Harnessing energy from the sun's rays can be accomplished in a variety of ways. Solar energy technologies provide electricity, hot water, and heat for homes businesses and industries. Electricity is produced when photons (particles of light strike the surface of a photovoltaic (PV) panel. Water or air can be heated by the sun either directly or indirectly, for use in a home, commercial, or industrial application. Finally, the sun can also be used to directly heat homes and buildings in the winter through appropriately placed windows and building orientation, a concept known as passive solar design. Below is a brief description of each solar technology mentioned.

Photovoltaics (photo = light, voltaic = electric) is a means of utilizing the sun's energy to produce electricity. This is accomplished by using a semiconductor material, similar to a computer chip. When high-energy photons from the sun (the same kind that cause sunburn) strike the surface of a PV panel, electrons in the PV cells are dislodged and begin to move, generating electricity. The electrons flow into a wire, creating a current of electricity.

In order to produce electricity, PV cells must remain unshaded throughout the day, face south, and be tilted to a certain angle to take advantage of the sun's path, both on a daily and seasonal basis. In order to utilize the electricity that the photovoltaic panels produce, there are a few other components that make up a typical PV system: charge controller, batteries, inverter, and wiring (see Figure 1).

Figure 1. Most household appliances operate on alternating current (AC). This illustrates a basic configuration of the PV modules and equipment in an AC system. (Circuit breakers and safety fuses are not shown.)
Please click to enlarge

Charge controller - regulates the flow of electricity from the PV panels to the inverter/battery bank. The charge controller ensures that electricity does not flow backwards, from batteries to PV panels, especially at night when PV panels are not producing power. It also prevents the batteries from being over - or under-charged.

Battery - stores electricity produced by PV panels, so electricity is available at times when the sun is not shining. A home that is not connected to the electric utility grid and depends upon the power stored in the batteries is known as a stand alone system, since it is independent of electricity produced by power plants.

Inverter - changes the electricity produced by photovoltaic panels from DC (direct current) to AC (alternating current). DC electricity is the type of electricity produced by batteries, whereas AC is what typically use in the United States. Inverters can also safely send the electricity produced by the PV panels back into the electric utility grid, if the building is connected to existing power lines. This type of PV system is known as grid connected or grid tied, since it is interactive with the utility grid. This type of system does not need batteries to store the electricity produced by the PV panels; it simply uses the utility grid as a means to "store" excess power.

In the 1890s solar water heaters were being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves. Artificial gas made from coal was available too to heat water, but it cost 10 times the price we pay for natural gas today. And electricity was even more expensive if you even had any in your town! Many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, just east of Los Angeles, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida and many other sunny parts of the United States. The picture shown here is a solar water heater installed on the front roof of a house in Pomona Valley, California,?in 1911 (the panels are circled above the four windows).

By 1920, ten of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. As these low cost fuels became available, solar water systems began to be replaced with heaters burning fossil fuels. Today, solar water heaters are making a comeback. There are more than half a million of them in California alone!

Heating water with the sun is one of the most cost effective applications of solar energy. There are many uses for hot water in residential and commercial applications. Described below are the two most common: hot water for swimming pools and hot water for indoor use.

Low temperature hot water is used primarily for heating swimming pools. Solar water heating ?systems for these applications are among the most cost effective, often with a payback of less than two years. These relatively simple systems are usually mounted on the roof of the house like the one pictured on the right. The existing pool pump circulates water from the pool, through the solar collector, and then back into the pool. Solar pool heating systems can extend the swimming season by several months for a fraction of the cost of heating with electricity or natural gas.

Medium temperature hot water is used for daily, indoor uses such as bathing, cleaning, and sometimes heating of buildings. There are a variety of solar water heaters that can be used to preheat water for use in buildings.

Passive Systems-rely on water pressure in the main water line or the natural tendency for hot water to rise (known as thermo siphoning systems). These systems are among the least costly and have no moving parts that may wear out over time. The simplest system, known as a batch or "breadbox" water heater, is something the average do-it-yourselfer can construct at minimal cost. Passive systems consist of a collector, usually a glazed box with a metal tank or piping inside which is painted black, and a storage tank which can be an existing water heater.

Active Systems - rely on pumps which circulate water or other liquid through a solar collector. The hot water from the solar collector is usually stored in a typical water heater, which functions as a backup system for when the sun is not shining. Although these systems tend to be more expensive, they have higher efficiencies that usually offset the higher first cost.

A recent study shows that installing a solar water heater in conjunction with an existing electric water heater would result in a savings of $237 per year in electricity bills (Environmental Building News, July/August 1999). The potential for pollution reduction through the use of solar water heaters is staggering. Each system would reduce pollution by over 3 tons per year, as compared to a water heater run by fossil fuels.