Solar energy has a problem.

It’s available only in the daytime. Thus, it must be stored until nightfall when it is most needed. But that’s not the real problem.

It’s abundant in Las Vegas and scarce in Rochester. Thus, it must be transported to where it is most needed. That’s not the real problem either.

It’s heat, thermodynamically the least valuable kind of energy. Thus, it must be transformed to be useable.
 

Not the real problem, though.

The real problem with solar energy is that it is weak.

In Barstow, California, there's a huge solar energy demonstration plant. It takes up scores of acres. It uses hundreds of giant heliostats, computer-controlled mirrors, that reflect sunshine from all over the field onto a 300-foot tall, "black-body" absorbing tower in the center of the complex. (Pity some hapless bird that inadvertently flies near that beam.) 

Inside the tower, super-heated to nearly one thousand degrees by the concentrated rays, is the steam that drives turbines for the generation of electricity. 

The requisite investment for the project reflects somebody's grand vision (and wishful thinking) about the centralized use of solar energy. 

If you chance to pedal by this wondrous solar energy farm, by all means get off your bicycle and take a good look. It may be the only one of its kind in the world. Ever. 

If you look beyond the tower and the heliostats, you will see nearby a conventional, fossil-burning power plant. That one feeds electricity into the grid for the whole Southwest. 

As for the solar demonstration complex in the foreground, its total output is less than what's called the "parasitic load" for that conventional power plant. 

Which isn't much, just the power required to operate some pumps and office lights.
 
 

Energy is measurable in various units -- Btu's (British thermal units), kilowatt-hours, acre-feet. 

Acre-feet? 

That's right. The energy content of water stored behind a dam is conventionally expressed in acre-feet. To calculate this unit, you take the height of the water behind the dam, measured in feet, and multiply that by the size of the reservoir, measured in acres. 

Water flowing through a hydro-electric station under a dam produces some amount of kilowatt-hours from the acre-feet. The conversion efficiency has more to do with the "feet," than with the "acres." 

Dams should be few and high rather than many and not so high. 

That's why tidal energy conversion is not as practical as wishful thinkers think. Although damming the ocean puts vast tidal acres into storage, there are just not that many tidal feet to work with. 

The resulting available energy is so little that a dike around the entire U.S. would produce barely enough kilowatt-hours to light the city of Boston, Mass.

Some places in the ocean are saltier than other places. 

That's because the sun shines brighter in some places causing more evaporation and leaving behind a more concentrated salt solution. In other places, it rains more. The rain, being distilled water, dilutes the ocean. 

Places where evaporation exceeds rainfall are "zones of divergence," and the rainier places are "zones of convergence." Water is transported from the former to the latter in the atmosphere, under the influence of long-term weather patterns. 

Zones of convergence also occur on land. Rain runs downhill, eventually to the ocean. Bodies of fresh water occasionally accumulate along the way, and some rainwater soaks into the ground forming aquifers. That's where we get much of our fresh water. 

Can zones of divergence ever occur on dry land? 

Surprisingly, the answer is yes. And on some of the driest land in the world! In parts of the Sahara, for example, evaporation exceeds rainfall by quite a lot, which raises the question, where does that water come from? 

It seeps into the desert's aquifers from some distant zones of convergence. Evaporation takes place right through the porous desert soil, and the atmosphere carries the water vapor away, to be condensed into rain -- elsewhere. 

Thus, water is a renewable resource. In the hydrologic cycle, all the loops close. The earth's waters are constantly evaporating somewhere and condensing somewhere, blowing and flowing away, but always returning. Always in balance. 

Not quite.

"Dry farming" is a misnomer. It means farming without irrigation. But it isn't exactly dry. Where dry farming is done, there's usually plenty of rainfall. Where there isn't enough and irrigation is required, that should be called dry farming. 

Irrigation can be necessary even in places that have plenty of rainfall. Certain types of soil permit water to soak through too rapidly. Crops would dry out unless extra amounts of water are applied. 

Irrigation requires the transportation of water from somewhere, preferably from a zone of convergence. Or, it requires the pumping of water from underground. Unlike petroleum which is non-replenishable, water pumped from the ground will be replaced in due course by natural mechanisms in the hydrologic cycle. 

