A review of “An Evaluation of the U.S. Department of Energy’s Marine and Hydrokinetic Resource Assessments. 2013. Marine & Hydrokinetic Energy Technology Committee; National Research Council” by Alice Friedemann, July 7, 2014.
The U.S. Department of Energy (DOE) hired contractors to evaluate five Marine and Hydrokinetic Resources (MHK) globally: 1) Ocean tides 2) Waves 3) Ocean Currents 4) Temperature gradients in the ocean (OTEC) and 5) Free-flowing rivers and streams.
Then DOE asked the National Academy of Sciences (NAS) to evaluate the results, so NAS assembled a panel of 71 experts to write this assessment.
The NAS replied it was a waste of time for DOE to ask the contractors what the global theoretical maximum power generation from MHK resources might be. For example, solar power plants provide less than .1 % of electricity in the United States, even though the theoretical amount would be staggeringly enormous if you plastered the entire continent with them. But you can’t do that.
Nor can you fill the world’s ocean and rivers with devices to harvest the power in waves, tides, ocean currents, rivers, and temperature gradients (OTEC).
NAS says DOE should have asked was how much power could be generated locally at specific sites in the United States after taking into account technical and practical resource limits. For example:
The GIS database of MHK resources has a 100 MW resource. But after evaluating the location further, it turns out to be a 2.7 MW resource because of 1) technical resource limits (turbines 30% efficient, only 20% of the area can be used, the efficiency of connecting the extracted energy to the electric grid is 90%), and 2) practical resource issues: 50% of the remaining area interferes with existing fisheries and navigation routes, leaving a practical resource of 2.7 MW (100 MW * .30 * .20 * .90 * .50 = 2.7 MW).
Here are some more practical barriers to developing MHK:
- Impacts on marine species and ecosystems (e.g., rare or keystone species, nursery, juvenile and spawning habitat, Fish, Invertebrates, Reptiles, Birds, Mammals, Plants and habitats)
- Bottom disturbance
- Altered regional water movement
- acoustic, chemical, temperature, and electromagnetic changes or emissions
- Physical impacts on the subsurface, the water column, and the water surface, scouring and/or sediment buildup, changes in wave or stream energy, turbulence
- Endangered Species Act; Coastal Zone Management Act; Marine Mammal Protection Act; Clean Water Act; Federal agency jurisdictions: National Oceanic and Atmospheric Administration (NOAA), U.S. Army Corps of Engineers (USACE), Federal Energy Regulatory Commission (FERC), State Department, U.S. Fish and Wildlife Service (FWS), Environmental Protection Agency (EPA), Bureau of Ocean Energy Management (BOEM), U.S. Coast Guard
- Overlapping jurisdiction of state and federal agencies: FERC (within DOE) has jurisdiction over hydroelectric development; leases on the U.S. outer continental shelf require approval by BOEM (Dept of the Interior; NOAA (Dept of Commerce) is responsible for licensing commercial OTEC facilities; FWS (Dept of the Interior) and NOAA coordinate protection of marine mammals from potentially harmful development; NOAA also protects essential fish habitats. Projects in navigable waters fall under the jurisdiction of USACE and may also require involvement of the U.S. Coast Guard. USACE permits may be required for projects involving dredging rivers or coastal areas. The Coastal Zone Management Act involves coordination among local, state, and federal agencies to ensure that plans are in accordance with a state’s own coastal management program.
