How Hydrokinetic Energy Works

The power of moving water is obvious to anyone who has stood amidst breaking waves or struggled to swim against a river’s current. New technologies can enable us to harness the might of moving water to help light our homes and keep our ice cream cold in the freezer without building new dams that can have major impacts on wildlife and water quality.

The use of water power dates back thousands of years to the water wheels of Ancient Greece, which used the energy in falling water to generate power to grind wheat. We now are presented with an opportunity to develop a new generation of water power, one that will harness the abundant energy of our oceans and rivers.

Hydrokinetic technologies produce renewable electricity by harnessing the kinetic energy of a body of water, the energy that results from its motion. Since water is 832 times denser than air, our tides, waves, ocean currents, and free-flowing rivers represent an untapped, powerful, highly-concentrated and clean energy resource.

Estimates suggest that the amount of energy that could feasibly be captured from US waves, tides, and river currents is enough to power over 67 million homes. [1] Based on current project proposals, experts predict that the country could be producing 13,000 MW of power from hydrokinetic energy by 2025. [2] This level of development is equivalent to displacing 22 new dirty coal-fired power plants [3], avoiding the annual emission of nearly 86 million metric tons of carbon dioxide, as well as other harmful pollutants like mercury and particulate matter. The avoided carbon emissions in 2025 would be equivalent to taking 15.6 million cars off the road. [4] 

All energy technologies impact the environment, but all impacts are certainly not the same. As we choose which energy resources to develop, we must weigh their varied costs within the context of the existing hazard of global climate change. Studies are underway to investigate the potential impacts of harvesting wave and current energy on wildlife and the environment; however, it is clear that these technologies could help reduce the greenhouse gas emissions that are causing dangerous global warming. We need to weigh the environmental impacts of hydrokinetic technologies against the environmental and impacts of other available energy technologies, keeping in mind the costs of fossil fuels on air pollution and water pollution as well as global warming, and the need to have sufficient low-emission alternatives.

The hydrokinetic resource

There are a number of types of water resources from which it is possible to generate electricity from kinetic energy. Capturing the energy contained in near and off-shore waves is thought to have the greatest energy production potential amongst these hydrokinetic options. The rise and fall of ocean waves is driven by winds and influenced by oceanic geology. The promise of waves as a power source comes from both sheer resource availability and a relatively advanced technological development status. Extracting only 15% of the energy in US coastal waves would generate as much electricity as we currently produce at conventional hydroelectric dams. [5] Much of this wave potential is found along our Pacific Coast, near big cities and towns.

In addition to waves, researchers believe that ocean tides hold promise as an energy resource. Each change in the tide creates a current, called a tidal stream. These predictably regular tidal streams have the potential to provide us with a reliable new source of clean electricity without building the dams, or barrages, that have been part of the few existing tidal projects developed in some other countries.

Other stream resource prospects for hydrokinetic energy development include ocean currents, such as the Gulf Stream, which result from winds and equatorial solar heating; free-flowing rivers, from which energy capture is possible without any intrusive dams; and even constructed waterways, such as irrigation canals.

While stream-based hydrokinetic energy research is not as evolved as its wave energy counterpart, initial estimates expect these water resources could fulfill all of the electricity needs for an additional 23 million typical homes. [6] Stream hydrokinetics could prove to be a particularly valuable resource for regions with lower wind energy potential, especially in the US Southeast; as of March 2008, 36 preliminary permits have been granted by the Federal Energy Regulatory Commission (FERC) allowing pilot project development and technology impact research along the lower Mississippi river. [7] Additionally, capturing just 0.1% of the available energy in the Gulf Stream could supply Florida with 35% of the state’s electricity needs. [8]

Beyond the sheer size of the resource, hydrokinetic energy is attractive for its predictability; wave patterns can be predicted days in advance, and tides for centuries. Additionally, while waves and ocean currents are variable, they can provide continuous power, [9]  which is not possible from variable-output renewables like tidal streams, wind, or solar power. Since the kinetic energy held in a stream is related to its speed cubed, extracting the most electricity from each hydrokinetic project will depend heavily on site selection. A water current with double the speed contains eight times as much energy as one moving just half as fast!

