Scaled generations of all-glass, glass-making solar furnaces

Specifications for scaled generations of all-glass, glass-making solar furnaces.

A classic science-kid's demonstration is making glass from sand with a solar furnace, and, in fact, the glass-making capacity of a solar furnace is prodigious. A solar furnace of any size, even if it is 100% glass, can make all the glass needed to replicate itself in a matter of weeks. The glass replication time of a small backyard solar furnace may be only a matter of days. More likely at large scale, the glass for a furnace would be produced by a smaller solar furnace that produces at a rate that does not outstrip the capacity to fabricate and assemble glass parts. That smaller furnace, in turn, may have been made by an still smaller furnace; and so on, through multiple generations of scale.

Starting at a very big scale (a solar furnace on 9 square miles of land) and working for the most part within areal units of the Public Land Survey System of the western United States, the table above shows specifications for six scaled generations. If these calculations can be taken seriously, the time from the completion of the room-sized 9-square meter furnace to the completion of its descendant quarter-township furnace is 6.3 years.

Quarter-township: the natural size of a replicable solar plant in the U.S.

The natural size of a replicable solar plant in the western United States is a quarter-township, or 9 square miles.

A solar power plant worthy of replication needs to be a good fit to its circumstances. In the western United States (Texas excepted) one part of those circumstances is the Public Land Survey System (PLSS,) a rectilinear grid to which property lines conform. Surveyed from reference points named for their meridians (e.g. San Bernardino Meridian, Gila and Salt River Meridian) the Public Land Survey System is a grid of six-mile by six-mile squares called townships.

In most of the American West, property lines conform to a national grid of 6 mi x 6 mi townships.

Another pre-existing circumstance for a solar thermal plant is the technology of steam-electric generating equipment. For many decades, electric utilities have preferred to build coal plants with multiple, separately-fired, turbine-generator units that are rated in the range of 600 MW to 1200 MW (see the chart below of the latest advanced high-temperature turbines installed by Siemens.)

Siemens' advanced steam turbines are commonly manufactured in the range of 600 to 1200 MW

New coal plants might operate at 80% capacity factor—i.e., their annual output is equivalent to running full power for 80% of the time—a statistic dependent not only on equipment reliability, but also on steady demand for electricity at a price that exceeds fuel costs. Capacity factors tend to come down in competition with wind turbines and other sources that do not pay for fuel, but a utility would certainly expect a new generating plant, solar or not, to be running full power most of the time. The Gemasolar plant in Spain has demonstrated that a solar plant with thermal storage can achieve a 75% capacity factor.

Taking advantage of an excellent solar climate, California's Ivanpah Solar Electric Generating System Unit #1 achieves a capacity factor of 0.32 without any energy storage at all. It has a rated power of 126 MW from a land area of 3,800,000 square meters or 33 W/m2 of land area. The mirror-area/land ratio for this plant is only 0.21. Upgrading to telescopic heliostats would bring the mirror-area/land ratio to 0.70, boosting the rated power to 33 W/m2 * 0.70/0.21 = 110 W/m2 of land area.

Based on Google imagery, the heliostat field of Ivanpah 1 is very nearly a square, 1995 m on a side, with three corner truncations, giving a heliostat field land area of about 3,800,000 square meters. This unit has 53,527 heliostats each with a mirror area of 15 square meters, giving a total mirror area of 803,000 square meters. The mirror/land ratio is 0.21.

Raising the 0.32 capacity factor of Ivanpah to 0.75 requires some amount of thermal storage (half-a-day, roughly) and a de-rating of the plant to 110 W/m2 * 0.32/0.75 = 47 W/m2 .

A quarter-township plant occupies a square of land 3 miles (4,828 m) on a side, having a land area of 9 square miles or 23,000,000 m2. Therefore, in Ivanpah's excellent solar climate, a quarter-township solar highbeams plant would be rated about 47 W/m2 * 23,000,000 m2 = 1.1 GW at a capacity factor of 75%.

Of course, the precise rating of a quarter-township plant would depend on many details including the quality of the local solar resource, but the size constraint will be the same throughout the West: a quarter-township.

In round metric numbers, a quarter-township solar plant is 5 km x 5 km. The central optics will be about 190 m tall— not that much taller than the 160 m power tower at Crescent Dunes, Nevada, or the 169 m (all-masonry) Washington Monument.