Social and economic:
- Spatial conflicts (e.g., ports and harbors, marine sanctuaries, navigation, shipping lanes, dumping sites, cable areas, pipeline areas, shoreline constructions, wreck points, mooring and warping points, military operations, marine sanctuaries, wildlife refuges, Traditional hunting, fishing, and gathering; commerce and transportation; oil and gas exploration and development; sand and gravel mining; environmental and conservation activities; scientific research and exploration; security, emergency response, and military readiness; tourism and recreational activities; ocean cooling water for thermoelectric power plants that use coal, natural gas, or nuclear fuel; aquaculture; maritime heritage and archeology; offshore renewable energy; view sheds, commercial and recreational fisheries, access locations such as boat ramps, diving sites, marinas; national parks, cultural heritage sites
- Interconnection to the power grid (e.g., transmission requirements, integrating variable electricity output, shore landings; Capital and life-cycle costs (e.g., engineering, installation, equipment, operation and maintenance, debris management, and device recovery and removal
TABLE 1 Issues That Impact the Development of the Practical MHK Resource
No Commercial scale MHK plants exist because:
Once installed, MHK devices are subject to mechanical wear and corrosion that is more severe than land-based equipment
Corrosion-related problems (i.e. galvanic, stress, fatigue, biocorrosion) and marine fouling are key challenges for all MHK devices. Advanced structural materials with appropriate coatings and paints still need to be identified in order to construct the robust, corrosion-resistant components for MHK energy generation.
Survivability in hurricanes, tides, storms, large waves, and so on
This is another challenging problem, especially in shallow water. Devices can be destroyed, damaged, or moved from their moorings under the actions of rough seas and breaking waves
Making MHK devices rugged enough is expensive
Rugged MHK devices require huge amounts of steel and concrete, which is inherently expensive, and many use expensive exotic materials or engineering. The power electronics on MHK devices will be a challenge to implement and operate reliably. In shallow tidal and riverine areas, there is a great concern that debris will affect both the efficiency and durability of any installed devices.
Capital and Life-Cycle Costs
As with any energy device or power plant, there are costs such as design, installation, operation and maintenance, removal, and replacement. The largest of these costs, and potentially the greatest barrier to MHK deployments, is the capital cost. An earlier NRC committee concluded that it will take at least 10 to 25 years before the economic viability of MHK technologies for significant electricity production will be known. A 2008 report evaluating the potential for renewable electricity sources to meet California’s renewable electricity standard found that the cost of electricity from waves and currents was higher than that from most other renewable sources and had a substantially greater range of uncertainty.
The best places for MHK are often far from urban centers
- In-stream power: Alaska is by far the largest resource but it’s questionable whether it would work because rivers freeze up, the scour incurred during spring ice break-up would make year-round deployment a challenge and possibly require seasonal device removal.
- Tidal resource: Alaska’s Cook Inlet
- OTEC: only feasible near Hawaii, Puerto Rico, U.S. Virgin Islands, Guam, Northern Mariana Islands, and American Samoa.
These challenges affect not only installation, maintenance costs, and electricity output, but also MHK scalability from small to utility applications
Time and Regulation
The time to get all the regulatory agencies at federal, state, and local levels to agree to a project is formidable and time-consuming. MHK devices are far from being ready to scale-up to commercial levels.
Most of the ocean and rivers are too far to connect to the electric grid
The distance required to interconnect into the electricity system is critical, as it directly impacts the economic viability of a project. Often the MHK device needs to be placed far from areas close to the grid because ports, cities, and other users already occupy prime grid-connection locations.
Connection to the grid is challenging and requires extra equipment due to harsh environmental conditions, intermittent and unstable load flows, variable energy output, lack of electrical demand near the generation, the length of cable from a device or array to a shore terminus, potential environmental impacts from the cable, permitting issues, and the reliability of the equipment.
The situation is even more complicated if there are large numbers of offshore generators, because connecting a large number of devices together with no load demand along the path of the network cable could produce an unstable system.
The potential of tidal power has long led to proposals of a barrage (a dam that lets water flow in and out) across the entrance of a bay that has a large range of height between low and high tides. It would generate power by releasing water trapped behind the barrage at high tide through turbines similar to a hydro-power facility. Or this could be done with in-stream turbines similar to the way that wind turbines work.
Scale: A tidal amplitude of 3.3 feet would require over 110 square miles to produce 100 MW (enough to power about 70,000 homes). This is why tidal power is limited to regions with very large tides (which tend to be in the northern latitudes, far from any cities that could use the power). Even with a current speed of 3 meters per second, a 100 MW project would need a flow of nearly 40,000 cubic meters per second, which requires 120 turbines, each having a cross-sectional area of 120 square yards, or 24 turbines of 82-foot diameter. Many more turbines would be needed for more typical, smaller currents. This many large turbines are likely to interfere with existing water uses, and an array this large would have near-field back effects that reduce the current each individual turbine experiences.