State and federal policymakers across the US have taken notice of the potential of hydrokinetic energy, and have begun to support its development through legislative and monetary means; ocean energy is an eligible resource for credit under renewable electricity standards in sixteen states, and for federal renewable energy production tax credits, as expanded in the Energy Policy Act of 2005. Furthermore, hydrokinetic energy development was marked for increased research funding appropriations in the 2007 Energy Independence and Security Act.

The technologies developed to generate energy from waves and currents, called hydrokinetic energy conversion devices, are generally categorized as either wave energy converters (WECs) or rotating devices. Today, each type of device remains in development, though some fully permitted pilot devices are already deployed on-site.

The industry is rapidly progressing and hopes to build full “wave parks” and turbine arrays capable of delivering clean, renewable electricity to the grid on a commercial scale within the coming decade. There are numerous promising configurations within each of these technology categories, and the lack of a clear leader today emphasizes the need to support further engineering studies and pilot deployments to establish the most cost-effective and environmentally sound options.

Wave Energy Cf Devices

WECs utilize the motion of two or more bodies relative to each other. One of these bodies, called the displacer, is acted on by the waves. The second body, the reactor, moves in response to the displacer. While there are a number of designs and configurations of WECs, the four most commonly discussed are the:

• Oscillating Water Column: Waves enter and exit a partially submerged collector from below, causing the water column inside the collector to rise and fall. The changing water level acts like a piston as it drives air that is trapped in the device above the water into a turbine, producing electricity via a coupled generator. To see an OWC from the inside out, check out an animation of the technology.

• Point Absorber: Utilizes wave energy from all directions at a single point by using the vertical motion of waves to act as a pump that pressurizes seawater or an internal fluid, which drives a turbine. This type of device has many possible configurations. One configuration, called a hose pump point absorber, consists of a surface-floating buoy anchored to the sea floor, with the turbine device as part of the vertical connection. The wave-induced vertical motion of the buoy causes the connection to expand and contract, producing the necessary pumping action. Through engineering to generate device-wave resonance, energy capture and electricity generation by point absorbers can be maximized.

• Attenuator: Also known as heave-surge devices, these long, jointed floating structures are aligned parallel to the wave direction and generate electricity by riding the waves. The device, anchored at each end, utilizes passing waves to set each section into rotational motion relative to the next segment. Their relative motion, concentrated at the joints between the segments, is used to pressurize a hydraulic piston that drives fluids through a motor, which turns the coupled generator. You can watch a video of an actual attenuator at work.

• Overtopping Device: A floating reservoir, in effect, is formed as waves break over the walls of the device. The reservoir creates a head of water—a water level higher than that of the surrounding ocean surface—which generates the pressure necessary to turn a hydro turbine as the water flows out the bottom of the device, back into the sea.

Rotating Devices

Rotating devices capture the kinetic energy of a flow of water, such as a tidal stream, ocean current or river, as it passes across a rotor. The rotor turns with the current, creating rotational energy that is converted into electricity by a generator. Rotational devices used in water currents are conceptually akin to, and some designs look very similar to, the wind turbines already in widespread use today—a similarity that has helped to speed up the technological development of the water-based turbines. Some rotational device designs, like most wind turbines, rotate around a horizontal axis, while other, more theoretical concepts are oriented around a vertical axis, with some designs resembling egg beaters.

Environmental impacts and facility siting

While the generation of electricity by hydrokinetic devices does not produce harmful air emissions, like the greenhouse gases linked to global warming, further research is necessary to determine what other types of environmental impacts may result from tapping the energy in waves and currents. The extent of these local impacts is important to evaluate, and appropriate caution should be taken in the development of regulations surrounding hydrokinetic energy development.

Scrutiny of hydrokinetic energy projects must be placed in the context of our wider energy and electricity generation mix. Specifically: the US must shift its energy resources away from the fossil fuels that are contributing to global warming, price volatility, and energy insecurity, and towards a clean, renewable, stable, homegrown energy supply that will create domestic jobs and save consumers money.

