Wave-Hydro

Wave-Hydro

Oceans and rivers possess tremendous amounts of energy in the form of waves, currents, tidal flows and temperature gradients. Estimates for the energy contained in currents and tidal flows alone range from 280,000 Terawatt-hours (TWh) to over 700,000 – many times over the electric power generation of the entire world, which is approximately 16,000 TWh. Prevailing ocean currents hug populous continental coastlines, making approximately 4,000 TWh—one quarter of the world’s current electricity demand – accessible.

Likewise, similar power resides in major rivers.

Wave Energy

Wave energy is produced when electricity generators are placed on the surface of the water. Energy output is determined by factors such as wave height, wave speed, wavelength, and water density. Wave energy can be harnessed through a number of devices that operate on different principles:

  • Pneumatic devices use wave motion to compress air and drive a turbine.
  • Buoyant devices move on the surface with the waves, exerting tension against an anchor line to harness energy.
  • Raft devices tethered to each other or to pontoons use the relative motion of each other moving with the waves to harness wave energy.
  • Spillover devices capture water and power from breaking waves.
  • Commercial energy from waves and tides is still under development. Technology to support it is costly to develop, and unlike solar, geothermal, or even wind, it is not a solution that a homeowner can implement.

R&D is also underway to use these devices to provide a mounting platform for wind turbines – offering the possibility of a hybrid wind-wave device. Hybrids have the potential to maximize the use of sub-sea power connections and therefore increase their cost efficiency, paying back energy dividends more quickly.

Tidal Energy

Tidal stream devices operate below the surface to extract energy from the tides. Unlike wave power, tidal streams are predictable – though cannot always be matched to hours of peak demand. Current tidal technology involves dam projects, which are expensive to build and disrupt the environment.

New technologies include tidal turbines, which resemble wind turbines and are mounted on the sea floor. Unlike dams, they operate in freely flowing current and require no large-scale construction, lowering the cost of development and minimizing the impact on the marine ecosystem. But in order to capture the diffuse nature of tidal energy, this also means that large numbers of turbines spread over large areas are required to generate significant power. R&D is underway to test large grid-connected systems to unlock further potential in this technology.

Ocean Thermal Energy

This form of energy uses the temperature difference that exists between deep and shallow ocean waters to run a heat engine for power. Rather than using fossil fuels to run the engine, ocean thermal energy uses the sun’s warming of the surface water.

The total energy available from this method is perhaps twice as high as wave energy, but in order to generate the highest output, large temperature differences between the surface and deep waters must exist. These differences are greatest in the tropics, and the most promising projects are operating in Hawaii and the Pacific rim, where deep water is more accessible and surface waters are consistently warm.

Challenges with Wave and Tidal Power 

Commercial energy from waves and tides is still under development. Technology to support it is costly to develop, and unlike solar, geothermal, or even wind, it is not a solution that a homeowner can implement.

Considerations and challenges for implementing wave-hydro systems include:

  • Efficiently converting wave motion into electricity. Waves have intermittent power surges, while most turbines operate on steadily streaming power;
  • Constructing affordable devices that can survive storm damage and saltwater corrosion;
  • Reducing the high total cost of electricity—including the primary converter, the power takeoff system, the mooring system, installation& maintenance cost, and electricity delivery costs;
  • Impacts on the marine environment, such as noise and visual appearance;
  • Potential for some installations to displace commercial and recreational fishermen from productive fishing grounds, can change the pattern of beach sand nourishment, and may represent hazards to safe navigation. 

Energy from Water 

Texas has limited potential for generating significant amounts of additional power and energy from water resources. Most good hydropower generation sites in Texas have already been developed. There are numerous sites for new hydroelectric sites, some with a potential of greater than 10 MW, but the hurdles related to siting and low generation potential will prevent most of them from development. Saline gradient solar ponds could prove to be a beneficial energy resource for the western region of Texas if there is a need for low grade hot water to assist in desalination or aquaculture temperature regulation. 

Texas currently has 675 MW of conventional hydroelectric power, less than one percent of the state’s total electric generating capacity. A 2006 assessment by the U.S. Department of Energy estimated that Texas had 18,000,000 MWh/yr of potential new hydropower generation although only 2,900,000 MWh/yr of this electricity is actually considered feasible. Much of this additional hydropower may never be developed due to economic and environmental constraints. Texas’ existing hydropower plants could act as “pumped storage” facilities using inexpensive off-peak electricity to pump water behind the dam, then used later to generate power during high cost peak demand periods. This small, but possibly valuable, peaking resource capacity could complement intermittent wind power output

The total cost of hydropower production is low because there is no fuel cost. The average production cost in the US is less than 0.9 cents per kWh. Hydropower does not directly produce air pollution although it can result in other environmental impacts. Hydropower development may face regulatory impediments, including environmental protection, economic regulation of water and electricity, safety, and land use.

Texas has very limited potential to extract energy or electricity from ocean waves, ocean thermal gradients, currents, and tides. Wave energy systems require relatively large installations along the shoreline that could pose obstacles to development by interfering with marine animals, as well as boating and shipping traffic. Viable electricity costs have been estimated for wave farms along the California and Oregon coasts, however, Texas’ offshore wave power densities are typically well below those considered to desirable. Power can also be produced from the ocean as the temperature differences between the surface and depths below 100 meters can drive a heat engine to produce electricity. Texas’ ocean thermal energy potential is limited because the ocean depth near the Texas Gulf Coast is less than what is optimal for its development. Tides and ocean currents have also been explored for their energy potential but have not proven to be viable energy resources in Texas.

Useful energy can be produced using salinity gradients, through pressure retarded osmosis (PRO) and reverse electrodialysis (RED), or salinity gradient solar ponds (SGSP) that capture and store solar thermal energy. PRO and RED systems could be used at the saline gradient between Texas river mouths and bays, but only for very limited quantities of electricity. SGSP has the advantage of providing energy on demand and being able to use reject brine, often considered a waste product. Research has established the technical viability of using SGSP technology for electricity and water desalination, particularly in desert areas or where freshwater is not otherwise abundant. However, the demand for increased volumes of freshwater might promote
the development of technologies that could reduce their cost for electric generation.

The potential for additional energy production from water resources in Texas is minimal and a substantial economic benefit is not anticipated for the state. However, some technologies, such as the use of SGSP for desalination or aquaculture enhancement, could prove beneficial to specific projects and locales.

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