More than 1 channel: Power is reduced if there’s more than 1 channel, which also tends to divert flow to other channels.
Engineering challenges: Corrosion, biofouling, and metal fatigue in the vigorous turbulence typically associated with strong tidal flows.
Conflicting uses: Some of the locations with the highest tidal energy density are also estuaries having ports with heavy commercial shipping traffic. It is likely that there will be limitations to the number and size of turbines and the depth at which they can be deployed so as not to interfere with established shipping lanes.
Tides only generate power two to four times a day.
Power in ocean waves originates as wind energy transferred to the sea surface when wind blows over large areas of the ocean. The resulting wave field consists of a collection of waves at different frequencies traveling in many directions.
If energy is removed by a wave energy device from a wave field at one location, less energy will be available in the shadow of the extraction device, so a second row of wave energy devices won’t perform as well as the first row. The planning of any large-scale deployment of wave energy devices would require sophisticated, site-specific field and modeling analysis of the wave field and the devices’ interactions with the wave field.
One theoretical study on wave-device interaction modeled the Wave Dragon Energy Converter deployed in the highly energetic North Sea. They concluded that capturing 1 GW of power would require the deployment of a 124-mile-long single row of devices or a 5-row staggered grid about 1.9 miles wide and 93 miles long. This doesn’t take into account that the recovered power must be transformed into electricity and then transmitted. Because of the high development and maintenance costs, low efficiency, and large footprint, such devices would be a sustainable option only for small-scale developments considerably less than 1 GW close to territories with limited demand, such as islands.
It would take about 81 miles of wave machines to produce as much power as a typical power plant (1000 MW). Even if you built wind machines as far north as Canada and as far south as Mexico along both coasts, you’d only get 9% of the electricity we use now (Hayden).
Wave Power Efficiency
None of these systems are likely to operate at efficiencies over 90% and will probably have more realistic efficiencies of 50-70%. This calls into question claims of wave energy facilities that capture 90% or more of the available energy.
Other Wave Power Issues
- Waves are intermittent, which means energy production is spotty
- Waves have a low potential energy that varies with the weather and only a small hydraulic head of 2 or 3 meters. Hence large volumes of water have to be processed which means large structures relative to power output
- The waves are a challenge for energy harvesting since they not only roll past a device but bob up and down or converge from all sides in confused seas, plus have to cope with the period of the wave (Levitan)
- No design that’s been investigated is very good at capturing a very large fraction of the energy over a range of wave conditions. If they’re designed to efficiently capture wave energy in “average” sea conditions, they’ll be totally overwhelmed in high sea conditions. If they’re designed for efficient energy capture in high sea conditions, they’ll be almost totally insensitive to the energy present in average conditions (HED).
- These devices typically produce what’s known as low-frequency power, which can be difficult and expensive to convert to high-frequency electrical grids
- Wave technologies have lots of electrical components, hydraulic fluids and oils — all presenting a pollution risk
- So far about 30 wave power ventures have failed, such as Denmark’s “Wave Dragon”, the UK “Salter Duck”, Netherlands “Archimedes Wave Swing”, The Sea Clam, the Tapchan, the Pendulor, Finavera Renewables “AquaBuOY” in Oregon, Pelamis Wave Power in Portugal, Verdant Power’s East River project ($30 million spent so far), Pacific Gas & Electric’s wave energy testing program, Oceanlinx in Sydney, and Ocean Power Technologies in July 2014 canceled plans to build a wave energy project off the coast of Australia, saying it says is no longer commercially viable and will repay what it has received of a A$66.5M government grant, which was intended to be used toward the A$232M proposed cost of building the project.
