Measuring PV Cell/Module Voltage and Current

Measuring PV Cell/Module Voltage and Current

Rationale: Knowing the electrical characteristics (voltage and current) of a PV cell or module provides necessary information for an engineer to design a practical energy conversion system. Relating PV cell characteristics to commonly used sources of DC electricity, such as cells and batteries, provides a meaningful reference for understanding their capabilities and usefulness.

Objectives:

1. Use a digital multimeter to measure voltage and current.

2. Characterize a PV cell/module in terms of its voltage and current capability using various light sources.

3. Explain why connecting a load resistance to a PV cell is necessary when making measurements to determine electrical power output.

3. Demonstrate how two or more PV cells/modules can be electrically combined to produce useful amounts of electricity.

4. Describe several practical energy conversions related to the use of PV cells and modules.

Frameworks Addressed:

Technology/Engineering – High School Energy and Power Technologies—Electrical Systems

5.1Explain how to measure and calculate voltage, current, resistance, and power consumption in a series circuit and in a parallel circuit. Identify the instruments used to measure voltage, current, power consumption, and resistance.

5.2Identify and explain the components of a circuit, including sources, conductors, circuit breakers, fuses, controllers, and loads. Examples of some controllers are switches, relays, diodes, and variable resistors.

Guiding Questions:

• Where does our electricity come from?

• How many cells/batteries do I, or my family use each year

• What is the difference between a cell and a battery? What materials are in cells/batteries?

• Where do the raw materials come from and where do they go when the battery is “used-up”?

• How much energy is stored in a cell/battery?

• How much energy is produced by the SSL on a full-bright sun day?

• What is the same, and what is different about AAA, AA, C and D cells ?

• What is different about rechargeable and non-rechargeable cells/batteries?

• How much voltage, current and power can a PV cell produce?

• Why are some PV arrays very small, and some very large?

• How do PV Cells “make” electricity? What materials are they made from?

©STEM Solar Labmaterials can be copied for instructional, non-commercial use.

Semiconductors and the Built-In Electric Field for Crystalline Silicon Photovoltaic Cells

To separate electrical charges, crystalline silicon cells must have a built-in electric field. Light shining on crystalline silicon may free electrons within the crystal lattice, but for these electrons to do useful work—such as provide electricity to a light bulb—they must be separated and directed into an electrical circuit.

PV Semiconductors

Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.

Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.

To create an electric field within a crystalline silicon photovoltaic (PV) cell, two silicon semiconductor layers are sandwiched together. P-type (or positive) semiconductors have an abundance of positively charged holes, and n-type (or negative) semiconductors have an abundance of negatively charged electrons. When n- and p-type silicon layers contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.

Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet"what is called the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

Creating P-Type and N-Type Semiconductors

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In a crystalline silicon PV cell, p-type silicon must contact n-type silicon to create the built-in electrical field. The process of doping, which is used to create these materials, introduces an atom of another element into silicon crystal to alter its electrical properties. The dopant, which is the introduced element, has either three or five valence electrons—which is one less or one more than silicon's four.

Phosphorus atoms, which have five valence electrons, are used to dope n-type silicon because phosphorus provides its fifth free electron. A phosphorus atom occupies the same place in the crystal lattice formerly occupied by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, having no bonding responsibilities. When phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.

The most common method of doping is to coat a layer of silicon material with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then reduced so the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique in which phosphorus ions are precisely driven into the surface of the silicon.

But the n-type silicon cannot form an electric field by itself. It also needs p-type silicon. Boron, which has only three valence electrons, is used for doping p-type silicon. Boron is introduced during silicon processing when the silicon is purified for use in photovoltaic devices. When a boron atom takes a position in the crystal lattice formerly occupied by a silicon atom, a bond will be missing an electron. In other words, there is an extra positively charged hole.

P-Layer Design

In a PV cell, photons are absorbed in the p-layer. It is therefore important that this layer be "tuned" to the properties of incoming photons so it can absorb as many as possible and, thus, free up as many electrons as possible. The design of the p-layer must also keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. To accomplish these goals, p-layers are designed to free electrons as close to the junction as possible, so that the electric field can help send the free electrons through the conduction layer (the n-layer) and out into the electrical circuit.

