There are three necessary elements in passive solar design: a collector, thermal mass and some sort of control system. Thermal mass is a body of material with a significant density, such as a concrete floor, brick wall or barrel of water. Sunlight strikes the thermal mass after entering through the collector — a window. The efficiency of a passive solar system is greater if the thermal mass has a dark surface so it absorbs more sunlight. A simple control system might be a window overhang or awning, blinds or curtains.

In direct gain systems, sunlight enters through a window and directly strikes the building's thermal mass. A direct gain system with a winter sun angle is shown in Figure 3.The solar energy is absorbed by a masonry floor and/or interior walls in the building, thus increasing their temperature, which stores the energy. At night, the energy stored in the thermal mass is released because of the temperature difference between the thermal mass and the surroundings, warming the building interior. If ventilation and overhangs or other shading devices are used, this thermal mass can cool the building in summer. Ventilating a building at night exposes the thermal mass to cooler air and the thermal mass loses energy to the air. This cools the thermal mass. During the day, the thermal mass absorbs energy from the surroundings, cooling the building.

Figure 3. Key components of a direct gain passive solar system. click for copyright

In indirect gain systems, such as the Trombe wall shown in Figure 4, sunlight strikes a dark wall that is placed between sun-facing (south in the northern hemisphere) windows and the building interior. The wall absorbs solar energy through radiation, stores it, and then releases it into the building when the indoor temperature falls below that of the wall's surface. These systems cannot be used for cooling. Trombe walls must be shaded in summer to avoid overheating the building.

Figure 4: Trombe wall in an indirect-gain passive solar system. click for copyright

In insulated gain systems (sunspaces), a separate room with a thermal mass floor or inside walls is designed to store thermal energy. Heat is distributed throughout the building through ceiling vents, floor vents, windows and doors. Sunspaces cannot be used in summer for cooling, except for improving ventilation. It is best to minimize or eliminate ceiling glazing in sunspaces because it causes overheating in summer.

For an in-depth discussion of thermal energy, heat transfer, and why some materials can store thermal energy better than others, see Energy unit, Lesson 6: How Hot Is It? and How Much Heat Will It Hold? activity.

• A few days in advance, gather boxes, cans and jars from the students.
• Divide the class into teams of four students each.
• A day or two before the activity, set aside ~30 minutes for teams to paint the outside of their cardboard boxes black. Spread out newspaper to protect table tops. Use masking tape and markers to label the boxes with their names (or team names).
• If using jars to measure insulation materials, have students brainstorm how to modify a jar to measure specific volumes. (Answer: Pour a known volume of water into a jar and mark the line to which it fills.) For this activity, use tape or markers to indicate a 1 cup (~250 ml) level on each measuring jar.
• Gather remaining materials and make copies of the Capturing the Sun's Warmth Worksheet.
• Set up insulation material stations for sand, salt, water and shredded paper, each with measuring cups or jars.

1. Have each team use masking tape and markers to label their four cans: sand, salt, water, shredded paper.
2. On their worksheets, have students predict which material they think will store solar thermal (heat) energy the best.
3. Have each team member place 1 cup (~250 ml) of test material in its labeled can and place a thermometer in the center. When filling a can with sand or salt, pour in half of the material, add the thermometer, and then pour in the rest of the material.
4. On their worksheets, have students record the initial temperature of each material.
5. Place the cans inside the box and close the lid. If necessary, tape the lid closed.
6. Move the cans and boxes outside to a sunny location.
7. Use an extra thermometer to record the nearby outside air temperature, too, as a control.
8. Leave the boxes for 30 minutes.
9. Have one or two students take spare thermometers outside to measure and record the outside air temperature (as a control), and share the data with all teams.
10. Have teams take turns opening their boxes. Have students record on their worksheets the temperature of the materials in each can. Then, close the boxes and leave them for another 30 minutes. Repeat this step one to three times. Use the back of the worksheet to record any data that does not fit in the data table.
11. Put on potholders or gloves. Carefully stir the contents of each can occasionally and watch to see which temperature falls the slowest and which falls the fastest. Record this on the worksheet.
12. Have teams complete the worksheet by graphing their results.
13. Conclude with a class discussion to compare results. What can we conclude from our data? What do the graphs show us? Which material stores thermal energy from the sun best? Who predicted this correctly? How did the material temperatures compare to the outside air temperature at each measurement time? Which test materials lost heat the fastest? The slowest? See additional post-activity discussion questions / answers in the Assessment section.