The longest running concentrated solar power plant in the US is about to reach its 30th birthday , and the end of its power purchase agreement – but its owners are not about to pack it up and take it home. They are now looking at the next 30 years, and storage is likely to form a major part of the equation.

CSP (also known as solar thermal) is often branded an emerging technology, but the first plants have been around for decades. The 14MW SEGS I and 30MW SEGS II plants near Daggett in the Mojave Desert in California were built in 1985. (SEGS stands for Solar Energy Generating System).

The plants use parabolic trough technology that uses curved mirrors to reflect sunlight onto a receiver containing fluids – in this case oils and synthetic fluids. That creates steam and drives a turbine. Now the owners, Cogentrix, owned by the Carlyle Group, are looking to upgrade the plant and possibly add storage as it seeks a new PPA to cover the next 20 years of operations.

Brad Bergman, the general manager of SEGS I and II, told RenewEconomy during a tour of the plants last week that CSP plants grow old gracefully and can continue to operate almost indefinitely. The initial capital cost has been depreciated, and any upgrades in equipment will be small in comparison.

“Coal and nuclear need to worry about pollution control and safety issues. We are not worried about emissions, and we don’t have a fuel cost, but we do need to focus on operations and maintenance costs.”

As part of that focus on O&M, and its attempts to secure an off-take agreement that better reflects the value of the electricity it produces, Bergman is currently testing a range of new trough designs that will improve efficiency, and he is also looking at storage options.

SEGS 1 actually had storage when it began in 1985, using the heat transfer fluid in large tanks. That ended about 12 years later, but the company is now looking at new cheaper and more efficient storage options such as molten salts. The infrastructure to support it is still in place, so it wouldn’t require large capital expenditure. SEGS II may also go with storage, although this would require a more expensive redesign of key equipment.

Bergman would not reveal the costs of the SEGS plants, other than to say that the general rule of thump for CSP plants is around 14c/kWh. It was recently reported, and a story republished on this site, that SEGS was receiving as low as 5.7c/kWh. Bergman, however, said that his plants were still on a fixed tariff PPA, and the price quoted simply reflected the wholesale cost in the winter period.

However, Bergman does want utilities to write a PPA that reflects the value of electricity, and time which it is delivered, which is in the daytime peaks. Adding storage, he says, would give it added flexibility – guaranteeing the electricity can be delivered at certain times, and/or extending into the evening.

The SEGS 1 plant is still using many of the mirrors that were installed nearly 30 years ago, although it is likely that these will be replaced in coming years with more efficient, new generation reflectors, which are growing in size with each generation. The SEGs II plant is currently testing a range of different products, including a mirror film. That will reduce breakage, but there is a question about durability.

As it is, Bergman says, the output from the parabolic troughs is quite smooth, “Once you build up heat, you get what we call solar inertia, so even if clouds come over, the effect is only gradual. We don’t get spikes like in solar PV plants.”

Daggett, in the heart of the Mojave desert, and right alongside part of Route 66, has one of the best solar resources in the world, at more than 1000 watts/square metre. It is also adjacent to transmission lines and land costs are minimal. The other 7 SEGS plants, which total another 300MW, are located nearby, as is a new plant being installed by Abengoa.