Ocean Current Power
Ocean currents (excluding tidal currents) are affected by Coriolis forces and mainly generated by winds that cause strong, narrow currents which carry warm water from the tropics toward the poles, such as the Gulf Stream, with an ocean current in the Florida Strait that can exceed two meters per second.
The ocean current power team estimated the Florida current could generate 14.1 GW, or 62% of the 20 GW maximum power obtainable.
NAS thought that figure was way too high for many reasons and concluded that maximum power that could be extracted is 1 and 2 GW at best.
Or it may be less than 1-2 GW:
- If the high turbine density in the water column diverted the Florida Current and forced the flow around the Bahamas
- Seasonal variability and meandering might limit the placement of turbines to just a few narrow areas where the flow was consistent
Ocean Thermal Energy Conversion (OTEC) Power
Ocean thermal energy conversion (OTEC) is the process of deriving energy from the difference in temperature between surface and deep waters in the tropical oceans. The OTEC process absorbs thermal energy from warm surface seawater found throughout the tropical oceans and ejects a slightly smaller amount of thermal energy into cold seawater pumped from water depths of approximately 1,000 meters. In the process, energy is recovered as an auxiliary fluid expands through a turbine.
NAS thought the study should have been limited to just the areas this could possibly work: the Hawaiian Islands, Puerto Rico, U.S. Virgin Islands, Guam, the Northern Mariana Islands, and American Samoa. Hawaii could generate 143 TWh/yr, the Mariana Islands (including Guam) 137 TWh/yr, and Puerto Rico and the U.S. Virgin Islands 39 TWh/yr. The majority of this resource is found far from the United States near Micronesia (1,134 TWh/yr) and Samoa (1,331 TWh/yr).
The efficiency is so low — just 3 to 4 percent — that it may take more electricity to pump the deep cold water to the surface than is generated by the process.
OTEC sinks a long pipe into cold waters (< 4°C) beneath warm seas, whose daily high temperatures are > 25°C, and uses the temperature difference to generate electricity. The 20°C difference in temperature between the hot and cold reservoir is too tiny to generate much power (Smil).
The continental U.S. resource is very seasonal and limited, and it is unlikely that plant owners would want to operate only part of the year.
OTEC plants are vulnerable to corrosion, strong currents, tides, large waves, hurricanes, and storms, and remaining anchored.
OTEC could cause environmental damage.
OTEC plants must be near tropical islands with steep topography to make it easier to reach deep cold water and transmit power to shore.
The committee estimated the global OTEC resource could be 5 TW (a 100-MW plant every 30 miles in the tropical ocean). In reality, this would never happen because you need to connect them to land-based electric grids.
OTEC needs very large equipment and very high seawater flow rates
OTEC systems are similar to most other heat engines. There are significant practical aspects that make it difficult to implement, mainly from the small available temperature difference of only ~20ºC between the warm and cold seawater streams. Because of the low efficiencies, OTEC plants require very large equipment (e.g., heat exchangers, pipes) and seawater flow rates (~200-300 cubic meters per second for a typical 100-MW design) that exceeds any existing industrial process to generate a significant amount of electricity.
OTEC needs to be near existing electric power systems
The cold-water pipe is one of the largest expenses in an OTEC plant. As a result, the most economical OTEC power plants are likely to be open-ocean designs with short vertical cold-water pipes, close enough to shore to connect to existing electric power systems.
Concerns with tides, variation in power output, shear current effects on the cold-water pipe
The committee is concerned about the variations in isotherm depth due to internal tides, which can be significant near islands. For example, deep isotherm displacements of as much as 50 or even 100 m are common near the Hawaiian Islands, which could induce a 5-10 percent variation in power output over the tidal cycle. In addition, areas with strong internal tides will also impose strong shear currents on the cold-water pipe. Seasonal variations could lead to a 20% variation in power output in Hawaii over the course of the year. Even more dramatic changes result from fluctuations due to El Niño or La Niña in the central tropical Pacific, where the committee estimates variations in power production as high as 50 percent. The assessment group largely fails to address the temporal variability issue.
