Utility scale solar thermal power is something you still don’t hear much about, but along with photovoltaic power, it is a big part of the reason solar power is possibly the only source of renewable energy that is not only absolutely clean and sustainable, but capable of exponential growth for decades to come. And Ausra, headquartered in Palo Alto, California, has perhaps the most promising solar thermal design we’ve ever seen.
|Ausra’s “solar field” of reflectors.
Note the heat exchanging tube overhead.
Solar thermal power uses mirrors to reflect sunlight onto heat exchangers, in order to heat a thermal transfer fluid to drive a turbine, which turns a generator to produce electricity. Only about 500 megawatts of solar thermal power capacity exist in the world today, most of it at Kramer Junction in California’s Mohave Desert.
Compared to total worldwide energy production, solar thermal electricity production is negligible, only about one 20th that of photovoltaic energy production, which itself represents less than one-twentieth of one percent of worldwide energy production. But this is about to change, and solar thermal technology will race photovoltaic technology neck and neck, as together they grow to a significant share of global energy production.
There are three basic ways to concentrate solar energy – one is a “power tower” where a boiler sits atop a tower surrounded by 2-axis tracking mirrors that each individually move each day to reflect the sun’s light onto the boiler. Another design is a field of parabolic mirrors, each of them equipped with a 2-axis mechanism to track the sun all day, with each of them having a heat exchanger positioned at a single reflective focal point a few feet away from the center of the dish. Finally, the most cost-effective design appears to be the parabolic trough, where only a single axis tracking mechanism moves curved, mirrored troughs each day from east to west, with a heat exchanging tube suspended at the reflective focal point above each trough. All of these designs have been tried with some success.
What Ausra has done is taken the parabolic trough concept, with the simpler single-axis tracking mechanism, but designed a solar field where one heat-exchanging tube, running north to south, is suspended several meters in the air above several lengthwise tracking mirrors. Because the heat exchanging tube is further away from the mirrors, they don’t need to be as curved, reducing costs. Because several mirrors share one tube, there is a greatly reduced need for plumbing. And single-axis rotation, simply moving east to west with the sun, requires far less mechanical elements, and far less maintenance.
Yesterday we had a chance to speak with John O’Donnell, a physicist who is now EVP of Ausra. He explained several additional reasons why Ausra’s design is destined to become the standard for solar thermal power stations. Each of the mirrors is designed just small enough to fit in a standard interstate multi-mode shipping container. Each mirror requires just eight minutes to be manufactured on an automated production line. They are light weight and primarily require only flat glass and raw steel in their manufacture. The heat exchanging tubes are single lined and require far less maintenance than earlier designs. The heat transfer fluid is water; not molten salt, or some other expensive, corrosive, toxic substance – just water, of which nearly 100% is recycled. Ausra’s elegant, least cost design, according to O’Donnell, “has generated explosive interest around the world.”
Something we never would have guessed is that the incremental costs for building an oversized steam storage unit are not significant; O’Donnell noted both that the storage unit would not consume very much space, and even if it were built out to allow 20 hours of operation per day, it would add less than 10% to the cost of the entire power station. A clear advantage of solar thermal power is this ease in stretching the hours of operation into the evening when power consumption is heaviest. O’Donnell stated the current designs have a steam storage unit sized to stretch the daily hours of electricity generation through 8 p.m., which is when peak demand typically begins to subside. Ausra believes they can sell electricity using their technology for $.10 per kilowatt-hour; a price that is definitely competitive with today’s rates, especially during peak hours.
A brief report on solar thermal power would not be complete without noting the space required to generate electricity using this technology. In “California Land Use Choices” we estimated you can get 130 megawatts from a one square mile solar thermal power station. A utility scale photovoltaic power station of the same size would generate about twice that, but would cost far more to build. According to O’Donnell, the plant they are in the permitting phase for right now, to be located just south of Paso Robles in sunny Central California, is going consume exactly one square mile, and it is designed to generate 175 megawatts. Unlike biofuel, the land required to power the world with solar thermal or photovoltaic energy is simply not significant.