Parabolic trough systems and their potential – talking storage
In my hubs about concentrating solar power (CSP) plants I focused on the parabolic trough design. These systems have their place in the power generation market, especially in the multi MW range. They have the oldest and most proven technology. Patents and first designs go back to the early past century. Even though the photovoltaic (PV) effect was discovered in 1839 by Becquerel, it was not until the1950s that the first modern PV cells were developed in Bell Laboratories.
Regions with the most direct normal solar radiation, like Southwestern USA, the North and South Africa region, Southern Spain, or the Middle East offer the most suitable climate for this kind of power plant. But even in the sunniest of regions, clouds can obstruct the sun, and thereby cutting off the heat/power source. Or obviously, the time of solar radiation is limited to day time.
Still, CSP plants can provide a more consistent power output as a result of their capability to store thermal energy . In this article, I would like to talk a little bit about the storage option. Storage systems can buffer the energy during cloudy weather conditions and hours after sunset and give thus the opportunity to deliver power fairly independent of solar irradiation. They can improve economics through heating up the storage media during times of low prices and deliver power during those peak periods when prices are usually high. So it is no surprise that many CSP plants utilize thermal energy storage (TES) systems.
Sunset over Lava
Types and Projects
Most common set ups are two tank direct (same fluid as heat transfer and for storage) and two tank indirect storage system. Types of storage media include the popular molten salt (usually a mixture of sodium nitrate and potassium nitrate), concrete, mineral oil, synthetic oil. Molten salt mixtures seem to offer the best benefit, as they are less pollutant, more cost effective, have a higher heat capacity, are non-flammable, etc. On the other side they are corrosive and require higher system temperatures.
A famous example for a two tank direct system includes the already mentioned SEGS 1 plant. It had two tanks with the heat transfer fluid (mineral oil) as storage fluid and a storage capacity of 3 hours, which was in operation till 1999. The 280 MW Solana plant in Gila Bends, Arizona, will utilize molten salt as storage medium with thermal storage of 6 hours. The 1 MW Saguaro plant facilitates 6 hours thermal storage, using oil. The Hualapai Valley Solar Project in Arizona is a 340 MW plant by Mohave Sun Power, with molten salt storage, with plans to get connected to the grid in 2014.
The Bell Independent Power Corporation is working on a 5 MW demonstration project with a single tank storage in Tuscon, Arizona. They are using a special type of salt, supposedly utilizing more of the stored energy than conventional two tank storage set ups. On the Island of Hawaii near Kona, Keahole Solar Power, LLC built a 2 MW plant ‘Holaniku at Keahole Point’ with 2 h of thermal storage using molten salt as storage medium.
In Europe, the three 50 MW Andasol plants in Spain each include molten salt tanks (indirect storage) with 7 to 8 hours storage capacity. In Italy, the 5 MW parabolic trough plant Archimede is the first plant to use molten salt for heat transfer and storage (storage capacity 8 hours). It is in operation since 2010. Kaxu Solar One in South Africa (Northern Cape) with 100 MW output is planned with 3 h of thermal storage.
Thermal energy storage highly increases the reliability and output consistency of a CSP plant. There is no doubt that parabolic trough systems certainly face lots of challenges, but they are a valuable option for complementing PV technology.