Spacing must be far apart given the huge seawater requirements
Clearly, a key question for determining the OTEC technical resource would be how closely plants could be spaced without interfering with each other or excessively disturbing the ocean thermal structure. At regional and global scales there could be a variety of impacts on the ocean arising from widespread deployment of OTEC.
There are many interesting physics, chemistry, and biology problems associated with the operation of an OTEC plant. Whitehead suggested that an optimal plant size would be around 100 MW in order to avoid adverse effects on the thermal structure the plant is designed to exploit.
In-Stream Hydrokinetic Power
In-stream hydrokinetic energy is recovered by deploying a single turbine unit or an array of units in a free-flowing stream. Estimates of the maximum extractable energy that minimizes environmental impact range from 10 to 20% of the naturally available physical energy flux.
There are many limiting factors that will reduce the in-stream hydrokinetic energy production
These factors include but are not limited to ice flows and freeze-up conditions, transmission issues, debris flows, potential impacts to aquatic species (electromagnetic stimuli, habitat, movement and entrainment issues), potential impact to sites with endangered species, suspended and bedload sediment transport, lateral stream migration, hydrodynamic loading during high flow events, navigation, recreation, wild and scenic designations, state and national parklands, and protected archeological sites. These considerations will need to be addressed to further estimate the practical resource that may be available.
Navigable waters are a resource for a number of sectors, and coordinating their use is an immense logistical challenge that will definitely impact in-stream energy development.
NAS criticisms of the DOE report
This is just a very small part of the criticisms scattered throughout the report, much of which criticizes the data, methods, and conclusions of each of the 5 contractors, such as:
The committee was disappointed by the resource groups’ lack of awareness of some of the physics driving their resource assessments, which led to simplistic and often flawed approaches. The committee was further concerned about a lack of rigorous statistics, which are essential when a project involves intensive data analysis. A coordinated approach to validation would have provided a mechanism to address some of the methodological differences between the groups as well as a consistent point of reference. However, each validation group (chosen by individual assessment groups) determined its own method, which led to results that were not easily comparable. In some instances, the committee noted a lack of sufficient data and/or analysis to be considered a true validation. The weakness of the validations included an insufficiency of observational data, the inability to capture extreme events, inappropriate calculations for the type of data used, and a focus on validating technical specifications rather than underlying observational data.
The committee is also concerned about the scientific validity of some assessment conclusions.
All five MHK resource assessments lacked sufficient quantification of their uncertainties. There are many sources of uncertainty in each of the assessments, including the models, data, and methods used to generate the resource estimates and maps. Propagation of these uncertainties into confidence intervals for the final GIS products would provide users with an appropriate range of values instead of the implied precision of a specific value, thus better representing the approximate nature of the actual results.
The committee has strong reservations about the appropriateness of aggregating theoretical and technical resource assessments to produce a single-number estimate for the nation or a large geographic region (for example, the West Coast) for any one of the five MHK resources. A single-number estimate is inadequate for a realistic discussion of the MHK resource base that might be available for electricity generation in the United States. The methods and level of detail in the resource assessment studies do not constitute a defensible estimate of the practical resource that might be available from each of the resource types. This is especially true given the assessment groups’ varying degrees of success in calculating or estimating the technical resource base.
Challenging social barriers (such as fishery grounds, shipping lanes, environmentally sensitive areas) or economic barriers (such as proximity to utility infrastructure, survivability) will undoubtedly affect the power available from all MHK resources, but some resources may be more significantly reduced than others. The resource with the largest theoretical resource base may not necessarily have the largest practical resource base when all of the filters are considered. It is not clear to the committee that a comparison of theoretical or technical MHK resources—to each other or to other energy resources—is of any real value for helping to determine the potential extractable energy from MHK.
Site-specific analyses will be needed to identify the constraints and trade-offs necessary to reach the practical resource.
