Contributed by Robert Lyman © 2017
A new paper prepared by the U.S. National Oceanic and Atmospheric Administration’s (NOAA) Earth System Laboratory and published in the Proceedings of the National Academy of Sciences (PNAS) is drawing attention in policy circles in the U.S. The paper critiques the claims of a study by Mark Jacobson et. al. that it is feasible, at low cost, to achieve 100% conversion of the U.S. electricity generation system to wind, hydroelectricity and solar energy by 2050 (the “WWS Vision”).
The authors of the critique include experts in the National Renewable Energy Laboratory, who could not be accused of being “climate sceptics”. Indeed, they have previously authored reports in which they concluded that an 80% decarbonisation of the U.S. electrical grid eventually could be achieved at “reasonable” cost, assuming that a broad suite of generation options and other technologies are employed. Their critique of the Jacobson et. al. study, instead, challenges its methodology, modelling and assumptions.
One of the most interesting parts of the critique concerns its challenge to the assumption that the variability of wind and solar generation will be readily addressed through the widespread and rapid installation of two grid-level energy storage systems. The Jacobson et. al. study assumes a total of 2,604 gigawatts (GW) of storage charging capacity, more than double the current capacity of all power plants in the U.S. The energy storage capacity would consist almost entirely of two technologies that “remain unproven at any scale”: 514.6 TWh of Underground Thermal Energy Storage (UTES) and 13.26 TWh of phase change materials (PCMs). Jacobson et.al. envision UTES systems being deployed in every home, business, office building, hospital, school, and factory in the United States.
In the “supporting information” published with the NOAA paper there are more detailed descriptions of the two storage technologies. The following is a summary that may be useful for the layperson.
Underground Thermal Energy Storage (UTES)
UTES systems use geothermal boreholes (i.e. holes drilled deeply into the earth) to store heat in the soil. To date, they have only been used in a handful of projects and at small scale. The largest UTES borehole system in the world is a project in Crailsheim, Germany that supplies seasonal thermal storage for 260 homes and two community buildings and has a total storage capacity of 0.0041 TWh. An even smaller ground hole heating system supplies Drake Landing, a master-planned community of solar-powered homes in Alberta. Both of these projects are supplemented by conventional fossil-fuelled heating systems. The performance and cost of UTES systems depends on the underlying geology of the site, such as the thermal properties of the soil and the absence of any groundwater flow (groundwater flow will remove stored heat over time). The two systems now operating supply only heating, whereas the Jacobson et. al. paper envisions 85% of U.S. residential air conditioning, 95% of commercial and industrial air conditioning, and 50% of commercial and industrial refrigeration being coupled with UTES and/or ice-based PCM storage systems.
The Jacobson et. al. paper states, but does not document, a wide range of costs for UTES varying from U.S. $0.71 to $1.71 * per kilowatt hour (kWh). The known capital costs for the Drake Landing system, according to the NOAA critique, suggest that a UTES installation cost of at least $1.8 trillion for the 100% wind, solar and hydroelectric system.. This excludes the cost of the requisite heating and cooling systems inside homes, businesses and industrial facilities capable of making use of stored energy in UTES systems. The cost estimates available also are based on installation of UTES at the construction stage; the costs of retrofitting existing buildings are likely to be higher.
(* updated/typo corrected June 27, 2017 – previously $.71)
Energy Storage in Phase-Change Materials (PCM)
The use of phase change materials in high-energy storage is still effectively at the research and development stage. To date, only a handful of concentrating solar power projects have been built worldwide with any thermal storage, and these systems exclusively employ more mature (and costly) molten salt storage systems. Phase-change materials, so-called due to their ability to store heat by transitioning from a solid to a liquid state, include paraffin wax and certain salts. Employing these materials could yield higher densities and lower costs than molten salt. However, several technical challenges must be solved before these storage systems would be ready for commercial use, including notably solving corrosion material degradation and thermal stress-related durability problems. As PCM remains pre-commercial, there is no reliable data for the current cost of PCM storage; it is anyone’s guess. A recent technical report from the International Energy Agency and the International Renewable Energy Agency reported a wide range of U.S. $11 to $55 per kWh.
The NOAA critique concludes that, “The relative immaturity of these technologies cannot be reconciled with the authors’ assertion that the solution proposed (in the Jacobson et.al. paper) are ready to be implemented today at scale at low cost and that there are no technological or economical hurdles to the proposed system.”
In fact, much of the case made by those who believe that the world can and should quickly transition away from fossil fuels rests on the thesis that the inherent variability problems associated with wind and solar energy can be solved by energy storage systems. The NOAA paper shows how this thesis rests on the assumed existence of energy storage systems that either do not yet exist, are not ready for mass application and/or would be extraordinarily expensive to implement.
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