By Casey Gutowski, SCHOTT Solar

After more than a decade of hibernation, parabolic trough concentrated solar power (CSP) has emerged from its slumber with a mighty roar. Large new parabolic trough CSP power plants have recently opened or soon will open in the United States and Europe. Arizona Public Service has announced plans to build the 280 MW Solana generating station outside Phoenix. The Western Governors Association, the Club of Rome and Scientific American have all made CSP plants major elements of their renewable energy development plans.

The reason for the sudden enthusiasm is simple: parabolic trough CSP is affordable and it works. With operating costs currently just over 3 cents/kWh, the California Solar Energy Generating System (SEGS) has been generating enough electricity to power 200,000 households for more than 15 years. Recent improvements to CSP technology, such as more efficient receivers and lightweight aluminum frames, are further improving CSP’s ability to generate power in a reliable and cost-effective manner. For instance, SCHOTT has developed new coatings for its receivers that enable them to absorb more solar radiation and lose less heat.

Moreover, the growing interest in CSP is helping encourage the development of other new technologies that could further lower its generation and transmission costs. Three new technologies in particular—new receivers that use molten salt as a heat transfer fluid (HTF), improved mirror systems and high voltage direct current (HVDC) transmission equipment—could help make CSP cost-competitive with natural gas-generated electricity before 2020, if not sooner.

Hydrocarbons (oils) have been the standard HTF, first heating to 750 F in the receivers and then transferring that heat to the turbines. A fluid that could be heated to a higher temperature would boost both turbine output and efficiency. Unfortunately, hydrocarbon HTFs don’t work well at such elevated temperatures. Molten salt, however, can be heated to 930 F or more, thus boosting generator output. Moreover, using molten salt as the HTF would allow for smoother integration between the plant and new heat storage systems that use molten salt to store energy for later use. These types of heat storage systems are already being deployed. At the Andasol 1 and 2 stations in Spain (set to open later this year) large reservoirs of molten salt enable the plant to run for up to seven hours after sunset.

Molten salt poses a challenge, though, as it is much more corrosive than hydrocarbon HTFs. To accommodate the medium, SCHOTT has begun developing a new generation of receivers with coatings and glass-to-metal seals that will be able to handle molten salt’s corrosiveness and higher temperature.

Whatever the HTF medium, receiver efficiency is growing. With new production methods, it’s possible to produce receivers of greater length and capacity to go with larger solar trough panels. This, in turn, helps create better economies of scale for CSP equipment manufacturers and improves the ability of CSP plants to operate at higher temperatures.

Even the mirrored parabolic troughs that capture and focus sunlight are getting a fresh look. Sometimes the seals that bond these mirrors’ glass to their metal substrates let in moisture, corroding the mirror and diminishing its efficiency. Using leaded paint in the seal had been one solution, but it is no longer allowed for environmental reasons. However, researchers are developing better seals that will bond the glass to the metal without letting in moisture. Also being studied is substituting other materials for glass. Stainless steel or metalized membranes (plastics) that have been used in household solar heating applications offer promise. Plastics resist corrosion and offer weight (and cost) savings as well.

Finally, the development of large-scale CSP generation poses the problem of getting the electricity from these plants to cities hundreds or even thousands of miles away. That’s difficult because today’s transmission grid uses alternating current. Over longer distances large amounts of power are lost. This has prompted many to reconsider using high-voltage direct current (HVDC) technologies for transmission. HVDC can transport electricity for long distances with little loss, though the electricity must then be converted from direct current to alternating current. Although this conversion equipment would require some investment, given the antiquated nature of today’s grid, the development of new HVDC transmission systems could be accompanied by a grid upgrade.

Today, more than 5,800 MW of solar CSP projects are under development around the globe. CSP’s inherent efficiency, reliability and affordability are increasingly making it a more important part of many countries’ renewable energy portfolios. With continued technological improvements (such as improved solar receivers, HTFs, parabolic mirrors and transmission equipment) we can expect CSP’s role to expand further and help move the world more quickly towards a clean energy future.

Author: Casey Gutowski is director of business development for SCHOTT Solar.