and

geographical location (along with the efficiency of the photovoltaic cell). Scientists estimate

that sunlight will provide useful solar energy for only about 6 to 7 hours per day, because during the

early hours and late hours of the day the angle of the sun's light is too low. So, for example, 6 productive

hours of direct sunlight a day on a square meter of land has the energy equivalent of 6 kilowatt-hours

(or 6000 watts/hour) of solar energy. Deserts, with very dry air and little cloud cover, receive the most

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sun—and has the energy equivalent of more than 6 kilowatt-hours (KWH) per day per square meter.

Northern climates, such as Boston, get closer to 3.6 kilowatt-hours. A solar insolation map provides

data indicating the number of productive hours of direct sunlight (or “peak hours”) for a particular area.

Additionally, photovoltaic output is directly correlated to the efficiency of the solar panels. Due to the

laws of thermodynamics, solar panels can never be made to be 100% efficient. That’s because when

you convert energy from one form (solar) to another (electricity), some energy will invariably be

converted into heat. On top of this, conventional photovoltaic panels absorb only a narrow band of the

solar spectrum, and the rest of the wavelengths get ‘wasted’ by being reflected or converted into heat.

Photovoltaic panels commercially available today have a relatively low efficiency, averaging only 15%.

That means that photovoltaic cells in the desert typically produce .9 KWH per square meter, and the

global average photovoltaic output is about .6 KWH per square meter.

Here’s how to use solar insolation maps to calculate how long it will take to fully charge a battery;

simply find the total number of peak sun hours per day at your location, and then multiply that number

by the maximum current that the solar module will produce in a peak sun. That will tell you the total Ah

that you can charge into the battery in a day. For example, a solar insolation map shows that Maine has

5-6 peak sun hours per day in May. The maximum power output of our solar array is 70 mA, so in one

day you can produce about 70 mA x 5.5 hours = 385 mAh. The battery capacity for each battery is 1000

mAh, and they are connected in a series circuit (which does not add current, so the battery capacity

remains at 1000 mAh). Therefore, it would take 1000 / 385 or about 2.6 days to completely charge both

batteries.

The math above aligns with the ‘field observations’ with the solar lanterns. They seem to operate best

when “let out in full sun” for a day or two to charge the batteries before switching on the LEDs.

Fortunately, the LEDs use comparatively little current; about 20 mA each (but current use varies by LED

color). The LEDs are in a parallel circuit, which adds current, making the total current necessary for both

LEDs 40 mA. Therefore, assuming 10 hours or nighttime LED light x 40 mA = 400 mAh per night.

Comparing solar power inputs to LED outputs, we have a relatively ‘balanced’ system, where the solar

lanterns should charge and discharge at about the same rate each day. However, several variables

including overcast days and outdoor temperatures may apply. Therefore, long-range field observations

of the solar lanterns in states throughout the U.S. are currently underway.

Solar insolation maps can be found online on different web sites, but the most common site is

http://www.nrel.gov/gis/solar.html.

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The Future of Solar

The amount of energy from the sun that falls on Earth's surface is enormous. If you factor the sun’s peak

hours by the area of the entire earth, you could calculate that the sun radiates more energy onto the

earth than the entire human population uses in one whole year. In fact, all the energy stored in Earth's

reserves of coal, oil, and natural gas is matched by the energy from just 20 days of sunshine! Tapping

solar power’s entire potential may not be a plausible goal, but there are emerging technologies being

developed which may drive down the cost and increase the efficiency of photovoltaic cells. Innovative

processes and designs are continually reaching the market and are sure to commence in the future. As

of the time this lab manual was printed, the following ‘next generation’ photovoltaic technology has

shown promise:

Thin film technology – Silicon is still the most popular material for solar cells, and is the second-most abundant element in the Earth's crust (after oxygen). However, to be useful as a semiconductor material in solar cells, silicon must be refined to a purity of 99.9999%, which is expensive and energy consuming to produce. Additionally, much of this purified silicon is cut away and wasted in the manufacturing process. Thin film technology, by design, drives down the cost of manufacturing by reducing the amount of silicon used, but also produces a material with greater versatility. Thin film solar panels are commercially available for installation onto the roofs of buildings, either applied onto the finished roof, or integrated into the roof covering. The advantage over traditional PV panels is that they are very low in weight, are not subject to wind lifting, and can be walked on (with care). Thin film technology is also being developed for producing semitransparent silicon solar cells which can be applied as a window glazing, thereby creating a window tinting that generates electricity. Thin film photovoltaic cells are currently less efficient than conventional solar panels, but their market price can be driven down even further by replacing sodium with cadmium telluride as a semiconductor.

