Offshore Solar Energy

Solar energy technologies potentially suitable for use in offshore ocean environments include concentrating solar power technology and photonic technology.

The sun radiates an enormous amount of energy through the process of nuclear fusion. The high pressure and temperature in the sun’s core causes electrons to be stripped from hydrogen atoms, freeing the hydrogen nuclei to combine (fuse) to form one helium atom, producing radiant energy in the process. The sun radiates more energy in one second than the world has used since time began. Every minute the sun bathes the Earth in as much energy as the world consumes in an entire year.

Oceans cover more than 70 percent of the earth's surface and most of the ocean's energy comes from the sun. Deep ocean currents, waves, and winds all are a result of the sun's radiant energy. Solar radiation can also be converted directly to usable energy through a variety of technologies. These technologies will be considered in the OCS Alternative Energy Programmatic EIS.

Offshore Solar Energy Technologies

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" and for supplying energy for activities such as hydrogen production or desalinization.

Concentrating Solar Power (CSP) Technology

CSP technology utilizes focused sunlight. 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. Within the United States onshore CSP plants have been operating reliably for more than 15 years. All CSP technological approaches require large areas for solar radiation collection when used to produce electricity at commercial scale.

CSP technology utilizes three alternative 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 in Figure 1. The mirrored reflectors focus sunlight on the pipes and heat the oil inside to as much as 750°F. The hot oil is then used to boil water, which makes steam to run conventional steam turbines and generators.


	Figure 1: Parabolic Trough System Schematic Diagram Credit: U.S. Department of Energy


	Figure 2: A land-based parabolic trough system. Credit: U.S. Department of Energy

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 in Figure 3, the receiver sits on top of a tall tower in which concentrated sunlight heats a fluid, such as molten salt, as hot as 1,050°F. The hot fluid can be used immediately to make steam for electricity generation or stored for later use. The thermal energy can be effectively stored for hours, if desired, to allow electricity production during periods of peak need, even when the sun is not shining.


	Figure 3: Power Tower Schematic Diagram Source: U.S. Department of Energy


	Figure 4: A land-based power tower system Source: Warren Gretz, National Renewable Energy Laboratory

Dish/engine systems use mirrored dishes (about 10 times larger than a backyard satellite dish) to focus and concentrate sunlight onto a receiver. As shown in Figure 5, 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.


	Figure 5: Dish/engine System Schematic Diagram Source: U.S. Department of Energy


	Figure 6: A land-based solar dish-engine system. Source: Sandia National Laboratories

Solar Photonic Technology


	Figure 7: Land-based Crystalline Silicon Photovoltaic System Source: Sacramento Municipal Utility District and National Renewable Energy Laboratory

Solar photonic technology converts solar energy into useful energy forms by directly absorbing solar photons-particles of light that act as individual units of energy-and either converting part of the energy to electricity (as in a photovoltaic (PV) cell) or storing part of the energy in a chemical reaction (as in the conversion of water to hydrogen and oxygen).

PV technology converts sunlight directly into electricity when a PV cell absorbs and transfers the energy of the light to electrons in the atoms of the cell. The energized electrons escape from their normal positions in the PV material (typically a semiconductor) and move from the PV cell into an electrical circuit. 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.

Environmental Considerations

All CSP technological approaches and PV systems require relatively large areas for solar radiation collection when used to produce dispatchable electricity at the multi-megawatt scale. On the OCS, this large surface area array of solar collectors would need to be supported on some type of offshore floating or fixed structures. Such structures can be expected to have impacts, including the following:

Interference with commercial and recreational fishing
Interference with recreational boating, surfing, and diving
Ecosystem disturbances from shading of the water surface
Introduction of avian perching opportunities.

Other potential environmental impacts from CSP systems include accidental or emergency releases of toxic chemicals that may be used in the heat transfer systems, interference with aircraft operations if reflected light beams become misdirected into aircraft pathways, ecosystem disturbances from discharges related to the maintenance of cooling water systems, discharges related to the operation and maintenance of recycled steam systems, and ecosystem disturbances from construction, operation, and maintenance of both the solar energy conversion systems and the systems that transport electricity to onshore customers. Structures used for both CSP and photonic technologies in offshore applications would cause visual impacts in visible areas.

For More Information

Download the solar technology white paper:

Technology White Paper on Solar Energy Potential on the U.S. Outer Continental Shelf. (474 KB)

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