Solar Power for Sustainable Transportation Systems


J. Richard Guadagno

August 24, 2000

Present population and energy consumption trends indicate that within the next fifty years, all of the world’s fossil fuel reserves will be exhausted. Petroleum will probably disappear first, around the year 2020, although domestic (North American) natural gas may become exhausted at about the same time. Coal will last only a few decades longer even if world population becomes stabilized, since it will probably have to be substituted for much of today’s oil and natural gas – at least temporarily – in addition to its present heavy use for electric power production and industrial applications. In any case, world civilization will inevitably become totally dependent on direct solar energy long before the 22nd century begins. Moreover, the most practical way in which the sun’s energy can meet the expected demand appears to be through its direct conversion into electricity by photovoltaic collectors.

It is somewhat ironic that the resource which maximizes our ability to gather solar energy is one which has always been regarded as worthless: uninhabitable desert wastelands. The eastern hemisphere is much better equipped in this respect than the western. The Sahara Desert is so vast and so empty that it alone can easily supply all of the needs for both Africa and Europe without displacing any settled populations. Other deserts scattered across southern Asia will suffice for that continent, providing that population becomes stabilized (either intentionally or through famine, disease, and war). Australia will have it easiest of all, with its vast interior deserts. But Americans, on both continents, will have to plan more carefully to be able to utilize their limited amounts of unused desert terrain.

Eventually every building in the world will probably be roofed with solar collectors. This will be adequate to provide all of our energy needs, and in a much simpler, cheaper, and more efficient manner than the complicated hodgepodge of energy exploitation which we utilize today. But until this happens, let’s be more specific and concentrate on a single, more immediate issue whose development will provide data and experience for other, currently less urgent future energy demands: the design of a nationwide ground transportation system for the United States which can be powered entirely by photovoltaic energy. To accomplish this goal, such a system must maximize the energy efficiency of all its various components. The most important of these, since it provides the majority of the demand for transportation energy consumed today – and is likely to continue to do so for the foreseeable future – is the means used for the propulsion of vehicles of all types for trips involving more than strictly local travel.

It is highly probable that only one propulsion system will be able to achieve this goal – a linear synchronous motor, or LSM. This device, with its proven capability of converting electrical energy into the kinetic energy of vehicles with efficiencies of up to 98%, exceeds the maximum achievable by its nearest competitor, a linear induction motor, by 40%. Moreover, LSMs offer many other advantages as well: (1) absolute control of the positions of all vehicles at all times; (2) traffic capacities which are several times as high as any other method, therefore minimizing the number of corridors necessary; (3) automatic regeneration of energy during braking and on downhill grades; and (4) an extremely simple computer control system capable of handling large volumes of traffic over a very wide area with the minimum chance for error.

If these LSM-driven systems are elevated above the ground, and if the vehicles are suspended beneath the track structure, more than half of the required energy – based on the average insolation values for the contiguous 48 states – can be collected by solar panels mounted directly on these track structures. The remainder can be gathered in most cases by installing similar collectors on the roofs of existing buildings adjacent to the tracks. Areas where few buildings are present are also those where such panels can most easily be installed in fields along the tracks, so this is another possible siting method.

But the sun doesn’t shine with equal intensity in all areas. Nor does it always shine in any area at the precise times that its energy is needed. Therefore two additional features must be added to provide a high-reliability supply of solar energy: means for transporting electricity from one area to another, and means for storing energy, sometimes for long periods of time. There are two regions of the United States which receive sufficient insolation to provide all the energy locally without the need for exchange with other areas: the Intermountain West and the Southern Midwest to Southeastern region. The Pacific coast and the northern Great Plains have such wide seasonal variations in insolation that either some exchange with other regions or large storage facilities will be necessary. The northern Central Plains and Northeast will probably have to depend on at least some importation of power from other regions. And the Southwest, with its vast excess of solar collection potential, will be the primary source of this power. Hawaii can readily achieve solar energy independence, but Alaska will need massive storage facilities to utilize the abundant excess collected during its long summer days for use in the dark of winter.

