Focus questions

How can the sun be used to heat water?

Essential materials

• Flat box 20.3 cm by 30.5 cm by 1.9 cm (8 inches by 12 inches by three-fourths inch) with fastener straps attached and two 6 mm (one-fourth inch) holes punched on opposite sides
• Aluminum foil 15 cm by 27 cm (6 inches by 11 inches)
• One five-meter (16.5 feet) length of tubing 6mm (one-fourth inch) outside diameter
• Plexiglas sheet 20.3 cm by 30.5 cm (8 inches by 12 inches)
• Two one-meter lengths of tubing 10 mm (three-eighths inch) outside diameter
• Collapsible funnel 
• Plastic beaker 400 ml (13 oz)
• Two bulb thermometers
• Two spring-loaded clothespins
• Transparent tape
• Solar Energy SEEDPACK component box
• Submersible water pump
• Coaxial adapter
• Coaxial DC power jack
• Solar cell backpack with two attached solar cells connected to junction box and Power Out cable
• One pair of alligator clip leads

Optional equipment for automated solar tracker:

• GoGo board
• Serial cable
• Two temperature sensors
• Connecting posts
• Alligator clips
• GoGo Monitor software (available as a free download)

Main ideas and background information

• Thermal (or internal) energy is the sum total of the energy of all molecules in an object.
• The thermal energy for a body of air or other gas is due to the molecular kinetic energy (energy of the molecules’ motion).
• The thermal energy of a liquid or solid object consists of intermolecular potential energy (energydue to forces between atoms or molecules) as well as molecular kinetic energy.
• In general, the larger an object (i.e., the more molecules it contains), the more thermal energy it has. This is because each molecule contributes its energy to the sum total.
• Heat is the thermal energy that is transferred from one object to another because of a difference in temperature. For this reason, we say that heat is the energy that flows from an object of a higher temperature to one of a lower temperature.
• Temperature is a measure of the average kinetic energy of the individual molecules that make up an object. If two objects have the same temperature, then the average molecular kinetic energy is the same.
• In this activity, the radiant energy from the sun goes through the transparent pane of the solar water heater panel as visible light and is absorbed by the water in the tubing as heat. This increases the thermal energy of the water molecules, as well as the temperature of the water.
• The heated water radiates heat energy. Much of this energy is in the form of infrared radiation, which cannot go back out through the transparent panel pane. Instead, the panel pane absorbs the infrared radiation and radiates it back into the panel. This helps to keep the energy from the sun concentrated in the panel. Radiant energy in the form of visible light goes into the panel, but infrared radiation in the form of heat cannot escape. This mechanism is called the greenhouse effect.
• Another factor is the insulating quality of the panel. The air in the panel cannot move away. It has to stay where it is. If the air around the tubing was not trapped by the panel container, it could move, taking away with it heat from the water in the tubing. The result would be cooler water in the tubing.

Procedural tips

• When inserting the tubing into the holes in the side of the flax box, pinch the end of the tubing for a slightly smaller diameter. Then push the pinched end of the tubing through the hole from the inside out.
• Do not stack the tubing when coiling it inside the flat box. Create as much of a single layer of tubing as possible.
• When filling the panel tubing with water, it is not necessary to stand the panel completely upright. However, the funnel must be at a higher level than the coil of tubing. We also suggest that you position a cup or paper towel beneath the downstream end of the tubing to catch the first drops of water that come through.
• To seal off the end of the tubing, fold the tubing back on itself and clamp the folded tubing together with a spring-loaded clothespin.
• Do not worry if you see a few air bubbles in the water. As long as the tubing contains mostly water, you will have enough to heat for this activity.
• Before you begin to drain heated water out of the panel, be sure to have a beaker containing a thermometer positioned beneath the end of the downstream tubing. Water will drain out when you remove the two clothespins. You may have to blow into the tubing to get the remainder of the water out. This is easy to do. Just wrap the end of the tubing with clean tissue paper, put it up to your mouth, and blow firmly.  
• As soon as you have all of the water out of the tubing, measure the temperature. The heated water will start to cool right away.
• For the circulating solar water heating system, sometimes called an active solar water heating system, do not be concerned if the water appears to be flowing through the tubing too slowly. The main idea is for water to be moving through the tubing continuously.
• In an actual water heating system, the reservoir is insulated so that the heated water does not cool quickly. For this simulation, students might want to insulate the reservoir with Styrofoam or other material.
• For the electrical hookup with the water pump, make sure all three solar panels are connected to the Solar Panels In terminals on the small black junction box. Power from the Power Out terminal is used for the water pump. Position the solar cells so that they are facing the sun directly for the maximum electrical power.
• The extension project is actually a model of a home water heater with a thermostat. An internal thermometer in the home water heater compares the desired water temperature with the actual water temperature and turns the heating mechanism on or off accordingly.

Discussion

1. How does the sun heat the water? [Radiant energy from the sun is absorbed by the water, increasing its thermal energy and its temperature.]
2. How does the transparent pane of the panel help to heat the water? [The transparent pane allows radiant energy from the sun in the form of visible light to pass through into the panel. However, it does not allow infrared energy in the form of heat to escape.]
3. What is the purpose of the pump? [The pump is used to circulate water through the tubing in the solar water heater. This makes it possible to heat more water than can be stored in the tubing at one time. However, this water must be collected in an insulated reservoir so that it does not cool appreciably.]
4. How might this activity be useful in everyday life? [For example, students could describe how the sun heats a body of water, such as a lake or a swimming pool, over the course of several weeks. They might also discuss how the water heater with the pump is like a home water heater. Perhaps they could also describe a new application that they would invent.]

Assessment

Are students able to describe how the sun can be used to heat water? [A coil of tubing filled with water is placed in a transparent enclosed panel and exposed to the sun. The sun heats both the water and the air in the panel. The air acts as an insulator, helping to keep the heated water hot. The transparent material reflects infrared (heat) energy back into the panel. If water is pumped through the coiled tubing, a larger quantity of water can be heated.]

Extensions and further investigations

Have students repeat the experiment using

• tubing that is not transparent.
• tubing of a different size.
• tubing that is not in an enclosed container.
• different times of exposure to the sun.
• different transparency material.

Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

Architecture
Hydraulic engineering
Environmental engineering

Correlations with standards 

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B - Physical Science: B1, B3, E1-E4

Grades 9-12
Standard B - Physical Science: B5, B6, E1-E4

Glossary/vocabulary

Greenhouse effect
Heat
Intermolecular potential energy
Molecular kinetic energy
Temperature
Thermal (or internal) energy

Resource links

Solar Hot Water from Wikipedia

Basic overview of solar water heating.

Solar Water Heating from Montana Green Power

Discussion of the different types of solar water heating systems.

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