This past October, in our report “Ausra’s Solar Thermal Power,” we reported on this newcomer in the rapidly growing solar thermal power industry, with at least two innovations that could make them a major participant in this sector which is finally taking off. One of Ausra’s innovations is the design of their solar field – they have taken the single axis parabolic trough design, and revised it so that instead of one linear heat exchanger being positioned at the focal point of each mirrored parabolic trough, ten mirrors focus on one shared linear heat exchanger. In terms of mirrors vs. heat exchangers, going from one-to-one to ten-to-one saves a lot of money in plumbing costs, and makes an awful lot of intuitive sense.
|SOLAR RADIATION – CONTINENTAL UNITED STATES|
|Average kWh per square meter per day – the yellow
areas absorb 7-8, the blue areas average around 1-2.
(Source: Sandia National Labs)
Ausra’s other innovation may be more significant, however, as well as harder to understand. Ausra claims they have a proprietary method to store steam from the solar field, so their solar thermal power plant will be able to continue to generate electricity after the sunlight has faded. This is important because the solar peak is between 11 a.m. and 1 p.m., and the demand peak is between 6 p.m. and 8 p.m. If you can store the surplus thermal energy captured during peak sunlight and discharge it during peak demand six hours later, you will eliminate the biggest challenge facing solar power, which is that grid demand for electricity doesn’t exactly track with the availability of solar energy.
Storing steam is not easy. Today we talked with Greg Kolb, who is a distinguished member of the technical staff for concentrating solar power at Sandia Labs in New Mexico. Kolb stated that to-date thermal storage has usually used molten salt. Kolb explained that salt has been easier to work with because it is cheap, with nitrate salt there are no corrosion issues, it has twice the density of water, the molten salt can run directly through the heat exchangers in the solar field (just like water), and unlike water, molton salt doesn’t require high pressure vessels. Kolb claimed that “to get any economics with steam you would need a 1,500 PSI vessel.” And if Ausra’s 1 square mile solar field feeds steam to a 175 megawatt turbine that they want to run for, say, two hours after the sun is way past peak, even at a modern turbine efficiency of 50%, they are going to have to store 700 megawatt-hours of energy in the form of steam. At 1,170 BTUs per pound stored in steam at 1,500 PSI, at a weight density of .277 cubic feet per pound, 700 megawatt-hours of steam storage will require a steam storage volume of 16,000 cubic meters. And if they want to produce electricity 24 hours per day, they’ll need a lot more than that.
A few weeks ago at the AlwaysOn’s Venture Summit West conference, an engineer explained the difficulties with steam. Basically, the greater the volume of storage you need to keep under pressure, the stronger the walls of the pressure vessel will need to be. To use a basic example, if you have a cubic meter of interior storage, the 1,500 PSI energy contained in a cubic meter of volume is pushing against 6 square meters of interior vessel surface (a surface area to volume ratio of 6 to 1). But if you double the dimensions to 2 meters on a side, you have 24 square meters of interior surface area to contain 8 cubic meters of 1,500 PSI energy (a surface area to volume ratio of 3 to 1). That is to say, using this example, if the amount of interior surface relative to the amount of energy stored inside has halved, the walls have to be at least twice as thick in the larger vessel. Based on all this, it appears likely Ausra’s solution will employ multiple steam storage units.
Clearly if Ausra has figured this out, then they will be offering a huge step forward to the solar industry. Solar and wind energy cannot realize their full potential until there are viable, economic, storage technologies. And steam isn’t the only method where you can make a case for an economically viable solar thermal storage alternative. Kolb made a convincing case for using molton salt storage technology, and this method is being used in other installations around the world, primarily in Spain. One of the biggest companies active today in solar thermal utility scale electricity is Acciona Energia, based in Spain with installations all over the world. While they have experimented with steam storage as a 30 minute buffer to assist a solar field to provide even power through “cloud transients,” the heavy lifting 24 hour storage technology they are betting on is molten salts.
The most salient observation we can think of regarding solar thermal today is its seeming inevitability. Whether utility scale solar thermal storage arrives using molten salt, steam, or some other technology, it appears to be solveable, and the prospect of sub $.10 kWh renewable electricity with virtually no externalities is going to be a good business proposition for a very long time.