In this lesson, students will first discuss where energy comes from, including sources such as fossil fuels, nuclear, and such renewable technologies as solar. After this initial exploration, students will investigate the three main types of heat transfer: convection, conduction, and radiation. Students will learn how properties describe the ways different materials behave, for instance whether they are insulators or conductors. Students will complete a crossword puzzle to reinforce their vocabulary in this content area. The class will then focus on the acquisition and storage of energy through the design, construction, and testing of a fully functional solar oven.

The design, construction, and testing of a solar oven is an engineering project combining materials science in the selection of materials with mechanical engineering in the use of heat transfer. energy solar energy solar oven heat transfer Explain the three types of heat transfer: conduction, convection and radiation. Identify materials that are good insulators or conductors of heat. To introduce this lesson, the teacher has a number of options. One option is to bring in a completed solar oven and ask the students how hot they think it can get. Let them discuss this and then challenge them to build ovens that will reach a certain temperature (200F is a reasonable challenge). If the teacher were to bring in the oven as well as something that had been made in it, for instance cookies, the ability to harness the sun's energy to cook food may be even more intriguing to the students. An alternate approach would be to simply have a discussion of the different energy sources available, and to explain that solar energy is the cheapest way to harness heat energy. It is also one of the few renewable resources available. The teacher could alternatively bring in a video or obtain statistics illustrating the actual use of solar ovens and solar energy throughout the world. Solar ovens are currently used in a number of developing nations. This lesson and activity are similar to a solar oven design project at Brigham Young University where solar ovens were designed for developing countries. More information on similar projects may be found at the Solar Cooking Archive. Introduce students to basic heat transfer concepts by asking them about situations in which they experience heat transfer in their daily lives. Why is the handle of a pot on a stove hot? Why does hot chocolate cool down when you blow on it? Why is asphalt pavement much hotter on a sunny day than a concrete sidewalk? Why do you wear a jacket in the winter? The teacher will introduce the idea of using these same concepts to heat an oven to about 200 degrees Fahrenheit to cook food. Students will learn that using heat from the sun is an inexpensive way to create heat without impacting the environment significantly. Students will look at pictures of various solar ovens and brainstorm how the heat transfer concepts enable them to work. They will evaluate the differences between some of the different designs and speculate about which ones might collect and store the most heat. The lesson covers the fundamental mechanisms of heat transfer: conduction, convection, and radiation. The specifics of each type of energy transfer can be seen below in the vocabulary/definitions section. The teacher should understand the differences between conduction, convection, and radiation. For instance, only via radiation can energy travel through a vacuum. The teacher should also understand the basic properties of a material that will affect its ability to collect energy from the sun, specifically a material's emissivity. A material with a high emissivity will absorb the majority of the sun's rays, for instance a black shirt, while a substance with a low emissivity will reflect the majority of the sun's rays, such as a white shirt. The teacher should also understand the concepts of heat flow. Heat will flow from an area of high temperature to an area of low temperature; therefore, insulators are needed to store heat. Specific examples of each of the above concepts can be found in the vocabulary list provided with this lesson. Conduction is heat flow from one part of a solid object to another or between two objects in contact with one another. Perhaps the best way to understand this is through a discussion of molecular motion. As an object is heated, its temperature increases. By definition, temperature is the average amount of kinetic energy in a system. As the kinetic energy of the system increases, the random motion of the molecules comprising that system begin colliding more with each other as well as any objects that the system is in direct contact with. By molecules of the system colliding with an adjacent object, the system can transfer its heat and impart more kinetic energy onto the object, thus increasing its temperature as well. (Examples: touching a hot or cold object, the handle of a soup spoon being hot because heat is conducted through it from the heated liquid). Convection is heat flow through fluid or gas movement or through a fluid or gas moving past an object. In conduction, heat flows through the material. In convection, heat flows as hot air or water flows away from a hot object. (Example: By blowing on soup, effectively the air above the soup is being swept by at an increased rate. This increase in