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The levels of renewable energy use described in the above scenarios may require some major energy infrastructure changes. For example, the number of renewable energy technologies available today to directly displace petroleum use in the transportation sector is limited largely to ethanol and, perhaps, biodiesel. To meet the growing worldwide demand for transportation services in the second half of the 21st century, when world petroleum production will probably be in decline, will require other options. In the next few paragraphs, we

7 The REPP reference to a progress ratio of 82% (1 — 0.18) is based on growth in installations. The progress ratio is applied here to production because current installations are not well quantified. Mathematically this is justified if the production and installations are in a steady-state growth pattern.

briefly introduce what some of these major market shifts might entail for all energy sectors. We introduce market concepts in which alternative transportation forms reduce the need for liquid fuels, hydrogen becomes the common energy carrier, electric vehicles represent a means to use renewable energy more extensively, and superconducting transmission and storage are used to address the intermittency of leading non-hydro renewable energy technologies.

The scope of this chapter does not allow us to treat any of these topics in the detail they deserve and have received in multiple books and articles in the open literature. We focus here on the role that renewables might play in their realization, how they might provide larger markets for renewables, and some of the principal considerations in their development.

5.6.1 Hydrogen as a major energy carrier

Because it must be produced from other energy sources, hydrogen, as a fuel, is generally classified as an "energy carrier", not as an energy source. It is not a renewable fuel, but can be made using electricity from renewable energy sources or by using other energy feedstocks including natural gas, coal, biomass, and methanol.

The principal advantage of hydrogen is that it can be used as a fuel with no emissions at the point of use other than water vapor. Hydrogen can be directly combusted to produce heat and electricity or used at even higher efficiencies to produce electricity directly in fuel cells, a technology that is beginning to move into the marketplace. The major obstacles to the use of hydrogen as an energy carrier today are its cost of production, lack of an inexpensive storage technology, and the cost of fuel cells.

While there are no carbon emissions associated with the use of hydrogen, carbon is emitted in the steam reforming of methane, the most economic production method available today. Fortunately, there are ways to separate out the carbon emissions from the reforming process and to sequester that carbon. Research is under way to improve these processes and reduce their costs. If successful, such research could allow continued use of fossil fuels in a greenhouse-constrained world in centralized applications where the carbon is more easily separated and sequestered (Ogden, 1999).

Research is also under way to improve the production of hydrogen with renewable energy. The most cost-effective route today is to use electricity from renewable energy technologies to power an electrolyzer that separates water into oxygen and hydrogen. Polymer electrolyte membrane separators now operate routinely above 85% efficiency (Greenwinds, 2000), but they have relatively high capital costs approaching $1000/kW of input electricity. However, the future is expected to be different both because R&D is bringing down the costs of these technologies and because higher value is being placed on reductions in carbon and local air emissions.

Not only is research reducing the cost of electrolyzers and electricity production from renewables, but novel uses of renewables to produce hydrogen are also under investigation. These include mutant algal strains that produce hydrogen at higher rates than natural strains, photocatalytic water-splitting systems using non-toxic semiconductors, and photoelectrochemical light-harvesting systems that can split water molecules and are stable in a water/electrolyte solution. Biomass feedstock alternatives for hydrogen production are also under investigation. Biomass pyrolysis produces a bio-oil that, unlike petroleum, contains many highly reactive, oxygenated components derived mainly from constitutive carbohydrates and lignin. These components can be thermally cracked and the product's steam reformed at 750° to 850°C with Ni-based catalysts, high heat transfer rates, and appropriate reactor configurations to produce hydrogen with minimal char deposits (Padro, 1998).

This research is under way at least partially because hydrogen can greatly increase the opportunities to use renewable energy. The advantages hydrogen presents to renewable energy are twofold. First, it presents a means to convert electricity from renewable energy systems to a hydrogen fuel that can be used for transportation. Thus, renewable energy contributions in this sector would not be limited to only biomass-derived fuels, but could be made by all renewable electric technologies. Of course, this would require improvements in hydrogen storage, delivery systems, and other infrastructures. Secondly, hydrogen can be used as a storage medium allowing electricity produced at off-peak load times by intermittent renewable energy systems to be used later for on-peak loads. This would eliminate reliability and dispatchability constraints on the use of intermittent renewable electric sources like wind and solar. More importantly, it would allow these intermittent renewables to receive firm capacity payments for the electricity they provide, greatly enhancing their economic competitiveness. These two enhancements would provide a route by which renewables could greatly increase their contribution to the reduction of carbon emissions worldwide.

