Offshore Solar Energy
Solar energy technologies potentially suitable for use in ocean environments include concentrating solar power technology and photonic technology.
Solar energy reaches the United States at an average rate of about 6 million BTU/m2 (about 6,330 megajoules/m2) per year. Every minute the sun bathes the Earth in as much energy as the world consumes in an entire year.
Since oceans cover more than 70 percent of the earth's surface, they receive an enormous amount of solar energy. Deep ocean currents, waves, and winds all are a result of the sun's radiant energy and differential heating of the earth’s surface and oceans.
Solar Energy Technologies
Solar radiation can be converted directly to usable energy through a variety of technologies. While there are no commercial solar energy facilities operating offshore at this time, solar energy technologies potentially suitable for use in offshore ocean environments include concentrating solar power (CSP) technology and photonic technology. CSP is a thermal solar technology that concentrates the sun's rays to heat fluids or solids, and the heat is used to drive steam turbines or other devices to generate power. Photonic technologies convert the sun's radiant energy directly to electricity or other useful forms of energy. Selection of the appropriate solar technology for a given situation depends in part on the intended use of the energy to be generated. CSP technologies might be more appropriate for generating and delivering electricity to shore, while photonic technology might be more appropriate for generation of electricity to be used "on-site" (such as offshore platforms) and for supplying energy for activities such as hydrogen production or desalinization.
Concentrating Solar Power (CSP) Technology
CSP plants generate electric power by using mirrors to concentrate (focus) the sun's energy and convert it into high-temperature heat. That heat is then channeled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts the heat energy to electricity. In the United States, onshore CSP plants have been operating reliably for more than 15 years. This approach requires large areas for solar radiation collection to produce electricity at commercial scale.
CSP utilizes three technological approaches: trough systems, power tower systems, and dish/engine systems.
Trough systems use large, U-shaped (parabolic) reflectors (focusing mirrors) that have oil-filled pipes running along their center, or focal point, as shown below. The mirrored reflectors focus sunlight onto the pipes and heat the oil inside to temperatures as high as 750°F. The hot oil is used to boil water, which produces steam to run conventional steam turbines and generators.
|Land-based parabolic trough system.
(Credit: Argonne National Laboratory)
Power tower systems, also called central receivers, use many large, flat heliostats (mirrors) to track the sun and focus its rays onto a receiver. As shown below, the receiver sits on top of a tall tower in which concentrated sunlight heats a fluid, such as molten salt, to temperatures as high as 1,050°F. The hot fluid can be used to boil water, which produces steam to run conventional steam turbines and generators. Or, the thermal energy can be effectively stored for hours, if desired, to allow for electricity production during periods of peak demand, even when the sun is not shining.
|A solar power tower in operation. Notice the mirrors are concentrating sunlight at the top of the tower.
Dish/engine systems use mirrored dishes (about 10 times larger than a backyard satellite dish) to focus and concentrate sunlight onto a receiver. The receiver is mounted
at the focal point of the dish. To capture the maximum amount of solar energy, the dish assembly tracks the sun across the sky. The receiver is integrated into a high-efficiency "external" combustion engine. The engine has thin tubes containing hydrogen or helium gas that run along the outside of the engine's four piston cylinders and open into the cylinders. As concentrated sunlight falls on the receiver, it heats the gas in the tubes to very high temperatures, which causes hot gas to expand inside the cylinders. The expanding gas drives the pistons. The pistons turn a crankshaft, which drives an electric generator. The receiver, engine, and generator comprise a single, integrated assembly mounted at the focus of the mirrored dish.
Solar Photonic Technology
Solar photonic technology absorbs solar photons (particles of light that act as individual units of energy), and converts the energy to electricity (as in a photovoltaic (PV) cell) or stores part of the energy in a chemical reaction (as in the conversion of water to hydrogen and oxygen).
PV technology converts sunlight directly to electricity. Concentrated PV (CPV) systems, which must track the sun to keep the light focused on the PV cells, use various methods to concentrate sunlight such as mirrors or lenses. The primary advantages of CPV systems are high efficiency, low system cost, and low capital investment to facilitate rapid scale-up; reliability, however, is an important technical challenge for this emerging technological approach.
For More Information
Download the Technology White Paper on Solar Energy Potential on the U.S. Outer Continental Shelf.
For more information on solar power technologies, visit DOE’s solar webpage, EERE’s solar webpage, or NREL’s solar research webpage.