air flow causes a decrease in the temperature of the air directly above the soup thus allowing the warm liquid to dissipate some of its heat). Convection can also be thought of in terms of the transfer of energy due to density differences. If a gas or liquid expands and becomes less dense, it therefore becomes lighter as well. Lighter materials will rise while heavier materials will fall. If a cooler material is above a warm material, the warmer material will raise though the cooler material to the surface dissipating its energy (heat) to the surrounding environment. Through this process, heat is transferred from an area of high heat to one of low heat as would be expected. Radiative heat transfer is heat energy transferred through the movement of electromagnetic waves. Heat transfer via radiation is dependent on the temperature as well as the emissivity of the objects that are heating or cooling. Emissivity is a material property that is related to the color of the object. Radiation is the only type of energy transfer that does not require a medium to flow through. Radiation transfers energy via photons that impart their kinetic energy into a system when they strike its surface. This transfer of kinetic energy thus heats whatever substance the radiation is striking. Keep in mind that radiation can be either in the visible or invisible regions, with shorter wavelengths carrying more energy. All energy from the sun initially strikes the Earth as solar radiation. In an engineering and scientific sense, power and energy are not the same. Energy is the ability to do some sort of work and is measured in Joules (J) in the SI system of measurement. This work can be moving something or it can be heating something. Power is the rate at which energy is used to do work or in this case transferred in the form of heat. Power is measured in Watts (W) which are defined as 1 Joule per second (J/s). The flow of heat is generally measured in units of power because it is the rate at which energy is transferred from one place to another. These concepts of energy and power can be difficult for students to grasp. For the purpose of this lesson, it is not crucial that students come away with an understanding of the difference between power and energy, but it is important that the teacher presents these concepts accurately. Heat flow due to the direct contact of two objects or within a solid object Heat flow due to fluid movement such as the movement of air or water Energy transferred through the movement of electromagnetic waves; heat transfer not requiring a medium The property of a material that determines how well it conducts/transmits heat (Examples: metal generally has a high thermal conductivity, plastic generally has a low thermal conductivity) A material that does not conduct heat very well and has a low thermal conductivity (Examples: a good jacket, fiberglass insulation, a sleeping bag, anything with air trapped in it) The metric (SI) unit for power. (1 horsepower (hp) = 746 Watts (W)) Property of the surface of an object that determines how much electromagnetic energy is reflected and how much is absorbed by an object in the form of heat. Emissivity is very dependent on color. (Examples: Aluminum foil has a low emissivity because it reflects a majority of heat and a black surface has a high emissivity because it absorbs a lot of heat; a black shirt gets hotter than a white one in the sun) The amount of power received on the earth's surface per unit area (Watts per square meter in the SI unit system) A resource that is inexhaustible or replaceable by new growth; limitless supply (Example: Solar energy) A resource that is not replaceable after it has been used (Examples: fossil fuels such as oil or natural gas, iron ore) Cooking with the Sun: Creating a Solar Oven Discuss other ways that solar energy and heat transfer concepts can be used in our daily lives to solve practical problems. One recommended way to prompt this is by talking about the concept of a solar house. North Carolina State University features a solar house and provides a great deal of information at http://www.ncsc.ncsu.edu/solar_house/NCSU_solar_house.cfm. Ask students to evaluate the advantages and disadvantages of using renewable resources. Class discussion to assess students' ability to accurately discuss the covered material. Ensure that the students can adequately describe the three basic forms of heat transfer (conduction, convection, and radiation). The provided crossword puzzles can serve as a quiz to determine how well students understand the vocabulary. Presentations of solar ovens to the class, in which each student describes how it works and why they made certain material/design choices, such as what type of reflectors and insulation were used. The Solar Cooking Archive This website has lots of good information on solar cooking and building solar ovens. There are recipes, places solar ovens are used, and plans to build solar ovens. Solar Cooking International, "The Solar Cooking Archive," <www.solarcooking.org>, 6/2/2004 The Solar Cooking Archive Making and Using a Solar Cooker Radabaugh, Joe. "Making and Using a Solar Cooker." Backwoods Home Magazine. Iss. 30, 1998. <http://www.backwoodshome.com/articles/radabaugh30.html>, Accessed 6/2/2004. Making and Using a Solar Cooker Time Schedule for Lesson Crossword Puzzle #1 Crossword Puzzle #2