resource development, and direct MHK device and/or project developers to locations of greatest promise (Battey, 2011). In terms of resource assessments, the Energy Policy Act of 2005 (Public Law 109-58) directed the DOE to estimate the size of the MHK resource base. Earlier estimates (EPRI, 2005, 2007) of the amount of energy that could be extracted from MHK resources are based on limited and possibly inaccurate data regarding the total resource size and on potentially dated assumptions related to the amount of each resource that might ultimately prove extractable. To improve these estimates, the DOE contracted with the five assessment groups referred to above to conduct separate estimates of the extractable energy from five categories of MHK resources: waves, tidal currents, ocean currents, marine temperature gradients, and free-flowing water in rivers and streams (DOE, 2010). Performing these assessments requires that each group estimate the average power density of the resource base, as well as the basic technology characteristics and spatial and temporal constituents that convert power into electricity for that resource. Each assessment group is using distinct methodologies and assumptions. This NRC committee is tasked with evaluating the detailed assessments produced for the DOE, reviewing estimates of extractable energy (typically represented as average terawatt-hours [TWh] per year)1 and technology specifications, and accurately comparing the results across resource types.
In reviewing the initial methodologies from the five U.S. MHK resource assessment groups contracted by the DOE, the committee observed that the groups all employed different terminology to describe similar results. Thus, besides providing its review comments on each individual assessment, the committee is also taking on the role of providing a forum for comparing and contrasting the approaches taken by the respective assessment groups. To that end, the committee developed the conceptual framework of the overall MHK resource assessment process, presented in the section below, in order to help develop a common set of definitions and approaches.
1 Note that terawatt-hours per year can be translated into units of power, such as gigawatts, and used to represent the average power generation over the time period indicated. However, a unit such as terawatt-hours per year (or, as shown in an electricity bill, kilowatt-hours per month) is a standard unit for the electricity sector. Energy units such as kilowatt-hours or terawatt-hours measure the commodity that is generated by power plants and sold to consumers. For example, the Energy Information Agency’s table of total electricity generation (see http://www.eia.doe.gov/aer/pdf/pages/sec8_8.pdf) is given in billions of kilowatt-hours per year.