Light Harvesting – Another great disadvantage of conventional photovoltaic cells is that they tap only a narrow band of solar energy, and the rest of the energy is reflected or converted into heat. New developments are currently underway to employ different methods to concentrate the solar energy, filter it for its target wavelength, or pass it through the solar panel several times by using devices that act like mirrors, lenses, or filters. Reflective metal troughs directed at a specific angles can shuttle the light into multiple solar cells, each made from one of six to eight different semiconductors. Optical filters can allow only a single color to hit a solar panel specifically designed to receive it; the remaining colors are reflected toward other filters designed to let them through. Additionally, designing a 3-dimensional solar module has obvious advantages over conventional flat solar panels. This could help to solve some of the most serious drawbacks of solar electricity, such as the angle of the sun’s rays and need for large surface areas. MIT researchers have demonstrated that stacking them in three dimensions can help produce 20 times more power per square foot than using flat panels.

Multijunction photovoltaic cells – Another way to skirt the issue of the narrow bandwidth of solar energy tapped is to create multi-junction solar cells or tandem cells containing several P-N junctions. Each junction is tuned to a different wavelength of light. Traditional single-junction cells have a maximum theoretical efficiency of 34%, a theoretical "infinite-junction" cell would improve this to 87% under highly concentrated sunlight. Currently, the best lab examples of conventional silicon solar cells have efficiencies around 25%, while lab examples of multi-junction cells have demonstrated performance over 43%. Commercial examples of tandem cells are widely available at 30% under one-

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sun illumination, and improve to around 40% under concentrated sunlight. However, this efficiency is gained at the cost of increased complexity and manufacturing price.

Advancements in Nanotechnology – Nanotechnology, the science of manipulating materials at an atomic or a molecular scale, has the potential to reduce the amount of materials used to make photovoltaic panels, make them more flexible, allow more light to be absorbed, and allow them to tap into a greater bandwidth of energy coming from the sun. And the amount of different ways that nanotechnology can accomplish this is almost too numerous to mention. For example, nanotechnology can simply enhance our current photovoltaic cells by simply covering them with a coating of silica nanoparticles. This increases efficiency by tapping some of the energy available in the ultraviolet range; radiation which would otherwise only be converted to heat. Existing panels could also be coated with nanoparticles designed to ‘tune’ the solar energy to one wavelength. Nano-sized holes smaller than a wavelength of light can be drilled into each square centimeter of a solar cell’s surface which force the light to be absorbed rather than reflected. Nanowires can be grown to be significantly thinner than conventional wires within the panel, reducing the amount of shading to the silicon, and additionally having their own capability of converting light into energy.

Organic (carbon-based) solar cells would be cheaper and easier to make than current silicon-based solar cells. Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds. Carbon nanotubes have been developed, possessing a wide range bandgaps, strong photoabsorption from the infrared to the ultraviolet end of the spectrum, and high carrier mobility and reduced carrier transport scattering. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces.

The growing field of nanotechnology is said to be ‘limited only by the designer’s imagination”; it could

even lead to the development of a photovoltaic outdoor paint which could convert the surface of your

house into a generator!

STEM Application: Developing Solar Lanterns for South Africans

Excerpt from Pamela Ulicny, Biology and Environmental Science teacher in Tri-Valley Jr/Sr High School in

Hegins, Pennsylvania:

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“The inspiration for developing this solar-powered lantern was originally spurred by Mark

Gamble, CEO of Educo Africa. I was fortunate enough to be granted a professional development

trip to South Africa through the Toyota International Teacher Program, funded by Toyota Motor

Sales and the Institute for International Education. The trip was the experience of a lifetime and

an invaluable experience to share with my students. I was particularly moved by the plight of

poverty in South Africa, and how Educo Africa works to restore the dignity, power, and self-

worth of South African youth from disadvantaged communities. The solar-powered lanterns

were designed with the eventual and ultimate purpose of teaching South Africans science,

technology, math, sustainability, and job skills as they are trained to build their own solar-

powered lanterns. The enterprise piloted by Educo Africa would eventually build and sell solar

lanterns to those who do not have access to electricity. Families without electricity currently

use kerosene lanterns, which pose a fire hazard and have already resulted in severe burns,

fatalities, and the destruction of many homes. Additionally, using kerosene lanterns habitually

is equivalent to smoking two packs of cigarettes a day, and inhaling its fumes increases the

chance of cataracts, respiratory infections, tuberculosis or lung and throat cancers.”

