Design an investigation

Tilting solar panels

Units 4 and 6
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

A high school at 40° N latitude plans to install solar panels on its flat roof. Before the panels are mounted, students tested how the tilt angle of a panel affects how much electricity it produces.

Figure 1: Diagram of the test setup

The diagram shows five identical small photovoltaic (PV) panels on adjustable stands on the roof, all facing due south. A protractor on each stand shows its tilt from horizontal: 0° (lying flat), 20°, 40°, 60° and 80° (nearly vertical).

Each panel is connected to a meter that records the total electricity it produces from sunrise to sunset. The students ran the test on a clear day in December and again on a clear day in June. No panel was shaded on either day.

Source: Hypothetical study design

Table 1: Electricity produced in one clear day (watt-hours)

Tilt angle	December	June
0°	95	280
20°	150	290
40°	185	270
60°	200	225
80°	190	130

Source: Hypothetical data

Suggested time: 23 minutes

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Part (a)

1 point

Identify the independent variable in this investigation.

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Part (b)

1 point

Identify the dependent variable in this investigation.

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Part (c)

1 point

Identify one variable, other than the direction the panels face, that the students kept the same for all five panels.

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Part (d)

1 point

Using Table 1, identify the tilt angle that produced the most electricity in December and the angle that produced the most in June.

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Part (e)

1 point

Explain why the best tilt angle is steeper in December than in June.

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Part (f)

1 point

Explain why the panels tilted at 0° to 60° all produced less electricity in December than in June.

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Part (g)

1 point

Explain why the students pointed all the panels toward the south.

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Part (h)

1 point

Describe one environmental advantage of generating the school's electricity with PV panels instead of electricity from a coal-fired power plant.

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Part (i)

1 point

The school is also designing a new classroom wing. Describe one passive solar design feature that would reduce the energy needed to heat the wing in winter.

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Part (j)

1 point

The school will mount all of its panels at one fixed angle for the whole year. Using Table 1, explain why 40° is a reasonable choice.

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