to estimate the size of the MHK resource base. In order to assess the overall potential for U.S. MHK resources and technologies, DOE funded detailed resource assessments for estimating what it terms the “maximum practicably extractable energy” or “maximum practical, extractable energy” for each resource (see Appendix A for the funding announcements), as well as projects for generating the technological data necessary to estimate the expected performance of several MHK device designs currently under consideration (DOE, 2008 and 2009). The objective of DOE’s MHK resource assessment work was to help prioritize its overall portfolio of future research, increase understanding of MHK’s potential for generating electricity, and steer the developers of MHK devices and/or projects to locations of greatest promise.2 Earlier estimates (EPRI, 2005 and 2007) of the potential MHK resource are based on limited, possibly inaccurate data and assumptions related to the total resource and the fraction that might prove extractable.
DOE contracted with five assessment groups to conduct separate estimates of the extractable energy from five categories of MHK resources: waves, tidal currents, ocean currents, marine temperature gradients (also known as ocean thermal energy conversion [OTEC]), and free-flowing water in rivers and streams (DOE, 2010). The resource assessment groups are listed in Table 1-1. Each group was tasked with estimating the average power density of the resource base, as well as basic technology characteristics for potential devices and spatial and/or temporal variability of the resource. DOE requests for proposals did not offer a unified framework for the efforts, nor was there a requirement that the contractors coordinate their methodologies. As a result, each assessment group used distinct methodologies and assumptions, although there is some commonality between assessments being overseen by the same groups. The DOE contracts did specify that each assessment would have a validation component; those groups are also listed in Table 1-1.
DOE asked the National Research Council (NRC) to convene a committee of experts to evaluate the detailed assessments produced by each group, review the estimates of extractable energy, typically represented as average terawatt-hours per year (TWh/yr),3 and technology specifications,
2 H. Battey, U.S. Department of Energy, “DOE Water Power Program,” Presentation to the committee on February 8, 2011.
3 Note that TWh/yr is a unit of power and may be used to represent the average power generation over the time period indicated (1 gigawatt [GW] = 8.8 TWh/yr, 1 TWh/yr = 0.114 GW). However, a unit such as TWh/yr (or, as shown in an electricity bill, kilowatt-hours [kWh] per month) is a standard unit for the electricity sector. Energy units such as kWh or TWh measure the commodity that is generated by power plants and sold to consumers. For example, the Energy Information Agency’s (EIA’s) Annual Energy Review 2011 includes a table of total electricity generation that is given in billions of kWh/yr (EIA, 2012, Table 8.2a).