Transition to a worldwide hydrogen energy market could be a massive enterprise. However, with renewables it could be accomplished in stages. Initially, hydrogen and fuel cells could be used to simply firm renewable electricity, increasing the capacity value of intermittent renewable sources. In this case, the hydrogen would be consumed by a stationary fuel cell producing power at the time of peak loads at the same location where it is produced by an electrolyzer, eliminating any need for pipelines. Subsequently, additional hydrogen could be produced for transportation use in local fuel cell vehicles, delaying the need for extensive pipelines. Inasmuch as renewables are widely distributed, a large portion of the transportation market might be served in this decentralized fashion.

While a hydrogen-dominated energy market could produce some unique benefits for renewable energy, it might also afford an opportunity to separate and sequester carbon from fossil fuels, allowing their continued use in a climate-concerned world. We can be assured that technology developments will determine the future routes followed. We can also be fairly confident that a hydrogen economy will not evolve under the market conditions of today. Natural gas currently offers nearly all of the benefits of hydrogen except complete carbon reduction, yet costs less than one-fourth of that of hydrogen from electrolysis.

5.6.2 Electric vehicles

Another route by which the contribution of renewable energy might be greatly expanded to further reduce carbon emissions would be through the successful development of cost-competitive electric vehicles (EVs). As with the hydrogen economy described above, the use of EVs is not tied only to renewables; the electricity to charge the batteries of EVs can come from any generation source. But EVs would provide some extra advantages to intermittent renewables. As with hydrogen, EVs afford renewable electric technologies the opportunity to serve the transportation sector as well as other end-use sectors.

Secondly, EVs afford renewable electric technologies a storage medium, reducing the impact of intermittent generation from renewables. There are several infrastructure routes that might be followed. They vary primarily in terms of who owns and controls the recharging of the batteries used in the EVs. If consumers own their own batteries and charge them with no real-time input as to when electricity is most available and least expensive, then EVs will provide no real advantage to renewable electric technologies other than increasing the overall load and allowing for more electric capacity growth, which renewables could provide. However, if electric system operators have some control over when the batteries are charged, either directly or through real-time price signals to consumers, then there is a limited opportunity to charge the batteries when the intermittent renewable energy sources are available.

However, most vehicle owners will insist that their batteries be charged some time during the course of the night so as to be available for use on the following day. This limits the length of time one can wait for the intermittent renewable energy source to be available. If all owners waited to just before they needed the battery in the morning, a new peak load would be created.

Greater use of intermittent renewables might be achievable if a central repository owned the batteries, and vehicle owners simply exchanged batteries as required, similar to filling one's car at a gas station today. This scheme would provide more opportunities for charging the extra batteries when the intermittent renewables are available. Furthermore, it might provide a source of central storage to the electric system (i.e., the batteries not in use in a vehicle could be tied to the grid) that could reduce the need for peak generation and help smooth out transients introduced by intermittent renewable electric sources.

Regardless of the infrastructure developed, EVs, together with renewable electric technologies, could allow the displacement of much of the petroleum currently used for transportation and the carbon emissions associated with its use. Their use will depend primarily on resolving issues associated with EV batteries - cost, weight, life, performance, and driving range (National Laboratory Directors, 1997).

5.6.3 Superconductivity

Superconductivity is the ability of certain materials to conduct electrical current with no resistance and extremely low losses. Recent developments allow superconductivity to be maintained at relatively high temperatures using liquid nitrogen. Electric power applications include wires, motors, generators, fault current controllers, transformers, and superconducting magnetic storage.

Superconducting components could provide several advantages to renewable energy. With superconducting wires, it should be possible to transmit power for long distances with few energy losses. Cross-continent lines and even intercontinental lines could ensure that intermittent renewable electric resources spread across the continent(s) are always available to meet a region's peak load. Superconducting lines might be another route for transmitting hydropower from huge resources in the unpopulated areas of Africa and Siberia to loads thousands of kilometers away. Furthermore, superconducting magnetic energy storage might allow the off-peak generation by intermittent renewables to be stored and used on-peak. Intermittent renewable energy generators could then be credited with firm capacity.

More R&D is needed to make superconducting transmission lines and magnetic storage a physical reality, especially to make it a cost-effective option. If it does materialize as a significant factor in a carbon-constrained electric system, it will give an advantage not only to renewables, but also to conven-

tional generation technologies through more efficient generators, transformers, and transmission lines (DOE, 2000).

We have briefly discussed three areas of innovation that might lead to greater use of renewable energy. There are others, probably many that we haven't thought of at this time. The areas discussed above are not mutually exclusive, i.e., we could enjoy the benefits of both superconductivity and electric vehicles. There may even be synergisms between them.

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