Renewable Energy
Solar Energy
The term solar energy refers to the conversion of solar radiation into useful forms of energy, such as electricity or heat, through various technologies. The amount of solar radiation a location receives depends on a variety of factors including geographic location, time of day, season, local landscape, and local weather. Because of our location in North Carolina, we have excellent solar resources. Technologies relying on solar radiation include photovoltaic cells; passive solar heating; concentrating solar power; and solar heat collectors.
Photo courtesy of DOE/NREL.
Photovoltaic (PV) Cells
The process of converting light into electricity is called the photovoltaic (PV) effect. Within PV cells, semiconductor materials absorb sunlight which knocks electrons from their atoms, allowing electrons to flow through the material to produce electricity. An individual PV cell – averaging about 4 inches per side – typically converts 15% of the available solar radiation into about 1 or 2 watts of electrical power. PV cells are combined to form a module, which in turn are connected in arrays that vary in shape and size. For a more detailed discussion on PV physics, PV devices, and PV systems, visit the Department of Energy website.
Source: DOE/EERE
PV cells have historically been used in remote, off-grid settings. More recently, the PV industry has focused on home, business, and utility-scale systems. This transition stems from lower costs arising from economies of scale, establishment of net metering policies, and strong financial incentives. As a result, the installation of grid-connected PV cells exceeded the installation of off-grid PV cells for the first time in United States in 20051.
Passive Solar Heating
Passive solar heating refers to building designs that provide natural heating to a home or business. A well-designed building can capture heat in the winter and minimize it in the summer. One common technique is the use of large south-facing windows and building materials that absorb the sun’s heat during the day and slowly release it at night. Natural daylighting – the use of natural light to brighten a buildings interior – is an added benefit of many passive solar designs.
Concentrating Solar Power
This type of technology uses reflective mirrors and lenses to concentrate the sun’s energy to be used in industrial applications or to generate electricity. The most common concentrating technologies, parabolic-trough systems, consist of long, curved mirrors that focus sunlight on liquid in a tube running the length of the system. The liquid is heated to about 300 degrees Celsius and used to produce stream that drives an electric turbine
Parabolic-trough system. Photo courtesy of DOE/NREL.
Other technologies include parabolic-dish concentrators and central receiver concentrators. Parabolic-dish systems are similar to parabolic-troughs, but focus sunlight on a single point where fluid is heated in order to drive a generator. A central receiver concentrator involves a field mirrors that focuses sunlight on the top of tower where fluid is heated to produce electricity with a generator.
Solar Heat Collectors
Situated on the roof of a home or building, solar heat collector technology is used for water heating, space heating, and space cooling. Hot water solar collectors are often thin, flat, rectangular boxes with black transparent covers that face the sun. In the box, small tubes carry water or an antifreeze solution. Heated water can be transferred to storage tanks or antifreeze liquids can pass through coils in water tanks, thereby transferring the heat to the water.
Solar hot water heater. Photo courtesy of DOE/NREL.
Solar collectors can also be used be used space heating and even space cooling. A simple heating design would involve heated liquid passing through a radiator that heats the air. Larger, commercial applications could involve a ‘transpired collector’ to pre-heat ventilation air. This thin, black metal panel is mounted on a south-facing wall and absorbs the sun heat. Air passing through the panel is heated and then used in a building’s ventilation system. When cooling a building, heat from a solar collector can be used with ‘dissector evaporators’ to pull out warm, moist air or with ‘absorption chillers’ to heat refrigerants under pressure.
Advantages of Solar Energy
A major advantage of solar energy is that the fuel – solar radiation – is free, abundant and inexhaustible. Further, a wide variety of technologies and design principles can be used to capture this prominent North Carolina resource. Another advantage is that solar radiation is most abundant during peak demand periods – in the middle of the day and during the summer months.
The use of solar energy leads to environmental and health benefits by reducing pollution emissions. Solar energy technologies do not produce any air pollutants or greenhouse gases. Solar energy also offers diversification in an energy portfolio, increasing energy security and independence in North Carolina.
The downside of solar energy is the expense of some technologies and the variability of solar radiation. For instance, PV cells remain expensive relative to other renewable technologies and cloudy weather can reduce their effectiveness.
Additional Resources
North Carolina Solar Center
The North Carolina Solar Center acts as a clearinghouse for solar and other renewable energy programs, information, research, technical assistance, and training for the citizens of North Carolina and beyond.
Passive Solar Home Design
This resource, available from the Energy Efficiency and Renewable Division of the Department of Energy, provides information on passive solar home design.
Small Solar Electric Systems
This resource, available from the Energy Efficiency and Renewable Division of the Department of Energy, provides information on small solar electric systems.
Solar Hot Water Heaters
This resource, available from the Energy Efficiency and Renewable Division of the Department of Energy, provides information on solar hot water heaters.
1. The Prometheus Institute. 2006. U.S. market analysis. PV News 25(5): 4-5.
