The Physics of Heat Movement

Your home's windows, walls, and floors can be designed to collect, store, and distribute solar energy in the form of heat in the and reject solar heat in the summer. This is called passive solar or climatic design because, unlike active solar heating systems, it doesn't involve the use of mechanical and electrical devices, such as pumps, fans or electrical controls to move the solar heat.

To understand how passive solar design works, you first need to understand how heat moves.

Heat Movement Physics

As a fundamental law, heat moves from warmer materials to cooler ones until
there is no longer a difference between the two. A passive solar makes use of this law through three heat movement mechanisms—conduction, convection, and radiation—to distribute heat throughout the living space.

Conduction is the way heat moves through materials, traveling from molecule to molecule. Heat causes molecules close to the heat source to vibrate vigorously, and these vibrations spread to neighboring molecules, thus transferring heat energy. For example, a spoon placed into a hot cup of coffee conducts heat through its handle and into the hand that grasps it.

Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior.

Radiant heat moves through the air from warmer objects to cooler ones. There are two types of radiation important to passive solar design: solar radiation and infrared radiation. When radiation strikes an object, it is absorbed, reflected, or transmitted, depending on certain properties of that object.

Opaque objects absorb 40 to 95 percent of incoming solar radiation from the sun, depending on their color—darker colors typically absorb a greater percentage than lighter colors. This is why solar absorber surfaces tend to be dark colored. Bright white materials or objects reflect 80 to 98 percent of incoming solar energy.

Inside a home, infrared radiation occurs when warmed surfaces radiate heat towards cooler surfaces. For example, your body can radiate infrared heat to a cold surface, possibly causing you discomfort. These surfaces can include walls, windows, or ceilings in the home.
Clear glass transmits 80 to 90 percent of solar radiation, absorbing or reflecting only 10 to 20 percent. After solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation. Although glass allows solar radiation to pass through, it absorbs the infrared radiation. The glass then radiates part of that heat back to the home's interior. In this way, glass traps solar heat entering the home.

The term thermal capacitance refers to the ability of materials to store heat, and thermal mass refers to the materials that store heat. Thermal mass stores heat by changing its temperature. This can be from a warm room or by converting direct solar radiation into heat. The more thermal mass, the more heat can be stored for each degree rise in temperature. Masonry materials, like concrete, stones, brick, and tile, are commonly used as thermal mass in passive solar homes. also has been successfully used.