What Is Solar Energy?

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 (PV)

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. For a more technical description of how photovoltaic cells work, read Practical Photovoltaics by Richard Komp.

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, battery or 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.)

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 we 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.

Solar Water Heating

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, consist of plastic tubes usually no more than a quarter inch in diameter, and are colored black to absorb heat from the sun. 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 thermosiphoning 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 in Georgia 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.

Active Solar Space Heating

By capturing the sun's heat in glazed collectors and using fans or pumps to distribute heated air or water throughout a building, an active solar space heating system can cut energy use for winter heating by as much as 50 percent. Active solar space heating may be most cost effective in large warehouses and industrial buildings that often lack insulation and typically use inefficient electric resistance heaters during the heating season.

Passive Solar Design

Passive solar homes are intended to work with the inside and outside climate to minimize energy bills and maximize comfort. The key features that separate passive solar homes from more traditional homes are:

Orientation and site selection. The passive solar windows must face within 30 degrees of due south to maximize solar gain in winter and minimize overheating in summer. Trees on the site reduce summer cooling bills, but should not shade south-facing windows in winter.

Energy efficient design. This includes proper installation of recommended levels of insulation, air-tight design, and efficient heating and cooling systems.

Increased south-facing glass area. South windows receive about three times as much sunlight as east and west windows in the winter and one-third less sunlight in the summer. This provides a portion of winter space heating needs..

Reduced east and west glass area. Reduces summer cooling needs.

Thermal storage mass. Materials such as concrete floors, interior brick walls, brick pavers, and tile to store heat and modulate interior temperatures in both winter and summer.

Effective window shading and ventilation. Reduces summer cooling needs.

Moisture control systems. Increases the home's durability, improves indoor air quality, and provides comfort in both summer and winter.

Heat distribution. Maximizes the number of rooms heated by the passive solar features.

Figure 2. Annual Average Daily Peak Sun Hours for the continental United States

Solar Energy Resources in Georgia

Georgia receives more solar radiation than most states (see figure 2). However, states such as Maine, Massachusetts, Pennsylvania, Wisconsin, and others receiving less direct sunshine on an annual basis have embraced solar and other renewable energy technologies on a broader scale than Georgia. Investment in solar energy technologies within Georgia would bring new jobs and companies to the state, reduce air pollution (especially important in Georgia's urban areas), and create savings on utility bills for homeowners and businesses.

States in the southeastern U.S., such as North Carolina, Virginia, and Florida, are leading the way for encouraging investment in solar and renewable technologies. For example, both North Carolina and Virginia offer tax credits to PV manufacturers. As a result, Virginia is home to a manufacturing facility for one of the largest producers of photovoltaic panels in the world, strengthening the local and state economy. Georgia is one of the few states in the U.S. which currently does not offer some type of incentive for solar or renewable energy technologies to businesses or homeowners. However, there are a few federal tax incentives and low interest loans which are currently available. Many lending institutions are now offering low interest loans or specialized mortgages for energy efficient homes, as well as for solar technologies-primarily PV and solar water heating.

Consumer education about the positive attributes of solar energy is one step for furthering the solar industry in Georgia. Many consumers were victims of fly-by-night businesses during the 1980's that took advantage of solar tax credits to install inferior solar energy systems. Today, however, organizations such as the National Renewable Energy Laboratory (NREL) and the Florida Solar Energy Center (FSEC), can certify the reliability of most solar products and installers through testing and training procedures.