The extra solar collectors cannot simply be sited at random, but must be located with due respect paid to environmental and social concerns. But this can be accomplished. In the Southwest, for example, there are three types of terrain where many collectors can be built without adversely affecting the environment – and perhaps can even improve it. On a solo backpack across Arizona several years ago, I traversed many miles of wild and spectacular desert mountains and canyons. But in between these magnificent areas, I also had to cross vast flats or gently sloping regions where there seemed to be only one plant species – creosote bush – present, and where wildlife was almost totally absent. The few animals and birds which are there all seem to be concentrated along the occasional dry washes which collect the sparse runoff and allow other, more beneficial plants to grow.

If solar collectors were to be arrayed above the ground (and the creosote bushes) in these areas in properly spaced north-south rows, more than enough sunlight would still filter through to supply any plant species capable of growing there. At the same time, the decrease in the intense insolation would reduce the evaporation rate of the small amount of water which does fall, to the point where other, greener, and more varied plants could also grow, thus providing habitat for wildlife more characteristic of that along the washes today. The same procedure could also be followed at the numerous dry lake beds – some of them quite large – scattered from California’s Mojave desert across portions of Nevada, Arizona, and western Utah. Here one must make sure that the collectors are mounted high enough to remain above the water level on those very rare occasions when flash floods fill the lakes temporarily.

The third opportunity for environmentally benign installation of large solar cell arrays lies in the fact that a number of huge irrigation reservoirs have been constructed in the Southwest. Most of these have been located at low elevations where both temperatures and evaporation rates are very high. In addition, the reservoirs’ sizes allow large waves to propagate during the frequent periods of high winds. The resulting whitecaps cause even more water to be vaporized into the dry desert air. This effect is so great, in fact, that the amount of water lost from Utah’s Lake Powell alone exceeds all of the Colorado River water consumed for irrigation and domestic purposes in the states of Colorado, Wyoming, and Utah combined. Ironically, this unconscionable waste is taking place in that portion of the United States where underground water resources are being depleted at the fastest rate. Southern Arizona, one of the most rapidly growing places in the country, will be facing a water crisis within a decade or so which threatens to leave millions of people without reliable sources of water.

This problem could be alleviated to a significant degree if solar panels were to be mounted on large rafts of interconnected barges floating on the surfaces of the reservoirs and anchored to the bottom. The evaporation rate from that area of water covered by the barges would be reduced to near zero. Moreover, if each such raft was also surrounded by a skirt serving as a breakwater, the wave action generated by wind could also be controlled. The resulting water savings – plus those from similar treatment of California’s Salton Sea – could easily be sufficient to supply the domestic needs of the area’s entire population. An additional benefit, especially to Mexico, which lies at the end of the Colorado’s water delivery system, would be a reduction in the salt content of the water, which is greatly enhanced through excess evaporation.

Portions of each reservoir’s water surface could be left open to provide sufficient navigation channels for the many recreational boaters on the reservoirs. Fishing in the lakes would, in all probability, actually be improved. Some people may complain about the alteration of some of the scenic vistas, but if they knew what lies beneath the water, they would realize that most of the damage has already been done by the construction of the reservoirs themselves.

The same technique could also be applied to the huge reservoirs built along the Missouri River in Montana and the Dakotas. These reservoirs enjoy the advantage of being located closer to the regions of maximum energy demand. In this case, however, the reservoirs freeze over during winter, and energy collection would also decrease at that time of year. Perhaps the barges here could be designed to remain imbedded in the ice, but they may instead have to be hauled ashore for the winter.

Transferring electrical energy from areas where an excess can be produced to other areas – sometimes as far as 2000 miles away – where there is a deficit, is not an easy task. It will, of course, be much easier if superconductors can be used. At the present time, the materials making up these wires (or other geometries) must be cooled to a very low temperature before they become superconducting. The ultimate goal of researchers in this field is to produce materials which are superconducting at room temperature. Whether or not this will ever happen cannot be predicted, but much progress has already been made in this quest, and we can expect some more to be made in the future.

Therefore we will assume that superconductors will always need some cooling, and see how future transportation systems are likely to deal with that need. Again, if overhead structures are used, with the vehicles being suspended beneath them, it should be quite easy to leave space within these structures where superconductive lines can be installed. This space must provide room not only for the wires, but also for the cooling elements and surrounding insulation. The fact that superconducting wires for highly efficient systems are inherently small will make this task easier. Moreover, the placing of such conductors within strong, continuous structural members will shield them from damage, both mechanical and meteorological. This is especially true when compared with today’s power lines – exposed wires suspended in the air – which are subject to such a variety of abuses that power outages are frequent occurrences. A properly designed system of this type should require energy needs which are but a fraction of those of the LSMs, and the wires themselves can supply these needs.