As we continue to monitor the environmental and competing-use impacts of the hydrokinetic technologies producing electricity in our waters, we must also give proper recognition to the current and ongoing problem we face of climate change.

Choosing the most suitable locations for wave parks and turbine arrays depends on a number of impact considerations and site characteristics, including:

• Environmental – the effects of bottom-moored devices on the habitats of benthic animals and plants, like oysters, clams and sea grass; the potential for fish strike or impingement on a device; and whether a full-scale array of devices could create significant noise, hinder the movement or migration of aquatic animals, or even alter hydrologic and sediment regimes. [10] However, initial studies indicate that these impacts are likely to be minimal where appropriate care has been taken in site selection and project design. Further, any adverse impacts can be minimized through experience with pilot projects, and by drawing upon our experience with other marine fixtures, such as oil platforms. In fact, the first wave energy project license to be granted, after the completion of a thorough environmental assessment that found no significant impact would result, covers a project that will be developed within a National Marine Sanctuary.
• Economics – the cost of electricity generated will be a function of the power density of the stream (kW/m²) or wave crest (kW/m crest height), the distance the electricity must be transmitted to reach consumers, the ease of access to a site for ongoing maintenance and monitoring, and the availability of state or federal tax incentives for project financing and electricity production. Fundamentally, sites with manageably stronger currents and larger wave heights, all else equal, will provide the lowest cost hydrokinetic electricity.
• Competing uses – Fishermen, shipping vessel operators, recreational boaters, and coastal community groups have expressed concerns about the effects of hydrokinetic energy developments on their own usage and enjoyment of our waters. Their representatives have begun to take part in the negotiation process as potential development sites are explored, and will continue to play an important role in project placement and design. Our water resources already accommodate a wide range of uses. Subsequent to proper environmental and siting review, hydrokinetic energy is poised to be safely added to this mix, generating much-needed clean electricity without hindrance to other usages.

Commercialization

Currently, each prototype buoy and pilot turbine requires millions of dollars in research, development and deployment funding. However, a series of reports used cost of electricity (CoE) models to assess six of the leading wave energy project proposals. [11]  They concluded that the CoE in 2010 from the first utility scale project would be as low as 11.1¢/kWh—before accounting for any tax incentives for renewable energy investment or generation—and with opportunities for significant economies of scale to follow as the industry matures.

Additionally, models of commercial scale tidal energy project development proposals found a CoE of 4.8-10.8¢/kWh. In comparison, when wind energy entered the market over 20 years ago it had a CoE of over 20¢/kWh, [12] which fell to 4.7-6.5¢/kWh in 2006. [13] Recently, however, costs have escalated for all energy sources as specific construction materials and expertise have been in high demand globally. With proper support of project development and deployment, hydrokinetic electricity can before long become economically competitive with, or superior to, both conventional and advanced fossil fuel-based electricity sources—all the more so with the enactment of a climate change policy that puts a price on carbon pollution.

Despite the promise of hydrokinetic technologies to contribute significantly to our clean energy mix, there are barriers to the speedy development and delivery of this technology. Most pressing of these barriers are the current regulatory structure, and a need for additional financing to support environmental research and project deployment.

Despite the many differences between siting and impact issues for conventional dams and hydrokinetics, the regulatory process for both energy producers is the same, making it just as difficult to obtain a license to deploy a temporary pilot test turbine as it is to permanently dam a major river. [14] Furthermore, there is significant conflict over what agency and level of government has or should have the authority to approve hydrokinetic projects. The Federal Energy Regulatory Commission (FERC) is working to streamline federal approval for temporary projects under its Hydrokinetic Pilot Project Licensing Process, released in the fall of 2007. Permitting hurdles make it difficult to conduct on-site tests, and without field-tested evidence of a particular technology’s promise, investors are hesitant to provide the essential financing to jumpstart the widespread development of a hydrokinetic energy industry.

