In its funding opportunity announcements (DOE, 2008 and 2009), DOE requested that the assessment groups determine the “maximum practicably extractable energy,” which the committee originally interpreted as equivalent to the practical resource called for in the conceptual framework. After discussion with both DOE and the assessment groups, the committee concluded that the groups had interpreted “maximum practicably extractable energy” to mean the technical resource and that DOE did not expect the assessment effort to incorporate site-specific information needed to quantify the practical resource.
While a determination of the practical resource is beyond the scope of the tasks assigned by DOE, the committee sees the constraints represented by the socioeconomic and environmental filters as being among the most important considerations influencing future MHK investments. Box 1-3, which discusses these types of constraints on the development of solar energy, is presented as an example of what might be needed to assess the MHK practical resource. These filters are also central to evaluating the potential maximum contribution of MHK to U.S. electricity generation. The socioeconomic and environmental filters that need to be considered in an assessment of the MHK resource are described further in Chapter 7.
Assessing the potential for a particular renewable technology to address U.S. energy needs based on the theoretical resource alone would be inappropriate. As an example, solar power plants (which were first constructed nearly 30 years ago) currently provide less than 0.1 percent of the electricity consumed in the United States despite having a theoretical resource base that is orders of magnitude larger than current U.S. electricity consumption (EIA, 2012). While national-scale resource assessments may be useful for identifying geographic regions of interest for a particular MHK extraction technology, the practical resource will depend on a host of technical and environmental factors and may be significantly lower than what the assessments indicate is regionally or locally available. A survey of annual total energy outputs from several existing solar plants indicates that the ratio of plant outputs to the locally available theoretical resource ranges from as little as 2 percent for photovoltaics to as much as 12 percent for concentrated solar (National Renewable Energy Laboratory [NREL], available at http://www.nrel.gov/gis/solar.html; EIA, available at http://www.eia.gov/electricity/data/eia923/index.html). It is not possible to predict the practical MHK resource from national resource assessments until the constraints posed by both the technical extraction filters and the practical socioeconomic and environmental filters are better quantified for each of the specific resources.