The Real Path to Green Energy: Hybrid Nuclear-Renewable Power

Since the beginning of the industrial revolution, the world has developed an energy monoculture based on fossil fuels. And for good reason: Fossil fuels are easily transported; easily stored in coal piles, oil tanks, or underground; and easily scalable, from small home furnaces to massive factories.

Yet the emissions generated from burning fossil fuels are imperiling the planet. As such, the Group of Eight has pledged to reduce greenhouse gas emissions 80 percent by 2050 to combat the growing threat of climate change. Meanwhile, the developed world desperately wants to end its dependency on increasingly unstable oil resources that primarily come from a highly unstable region of the world–the Middle East. Oil, however, accounts for 35 percent of total world energy consumption and 39 percent of U.S. energy consumption. So weaning the developed world, and especially Americans, from it will not be easy.

Nuclear power has been proposed as a solution. And, again, for good reason: Nuclear power generates electricity without releasing carbon dioxide and offers a stable source of base-load power, or power night and day without interruption. But beyond directly producing base-load electricity, nuclear power also may prove to be surprisingly useful when paired with renewable energy sources such as biofuels, wind, and solar. In such a hybrid nuclear-renewable energy system, heat and hydrogen from nuclear reactors could provide the necessary carbon-emission-free energy to operate bio-mass-to-liquid fuel plants and provide backup electricity for intermittent wind and solar energy. In short, a hybrid system would take advantage of the complementary strengths of nuclear, wind, and solar power, along with biofuels, to become more economically viable and efficient.

The benefits of biofuels. Gasoline and diesel are the fuels of choice for the transportation sector because of their high energy densities, relative nontoxicity, and ease of storage. Existing technologies, such as the hybrid Toyota Prius and other high fuel efficiency vehicles, and near-term technologies, such as plug-in hybrid vehicles, may eventually reduce oil demand by one-half, but the transport sector will still need a transportable fuel. 1 Hence the interest in biofuels such as ethanol and biodiesel.

The problem is that producing liquid fuels from fossil fuels or biomass is itself an energy-intensive process–U.S. oil refineries constitute roughly 7 percent of the country's total energy demand. The fuel cycle for liquid fuels includes obtaining the feedstock, or raw materials; converting that feedstock to liquid fuels; transporting the liquid fuels to the user; and burning the liquid fuel in a car, truck, or airplane. Each of these steps consumes energy and releases carbon dioxide.

Liquid fuels can be made from any feedstock containing carbon, but the less similar the feedstock is to gasoline or diesel fuel, the more energy it takes to convert it. This applies to crude oil, natural gas, heavy oil, shale oil, coal, and biomass to differing degrees. Using coal as a feedstock, for example, is so inefficient that the liquefaction process consumes more energy than is available in the resulting fuel. Converting biomass into liquid fuels also is energy intensive. It differs from fossil fuels, however, in one key way: In theory, burning the resulting ethanol or diesel does not increase the total amount of carbon dioxide in the atmosphere. (Instead, it simply returns carbon dioxide to the air that had been removed by whatever crops are used as a feedstock.) With the caveat, of course, that non-fossil energy sources are used to produce it.

Using nuclear power to create biofuels. Today, most fuel ethanol is made from corn. So much energy is used to grow, transport, and convert that corn into ethanol that the production process consumes 70-80 percent of the final energy available in the fuel (and most of that energy comes from burning fossil fuels). But about one-half of this fossil fuel energy input could be replaced by supplying low-pressure steam from nuclear power plants to ethanol plants–an economically feasible way to reduce the greenhouse gas emissions from ethanol production. Already, this has been done commercially in Canada. And in Switzerland, Russia, and a few other countries, nuclear power plants sell low-pressure steam to nearby industrial customers–a strategy that should (and likely will) be adopted in the United States for biofuels production.

The combination of nuclear energy and biomass could produce enough gasoline and diesel fuel to replace oil in the U.S. transportation system.

Ethanol is by no means the only available liquid fuel production method for biomass. The energy content of liquid fuels per ton of biomass can be further maximized by converting the organic feedstocks to gasoline or diesel fuel rather than ethanol. The current way to do so is the Fischer-Tropsch Process–the same process used to convert coal into liquid fuel. (It dates back to the 1930s.) Studies, such as one by Idaho National Laboratory, have described how nuclear energy could be used to convert all of the carbon atoms in coal into liquid fuel without creating carbon dioxide emissions from the coal liquefaction process. 2 In such a system, nuclear reactors would be used to produce hydrogen from water, which would be combined with carbon monoxide from coal in the presence of a catalyst (e.g., iron) to produce synthetic liquid fuels. The same approach can be used for converting biomass into gasoline and diesel fuel, where the biomass is the feedstock while nuclear energy provides the heat and hydrogen to the biorefinery.

Biofuels by themselves could meet perhaps one-third of our liquid transportation fuel needs when biomass is used as both the feedstock and the energy source for a biorefinery. The combination of nuclear energy and biomass, however, could produce enough gasoline and diesel fuel to replace oil in the U.S. transportation system. 3 Furthermore, assessments indicate that countries around the world have similar access to biomass as does the United States. 4 Thus, the ability of biofuels to replace oil for global transportation needs depends upon the availability of external energy sources to power biorefineries. While some countries have more biomass than others, wide-scale biofuels production theoretically could disrupt energy monopolies that exist today based on nothing more than geology and geography.

Using nuclear power as a backup to renewable energy sources. A major issue in the large-scale adoption of renewable energy sources (e.g., wind and solar) for electricity production is the need for backup electricity. In addition to short-term variations in output (if the sun isn't shining or the wind isn't blowing), renewables have seasonal production patterns that don't match U.S. electricity demand. Wind production peaks in the spring–a time of low electricity demand. Likewise, peak solar electricity production is from April through August–a time of both some of the lowest and highest months of electricity demand.