The deployment of hydrokinetic energy would be facilitated by:

• Appropriation of adequate government funding for research, development and deployment of pilot devices. Although Congress has authorized increased funding for hydrokinetic energy in recent years, sufficient funds must be both authorized and appropriated in order for researchers to receive anything; [15]
• Provision of dedicated funding for site-by-site research and modeling to evaluate environmental impacts;
• Supportive federal economic and energy policies, such as loans or tax credits for hydrokinetic energy development, similar to those for wind and solar energy production and investment; and a federal renewable electricity standard to create demand and a secure market for additional renewable energy capacity;
• Reevaluation of the regulatory process to assist timely project development, while giving appropriate attention to environmental and community safeguards;
• Resolution of the permitting and licensing jurisdiction conflict in which claims to project approval powers have been made by different federal, state and municipal agencies; and
• Increased discussion and collaboration among public and private entities including the electricity industry, research engineers, aquatic scientists, environmentalists and community stakeholders.

Stay tuned! Hydrokinetic energy development is rapidly progressing, both technologically and with the help of supportive policies that recognize the critical role this renewable energy resource can play in a warming world. Harvesting the motion of our tides, rivers and oceans can be a part of an affordable and sustainable solution to reducing our dependence on fossil fuels, and the impact they have on environmental and public health.

References

[1] Bedard, Roger, et al. North American Ocean Energy Status – March 2007. 2007. Proceedings of the 7th European Wave and Tidal Energy Conference. 11-13 September 2007. Porto, Portugal.  Calculations include 260TWh of wave-generated electricity and 140 TWh from tidal and in-stream electricity. The estimates cited in the Proceedings assume a 15% conversion rate of hydrokinetic energy to mechanical energy, power train efficiencies and conversion availability of 90%. Our calculation assumes electricity use of 6,000 kWh per year for a typical non-electric heating U.S. household. 

[2] Dixon, Douglas. EPRI. “The Future of Waterpower: 23,000 MW+ by 2025.” June 2007. Environment and Energy Study Institute briefing. Washington, DC. And Personal communication, R. Bedard, EPRI. April 2008. Online at: http://www.hydro.org/hydrofacts/
EPRIEESITheFutureofWaterpower060807.pdf

[3] Assumes an average new coal plant generating capacity of 600 MW.

[4] Assumes a heat rate of 8,870 Btu/kWh for a new supercritical pulverized coal plant based on MIT data (Future of Coal, 2007), a carbon content for coal of 220 lbs/million Btu based on EIA data, and tailpipe emissions of 12,100 lbs/year for an average car based on EPA data.

[5] 260 TWh/year. Data source: Bedard, R., et. al. 2007.

[6] 140 TWh/year. Data source: Bedard, R., et. al. 2007.

[7] FERC. Issued hydrokinetics projects preliminary permits. Online at: http://www.ferc.gov/industries/hydropower/indus-act/hydrokinetics/permits-issued.asp

[8] Minerals Management Service. 2006. Technology Whitepaper on Ocean Current Energy Potential on the US Outer Continental Shelf. US Department of the Interior, Renewable Energy and Alternate Use Program. Pg. 3. Online at: http://ocenergy.anl.gov

[9] Minerals Management Service. 2006. Technology White Paper on Wave Energy Potential on the U.S. Outer Continental Shelf. U.S. Department of the Interior Minerals Management Service Renewable Energy and Alternate Use Program. Online at: http://ocsenergy.anl.gov

[10] For a more in depth discussion of environmental concerns, see: Cada, et al. 2007. Potential Impacts of Hydrokinetic and Wave Energy Conversion Technologies on Aquatic Environments. Fisheries 32:4, pp 174-181. Online at: http://hydropower.inel.gov/hydrokinetic_wave/pdfs/
cada_fisheries_reprint.pdf

[11] Bedard, R., et. al. 2007.

[12] Previsic, M., B. Polagye, & R. Bedard. 2006. EPRI. EPRI-TP-006- SF CA. System level design, performance, cost and economic assessment – San Francisco tidal in-stream power plant. Online at: http://oceanenergy.epri.com/streamenergy.html#reports

[13] Bedard, R., et. al. 2007.

[14] Ibid.

[15] Hydrokinetic energy was included as an eligible renewable energy resource by the Energy Policy Act of 2005. Various funding authorizations for research and development were also included in this Act as well as the Energy Independence and Security Act of 2007.