Quantifying the interaction between MHK installations and the environment was a challenge for the assessment groups. Deployment of MHK devices can lead to complex near-field and/or far-field feedback effects for many of the assessed technologies. Analysis of these feedbacks affects both the technical and practical resource assessments (and in some cases the theoretical resource) and requires careful evaluation. The committee noted in several instances a lack of awareness by the assessment groups of some of the physics driving their resource assessments, such as the lack of incorporation of complex near-field and/or far-field feedback effects, which led to simplistic and sometimes flawed approaches. The committee was further concerned about a lack of rigorous validation.
As part of the evaluation of the practical resource base, there seemed to be little analysis by the assessment groups of the MHK resources’ temporal variability. The committee recognized that the time-dependent nature of power generation is important to utilities and would need to be taken into account in order to integrate MHK-generated electricity into any electricity system.
DOE requests for proposals did not offer a unified framework for the efforts, nor was there a requirement that the contractors coordinate their methodologies. The differing approaches taken by the resource assessment groups left the committee unable to provide the defensible comparison of potential extractable energy from each of the resource types as called for in the study task statement. To do so would require not only an assessment of the practical resource base discussed by the committee earlier but also an understanding of the relative performance of the technologies that would be used to extract electricity from each resource type. Simply comparing the individual theoretical or technical MHK resources to each other does not aid in making such a comparison since the resource with the largest theoretical resource base may not necessarily have the largest practical resource base. However, some qualitative comparisons can be made, especially with regard to the geographic extent and predictability of the various MHK resources. Both the ocean current and OTEC resource bases are confined to narrow geographic regions in the United States, whereas the resource assessments for waves, tides, and in-stream show a much greater number of locations with a large resource base. As for predictability, while there is multi-day predictability for wave and in-stream systems, especially in settings where the wave spectrum is dominated by swells or in large hydrologic basins, the predictability is notably poorer than for tidal, where the timing and magnitude of events are known precisely years into the future.
Overall, the committee would like to emphasize that the practical resource for each of the individual potential power sources is likely to be much less than the theoretical or technical resource.
Tidal resource NAS criticisms
Based on the final assessment report, the assessment group produced estimates of the total theoretical power resource. However, this was done for complete turbine fences, which essentially act as barrages. The group did not assess the potential of more realistic deployments with fewer turbines, nor did they incorporate technology characteristics to estimate the technical resource base. It is clear, however, that the practical resource will be very much less than the theoretical resource.
Because power is related to the cube of current speed, errors of 100% or more occur in the prediction of tidal power density in many model regions. In the Pmax scenario, the fence of turbines is effectively acting as a barrage, so that Pmax is essentially the power available when all water entering a bay is forced to flow through the turbines. Pmax is thus likely to be a considerable overestimate of the practical extractable resource once other considerations, such as extraction and socioeconomic filters are taken into account.
Allowing for the back effects of an in-stream turbine array deployed in a limited region of a larger scale flow requires extensive further numerical modeling that was not undertaken in the present tidal resource assessment study and is in its early stages elsewhere. However, a theoretical study by Garrett and Cummins (2013) has examined the maximum power that could be obtained from an array of turbines in an otherwise uniform region of shallow water that is not confined by any lateral boundaries. The effect of the turbines is represented as a drag in addition to any natural friction. As the additional drag is increased, the power also increases at first, but the currents inside the turbine region decrease as the flow is diverted and, as in other situations, there is a point at which the extracted power starts to decrease. The maximum power obtainable from the turbine array depends strongly on the local fluid dynamics of the area of interest. Generally, for an array larger than a few kilometers in water shallower than a few tens of meters, the maximum obtainable power will be approximately half to three-quarters of the natural frictional dissipation of the undisturbed flow in the region containing the turbines. In deeper water, the natural friction coefficient in this result is replaced by twice the tidal frequency. For small arrays, the maximum power is approximately 0.7 times the energy flux incident on the vertical cross-sectional area of the array (Garrett and Cummins, 2013). Estimates of the true available power must also take into account other uses of the coastal ocean and engineering challenges.