Currently, the primary fuel to generate such backup electricity in the United States is natural gas. Its use as a backup power source, only fired up when demand spikes, makes sense economically–most of the cost is associated with the purchase of fuel and plant operation, not the initial plant construction. In contrast, for capital-intensive nuclear, renewable, and fossil fuel-burning power plants equipped with carbon-sequestration technology, most of the cost of electricity is associated with paying for the plant's construction. So if such plants operate only one-half of the time as backup power sources, electricity costs will almost double because the capital costs remain fixed while electricity production is cut in half. In other words, operating capital-intensive, electric-generating technologies at anything but full capacity results in high electricity costs. That would seem to make them useless in any hybrid energy system. Yet there are solutions that allow nuclear power and renewables sources to work together to maximize efficiency and economic viability.

Hydrogen's role in hybrid nuclear-renewable systems. One set of options involves the production and use of hydrogen. 5 Hydrogen can be produced from electricity when excess electricity is available. This hydrogen then can be used later to produce liquid fuels from biomass and to cover any electricity shortfalls. The important characteristic of hydrogen in the context of the electrical grid is that it can be stored inexpensively for days, weeks, or months in large underground facilities using technology originally developed to store natural gas. In the United States, commercial technology currently exists to store hydrogen on a scale sufficiently large enough to cover seasonal variations in electricity demand.

There are major thermodynamic incentives to produce hydrogen with electricity in tandem with nuclear reactors as opposed to using electricity alone. In traditional electrolysis, electricity is used to break water's chemical bonds and release hydrogen and oxygen. Today, high-temperature electrolysis is being developed where heat is used to convert water to steam; then it is electrolyzed to produce hydrogen with much lower inputs of electricity. If high-temperature electrolysis and similar technologies are commercialized (high-temperature electrolysis is currently in a pilot phase), energy sources that produce heat and electricity will have significantly lower hydrogen production costs than energy sources that solely produce electricity. Thus, nuclear reactors, which produce high-pressure, high-temperature steam at about one-third the cost of electricity, and solar thermal power plants, where mirrors concentrate sunlight on a central point to heat up a fluid and produce steam, would have economic advantages in hydrogen production relative to “cooler” energy technologies such as wind farms and solar cells.

In the long-term, as biofuels and hydrogen production technologies are developed, hybrid nuclear-renewable energy systems can become the prominent energy source to maximize biofuels production per ton of biomass.

High-temperature electrolysis may create other incentives for hybrid nuclear-renewable energy systems as well–for example, take wind farms, where relatively small differences in wind velocity can have large impacts on electricity production costs. In the United States, most of the high-quality wind resources are on the Great Plains, with the Dakotas having the strongest and most sustained winds. 6 Yet long-distance transport of electricity or hydrogen for liquid fuels production is expensive because the intermittent characteristics of wind means that pipelines operate at partial capacity for most of the year and power lines suffer long-distance transmission losses. In a hybrid system, however, when significant electricity output is coming from wind farms, the steam from nuclear plants would be diverted to the high-temperature electrolysis system with wind providing electricity for high-temperature electrolysis and to the grid. The nuclear and wind systems would each do what they do most economically to maximize efficient high-temperature electrolysis hydrogen production. If electricity is needed when windmills are still, the nuclear reactor and hydrogen from storage would be used to produce electricity. The economic feasibility of the system would be based on geographically isolated, low-cost renewable energy resources (such as remote wind farms), the economic gains of the high-temperature electrolysis process relative to traditional electrolysis, and the more efficient use of energy transmission facilities by maximizing transmission even when the wind farm is not operating.

A hybrid nuclear-renewable system that uses hydrogen at times of low wind or sun could incorporate other technologies to further maximize efficiency. Solid-oxide, high-temperature fuel cells are being developed by Siemens and other companies for electricity production as stand-alone units and are being integrated into combined-cycle gas turbines where the fuel cell functions as a “burner” that produces electricity. For peak-power applications, this technology has an advantage because a fuel cell operates as a high-temperature electrolysis unit producing hydrogen when operated in reverse. By using the same piece of equipment for both electricity and hydrogen production, the system's capital costs are minimized. This combination would allow the nuclear power station at times of excess renewable electricity production to be a consumer of electricity (nuclear steam plus grid electricity for hydrogen production and storage) and a producer of peak electricity (nuclear and fuel cells) at times of high electricity demand. It is a non-fossil method allowing for rapid responses to demand fluctuations.

Replacing fossil fuels. If biofuels are to replace oil, biofuel production will require massive quantities of heat and hydrogen. Today, steam from existing nuclear power plants could help biofuel production reduce its reliance on fossil fuels. In the long-term, as biofuels and hydrogen production technologies are developed, hybrid nuclear-renewable energy systems can become the prominent energy source to maximize biofuels production per ton of biomass.

Currently, hydrogen is used to produce liquid fuels from oil, tar sands, and coal. In the future, its primary use likely will be to maximize liquid fuels production from biomass. How hydrogen is produced will have major impacts on the electricity grid (not to mention future greenhouse gas emissions). If hydrogen is produced using electricity from nuclear and renewable sources and heat from nuclear reactors, its production can be easily varied. It also allows for the full utilization of capital intensive nuclear and renewables.

We are in a transition from a fossil-fuel dominated economy to a more diverse energy system. Such a future, where hybrid nuclear-renewable energy systems exist, will allow the different characteristics of each energy source to work best and allow for the ultimate replacement of fossil fuels.