The storage of electrical energy, whether it is from day to night, from sunny days to cloudy, or from summer to winter, is likely to require the development of entirely new technologies. Today the only widely used technique is that of pumped storage reservoirs. Water stored in a reservoir at one elevation is pumped up to another reservoir at a higher level during periods when power demand is low and extra capacity high. This water then runs through turbines back down to the first reservoir when additional power is needed.

This is a rather inefficient process, with relatively high losses for each diurnal cycle. Moreover, it can be used only in those areas where both cheap land and significant elevation differences are present. Since it employs gravitational energy, it requires a lot of material (water) to produce a relatively small amount of energy, and must therefore be regarded as a very low-grade energy storage mechanism.

We can expect future energy storage techniques to be quite different, and to be of a much higher grade, probably employing chemical instead of mechanical energy. Of those which have been proposed so far, the most promising appear to be (1) the production of hydrogen and oxygen from water through electrolysis, with subsequent recombination of the two in hydrogen-oxygen fuel cells, and (2) the use of sodium-sulfur batteries.

Electrolysis ranks right up there with LSMs as another extremely efficient industrial process. Ordinary water, with only enough electrolyte dissolved in it to make it conductive, can be separated into hydrogen and oxygen gas through the application of direct current between two electrodes. The hydrogen is released at one electrode, and the oxygen at the other. These can then easily be collected and stored in separate (and, to insure safety, separated) containers. They can then be recombined when power is needed by feeding them both into hydrogen-oxygen fuel cells. Such cells are considerably more efficient than the hydrogen-air fuel cells planned by some for use as vehicle power generators. Since photovoltaic panels produce direct current, it can be fed directly to the electrodes.

Performing this operation at a stationary facility has many major advantages over using hydrogen to propel a vehicle directly. You can get much higher output without dragging the extra weight of an oxygen tank around in the vehicle. The oxygen and hydrogen tanks can be separated far enough apart that – unlike performing the same task in a vehicle – a single accident of any type is very unlikely to occur which would bring the two gases together and risk an explosion. And both the gas tanks and the other facilities of the plant can easily by armored by thick walls to prevent either intentional or accidental disruption of the process. The output power of the fuel cells will be fed through motor-generator units or other devices to convert the direct current into alternating current, which is needed to power LSMs. Such power storage facilities would be built along the tracks for easy transfer of both incoming and outgoing current. Only a tiny reservoir of water would be needed to produce the same power output as a many times larger pumped-storage facility.

Sodium-sulfur batteries have also been proposed as vehicle propulsion means, primarily because both constituents are much lighter in weight than the lead of lead-acid batteries. But other considerations make them impractical for mobile use. The energy-producing reaction between the two elements can take place only at temperatures high enough to maintain both of them in a liquid state. At these temperatures, both elements are highly reactive, and if either of them comes into contact with either oxygen or moisture in the air, it is likely to cause a fiery explosion. Moreover, the reactive products of such a reaction are sodium hydroxide and sulfuric acid, respectively, both of which are very toxic and corrosive. Thus if a vehicle powered by a sodium-sulfur battery were to be involved in an accident, it is highly probable that a serious explosion would ensue, complete with the dispersion of some very caustic gases and liquids.

In a stationary facility, however, all of these problems can easily be eliminated. Thick, heavy walls coupled with insulation can not only keep interfering influences at bay, but they can also make it quite easy for the necessary high temperatures to be maintained at all times. The entire building, containing myriad individual sodium-sulfur cells, would be filled with nitrogen gas to forestall any reactions from possible leaks. Since human workers cannot tolerate either the temperature or the atmosphere, all necessary activities which might have to be performed in case of a failure of any type, would be carried out by simple robotic devices.

Both of these techniques utilize only resources which are abundant, such as water (which is all recycled anyway), sodium, and sulfur. Thus their development would not depend on scarce materials like nickel, cadmium, or lead. Nor are these the only alternatives. Other kinds of high-grade, low volume energy storage media may prove to be just as feasible. The important thing is that there are means available for powering our future transportation systems on a sustainable basis, and that our past and present reliance on biological and fossil fuels can eventually be looked back upon as a minor aberration in man’s historical evolution.


Last modified: August 24, 2000