Conclusions & Recommendations. The assessment of the tidal resource assessment group is valuable for identifying geographic regions of interest for the further study of potential tidal power. However, although Pmax (suitably modified to allow for multiple tidal constituents) may be regarded as an upper bound to the theoretical resource, it is an overestimate of the technical resource, as it does not take turbine characteristics and efficiencies into account. More important, it is likely to be a very considerable overestimate of the practical resource as it assumes a complete fence of turbines across the entrance to a bay, an unlikely situation. Thus, Pmax overestimates what is realistically recoverable, and the group does not present a methodology for including the technological and other constraints necessary to estimate the technical and practical resource base. The power density maps presented by the group are primarily applicable to single turbines or to a limited number of turbines that would not result in major back effects on the currents. Additionally, errors of up to 30% for estimating tidal currents translate into potential errors of a factor of more than 2 for estimating potential power. Because the cost of energy for tidal arrays is very sensitive to resource power density, this magnitude of error would be quite significant from a project-planning standpoint. The limited number of validation locations and the short length of data periods used lead the committee to conclude that the model was not properly validated in all 52 model domains, at both spatial and temporal scales. Further, the committee is concerned about the potential for misuse of power density maps by end users, as calculating an aggregate number for the theoretical U.S. tidal energy resource is not possible from a grid summation of the horizontal kinetic power densities obtained using the model and GIS results. Summation across a single-channel cross section also does not give a correct estimate of the available power. Moreover, the values for the power across several channel cross sections cannot be added together. The tidal resource assessment is likely to highlight regions of strong currents, but large uncertainties are included in its characterization of the resource. Given that errors of up to 30% in the estimated tidal currents translate into potential errors of more than a factor of 2 in the estimate of potential power, developers would have to perform further fieldwork and modeling, even for planning small projects with only a few turbines.
The tidal resource assessment is likely to highlight regions of strong currents, but large uncertainties are included in its characterization of the resource. Errors of up to 30% in the estimated tidal currents translate into potential errors of more than a factor of two in the estimate of potential power. Although maximum extractable power may be regarded as an upper bound to the theoretical resource, it overestimates the technical resource because the turbine characteristics and efficiencies are not taken into account.
Waves. The theoretical wave resource assessment estimates are reasonable, especially for mapping wave power density; however, the approach taken by the assessment group is not suitable for shallow water and is prone to overestimating the resource. The group used a “unit circle” approach to estimate the total theoretical resource, which summed the wave energy flux across a cylinder of unit diameter along a line of interest, such as a depth contour. This approach has the potential to double-count a portion of the wave energy if the direction of the wave energy flux is not perpendicular to the line of interest or if there is significant wave reflection from the shore. Further, the technical resource assessment is based on optimistic assumptions about the efficiency of conversion devices and wave-device capacity, thus likely overestimating the available technical resource. Recommendation: Any future site-specific studies in shallow water should be accompanied by a modeling effort that resolves the inner shelf bathymetric variability and accounts for the physical processes that dominate in shallow water (e.g., refraction, diffraction, shoaling, and wave dissipation due to bottom friction and wave breaking).
The wave power team used a model that’s only accurate in water depth over 164 feet deep (50 m). Yet shallow-water regions are where developers might prefer to put wave machines to minimize the distance to connect to the grid, and would be easier and cheaper to build and maintain if close to shore. NAS recommended a model for shallow be used next time, one with much higher spatial resolution that includes shallow-water physics (e.g., refraction, diffraction, shoaling, wave dissipation due to bottom friction and wave breaking).
Nor did they capture how often very large waves or extreme weather events are likely to occur that might destroy or harm the wave power equipment, and the model was likely to double count part of the wave energy, and even when this was pointed out, continued to use this methodology even though it “clearly overestimates the total theoretical resource”.
The mechanical and electrical losses in the transformation processes and transmission significantly reduce the technical resource, typically to 15-25% of the recoverable power. So the Energetech prototype would have had a technical power resource of just 4.5% to 7.5% of the incident wave’s theoretical power.
Estimates of the current state of wave-energy technology are not based on proven devices.
