The various physical and chemical techniques for energy storage will all continue to be investigated and developed. Of the physical techniques, pumped hydro and compressed air energy storage are the most promising for peak-shaving and loadleveling within the electricity supply network, provided the terrain and other conditions are suitable. For smaller-scale storage, further research will be conducted on flywheels and on electromagnetic and electrostatic devices. Of these, electromagnetic storage is too expensive for general use. Flywheels may prove suitable for some specialized uses, but we doubt that they will find substantial widespread application. Electrostatic devices (electrochemical capacitors) complement batteries in being high-power, low-energy devices and show considerable promise for use in hybrid systems.
Hydrogen energy, the so-called ‘ultimate’ form of energy, is the Holy Grail for environmentalists – clean, abundant, non-polluting. This dream has been around for over 30 years. The principle of producing hydrogen in an electrolyzer (using a renewable source of electricity), storing it as a chemical hydride, and regenerating the electricity in a fuel cell when needed, sounds attractive at first acquaintance. The practice and the economics are quite a different matter. In the early days of the dream, cheap abundant nuclear power was to have been the most practical means of generating the hydrogen. As this no longer seems likely, it will be necessary to fall back on solar – or wind-generated electricity. The requirement for three separate devices (electrolyzer, hydride-store, fuel cell) merely to store and utilize small quantities of electricity is not at all efficient from an energy viewpoint. Such an approach would therefore be a gross misuse of renewable energy. Moreover, the activity would be capital intensive and there would be the added cost of the power-conditioning equipment.
We do not see hydrogen being produced from renewables on a significant scale in the next 20 years. Rather, hydrogen for fuel cells will be produced, as now, from fossil fuels. Meanwhile, electrolyzers will continue to be used mostly for the production and processing of chemicals and metals, and for life-support oxygen in submarines and manned spacecraft. Recently, the largest hydrogen production plant in the UK, based on natural gas and producing 32 000 t of hydrogen per year, has come on-stream at a chemical manufacturing site in North Teeside. From the point of view of greenhouse gas emissions, however, the use of fossil fuels to generate hydrogen for chemicals manufacture or for use in fuel cells is useful only if the carbon dioxide that is inevitably produced can be sequestered. Practical technology for this does not yet exist and its development is an area for immediate attention.
The realization of a ‘Hydrogen Economy’ is linked irrevocably with that of the fuel cell. There is no doubt that fuel cells work best on hydrogen and this requires any other fuel to be converted to hydrogen, at least for use in low-temperature cells. Unless the fuel reformer is tied directly to the fuel cell and produces hydrogen at exactly the rate that the fuel cell demands, as is proposed in some of the electric vehicle concepts, it is necessary to have a buffer store for hydrogen. This may be a metal hydride or a chemical carrier. The alternative concept is to have a much larger, industrial-scale reformer, divorced from the fuel cell, and to establish a distribution system for the hydrogen. Some proponents of FCVs favour this approach and are considering setting up a chain of service stations where hydrogen is supplied on tap. There is then the problem of storing the hydrogen on-board the vehicle. The two options are high-pressure storage in gas cylinders – bulky and heavy, through rapidly improving – or in a hydride storage bed. The latter is feasible in theory, but there are some complex heat and mass-transfer problems to solve. As mentioned earlier, we are pessimistic about fuel cells for cars, less so for buses and trucks. It should also be noted that automobiles (particularly diesels) are becoming increasingly efficient. Clearly, the fuel cell is aiming at a moving target.
Stationary fuel cells are quite another matter and it is possible that within 20 years these will be installed widely, with hydrogen piped in from a centrally sited reformer. From an environmental standpoint, however, such an arrangement would not be ideal. All the carbon atoms in the fuel used by a reformer finish up as carbon dioxide so that there is no saving in greenhouse gas emissions, except in so far as the fuel cell is more efficient than an engine.
Finally, we turn to the role of batteries for energy storage. Here, some real progress is being made. In the past 10-20 years, there have been major improvements in the lead-acid battery, the nickel-metal-hydride battery has been invented and commercialized, and the lithium-ion battery has made its debut. In small sizes, the last-mentioned battery is sweeping the electronics market, and much work is in progress worldwide to scale it up to larger units, to ensure its safety, and to reduce its cost. Provided these goals are achieved, the lithium-ion battery and its off-shoot, the lithium-polymer battery have a promising future. Some larger storage batteries also appears encouraging. For example, sodium- nickel-chloride is targeted at the market for battery electric-vehicles while sodium – sulfur batteries (still being developed in Japan) would be appropriate for the storage of distributed electricity. Technically, the battery scene is looking promising for small-scale electricity storage, although there is still the issue of cost to be faced.