Solar Homes

Solar power for homes and other buildings uses either an active or a passive form. Active solar devices collect energy from the Sun and store it or move it by the use of electrical controls, pumps, and fans. Passive solar power, by contrast, does not rely on any mechanical help. Passive solar

Houses with solar panels such as this one drastically cut the electricity they take from the municipal power grid.
(Gray Watson and Rosemary McCrudden)

energy employs windows, walls, and doors to collect, store, and distribute solar energy without the need for any other energy input. Passive systems and active systems conserve heat in the winter, but they also reject excess solar heat in the summer.

The main principle of solar energy relates to the concept of gain, which is the amount of usable solar energy that a structure captures. Solar homes can be designed to contain one of the following three types of gain:

1. direct gain—sunlight enters south-facing windows and strikes walls and floors, which store the energy as heat
2. indirect gain—solar heat strikes the home’s outside wall and is stored between the outer and inner wall from which it radiates as heat
3. isolated gain—a structure, such as a solarium, separate from the main house captures and stores solar energy as heat in its masonry

 Regardless of the type of solar gain a house has been built to use, designers and environmental engineers pay attention to five basic elements of solar energy systems, described in the following table.

 Solar homes distribute energy by three different means: conduction, convection, and radiation. In conduction, heat energy moves through solid matter by exciting molecules as it spreads through the matter. For example, a lit stove boils water in a pot because the metal pot conducts the stove’s heat energy to the water. Convection consists of heat circulation through

Solar energy can be equally valuable outside traditional neighborhoods, such as this passive solar farmhouse in
New York. (Green Rabbit Farm)

liquids or gases rather than solids. A lighted fireplace, for instance, heats a room by convection. Radiation may be solar or infrared radiation, both associated with the light spectrum, or nuclear radiation used in nuclear power plants. Solar radiation consists of the movement of heat through air from warm objects to cooler objects, such as a solarium warming up when exposed to bright sunlight. Infrared radiation consists of a warm surface transferring heat to a cooler surface. This type of radiation works when a person is chilly and snuggles under an electric blanket to warm up.

Solar homes contain design elements and building materials that aid in heat distribution by any of three different mechanisms: radiation is absorbed, reflected, or transmitted. Large glass windows transmit plenty of sunlight to the indoors. Glass absorbs only 10–20 percent of solar radiation and transmits the rest. The 80–90 percent of solar radiation that enters a house can then be absorbed by materials that absorb and hold heat better than others. Opaque solid materials such as stone floors and walls absorb 40–90 percent of the radiation entering a house; darker materials absorb more heat than lighter colored or white materials that reflect much of the radiation.

Passive solar energy works best if a building has been designed to make maximum use of the sunlight for heating and energy production. This diagram illustrates the ways in which passive solar energy collection is optimized— large windows, reflective surfaces—and stored in thermal mass materials, such as concrete and brick, which absorb heat then gradually release it.

The best-designed solar homes possess an attribute called thermal capacitance, meaning the capacity of materials to store heat. The following materials predominate in new solar homes because they have a high degree of thermal capacitance: stone, masonry, concrete, brick, tile, and water. These materials are named thermal mass materials and may also be thought of as insulating materials because they hold their temperature once they have heated up or cooled down. Thermal mass is so effective that in cities where hundreds of brick and concrete buildings fill each block, the buildings raise the outside temperature 2–8°F (1.1–4.4°C), an event known as the heat island effect.

Many solar homes offer all the luxuries of conventional homes (modern fully equipped kitchens, storage space, patios, porches, pools, fireplaces, skylights, fountains, and gardens), and they have become the most desirable type of new homes in many parts of the United States.

The energy expert Bernadette del Chiaro of the alternative energy advocacy group Environment California told the Los Angeles Times in 2007, “[House] builders are seeing that they’ll get more buyers coming to their developments when they have solar. They sell like hot cakes.” Solar homes in fact outsell homes in some communities such as San Jose, California. Solar homes nonetheless account for only a little more than 5 percent of all homes in California, a state that seeks to reach 50 percent solar in all new housing by 2015. Other smaller communities have similar goals of making all new public buildings solar-powered.

Solar devices installed on homes work best to save energy when used in combination with thermal mass building materials, as mentioned. Other alternative building materials sell at moderate cost, but the use of these materials requires a break from traditional methods of construction. Michael Funk helped design his solar-powered house in California’s Sierra Nevada. He admits that not all alternative materials were as readily available at building supply businesses as conventional materials. “When it comes to building with alternative and sustainable materials and using local artists and craftspeople, you can run into resistance,” he told Natural Home and Garden magazine in 2006. “You have to keep asking, keep digging, keep pushing. The products are out there. When you meet resistance, just push back. There’s a way to get everything you want. It just takes more work and persistence.” The table on page 114 lists common components and materials preferred for building new solar homes.

Existing houses and buildings can be switched to solar power with the installation of solar panels on the roof and connections to the house’s main energy input. The task can be done in a few days to weeks depending on the size of the building and number of panels to be installed. Many homeowners have been hesitant to install solar energy because of solar’s current high price. For the present, homeowners need about 50 years for the energy savings of solar energy to repay the installation cost.

Homeowners cope with three disadvantages to solar power in addition to expense: (1) periodic cutting of overhanging tree branches that block the sunlight from reaching rooftop collectors; (2) surrounding tall buildings that may make solar energy use difficult or impossible; and (3) solar collectors on the house that change the house’s appearance. Solar homes in very cold climates or places with heavy cloud cover may need a backup generator or occasionally draw energy from

the local municipal supply. Solar homes also offer many advantages that outweigh some of the concerns. First, they use an energy source that is free, sunlight. Second, they produce no carbon dioxide (CO2) emissions, and, third, produce no or little air, water, or noise pollution. Fourth, rooftop solar collectors install quickly, and, fifth, collectors do not upset the landscape. Finally, the energy cost savings drop to near zero or even below zero. In some U.S. states, energy utilities reimburse homeowners with solar systems that put energy back onto the community’s energy grid, also called a power grid.

Solar power has become mainstream throughout Europe, Australia, and the United States because it uses the simple principle of letting nature supply energy for human use. Ecosystems have always managed energy this way, but human civilization long ago adopted a different means of making energy: producing it by burning energy-containing fuels in engines. Environmental engineers are beginning to return to studying nature for clues about how to run systems at maximum efficiency from natural materials, the principle behind biomimicry.