Ocean Currents. The ocean current resource assessment is valuable because it provides a rough estimate of ocean current power in U.S. coastal waters. However, less time could have been spent looking at the West Coast in order to concentrate more fully on the Florida Strait region of the Gulf Stream, where the ocean current can exceed 2 m/s. This would have also allowed more focus on the effects of meandering and seasonal variability. Additionally, the current maps cannot be used directly to estimate the magnitude of the resource. The deployment of large turbine farms would have a back effect on the currents, reducing them and limiting the potential power. Recommendation: Any follow-on work for the Florida Current should include a thorough evaluation of back effects related to placing turbine arrays in the strait by using detailed numerical simulations that include the representation of extensive turbine arrays. Such models should also be used to investigate array optimization of device location and spacing. The effects of meandering and seasonal variability within the Florida Current should also be discussed.
The group chose to use a specific OTEC plant model proprietary to Lockheed Martin as the basis for its resource assessment, a 100-MW plant, a size generally considered to be large enough to be economically viable and of utility-scale interest yet small enough to construct with manageable environmental impacts. Since no plants this large have yet been built, there are many technical and environmental challenges to overcome before even larger plants are attempted.
The committee views the use of the HYCOM model for assessment of the theoretical resource to be inadequate and also regards the application of a specific proprietary Lockheed Martin plant model with a fixed pipe length to be unnecessarily restrictive.
The DOE funding opportunity for OTEC was the only one to specify that the assessment should include both U.S. and global resources, and the assessment group chose to focus on the global resource. The committee believed, however, that more emphasis should have been placed on potential OTEC candidates in U.S. coastal waters. To demonstrate this point, the committee evaluated equation 1 and used the National Oceanographic Data Center of the National Oceanic and Atmospheric Administration’s World Ocean Atlas data to map this function for a 1,000-m pipe length, a TGE efficiency of 0.85, and PL of 30 percent. This simple exercise shows that in USA territory, the coastal regions of the Hawaiian Islands, Puerto Rico and the U.S. Virgin Islands, Guam and the Northern Mariana Islands, and American Samoa would be the most efficient sites for OTEC.
The committee is also concerned that the 2-yr HYCOM run will not provide proper statistics on the temporal variability of the thermal resource. Although it does include both El Niño and La Niña events, 2 years is not sufficient to characterize the global ocean temperature field with any reliability. Longer datasets are widely available, so it is not clear why the assessment group limited itself in this way. Ocean databases that extend for more than 50 years are readily available; these data would allow assessment of the inter-annual variability in thermal structure due to El Niño/Southern Oscillation (ENSO) to be evaluated. The advantage of HYCOM’s higher resolution over earlier estimates from coarser climatologies may vanish if HYCOM is used without appropriate boundary conditions near the coasts, resulting in inaccurate seasonal and inter-annual statistics on thermal structure. Without these abilities, this study is not much more valuable than prior maps of global ocean temperature differences, which already identified OTEC hot spots.
The OTEC assessment group’s GIS database provides a visualization tool to identify sites for optimal OTEC plant placement. However, assumptions about the plant model design and a limited temperature data set impair the utility of the assessment. Recommendation: Any future studies of the U.S. OTEC resource should focus on Hawaii and Puerto Rico, where there is both a potential thermal resource and a demand for electricity
Rivers and Streams. The theoretical resource estimate from the in-stream assessment group is based upon a reasonable approach and provides an upper bound to the available resource; however, the estimate of technical resources is flawed by the assessment group’s recovery factor approach (the ratio of technical to theoretical resource) and the omission of other important factors, most importantly the omission of statistical variation of stream discharge. Recommendation: Future work on the in-stream resource should focus on a more defensible estimate of the recovery factor, including directly calculating the technically recoverable resource by (1) developing an estimate of channel shape for each stream segment and (2) using flow statistics for each segment and an assumed array deployment. The five hydrologic regions that comprise the bulk of the identified in-stream resource should be tested further to assure the validity of the assessment methodologies. In addition, a two- or three-dimensional computational model should be used to evaluate the flow resistance effects of the turbine on the flow
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