Photovoltaic Cells

Commonly called solar or PV cells, photovoltaic cells have powered satellites for decades. Solar cells are also located on calculators, road signs and outdoor lights. Currently, photovoltaic cells are made of certain materials, called semiconductors, of which silicon is the most common. Whenever light hits a PV cell, the cell absorbs some of the light's energy. This energy can knock electrons loose from the atoms of the semiconductor material. An electrical device on the PV cell forces these loose electrons to move in a particular direction, thus creating an electric current. Metal contacts at the top and bottom of a photovoltaic cell — like the terminals on a battery — connect the PV cell to an electric circuit. This circuit may be the electrical system of a building or a single device (like a solar calculator).

Solar electric systems for homes and businesses can be stand-alone systems (utility independent) or linked to the utility grid (called grid-tie or utility-interface systems). In stand-alone systems, the consumer produces all their own electricity with PV or a combination of PV other sources. Most stand-alone systems require batteries to store electricity for use during the night and cloudy periods. In grid-tie systems, the consumer uses electricity produced by their solar panels and can sell excess to or buy extra electricity from the utility company. Grid-tie systems may or may not have battery storage components.

The design, production, and installation of photovoltaic systems require many different types of engineers. Materials engineers, electrical engineers and physicists develop the materials and circuitry of the photovoltaic cells. Mechanical engineers, materials engineers and manufacturing engineers design the systems that produce PV cells. Electrical engineers and civil engineers design photovoltaic systems for homes and businesses. Engineers and scientists are working to make solar electricity feasible for everyone. One promising development is making PV cells from thin films rather than single silicon crystals. This technique may make photovoltaic systems more affordable.

Solar Concentrators

Engineers are also working on several other technologies (besides PV cells) to generate electricity from sunlight. One of the most efficient methods appears to be types of power plants that produce electricity by concentrating and converting the Sun's energy into high-temperature heat using various mirror configurations. The heat is then channeled through a conventional generator.

The most common type of solar concentrator is a power-tower system. The Sun's energy is concentrated by a field of hundreds or even thousands of mirrors (called heliostats) onto a receiver located on top of a tower. This energy heats molten salt flowing through the receiver, and the salt's thermal energy is then used to generate electricity in a conventional steam generator. Engineers use molten salt because it retains thermal energy efficiently, and it can be stored for hours or even days until it is needed to generate electricity. This is important since we use electricity even when we do not receive sunlight (at night or during overcast days).

A second kind of solar concentrator being developed by engineers is a trough system. This technology concentrates the Sun's energy with parabolic-curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator.

Solar Water Heaters

A solar water heater works by exposing a fluid to the Sun, which warms up and then harvests that energy in a collector. If a passive solar water heater is used, water that is heated directly by the Sun is stored for use as needed and then natural convection circulates the water.

Figure 2. Solar water heaters. click for copyright

Unfortunately, solar water heaters cannot always meet the hot water demand of a home. Conventional water heaters can provide additional heating of the stored water. Typical solar water heaters reduce the need for conventional water heating by about two-thirds. Today's solar water-heating systems can provide 40%-80% of a typical household's hot water demand, depending on the local climate, system size and type. Water heating systems that use a solar water heater to provide most of the heating with a gas water heater used to supply any additional heating are the most efficient and reliable.

• A few days before the activity, collect boxes and cans from students. The boxes should be large enough to accommodate four cans with thermometers.
• The day before the lesson, have each team paint the outside of a cardboard box black.
• Discuss the ideas of solar heating/cooling as a class. Ask students what they know about solar power and how it works.
• Set up stations for each filling material: sand, salt, water, shredded paper, measuring cups or jars.
• Make copies of the Solar Power Data Sheet.

1. Divide students into groups of 4.
2. Distribute four cans, five thermometers, and one pre-painted box to each team of students. Have teams put identifying tags or stickers on their box.
3. Ask students to rip off 8 3-4" pieces of masking tape. On four pieces of the tape, have students write their team name. Put one label on each of the cans. On the remaining four pieces of tape, groups should write the contents of the can (sand, salt, water, shredded paper, etc.). Put one piece of tape on each of the four cans.
4. Have each team member put one cup of filling in the appropriately-labeled can and carefully place a thermometer in the can. (Note: When filling a container with sand or salt, fill the container half way, add the thermometer, and then put in the rest of the filling.)
5. Have students record the initial temperature of each filling on their Solar Power Data Sheet.
6. Ask students to place all their containers inside their box, but leave the top off for now.
7. Next, if using shoe boxes, it may be necessary to cut a small hole for each thermometer in the top: have students mark the spot for each thermometer on the lid of the box and cut out as small of hole as possible for each thermometer. (Note: you may have to help kids with this task, as thermometers can easily break.)
8. Take the containers and boxes outside to a sunny location.
9. Have students carefully place the lid on their boxes. (Note: If it was necessary to cut holes in the box lid, be sure to remind students to thread their thermometers through the small holes that they previously created.) Use a piece of tape to secure the lid, if necessary.
10. As a class, measure and record the outside temperature. Have students record the temperature on their Solar Power Data Sheet.
11. Leave the boxes undisturbed for 30 minutes.
12. Take spare thermometers outside. Have teams take turns opening their boxes. Each team should take the temperature of the air inside the box (using the spare thermometer) and record the temperature of the contents in each can on their Solar Power Data Sheet. Remind them to replace the box lid when they are finished.
13. Leave the boxes undisturbed for another 30 minutes. Repeat the previous step at least two more times.
14. Have each student construct a graph showing how the temperature in the box and the temperature of each material changed with time.

Clean-up Reminder: the cans, newspaper, and shredded paper can be recycled. The salt and sand can be collected and reused. Any newspaper with paint on it should be thrown away.

Discussion Question: Discuss the ideas of solar heating/cooling as a class. Ask students what they know about solar power and how it works.

Graphing: Have each student construct a graph showing how the temperature in the box and the temperature of each material changed over time.

Roundtable: Have students work in their teams. Ask the class a question with several possible answers. Students on a team make a list, each one writing an answer and passing the paper on to the next person. Have the teams share their responses with the class.

• Have students discuss if any of the materials would be suitable as a thermal mass? Ask for details and supporting evidence from the activity. (Answer: Materials that are slow to heat up and cool down provide thermal mass because they store more energy per degree of temperature change.)
• Have students make a recommendation for which type of insulating material they would like to have as the insulation for their passive solar-heated home. Ask for details and supporting evidence from the activity. Let them know that this is one job of engineers who work with solar heating. (Answer: Insulating materials, such as shredded paper, can also be used in passive solar design because they slow the rate of heat transfer in and out of buildings through the walls, floors, and roof.)
• Do they think all materials would work equally well at keeping a home insulated for cooling purposes? (Answer: Students should answer no, based on their investigation.)
• Discuss why the boxes were painted black. How does color affect solar energy absorbed? (Answer: Darker colors absorb heat.) How could this affect what color clothing they might choose to wear in the summer and winter? (Answer: You may choose to wear light-colored or white clothing on hot days and dark colors or cold days.) How can you apply this information to the idea of solar energy collection by engineers? (Answer: Extend this concept to living in warm or cold areas.)
• Where else do you think engineers could design solar heating systems besides houses? (Answer: Schools, pools, cars, almost any where that requires heating.) Which of these would better for passive solar heating and which would be better for active solar heating?
• Engineers also can convert sunlight into electricity using solar photovoltaic arrays and concentrating mirrors. Where do you think this would be useful? (Answer: Anyplace that uses electricity or stores electricity.)