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a third more than the cheapest sulfur-removal approach.

2 Light-water nuclear reactor. This ordinary-water reactor uses uranium oxide fuel processed from sandstone ore and enriched with two to four per cent fissionable uranium-235. Boiling water carries the heat from the reactor core to a steam plant, which generates electricity with a thirty-two per cent conversion efficiency. The spent fuel cannot be

easily processed to make atomic bombs.

3 Ground-based solar thermal plants, now undergoing limited prototype development by the Energy Research & Development Administration (ERDA). A ten megawatt power plant at Barstow, Calif. will be operational by 1980. (A megawatt is one million watts or one thousand kilowatts.) This type of plant consists of a vast field of flat mirrors, or heliostats, that reflect solar energy and trap it in a central receiver. The heat produces steam as in a coal boiler or reactor core: the steam is expanded through a turbine that runs a generator to produce electrical power. Since the sun shines only in the daytime, storage capacity is required or else a coal or nuclear backup plant will have to be combined (gasified coal backup is compared in the cost analysis table below).

The alternative to the night-and-cloud problem is to place the solar converters in space where they will bask in sunshine one hundred

per cent of the time (or ninety-nine per cent if in geosynchronous orbit, due to daily eclipses).

4 Geosynchronous solar satellite stations. These are satellites

placed in an orbit some 22,000

miles high and traveling the same speed

as the earth rotates, so that they appear stationary relative to a fixed

point on the ground.

The concept was evolved for the National Aeronautics

& Space Administration (NASA) by a team consisting of Arthur D. Little

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Inc. (headed by Dr. Peter E. Glaser, vice president of engineering sciences), Grumman Aerospace Corp., Raytheon Co., and the Spectrolab division of Textron Inc. Giant arrays several miles wide form each satellite, which will look through a small telescope more like thin slices of football fields hovering in space instead of the conventional spherical satellites we have grown accustomed to. But at this altitude each satellite would appear to the naked eye as a star, albeit one you could see in the daytime. The satellites will collect solar energy, concentrate it slightly (two to one) onto thin "photovoltaic" cells that convert sunlight directly into electricity, collect the resulting direct current at voltages of about twenty kilovolts and carry it across a rotating joint to a transmitter that changes the direct current to

microwave energy.

A coherent microwave beam is transmitted to ground

receiving antennas called "rectennas."

These are spread out in circles

several miles in diameter on the outskirts of major cities.

The microwave transmission from space to earth will operate at least at an eighty-two per cent efficiency. We know this for certain because

that was the efficiency attained in a dramatic Jet Propulsion Laboratory

demonstration in the summer of 1975, in which microwave energy was

transmitted across one mile of air at Goldstone, Calif. and then con

verted to electricity. Transmission through space will be even more efficient, although the atmosphere attenuates, or weakens, the beam somewhat during that portion of its trip near the earth so that about ten per cent of the energy is lost.

In a variation of the photovoltaic satellite plan, designed by Gordon R. Woodcock and D. L. Gregory of the Boeing Co., thousands of acres of plastic film mirrors concentrate sunlight into a solar cavity.

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a set of helium gas turbines that turn alternators.

The turbine heat

engine converters require no technological improvements, as do photovoltaic cells, which need to be made thinner and more efficient for

serious consideration in space.

Once the electric power is generated--whether by heat engine or by solar cells--it is routed to a radio-frequency generator where it is converted to microwaves. The narrow microwave beam can be directed to any point on earth visible to the satellite simply by swivelling the transmitter antenna a few degrees. Microwave power can be broadcast day and night, rain and shine. Hence storage of the energy is unnecessary. The mircowave energy cannot harm people in airplanes flying through the beams because of the metal shielding and brief exposures. On the ground,

the energy is too diffuse to harm human beings in the area adjacent to the rectennas or under them; it is well within international standards of ten milliwatts per square centimeter imposed for microwave ovens. In fact, the big dish rectennas can be elevated for frost-free farming of the ground beneath. The rectennas are not solid dishes, but consist of open grids that cause only partial shading below; some crops, such as tobacco, prefer shade. Alternatively the rectennas can be integrated into a roof structure over a city of the geodesic type recommended by R. Buckminster Fuller for Manhattan in which, he calculates, the cost of a two-mile diameter dome could be recovered in ten years from the savings in snow removal alone, besides additional savings in heating and cooling due to the reduced surface area of the city. Some ornithologists, though, are concerned that the microwaves will distress or kill birds that fly

into the beams.

Birds, after all, don't have the protection of metal

airplanes to shield them.

But this possibility seems to be almost

4

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exactly balanced by other scientists who think that the birds will be attracted to the beams, especially in the winter, because the microwaves will impart a pleasant warming sensation. Either way, the problem is important since it could change the local ecology, but does not present

a serious obstacle because, if necessary, bird screens could be erected around the rectennas to the height of bird altitudes.

5 Low-orbit solar satellites, devised by Dr. James E. Drummond,

director of plasma engineering at Maxwell Laboratories Inc.

Instead of a

twenty-two thousand mile geosynchronous orbit, the powersats are orbited at about twenty-nine hundred miles. Enough satellites in two belts at

plus and minus forty-five degrees to the plane of the ecliptic can service any point on earth, unlike geosynchronous satellites, which are limited to

areas between seventy degrees north and south latitudes (which of course

isn't much of a limitation).

You would see the low-orbit powersats dimly

from the ground as widely spaced dotted lines of tiny squares following the path of the sun, each line forty-five degrees on either side of that

path,

the square dots changing into slivers as they recede into the

horizon.

At night they will be faint, for most of their sunlight will be

reflected back toward the sun.

Dr. Drummond also suggests a new conversion system now in the con

ceptual design stage called "cascaded dielectric power conversion." The system is a closed-cycle thermal converter similar to Woodcock's and Gregory's, but it eliminates their bulky and heavy mechanical turbines. Heat is converted directly to electricity by depolarizing a thin piece of dielectric, or non-conducting, material within a capacitor, or device for storing a charge of electricity. As the moments of the dipole (two mag

netic poles of opposite sign) become unaligned with the electric field,

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they release electrons held on one plate of the capacitor.

The electrons

are allowed to flow through an external circuit to replace a deficiency of electrons which the polarization had helped to maintain at the opposite

plate, thus generating current.

As might be expected, the direct cost of these five viable technologies increases with their scientific sophistication (except, possibly, for the last, which may turn out to be as relatively cheap as shown below). The costs are compared in the table that follows, adapted from a remark; able document issued in March and prepared by Mr. Richard Caputo, project engineer at NASA's Jet Propulsion Laboratory. It is called "An Initial Comparative Assessment of Orbital and

Terrestrial Central Power Systems."

Plant No. 5, below, was estimated by

the author on the basis of Dr. Drummond's submission to the House Commit

tee on Science & Technology last year. These cost figures are open to debate but derive from a well-reasoned set of voluminous assumptions. They will suffice for our purposes to show the kinds of economic and social costs associated with the five power systems selected:

Cost of Energy Proj. Research &

at Ground

Deaths Type of Power System Development Construction before Distrib. (30 Yrs) 1 Coal low-energy gasification $1.5-billion $1150/kW 0.6c/KWH

530 2 Nuclear lightwater reactor 1.4-billion 2280/kw 0.8°/KWH

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