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Solar energy systems use the sun's rays for electricity or thermal energy. In the United States, utility scale solar power plants are located primarily in the Southwest. However, smaller scale rooftop photovoltaic cells and hot water systems are effective in all regions. The United States has some of the best solar resources in the world, but solar made up only 0.4 percent of U.S. energy supply in 2014. The main barrier to widespread implementation has been the relatively high initial investment; however, costs have been decreasing in recent years. Federal tax credits are available, and many states offer incentives as well. Countries such as Germany have successfully used policy mechanisms such as feed-in tariffs to become the world leaders in solar energy production. The current U.S. industry is growing and employs 173,807 people.
There are two ways to harness solar energy. Passive systems are structures whose design, placement, or materials optimize the use of heat or light directly from the sun. Active systems have devices to convert the sun’s energy into a more usable form, such as hot water or electricity.
Passive Solar Space Heating: Passive solar heating systems are a simple and cost effective way to take advantage of the sun’s free, renewable energy and displace the need for electricity, natural gas, or other active energy systems. Strategic planning of building location, orientation, and materials provide great control over the inside temperature. Installing large south facing windows, planting shade trees, and using Trombe walls, which are made of absorptive materials that store heat during the day and slowly release it at night, are measures that can be adopted for new and old buildings alike. Sunspaces or glass rooms built on the south side of a building can provide up to 60 percent of a home’s winter heating. Regardless of climate, solar energy can be harnessed as long as the building has adequate insulation and ventilation.
Active Solar Space Heating: In an active solar space heating system, a collector holding a heat-transfer medium such as air or liquid captures the sun’s thermal energy, which is then distributed through the building via electric fans or pumps. Currently, there are no pre-fabricated residential solar heating systems, so interested customers must hire a specialized engineering firm to design an adequate system. The costs for such custom systems range from $3,000 to $10,000 depending on the size of the space. With savings in electricity or natural gas, active solar heating systems can pay for themselves in 7 to 10 years.
Solar water heaters (active) produce thermal energy to heat water for households, commercial entities, and swimming pools. These heaters are one of the most commonly implemented renewable energy technologies because of their cost effectiveness and relatively simple installation. With the proper model installed, they heat efficiently regardless of outside temperature. Solar water heaters typically need a backup conventional gas or electric water heater to account for cloudy days or unusually high water demand.
Solar water heaters consist of two parts: a solar collector and a storage tank. In warm climates, collectors heat water directly, but in cold climates, a denser fluid is heated and then transported to a water tank where it heats the water indirectly. The heater can be built to use an active or passive system for circulating warmed fluid depending on climate and the time of day when water demand is highest. The maximum heating temperature varies with collector model, but water temperature can exceed 200 degrees Fahrenheit, suitable for commercial purposes.
Solar water heaters can reduce conventional energy consumption for heating water by 60 percent in commercial applications and up to 75 percent in homes. Although initial home installation costs range from $1,500 to $3,000—at least double that of conventional heaters—the reduction in gas or electric bills realized over their 15-20 year lifespan allow solar water heaters to equal or better the long term cost of other water heaters.
Photovoltaic (PV) cells, or solar cells, are an active system in which small panels faced with semiconducting material turn sunlight into electricity. This material, usually made of silicon but potentially other polycrystalline thin films, generates a direct current when sunlight hits the panel. PV cells are effective in all regions of the country, from Alaska to Alabama. Commercially available PV panels are up to 22.5 percent efficient at converting sunlight into electricity in optimal conditions, but even in partly cloudy weather, they can operate at 80 percent of their maximum output. The United States is the leader in thin-film technology, which enables PV cells to be installed on windows and roof tiles. PV systems can be tailored to meet a building’s energy needs by adding concentrating or sun-tracking devices, DC-AC converters, and/or battery storage.
PV systems may or may not be connected to the electric transmission grid. PV systems linked to the transmission grid can supplement utilities’ energy supply during daylight hours, which normally include the peak energy demand periods. Independent PV cells can power a variety of individual items, from personal calculators and streetlights to water pumps on ranches and remote settlements far from power lines. A few utility-scale PV installations have been constructed although energy production is limited to daylight hours and they generally have higher upfront costs than fossil fuel plants.
Concentrated solar power (CSP) is an active system distinguished from other solar energy systems by its ability to function as a utility-scale power plant. CSP uses fields of mirrors to concentrate solar energy into channels holding heat-responsive fluid. The high temperatures excite the fluid to a point where it powers a turbine or engine, which in turn runs an electric generator. Without storage facilities, CSP systems can generate electricity for about eleven hours on a sunny summer day in the Southwest. However, CSP systems do have the potential to provide baseload power for utilities. A CSP system that uses oil or molten salt as a medium in the heat-transfer process can retain the thermal energy in thermos-like tanks for use when sunlight is not available. Another option hybridizes CSP with natural gas boilers, which heat the fluid when the sunlight cannot. Existing natural gas and coal power plants can be retrofitted with CSP technology.
Different models of CSP systems include: Linear Concentrating Systems, Parabolic Trough Systems (the cheapest and most common system in the United States), Linear Fresnal Systems, Dish/Engine Systems (which produce energy on a smaller scale, generally between 3 and 25 kilowatts), and Power Tower Systems (the most efficient CSP systems but also the most expensive and demanding on water and land resources).
CSP requires a substantial initial investment, and it typically uses more land and water than other solar technologies. The Southwest has excellent sunlight conditions for CSP, but the water supply needed—which can be as much as coal plants—places a heavy demand on the arid climate. Desalination and dry cooling systems can reduce the water demand by 97 percent but add to the cost of the plant.
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