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.