By optimizing these characteristics, the PV cell's conversion efficiency (how much light energy is converted into electrical energy) is improved.

Cell and Battery Data

The table below shows the battery volume for cell sizes AAA, AA, C, and D. The volume does not change with the type of battery, but the weight does, as shown here.

Battery
Cell Size / Volume
ml / Alkaline
Weight g / Carbon-Zinc
Weight g / NiCad
Weight g / NiMH
Weight g / Lithium Ion
Weight g>
AAA / 3.9 / 12 / 9.7 / 11 / 12
AA / 8.3 / 24 / 19 / 29 / 26 / 24
C / 26.5 / 65 / 48 / 85 / 82
D / 55.8 / 135 / 98 / 200 / 170

Energy storage in AAA batteries

Battery
Type / Avg. voltage
During discharge / milli-Amp
hours (mAh) / Watt-hours
Wh / Joules
J
Alkaline
Long-life / 1.225 / 1150 / 1.41 / 5071
Carbon-zinc / 1.1 / 320 / 0.35 / 1268
Nickel-Cadmium / 1.2 / 300 / 0.36 / 1296
NiMH / 1.2 / 800 / 0.96 / 3456

Energy storage in AA batteries

Battery
Type / Avg. voltage
During discharge / milli-Amp
hours (mAh) / Watt-hours
Wh / Joules
J
Alkaline
Long-life / 1.225 / 2122 / 2.60 / 9360
Carbon-zinc / 1.1 / 591 / 0.65 / 2340
Nickel-Cadmium / 1.2 / 1000 / 1.20 / 4320
NiMH / 1.2 / 2100 / 2.52 / 9072
Lithium Ion / 3.6 / 853 / 3.1 / 11050

Energy storage in C batteries

Battery
Type / Avg. voltage
During discharge / milli-Amp
hours (mAh) / Watt-hours
Wh / Joules
J
Alkaline
Long-life / 1.225 / 7800 / 9.56 / 34398
Carbon-zinc / 1.1 / 2172 / 2.39 / 8600
Nickel-Cadmium / 1.2 / 2500 / 3.00 / 10800
NiMH / 1.2 / 4500 / 5.40 / 19440

Energy storage in D batteries

Battery
Type / Avg. voltage
During discharge / milli-Amp
hours (mAh) / Watt-hours
Wh / Joules
J
Alkaline
Long-life / 1.225 / 17000 / 20.83 / 74970
Carbon-zinc / 1.1 / 4733 / 5.21 / 18743
Nickel-Cadmium / 1.2 / 5000 / 6.00 / 21600
NiMH / 1.2 / 9500 / 11.40 / 41040

Before actually working with the digital multimeter (DMM), please respond to the following question (circle one):

How comfortable are you with the use of a digital multimeter for measuring the voltage of cells and batteries?
not at all somewhat very
012345

Using Digital Multimeters

Digital multimeters typically have the following characteristics in common:

• They measure ac and DC voltage;
• They measure ac and DC current;
• They measure resistance;
• The black test lead connects to the “COM” input jack for all measurements;
• The red test lead connects to the V/Ω jack for voltage and resistance measurements, and is moved to the jack marked “200 mA”, “A”, or “10A” input (for high current measurements).

Functions (ac/DC voltage, ac/DC current or resistance) are separated into maximum value groups selected by the switch. Often DC values are indicated by a symbol similar to: ------, while ac values use: ~

Each range setting of the switch is typically labeled with the number 2, 20, 200 etc. and may also include a metric prefix such as: m (milli = x0.001),

µ (micro = x0.000 001), or k (kilo = x1000).

The maximum value that can be displayed in each range setting is 1.99, 19.9, 199 etc. There may be one more significant digit depending on how “good” the meter is, for example: 1.999, 19.99 etc.

If the quantity you are measuring exceeds the range setting (connecting a 9 volt battery to the test leads when the range switch is set on 2, for example) the display will show: 1 (followed by a blank readout). This indicates “over-range” and should not damage the meter if connected for a short time. To correct this, move the range switch to a higher setting (20, for example) until a meaningful display is shown.

