Adapted from "Consumer Guide to Solar Energy," K. Scheinkopf and S. Sklar, Bonus Books, Inc. and "Buildings for a Sustainable American Case Studies," Burke Miller Thayer, American Solar Energy Society.
Solar energy technologies can supply energy for many uses in your home. Passive solar design can provide heating, cooling and natural light; generate electricity; and solar water heating provides hot water for laundry, showers, and cooking.
It is important to consider the end use of each application when planning a solar home, and to use the most efficient, cost-effective technology for each application. For instance, a PV system could be used to operate an electric hot water heater, but a solar water heating system is much more efficient and less expensive for this particular application.
Passive Solar Designs
A "passive" solar house provides cooling and heating to keep the home comfortable without the use of mechanical equipment. This type of construction results in homes that respond to the environment.
For passive heating and cooling, the plan of the house, construction materials, building features, and other aspects of the home are designed to collect, store, and distribute the sun's heat in winter; and block the sun's rays in the summer. Passive solar houses can be built in any style and in any part of the country.
The following techniques use passive solar strategies to provide heat:
Direct Gain is radiant heat resulting from sunlight admitted
directly to the living spaces through south-facing windows, which
warms the interior surfaces (walls, furniture, floors, etc.).
For direct gain, the south-facing window area must be sized for
the climate, the type of window used and the amount of thermal
mass in the home.
Indirect Gain: In a design that employs indirect gain, an attached sunspace or Trombe wall collects heat from the sun before transferring it to other spaces within the home. The air heated in the sunspace circulates naturally or with the aid of fans to other rooms.
Thermal Mass: Any material in the home that absorbs and stores heat is considered Thermal Mass. Concrete, brick, tile, and other masonry materials are the most common choices for thermal mass in a passive solar home. These materials absorb and release heat slowly and are easily and inexpensively integrated into a house design. They are most effective when dark colored and located in direct sunlight. The addition of Thermal Mass allows saved solar energy to heat the house at night or on cloudy days. The combination increases the performance characteristics of the home, generally for only a modest cost increase.
Passive Cooling: Passive solar cooling can reduce or even eliminate the need for air conditioning in homes. At its simplest, passive cooling includes overhangs for south-facing windows, few windows on the west, shade trees, thermal mass and cross ventilation.
Using the principles of heat transfer, some of the same strategies that help to heat a home in the winter also cool it in the summer. For example; with a well-designed overhang, the south-facing windows that admit the low-angled rays of the winter sun are shaded from the high-angled summer sun.
Thermal mass, which stores heat in the winter to release in the evening, works in reverse in the summer. The mass cools down in the evening and retains that coolness the next day, moderating the effect of high daytime temperatures.
Energy Efficiency: Energy efficiency minimizes the need for heating, cooling and electricity, solar or otherwise. Designers of solar homes use insulation levels that are higher than those found in typical construction, and energy efficient appliances and lighting. Windows are up to twice as resistant to heat loss as those used in conventional construction. Air infiltration is also reduced by carefully sealing and caulking around window and door openings and under sill plates (which is rarely done in conventional housing).
Photovoltaics (also called solar cells), absorb sunlight and convert
it directly to electricity. Solar cells are very thin (about 1/100th
of an inch thick), rectangular or circular wafers, typically made
of silicon. When sunlight hits the cell, electrons are released
which then flow onto wires forming Direct Current (DC), the same
kind of current that flows from a regular battery. A four-inch
silicon cell can produce about one watt of DC electricity.
A number of cells (usually twenty or more) or a photovoltaic film can be mounted within a frame under a transparent glass or plastic covering to form a module. Modules can be connected to other modules to form an array. The larger the area of photovoltaics in each module, or the more arrays used, the more electrical power you will have.
The DC power produced by the photovoltaic cells will operate many devices, or it can be converted to alternating current (AC) for standard household appliances. For houses not connected to the electric grid, batteries store the DC electricity for later use. At present, photovoltaic solar cells convert between 12 and 20 percent of the sunlight that hits them into electricity. New cells have been tested at much higher efficiencies, and it is anticipated that they will be available in the near future.
Solar Water Heating
Heating water is one of the most common uses of solar energy.
Collector tubes inside an insulated box absorb the sun's heat
and transfer that heat to either water or another liquid flowing
through the tubes and is pumped in a continuous loop.In those
areas where freezing is not a threat, the tubes are water filled
and heated directly by the sun. This is called an "Open Loop"
system.
In those areas subject to freezing, the coils contain an antifreeze
liquid which is heated before transferring its heat to the water
by way of a heat exchanger. This system is called a Closed Loop"
system. When hot water is needed inside the house, the system
draws on this heated water.
Passive solar water heating is even simpler. There are two types of systems:
In the "Batch Water Heater" system, the unit is usually located on the roof or on the ground near the house so that the sun striking the collector goes directly into the storage tanks, where it heats the water. The hot water then flows downward into the home.
In the "Thermosyphon" system, the storage tank is located on the roof or in the attic, above the collectors. As the water is heated in the collectors, it becomes lighter and naturally rises into the storage tank above it. The heavier cold water sinks to the lowest point in the system, which is the collector.