Passive Solar Energy

Straw: A Renewable Resource

Passive solar heating and cooling represents an important strategy for displacing traditional energy sources in buildings. Anyone who has sat by a sunny, south facing window on a winter day has felt the effects of passive solar energy. Passive solar techniques make use of the steady supply of solar energy by building designs that carefully balance their energy requirements with the building site and window orientation. The term “passive” indicates that no additional mechanical equipment is used, other than the normal building elements. All solar gain is brought in through windows and minimum use is made of pumps or fans to distribute heat or effect cooling. All passive techniques use building elements such as walls, windows, floors, and roofs, in addition to exterior building elements and landscaping to control heat generated by solar radiation. Solar heating designs collect and store thermal energy from direct sunlight. Passive cooling minimizes the effects of solar radiation through shading or generating air flows with convection ventilation.

Passive Solar Heating

Passive solar heating of buildings occurs when sunlight passes through a window, heats an object, is absorbed and converted into heat. The most efficient window orientation for heat gain is due South, but any orientation within 30 degrees of due South is acceptable. Once the heat has entered the building, various techniques are used to keep and distribute it. To let the sun in, a ratio of roughly 8 percent window to floor area is recommended for South walls. Although this number is small, it is important to remember it comes from the floor area, which is much larger than the wall area. This controls over heating and is a significant issue. Once the heat is in, a well insulated and air tight building envelope helps prevent heat loss and allows solar heat to provide more of the heating needed. A crucial component of the energy efficient building envelope is the window system. Common double glazed windows let heat escape. High performance windows, with insulated frames, multiple glazing, low e-coatings, insulating glass spacers, and inert gas fills can reduce heat loss by 50 to 75 percent. Also, insulated window quilts will prevent heat from radiating out at night. Passive solar can supply up to 70 percent of a buildings heat requirements.

Passive Solar Building Design

The techniques of passive solar building design were practiced for thousands of years, by necessity, before the advent of mechanical heating and cooling. There is evidence that ancient cultures considered factors such as solar orientation, thermal mass, and ventilation in the construction of residential dwellings. Fully developed solar architecture and urban planning methods were first employed by the Greeks, Chinese, and Aboriginal peoples of the America’s Southwest, who oriented their buildings toward the South to provide warmth and light. Nearly two and a half millennia ago the ancient Greek philosopher Aeschylus wrote “Only primitives and barbarians lack the knowledge of houses turned to face the winter sun”. Similarly, Socrates said “Now, supposing a house to have a southern aspect, sunshine during winter will steal under the verandah, but in summer, when the sun traverses a path right over our heads the roof will afford and agreeable shade, will it not”? (Wikipedia).

Principals of Heat Transfer

To understand how a passive solar home design works, you need to understand how heat moves and how it can be stored. As a fundamental law, heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two. To distribute heat throughout the living space, a passive solar home design makes use of this law through the following heat-movement and heat-storage mechanisms:

Conduction

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

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.

Radiation

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% 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% 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% of solar radiation, absorbing or reflecting only 10% to 20%. 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.

Thermal capacitance

Thermal capacitance refers to the ability of materials to store heat. Thermal mass refers to the materials that store heat. Thermal mass stores heat by changing its temperature, which can be done by storing heat 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. Water also has been successfully used.(US Department of Energy).