Safety practices when using a meter include:

• Connect the meter to the device being tested before turning the power to the device “on”;
• Do not touch any metal parts of the test probe when making measurements;
• When measuring current, make sure that the test leads are in the correct input jacks, and that the range switch is on a setting greater than the expected reading before connecting the meter into the circuit.

Interpreting Display Readings (Practice)

Range/Function
Switch Setting / Display Shows / Quantity Measured (include units)
In decimal, power of 10, and metric prefix
Example:
DCA 20m / 15.9 / 0.0159A or 15.9x10-3A or 15.9 mA
DCA 200m / 26.5
DCV 200m / 63.7
DCV 200 / 1.2
DCV 20 / 0.02
DCA 200µ / 15.6

Reference Information for Chemical Cells and Batteries:

In order to make sense of the data you will collect later, it is helpful to be able to compare the electrical characteristics of the test PV cell/module with an electrical source that you are familiar with such as common cells and batteries.

1. With the test leads plugged into the Com (black lead) and V/Ω (red) jacks, and the range/function switch set to the 20V DC position, measure and record the voltage produced by each cell/battery.

2. Move the red test lead to the “A” jack, and set the range/function switch to the 10A position. Measure and record the current produced by each cell/battery.

AAA CellVoltage = ______Current = ______

AA CellVoltage = ______Current = ______

C CellVoltage = ______Current = ______

D CellVoltage = ______Current = ______

9 volt batteryVoltage = ______Current = ______

Question: What pattern do you see with this information? How might this be explained?

PV Cell Electrical Characteristics

I. PV Cell/Module Physical Design

Sketch a diagram showing the individual cells (front) of the module and the terminal connections:

1 2 3 4 5 6

How many cells are connected to terminals 1 and 2? ____ Terminals 3 and 4? _____ Terminals 5 and 6? ______Note that the silver “lines” are electrical conductors connecting cells to the terminals.

II. PV Cell Electrical Data

Using a voltmeter set for DC volts on the 20 volt range, measure and record the voltage in the table:

Terminal Pair / Voltage (volts) / Current (milliamperes)
1 and 2
3 and 4
5 and 6
1 and 4
1 and 6

Using the info from the physical sketch of the module and the voltage data, what is the (approx.) voltage produced by each cell? ______

Using an ammeter set for DC current, and on the 200 mA range, measure and record the current at the indicated (data table) module terminals.

Question: Based on both the physical arrangement of the PV cells, their wiring connections and the data recorded in the table, what conclusions can you make about voltage and current produced by this PV module?

Now that you have had practice using the digital multimeter (DMM), please respond to the following question (circle one):

How comfortable are you with the use of a digital multimeter for measuring the voltage of cells and batteries?
not at all somewhat very
012345

III. PV Cell Electrical Connections (Extension Activity)

Question: What connections need to be made to connect PV cells in series? In parallel? Draw a simple diagram to illustrate both of these connections using three cells.

Using the “rules” that voltages add when individual cells are connected in series, (current remaining equal to that of a single cell), and that currents add when cells are connected in parallel (voltage remaining equal to that of a single cell) predict and test the following combinations. Note that each group consists of 3 cells connected internally.

Groups of 3 Cells / Predicted Voltage
For Series Connection / Measured Voltages
For Series Connection / Predicted
Current For Parallel Connection / Measured
Current For Parallel Connection
1 / (single group) / (single group) / (single group) / (single group)
2
3

IV. Calculating Meaningful Power (Extension Activity)

Electrical power is found by using the relationship: P (watts) = V (volts) x I (amperes). An ammeter acts very much like a short circuit when making measurements. Connecting an ammeter across a cell/battery produces a zero voltage output during this test. A voltmeter has infinite resistance. During a voltmeter test, essentially no current flows from the cell/battery. Just connecting a voltmeter and ammeter to a cell/battery does not provide meaningful data for calculating power. In this demo/activity, a simple circuit, consisting of a PV module and resistor “load” will be used to obtain V and I data, for calculating power.

Data: V (volts) = ______I (amperes) = ______P (watts) = ______

Question: Each cell in this module measures 7 mm x 62 mm. How much power would a similar module with a surface area of 1 m2 produce under the same light-input conditions?

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