People around the world are using solar ovens to reduce reliance on firewood and coal. Even if you have electricity, a solar oven can be an effective, energy-saving way to cook your food. If you select this project, you are going to make a lightweight solar oven out of a cardboard box, which would be a portable oven for camping or your backyard.

There are numerous designs of a solar oven. You could check some designs here. You could also design your own solar oven.

The procedure of making a solar oven depends on the design you choose. Here, we provide an example.

• Cardboard boxes of different sizes
• Aluminum Foil Roll
• Flat Black Paint or Black Construction Paper
• Cotton Balls (used for insulation)
• Duct Tape/Glue
• Sunglasses (to protect your eyes)

You're free to choose the materials for your design. The above are suggestions.

• Use a thermocouple to measure temperature change and calculate the power
• Estimate of the solar power input using solar resource data
• Calculate thermal efficiency
• See how well you're able to maintain an elevated temperature in the oven; how well does it work overnight?

Heat is a form of energy that passes from a body at high temperature to a body at low temperature. Heat transfer studies the generation, use, conversion and exchange of the heat between physical systems. Three types of heat transfer:

• Conduction: Heat is transferred from one material to the other by direct contact
• Convection: Heat is transferred through a fluid caused by molecular motions
• Radiation: Energy is radiated or transmitted in the form of electromagnetic waves

Thermal conduction is the transfer of internal energy by microscopic collisions of particles and movement of electrons within a body. The colliding particles, which include molecules, atoms and electrons, transfer disorganized microscopic kinetic and potential energy, jointly known as internal energy. Conduction takes place in all phases: solid, liquid, and gas. The rate at which energy is conducted as the heat between two bodies depends on the temperature difference (and hence temperature gradient) between the two bodies and the properties of the conductive interface through which the heat is transferred.

The law of heat conduction, also known as Fourier's law, states that the rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows.

The thermal conductivity, k, is often treated as a constant, though this is not always true. While the thermal conductivity of a material generally varies with temperature, the variation can be small over a significant range of temperatures for some common materials.

Convection is the transfer of heat due to the bulk movement of molecules within fluids (gases and liquids). Convection includes sub-mechanisms of advection (directional bulk-flow transfer of heat), and diffusion (non-directional transfer of energy or mass particles along a concentration gradient). Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but that is called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other.

There are two major types of convection, namely

• Natural convection: fluid motion is caused by buoyancy (density), for example, a hot air balloon
• Forced convection: fluid motion is forced by external force, for example, by fans or by stirring

The basic relationship for heat transfer by convection is described by Newton's law of cooling:

Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. Unlike conduction or convection, radiation does not require the presence of a medium to take place. It can occur in the vacuum, such as solar radiation.

For the radiation to turn into heat, it must strike on a surface. So it is important to study the properties of surfaces. There are three important parameters of a surface, namely absorptivity(α), reflectivity(ρ) and transmissivity(τ). These three parameters represent fraction of incident radiation being absorbed, reflected and transmitted respectively. From energy conservation, it is straightforward to know that α + ρ + τ = 1. There are three special cases: (1) α = 1, called absolute black body, (2) ρ = 1, called absolute white body, and (3)τ = 1, called absolute transparent body. Among these three, the black body is more important in theoretical study of thermal radiation.

A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs radiation in all frequencies, not because it only absorbs: a black body can emit black-body radiation. A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone, not by the body's shape or composition. An ideal black body in thermal equilibrium has two notable properties

• It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.
• It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.

An approximate realization of a black surface is a hole in the wall of a large insulated enclosure (an oven, for example). Any light entering the hole is reflected or absorbed at the internal surfaces of the body and is unlikely to re-emerge, making the hole a nearly perfect absorber. When the radiation confined in such an enclosure is in thermal equilibrium, the radiation emitted from the hole will be as great as from any body at that equilibrium temperature.

The radiation energy emitted by a black body is described by Stefan-Boltzmann law (total emissive power) and Planck's law (spectral distribution of emissive power).

Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. The emissivity is a function of surface properties and direction of radiation, which makes it difficult in the real applications. Assumptions are made, such as diffusive emitter (radiation is uniform in all directions) and grey body (independent of frequency), to simplify real problems. The emissivity is only a function of temperature, and is constant in all directions and all wavelengths.