Nuclear Energy for Variable Electricity and Liquid Fuels Production: Integrating Nuclear with Renewables, Fossil Fuels, and Biomass for a Low- Carbon World
Author(s)Forsberg, Charles W.
Massachusetts Institute of Technology. Nuclear Energy and Sustainability Program
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The world faces two energy challenges: (1) the national security and economic challenge of dependence on foreign oil and (2) the need to reduce carbon dioxide emissions from the burning of fossil fuels to avoid climate change. Nuclear energy as a low-carbon domestic source of energy can address both challenges. However, nuclear energy in the United States is only used for base-load electricity production—about a quarter of the total energy demand. To address the two energy challenges, we have initiated a series of studies to understand long-term nuclearrenewable energy futures for a low-carbon world that can meet all energy demands. This includes liquid fossil fuel options with low greenhouse gas releases. This is a first effort to synthesize what has been learned about hybrid energy systems. The electricity challenge is to provide variable electricity production to match demand. Today this is primarily accomplished with variable-load fossil plants burning stored coal, oil, and natural gas. It is an economic option because of the low cost of storing fossil fuels and the relatively low cost of fossil power plants. The output of nuclear and renewable electricity sources do not match electricity demand. In a low-carbon world it would be required to store electricity when excess electricity is available to meet demand at times of low electricity production. If there are restrictions on carbon dioxide emissions, economics favors nuclear for most electricity production unless renewable electricity production costs are significantly lower than nuclear electricity production costs. This is because the amount of electricity that has to be stored to match electricity production with demand is much smaller in an all-nuclear system than any renewable system1. About two-thirds of all electricity demand is base-load electricity where the steady-state electricity output of a nuclear plant matches customer demand. While there are many electricity storage technologies to help match electricity production with demand over a period of a day (smart grid, pumped hydroelectric storage, batteries, etc.), only two seasonal energy storage technologies were identified2: nuclear geothermal heat storage and hydrogen. A nuclear renewables electricity system that also produces hydrogen for industrial markets may enable an economic system for variable electricity production where a larger fraction of the electricity can be produced by wind and solar energy sources. Nuclear energy can reduce greenhouse emissions from gasoline, diesel and jet fuel by replacing fossil fuels used in the production and refining processes. In the context of increasing U.S. oil production, a primary need is for heat to recover heavy oil and shale oil. U.S. shale oil resources exceed total oil produced worldwide to date and thus their use could eliminate U.S. dependence on foreign oil. The recovery and conversion of shale oil into liquid fuels using heat from nuclear reactors may have the lowest carbon dioxide releases per liter of fuel of all the fossil fuel alternatives to conventional crude oil production. Unlike almost all other industrial processes, shale oil and heavy oil production do not require steady-state heat input. That characteristic would allow nuclear plants coupled to shale oil and heavy oil production to operate at base-load with variable heat and electricity production. The variable electricity production could help match electricity production to demand and enable the larger-scale use of renewables. Heavy oil and shale oil production are the only potential industries large enough where variable heat demand is the alternative to energy storage to match electricity production with demand. Very little research has been done on these options. There is the potential for nuclear biofuels to supply a major fraction of the liquid fuels demand. This option results in no net addition of greenhouse gases to the atmosphere. Liquid fuels from biomass are limited by the availability of biomass. Synergisms between nuclear and biofuels can enable up to three times as much liquid fuel to be produced per ton of biomass. This is achieved by using nuclear to provide heat and hydrogen to operate the biorefinery and thus avoid the use of biomass as a fuel for the biorefinery. Liquid fuels can also be made from air and water with heat and electricity from nuclear power plants. This option can provide unlimited liquid fuel and places an upper cap on the cost of liquid fuels—2 to 3 times that of the cost of electricity on a unit heat basis. Key enabling technologies for a low-carbon nuclear-renewable energy system include nuclear-geothermal gigawatt-year energy storage, high-temperature electrolysis for hydrogen production, use of nuclear heat for reservoir heating of heavy oils and shale oil, conversion of lignin (the non-cellulosic component of plants) to liquid fuels, and densification of biomass for economic transport of biomass to large biorefineries. Most applications can be met with light water reactors; but some applications require the commercialization of high-temperature reactors. A nuclear renewables energy future is possible and potentially economic. Nuclear and renewable energy sources have different characteristics and in some systems are synergistic. Allnuclear or all-renewables energy futures are more expensive and difficult to achieve. Wind and solar economics are strongly dependent on location—particularly latitude because (1) it drives variable seasonal energy demands and (2) wind and solar inputs are functions of latitude. The analysis herein is for the United States but would be generally applicable for countries at similar or higher latitudes.3 Little work has been done to develop credible low-carbon energy futures for a prosperous world of 10-billion people. The uncertainties are very large.
Massachusetts Institute of Technology. Center for Advanced Nuclear Energy Systems. Nuclear Energy and Sustainability Program