The US Department of Energy (DoE) recently announced a US$20 million CSP-specific solicitation. “The US DoE SunShot programme embraces PV and CSP with equal intensity,” Ramamoorthy Ramesh, Director of the SunShot Initiative, says. “Looking at today's cost structure, they are basically the same at approximately US$0.15–0.18/kWh. There is no reason why going from a photon to an electron is any different than photon to thermal energy.”

At utility-scale, Ramesh says that CSP has a lot of advantages, especially with thermal storage, which PV does not have. “But, essentially these are two fairly equal siblings. When we started SunShot in January 2011 we made the commitment that we will have good investment in both areas and we do,” he says.

CSP's edge: Storage

Ramesh notes that when the sun is shining PV works very well, but every time a cloud passes, electrical output is reduced: “You can't have effective utility-scale PV without some kind of backup. You could have a 50% change in energy output using only PV which is something that utilities are very concerned about, and rightfully so.

“On the other hand, with solar thermal which is what CSP is, you convert the energy from sunlight into thermal energy - molten salt for example - which is stored at around 800 degrees c. This can't be stored indefinitely, but it can easily be stored for 4–8 hours. It is like having a big bank account that gives you a huge nest egg to keep in the background if you need to use it. But you don't touch it unless you have to. Not all older CSP plants have storage, but all modern ones do at various levels. Storage is important. In our PV programme we are pushing R&D activities to develop methods of energy storage for that solar platform too,” he adds.

“You can't even compare PV to CSP because if you put in the storage, the capacity to generate power jumps 40–50% immediately for CSP. For PV without storage, it is only 18–20% which is the normal amount of electricity you can generate from the panels,” he adds. “But, even that said, you have to keep in mind that the cost of doing CSP is very different from the cost of generating power from PV or CPV because CSP is, and always has been, geared to the large utility-scale – the hundreds of megawatts.”

“CSP can also fit into other areas. Hybrids are the next generation. Natural gas utility plants can take advantage of CSP. The same goes for clean coal. This can boost the electricity to overcome fluctuations in power generation without adding emissions issues. These are very alive and very vibrant topics,” Ramesh says.

Getting costs down

When DoE talks about all technologies, its language is now shifting across the board to ‘levellised cost’ of approximately US$0.06/kWh for all types of solar, including installation and ongoing maintenance. According to Ramesh, PV currently has the same levellised cost and the same parity as CSP. “So the question is not who is cheaper right now. Nobody has yet attained the operating levellised cost of US$0.06/kWh. The question is who will get there by 2020?”

Pratt & Whitney Rocketdyne (PRW), the Canoga Park, CA, firm best known for the development of rockets that took us to the moon and beyond, is currently working on creating better and lower cost thermal storage and delivery for CSP plants. “Our technology makes CSP much more competitive in cost, with higher value power that can be dispatched when needed,” Randy Parsley, Programme Area Manager of PWR's Renewable and Alternative Energy division, says.

It is a molten salt technology, but said to be of a much higher standard than any other that is being tested in labs worldwide today. It is not merely the drawing board dream, but is being installed in a power plant currently under construction – the SolarReserve/PWR partnership project in Nevada, USA. The plant has broken ground and is set to be producing power by December 2013.

The salt is heated to 550°F, it transforms into a liquid that looks and behaves somewhat like water, except it remains a liquid at low pressure to over 1050°F, at which point it creates high-temperature steam, which drives a more-efficient, standard steam generator. “In the receiver, the sun's concentrated solar power heats the liquid salt to over 1050°F. That is the key. That is where Pratt & Whitney Rocketdyne has the knowledge and expertise because of former applied aerospace and rocket technology. That's the difficult part of the design,” Parsley explains.

It is a completely closed loop system and requires little or no replenishment. Parsley says that the salt does not ever change its basic composition: “Salt does not ever change its phase from liquid to gas. It's pure liquid salt at 550°F and when it is heated to 1050°F, it is just the same liquid salt – only hotter. Also, since there is no phase-change, there is no significant change in pressure or volume of the salt. This allows overall system pressures that are much lower. You just use the same salt over and over, every day, 365 days a year.

“By simply optimising the solar field for more storage, enough energy can be stored to easily run all the way through the night without the sun. That is our expertise. That's the technology we exclusively licensed to our customer, SolarReserve,” Parsley adds.

Along with its receiver and heat-control technology, PWR supplies the software and control system for the collector field.

Commenting on PWR's technology, SolarVision's Skumanich has to admit that if Rocketdyne can prove its technology with this installation over the course of the next few years, and the costs really do look lower, this will breathe new life into CSP.

A dual perspective

SCHOTT Solar is both a PV panel and CSP component manufacturer, and can therefore offer a unique insight into the PV vs. CSP question. Casey Gutowski, Vice President, Sales & Marketing, says: “The global market continues to move toward a diversified energy portfolio. We believe both PV and CSP play a significant role in that portfolio.”

On the PV side, SCHOTT Solar produces c-Si solar modules at its Albuquerque, NM, USA, facility. At the same location, the company produces what is considered the heart of the CSP solar plant – the parabolic trough receiver (PTR).

The SCHOTT PTR features coatings on both the inner steel tube and the outer glass envelope that allows the receiver to absorb and trap as much sunlight as possible. Because the receivers operate at very high temperatures (400°C/750°F) the coatings are key to high plant operating efficiencies and to keeping heat loss at a minimum.

Gutowski says: “Our goal is to work with the industry to drive costs down, and very small improvements yield very big gains. A recent example of a product improvement to the PTR receiver is a new coating that is being used for the SCHOTT PTR 70, which increases the absorption rate of the receiver to over 95.5%. At the same time, the emission level of the heat radiation sinks to under 9.5%, which increases the overall efficiency of both the receiver and the solar field. These improvements directly result in reducing the energy production cost to the end user. Plants with SCHOTT Solar CSP receivers include the Andasol CSP plants in Spain and Nevada Solar One in the USA.

CSP to rise again

Gutowski concludes: “The global market demand for CSP has increased significantly in the last few years, with specific focus on the US and Spain, both set in motion by specific incentives from the governments. We are also seeing new markets developing, in the MENA region, Southeast China, India, South Africa, and Australia.”

About the author:
Joyce Laird has an extensive background writing about the electronics industry; semiconductor development, R&D, wafer/foundry/IP and device integration into high density circuit designs.