Let’s say that we want to create a solar powered lantern that will light up a small room for an Educo

Africa participant whose home is without electricity. Cape Town, South Africa (where the headquarters

of Educo Africa is located) has an average solar radiation of 4.9 peak sun hours per day. Let’s say that

you want to charge a 6V 1600mAh NiMH battery for a solar powered lantern. The solar panel produces

a maximum power output of 6V at 330 mA. So in one average day you can produce 4.9h x 330 mA =

1,617 mAh, which is enough to fully charge the battery. In order to get enough lumens (brightness) to

read, let’s get an LED light that gives off 500 lumens (equivalent in brightness to a 50W incandescent)

and uses 8 watts. Since power (watts) is the product of volts (V) times current (Amp), then the amount

of time that the light bulb could run on this solar module would be calculated as follows:

Watts = Volts x Amps

8 = 6 x A

A = 1.333 = 1333 mA

1600 mAh (in battery) ÷ 1333 mA (needed for light bulb) = 1.2 hours = 72 minutes.

Unfortunately, this may not be enough time necessary for nighttime reading, housework, and

schoolwork. So in this example, a larger solar panel and a larger capacity battery may be necessary.

Another possibility is to find an LED light that gives off an equivalent amount of lumens but uses less

watts.

Glossary:

alternating current - An electric current that reverses its direction many times a second at regular

intervals, typically used in power supplies.

anode - The negatively charged (electron supplying) electrode of a device supplying current such as a

battery.

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band gap energy - an energy range in a solid where no electron states can exist, or the energy required

to free an outer shell electron from its orbit about the nucleus of an atom.

cathode- The positively charged electrode of battery that receives electrons, supplying current.

current – A flow of electric charge. It is how ‘crowded’ the electrons are as they flow in a circuit. The

unit that measures the amount of current or ‘electrical flow’ going through a material is called an amp

(ampere, Amp or I).

diode – A semiconductor device with two terminals, typically allowing the flow of current in one

direction only.

direct current - An electric current flowing in one direction only.

electrolyte - A liquid or gel that contains ions and can transfer an electric current.

ohm – a unit that measures a material’s resistance to electrical flow.

Ohm’s Law - The law stating that the direct current flowing in a conductor is directly proportional to the

potential difference between its ends. It is usually formulated as V = IR.

LED – Light Emitting Diode - A semiconductor diode that converts applied voltage to light and is used in

lamps and digital displays.

parallel circuit - a closed circuit in which the current divides into two or more paths before recombining

to complete the circuit. This type of circuit adds current but does not add voltages.

photon - A particle representing a unit of light or other electromagnetic radiation, also defined as a

“packet” of solar energy of a particular wavelength.

photovoltaic effect – the creation of voltage or electric current in a material upon exposure to light.

power - is the rate of doing work, measured in watts, and represented by the letter P.

resistance – a material's opposition to the flow of electric current; measured in ohms.

semiconductor – materials that have properties in-between insulators (which does not allow charges to

flow freely) and conductors (which permits the flow of charges in one or more directions), thereby

allowing electrical conductivity in varying degrees.

series circuit - An electric circuit connected so that current passes through each circuit element in turn

without branching. This type of circuit adds voltages but does not add current.

transistor - A small electronic device containing a semiconductor and having at least three electrical

contacts, used in a circuit as an amplifier, detector, or switch.

voltage - electric potential or potential difference expressed in volts. A volt (V) is a unit that measures

the amount of ‘push’ that moves the electrons in a circuit.

watt – the standard unit for measuring power (P), defined as a joule of energy per second. Power (W) is

the product of voltage (V) times current (Amp or I).

References and Further Reading

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http://science1.nasa.gov/science-news/science-at-nasa/2002/solarcells/ - How Do Photovoltaics Work?

http://science.howstuffworks.com/environmental/energy/solar-cell.htm - How Solar Cells Work

http://electronics.howstuffworks.com/led.htm - How Light Emitting Diodes Work

http://phet.colorado.edu/en/simulation/photoelectric - PhET: an interactive simulation of the

photoelectric effect

http://electronics.howstuffworks.com/everyday-tech/battery5.htm - How Batteries Work

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf - NiMH Handbook and Application

Manual

http://electronics.howstuffworks.com/transistor.htm - How Transistors Work

http://www.youtube.com/watch?v=IcrBqCFLHIY – Video: How Does a Transistor Work?

http://www.nrel.gov/gis/solar.html - National Renewable Energy Laboratory: Solar Insolation Maps

A portion of the sale of these solar lantern kits will be donated to Educo Africa in Cape Town, so

that we can do our part to put an end to energy poverty. Our dream is to offer a safe and

sustainable light source so that disadvantaged students get a greater chance to read, study,

complete their homework and chores, and achieve their goals. If your school, club, or religious

organization would like to learn more, please contact Pam Ulicny or Ed Bender (contact

information below) and ask about the “This Little Light of Mine” fundraiser.

Questions & comments:

Ed Bender ed@sundancesolar.com

Pam Ulicny psu@tri-valley.k12.pa.us