Replaced, that is, if you don't pump it out too fast. When that happens, the water table goes down. In effect, you are mining water. And the water you're using today didn't just arrive as rain yesterday. Or last year. As you pump out more and the water table goes down, you're irrigating today's crops with ancient rainwater. Wells must be dug deeper. Sound familiar?
 
 

To do all that pumping, you need to expend a great deal of energy. Wind, a derivative of solar energy, would seem to be an especially appropriate form of energy for pumping water out of the ground. But windmills produce only a minute fraction of the pumping energy for today's farms. Most of the rest is provided by, well, petroleum.
 
 

Consider the great cornfields of the Middle-West. When you look down on them from a jet plane, or a space craft, you see patterns of circles inside squares tesselating the landscape. What a curious sight! That's the result of a technology invented by a farmer in Columbus, Nebraska. It's called "center-pivot irrigation." 

There's a well in the center of each circle and a quarter-mile-long steel pipe strung out radially to the edge of the circle. Water is pumped out of the well and through the pipe to oversized sprinkler heads. Huge pneumatic-tired bogeys carry the pipe in its slow-moving arc, like the hour-hand of a gigantic terrestrial clock. The genius of this invention is in the synchronization of the wheels and in the linkages that keep the pipe straight. 

Center-pivot irrigation brings land unsuitable for "dry farming" into productive use. 

The law of unintended consequences also prevails. Water sprayed in massive quantities and in small droplets evaporates rapidly, creating a zone of divergence, which means that much of the water gets carried away in the atmosphere. 

Pumping is nearly continuous, and with all that evaporation, the water table keeps dropping. Farmers must dig their wells ever deeper, even in nearby areas where center-pivot irrigation is not needed. Along with water, the pumps bring up undesirable salts, and through repeated recirculation, those salts accumulate on the surface, gradually ruining the very land brought into cultivation by this technology. 

Finally, for each acre of farmland under center-pivot irrigation, more than 70 gallons per year of diesel fuel are consumed.
 
 

Among the "rediscoveries" in the post-petroleum age will be the commercial sailing ship. It isn't even fun to predict this anymore. Full-scale prototypes are already being developed on both sides of the Pacific. 

There is hysteresis in technology, though. The pathway forward will not retrace the pathway over which we came. We're not in a cul-de-sac. 

Sailing ships of the future will not resemble their ancestors in the previous century any more than the bicycles of today are fashioned after the high-wheelers. 

Look for titanium-steel masts, dacron sails, weather data from satellites, inertial navigation, and sophisticated computer systems for trimming the sails.

The only thing really wrong about the great dirigibles of the twenties was timing. They came along about a century too soon. 

Consider first the materials out of which the huge airships were built. Inside their metallized canvas coverings, each was held rigid by a fragile triangulated space frame made of laminated wood. 

Indeed, most of the 161 rigid airships that were ever built broke in two and crashed. Most of the rest, of course, burned up. That's because, the lifting gas was the notoriously flammable hydrogen. Helium is just about as effective and also inert. The technology leader, Zeppelin, built 130 airships and might have preferred to use helium, but couldn't get the stuff. The U.S. was the "OPEC" of helium. Still is. 

The lifting gas was sealed inside 16 or more cells made of -- now get this -- gold beaters' skin. That's a membrane taken from the intestines of cattle and goats. It took 50,000 skins to make just one cell. Today, we'd use polyethylene. Just think, your trash bags are made out of a far superior material to that used in the Hindenburg. 

Airships of the future would be made of "space-age" materials. A monocoque (single shell) hull has already been proposed. 

Laminations of fiber-glass and polyurethane, as in surf-boards, provide the highest strength and the lowest weight. Inside would be a "ballonet," a separate cell into which air can be pumped for ballast. To go up, just open a valve. 

Among the best proposed designs is one which calls for the hull to be shaped circular in plan-form and elliptical in elevation. A gigantic flying saucer. Computer models show how a 100-meter version will carry as much payload as the Hindenburg, about 100 tons, at speeds of over 100 MPH. 

Powered by the sun. 

Airships in the post-petroleum age will be covered by solar cells, and electric motors will turn their propellers. To be practical, remember, solar-powered things have to cast big shadows. The "sun ship" does that better than any other mode of transportation. 

Here is an opportunity for silicon technology, which has radically transformed our commercial and private lives in the information processing realms, to make a significant contribution to transportation as well.