Electricity from Ocean Wave Energy: Technologies, Opportunities and Challenges
- Written by Shalinee Kishore, Lawrence Snyder and Parth Pradhan
The energy from ocean waves is a largely untapped resource that could play an important role in our electricity future. It is more consistent and predictable than that of other renewable resources such as wind and solar. Although several pilot projects have been successfully deployed worldwide, and some of them are grid-connected, the economic production of electric power from wave energy remains to be demonstrated. A key path forward will be the integration of smart technologies that harness vast amounts of sensor and meteorological data to support wave farm operations.
With estimates of economically recoverable wave energy resources ranging from 140 to 750 TWh/year worldwide with existing technology, energy from ocean waves is a largely untapped resource that could play an important role in our electricity future. It is more consistent and predictable than other renewable resources like wind and solar. What is more, the maximum energy density of waves (between 40 and 60 degrees latitude) is found in both hemispheres—where the advanced industrial economies of Europe, the United States and Japan reside.
A key barrier to making wave energy a reality, however, is cost. According to current estimates, the levelized cost per MWh of wave energy production is more than 1.5 times that of wind and nearly three times that of coal-based power.
Wave energy is more expensive than wind energy in part because wave energy conversion is in a much earlier development phase. Looking forward, this barrier will have to be overcome for wave energy production to reach its full potential. A key to reducing costs will be predicting the characteristics of waves, which can be reliably determined days in advance. This predictability will give wave energy producers—with low operational costs and a non-polluting technology—attractive market opportunities in the near future.
Many different techniques have already been proposed and tested for both on-shore, near-shore and off-shore wave energy extraction. The process of energy generation at a wave energy converter (WEC) consists of a number of steps, which include energy absorption from ocean waves by a type of energy capture mechanism, transmission of mechanical power to the generator by a power take-off mechanism and controlling power output by means of suitable power electronics or arrays of similar WECs, or both.
Most current research and development efforts are focused on improving the mechanical designs of WECs’ energy capture and power take-off mechanisms. WECs can be categorized broadly into four groups based on their underlying operating principle:
- Oscillating water columns (OWCs) consist of a partially submerged chamber that contains both a water column formed by the ingression of ocean waves and an air column formed by the trapped air. A turbine located at the top of the chamber experiences the expansion and contraction of the air column as ocean waves continuously change the water levels within the chamber. The LIMPET wave energy project near the Scottish island of Islay and the Oceanlinx Port MacDonnell project, soon to be deployed in South Australia, are examples of oscillating water columns. The LIMPET plant employs a full-scale on-shore OWC with a total installed capacity of 500KW. This grid-connected project is coordinated by Queen’s University Belfast and supported by the European Union under the Joule III Programme. The plant has been serving the local economy since early 2000. More recently, the Australian company Oceanlinx received close to $4 million in funding from the Australian Centre for Renewable Energy to deploy a 1 MW commercial-scale OWC, which will be a concrete box-like structure located 4 km off the coastline from Port MacDonnell. Grid connection is expected in late 2013, at which point it is anticipated that the deployment may serve up to 1,000 households.
- Overtopping devices harness energy from incoming waves by capturing water in a central reservoir and releasing it back to the sea through a number of hydroelectric turbines. An example is the Wave Dragon project in Denmark. Over the past decade, Denmark has hosted smaller-scale Wave Dragon projects.For example, in 2003 a 1:4.5 scale prototype was deployed in Nissum Bredning in Northern Denmark. Results from these earlier deployments have led to the latest commercial-size deployment of a 1.5 MW plant, which will occur at the Danish Wave Energy Center, DanWEC, at Hanstholmn in the Danish part of the North Sea. A 4 MW plant is to follow at Ekofisk in the North Sea, with the longer-term goal of installing a fully fledged power plant of 4-11 MW with a production price of 0.04€/kWh (~ 5.33 U.S. cents/kWh).
- Attenuators are typically long, multi-segment floating devices that are positioned in the direction of incoming waves. The segments flex and move at hinged joints as waves pass along, and the mechanical motion of the flexing is converted to electrical energy. The foremost example, the Pelamis device developed by the Scottish company Pelamis Wave Power, has been deployed in a variety of locations in the UK and in Portugal. Pelamis was the world's first offshore wave energy converter to be grid-connected, in 2004, and supplied electricity from the first multiple-machine wave farm at Agucadoura, Portugal to the country's Enersis in 2008. Currently the company is working with partners to build a 10 MW project off the Shetland Islands using multiple current-generation Pelamis devices. The company ScottishPower Renewables and the utility E.ON are testing Pelamis devices in Orkney currently; the former plans to install 66 such devices for a 50 MW facility off Marwick Head in Orkney. Pelamis Wave Power has initiated development of the Farr Point Wave Farm, for which they have obtained a seabed lease to develop a 15 MW farm that could eventually be expanded to 50 MW.
- Point absorbers are oscillating bodies that absorb energy from waves coming from all directions but whose output power is small due to their small size. An array of such devices is required to deliver substantial power to the grid. Several pilot projects have utilized point absorber WECs, including the PowerBuoy, developed by Ocean Power Technologies, a company with offices in the UK, Australia and the United States. The PowerBuoy will be installed off the coast of Reedsport, Oregon this spring, making it the first commercial wave power station in North America. The initial deployment of one 150-KW buoy will be extended to up to 10 buoys, for a 1.5 MW power station. Current estimates are that it will generate 4,140 MWh/year, or enough to power 375 homes. The PowerBuoy has already been tested in Scotland, Spain and Hawaii, and future large-scale projects are underway for Portland, Australia, Cornwall, UK and Coos Bay, Oregon.
A number of these technologies have the potential to scale up to large power projects serving hundreds of thousands of homes and possibly industries along the coast. But wave power, like other forms of renewable energy, will be a distributed resource and produce fluctuating power. As such, it too will require smart electricity grid support for reliable power delivery. Just as large-scale wind farms are coming under the purview of the smart grid, requiring information models for the control and monitoring of power injection, wave power will have to receive the same attention in the very near future.
But because ocean waves are more predictable than winds and solar reception, wave energy represents somewhat lesser challenges to smart grid operations. For the same reason, wave energy may be exploitable in a somewhat greater range of grid applications, for example, voltage regulation.
How can we integrate predictions of wave characteristics in making electrical control decisions? In many test sites—DanWEC at Nissum Bredning and Wave Hub, in the UK, for example—under-water electrical infrastructure has been built with substations, transformers and switchgear, to evacuate power from WECs in a farm and bring it to shore. Equipment choice and ratings in part determine the reliability of the electrical power produced, but the quality and quantity of power output heavily depend on how well-informed each WEC is about the nature of incoming waves.