by Timothy Mundon, Kleinschmidt
|Wave energy device manufactured by Pelamis.|
Marine renewable energy sources such as wave and tidal power are poised to become an important part of the U.S. future clean energy mix. Recent Electric Power Research Institute (EPRI) and Georgia Institute of Technology studies have measured the annual technically recoverable energy from U.S. wave and tidal resources at 1,170 terawatt-hours (TWh) and 66 TWh respectively–a significant proportion of the 4,000 TWh of U.S. electricity demand. In practice, the realities of project development likely will result in a much smaller realized resource, but if 10 percent extraction can be reached, this would be equivalent to the output of 37 large fossil fuel plants.
Obstacles, however, exist in the route toward large-scale adoption of marine renewables. To understand some of the complexities that challenge the development of wave and tidal energy, it is important first to appreciate that these resources are separate and distinct.
Wave energy is the energy contained in water waves that propagate across the ocean surface. They are generated primarily by wind that blows across the ocean and can propagate with minimal energy loss, thus waves will combine and continue to gain energy over long, open ocean stretches. In the U.S., the best locations are the Pacific Coast states, including Alaska and Hawaii.
Extraction of this energy at useful scales is challenging. Emerging devices are the result of a significant body of knowledge and many cycles of research and development. Serious academic attention started to be directed at this problem in the early 1970s, yet real devices that provide low energy costs while retaining physical robustness are just starting to emerge.
Much of the challenge is created by the nature of waves’ being composed of elliptically moving water particles. As waves approach the shore, particle motion goes from circular to increasingly elliptical, the waves become steeper and the energy content reduces (see Figures 1 and 2).
Real ocean waves also comprise a distribution of many different heights and periods that create a particular sea state with a common period and height, varying over time as conditions change. As a result, devices tend to be oscillating systems, tuned to the wave period and limited to a band of wave periods where they will generate maximum efficiency. One simple way to think about the performance is to consider the wave height as the energy carrier while the wave period is an efficiency tuning parameter. Many concepts of energy extraction have been developed, and each tends to be suited to a particular regime, such as offshore: greater than 30-meter water depth; nearshore: approximately 10-meter depth; and coastline.
A number of devices are grid-connected and installed in high-energy environments.
These devices are the pre-commercial prototypes of units that will be built into utility-scale arrays during the next few years. At the moment, no devices have been grid connected in the U.S., although some have been tested at smaller scales. The complexities of extraction have kept energy costs high so far, therefore technologies that can reduce device capital costs while increasing reliability are a primary research focus.
Solar and lunar gravitational forces affect varying changes in sea level that although very small mid-ocean are amplified by basin resonances and coastline topography to create very large surface elevation changes in specific geographical locations. These can be exceptional where naturally high tides are further constrained, such as in the Bay of Fundy, which experiences tidal ranges exceeding 15 meters and flow speeds greater than 5 meters per second in certain places.
|Tidal device manufactured by ORPC.|
Although the U.S. does not have one of the highest total tidal energy resources in the world, some areas have potential for significant generation. Alaska contains more than 90 percent of the U.S. tidal resource, which provides an extensive clean energy potential. Tidal hot spots in Washington and Maine also provide good locations for larger-scale projects, but elsewhere in the U.S., the feasibility of utility-scale tidal energy is low.
There are two general approaches to tidal energy extraction: a barrage approach and a hydrokinetic approach.
The barrage approach is based upon general hydropower principles and requires a structure to enclose a large tidal body of water. As the tidal height varies outside of the structure, water is discharged either into or out of the enclosed area through conventional hydro turbines and creates power. A number of projects exist that use this principle worldwide, including 250-MW projects in France and Korea; however, the inevitable environmental consequences of such a scheme mean the feasibility of permitting a project of this nature in the U.S. is very low.
A more environmentally practical solution, especially in the U.S., is the hydrokinetic approach. This method does not require any impoundment or dam and uses devices that are designed to extract energy from moving water without a requirement for a hydrostatic head. This approach is considerably lower in energy density than a barrage approach, but it is gaining considerable support and development around the world because of its minimal environmental impact. Devices are typically representative of wind turbines and use the same principles of energy extraction, with designs modified to suit the much higher-density underwater environment. A three-bladed open turbine has become the most common approach, although other designs exist that use different principles, such as ducted turbines, vertical-axis turbines or reciprocating hydrofoils.
The principles of energy extraction from a linear flow are well-known, but a number of factors complicate the development of submerged turbines:
- High-energy density. The density of water is roughly 850 times that of air.
- Energy increases with the cube of the velocity. The energy in 3 meters per second is double that in 2 meters per second.
- Alternating flow direction. Tidal flows reverse a number of times per day.
- Highly limited accessibly. Installation operations can be done only in low currents, providing extremely short windows for on-site marine activities, possibly as low as 45 minutes per slack tide.
So far five companies have demonstrated megawatt-scale, grid-connected devices and have plans for utility-scale deployments. The U.K. is close to starting work on a 100- to 300-MW tidal array, and there are projects elsewhere in similar advanced stages. The U.S. is behind the development curve, but there has been significant recent support by the Department of Energy in a number of areas.
Although devices exist, infrastructure and installation processes have not been demonstrated effectively across large, utility-scale arrays of devices. Work remains in a number of areas, including development of practical solutions for fast installation and recovery, subsea electrical aggregation solutions, and modeling tools that can predict energy output accurately across an array. These developments will go a long way to reduce the cost of energy to become more competitive with conventional renewables.
The U.S. has attractive wave and tidal resources, and utility-scale devices exist near the point of being ready for large-scale array deployment; however, it is difficult to imagine sustainable project development in the U.S. without substantial changes.
Project permitting has been the primary obstacle for U.S. projects, with environmental and process uncertainties’ providing significant roadblocks. A lack of experience from responsible agencies and an unwillingness to accept some of the potential risks associated with new technologies has proved exceptionally challenging for developers.
The second obstacle is the electricity market; the cost of energy from the first plants will be high, and it will take time for technology, installation experience and economies of scale to allow project costs to drop. Unfortunately, these high costs come when the consumer price of electricity is kept low by natural gas prices and legacy generation assets. This creates an extremely difficult marketplace for clean energy to compete without some form of cost-leveling mechanism, and at present, outside of small state-level programs, there are no adequate federal market mechanisms that can support early wave and tidal projects through to technological maturity. Therefore, the U.S. has a choice: Either let the rest of the world take the lead and develop the experience, tools and technology or act now to develop policy to create a more sustainable cost model that includes the environmental consequences of energy generation, enabling a U.S. market for marine energy and its supply chain benefits.
Tim Mundon is a chartered marine engineer with a doctorate from the University of Edinburgh focusing on wave energy. He works as a senior engineer in Kleinschmidt’s Marine Renewables Group and has more than 10 years’ experience with wave, tidal and marine energy projects.