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## Maximum Power Point

The Principles of Optimizing Photovoltaic Cell Power Output

### Overview

In this lesson, students will investigate how to optimize the power output of a photovoltaic cell using a home-made gnomon stand. Data collected from students’ investigations will be used to create current-voltage and power-voltage curves to determine the “maximum power point ” (MPP) at which their PV cell operates.

### Content Objectives

Students will know that

• Photovoltaic (PV) systems have unique advantages over conventional power-generating technologies. PV systems can be designed for a variety of applications and operational requirements.
• The high cost of PV modules and equipment (as compared to conventional energy sources) is the primary limiting factor for the technology.
• Voltage is the measure of the force by which an electron is pushed through a circuit; its unit is the volt (V). Current (I) is defined as the flow of electrons, and its unit is the ampere (A). Power is the product of voltage times current; its unit is the watt (W).
• Ohm’s Law describes the relationship between current, voltage and resistance.
• The angle at which a solar cell is positioned in relation to the sun affects its power output.
• The amount of current produced by a PV cell is proportional to the amount of the light hitting the cell; therefore, increasing light intensity or increasing the size of the cell itself will increase the power output of the cell.
• In order to construct a solar-powered system that will work at maximum efficiency, numerous factors pertaining to the design must be considered.

### Process Objectives

Students will be able to

• Identify the main parts of a PV system.
• Wire a solar cell, resistor, and multimeter in series.
• Determine the optimal conditions for operating a PV panel in a circuit with known resistance.
• Graphically illustrate the relationship between power and voltage to describe a PV cell’s “maximum power point” (MPP).

### Assessment Strategies

• Evaluation of the completed student handouts and the student’s participation in class discussions.
• Observation of student’s participation and performance during an inquiry-based activity.

### Suggested Time

Two to three (2-3) 50 minute class periods.

### Materials

Part 1:

• Teacher computer with Internet access
• Student computers with Internet access–small groups
• Projection equipment
• Video Clips (see multimedia resources below)

Part 2: Per group

• Maximum Power Point Student Handout PDF Document

• Photovoltaic panel
• 2 Multimeters (or an ammeter and a voltmeter) (capable of reading at least 200mA of current)
• 2 insulated wires with alligator clips
• Several resistors of varying capacity (different for each group, if possible)
• 1 “gnomon stand” (Construction directions are included in the PDF below.)

Maximum Power Point Teacher's Notes PDF Document

• Compass
• Computer
• Microsoft Excel or comparable graphing software (optional)

Notes on Materials

Most materials should be available in a physics lab. However, RadioShack and other electronics stores will carry inexpensive multimeters and circuitbuilding equipment. A great compendium of suppliers for various science materials can be found at the following website: http://www.exploratorium.edu/snacks/snacksupplies.html.

A great resource for solar panels and solar car kits is Sun Wind.

### Part I: Investigate solar energy and how a PV cell works (30 minutes)

1. Introduce the investigation of solar power with a discussion of the major source of energy in the system—the sun. The supplemental video segment below may be used to introduce the array of solar applications in Pennsylvania.

PA Energy's Energy from the Sun QuickTime Video

Optional: The following series of movies filmed at Penn State’s Center for Sustainability may be used to highlight components of the photovoltaic system including the inverter and two different types of solar applications:

Power Inverter QuickTime Video

Solar Water Heater QuickTime Video

Passive Solar QuickTime Video

2. Allow students to work in small groups to manipulate thePV system animationand complete Part 1 of the student handout below.

Maximum Power Point Student Handout PDF Document

To take a look at the micro-scale of what happens in a solar cell, allow students to access and view the short video below from individual computers.

Photovoltaics (pv4) QuickTime Video

### Part II: Build and test a PV system (1 50 minute Class Period)

3. Distribute “Part II” materials to pairs or trios of students.

4. You may wish to demonstrate how to wire the solar panel, resistor, and multimeter in series (described in the PDF below) before taking your class outside. Utilizing two multimeters perfects the circuit science in this lesson. Realizing that access to such materials is limiting, the multimeter can be wired parallel to the solar cell and one lead moved to the resistor to proceed with the experiment.

Maximum Power Point Student Handout PDF Document

5. Circulate among student groups as they work together to wire their solar panel circuits and turn on the multimeters. [They should obtain a current reading of about 120-150mA. If not, make sure that they have their multimeters set to the 200mA, 20VDC setting.] Students should obtain a gnomon stand and attach their solar panel to the stand with duct tape.

6. Students should now head outside and follow the instructions for Part 2 in the student handout. In this section, students should experiment with the PV cell to investigate solar orientation and angle of incidence. Once they determine the angle producing the highest power output, they should investigate how varying the direction at which the panel is facing affects power.

Maximum Power Point Student Handout PDF Document

7. Once students have completed the outdoor portion, return to the classroom to discuss what conditions optimized the power output of the PV cell in students’ explorations and allow students to complete questions b-g of the student handout.

### Part III: Construct current-voltage and power curves (1 50 minute Class Period)

8. Create Table 2 (in Part 3 of the PDF below) by compiling each group’s data for the resistance, angle of incidence, current, and voltage that produced the most power in Part 2.

Maximum Power Point Student Handout PDF Document

9. Allow student groups to work through Part 3 of the student handout to calculate the power output for each resistance and create an I-V Curve and Power Curve for their solar panels by hand or using spreadsheet software like Microsoft Excel.

10. Lead students through question 6 in the student handout to analyze their graphs to determine the maximum power point for their solar panels.

11. Discuss the importance of operating at Maximum Power Point (MPP) to be able to get the most power out of the solar panel. A good example discussed briefly in the teacher notes is the use of PV panels in transportation.

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