Why Parabolic Trough?
30 Years of Experience
Parabolic trough systems have been in wide use for utility-grade power generation since the mid 1980’s. In recent years, other CSP geometries such as parabolic dish, linear Fresnel, and central receiver or “power tower” have received considerable attention in the marketplace to meet the need of delivering cost effective electricity. While each of these designs has particular attributes and performance characteristics that make it a potentially attractive option, only parabolic trough has 30 years of experience in commercial operation, and been bank-financed many times. Therefore, it was the ideal technology for SkyFuel to develop for widespread integration in the global energy market. With a laser-like focus on cost reduction while maintaining the parabolic trough’s high performance, SkyFuel developed the SkyTrough®.
The deployment, performance and operation of commercial, utility-grade parabolic trough solar thermal power plants are well-understood and proven. In the United States, more than 350 MWe of parabolic trough plants have been operating at the cluster of nine Solar Electric Generation Systems (SEGS) facilities in California since the 1980’s. An additional 64 MWe parabolic trough plant, Nevada Solar One (NSO) has been in commercial operation in Nevada since 2007, and the Martin Solar plant in Florida since 2009. In Spain, thanks to generous government subsidies for solar power in the form of feed-in-tariffs, more than a Gigawatt of parabolic trough solar power plants was built between 2007 and 2013. Most of these plants incorporate molten salt thermal energy storage.
The experiences at SEGS and NSO and new data coming out of the plants in Spain enable the ongoing advancement of parabolic trough design and performance. All trough components, particularly the frames and support structures, the reflector surfaces, receiver tubes, sun-tracking drive and control system, and heat transfer fluid system have been evaluated and optimized over many design generations. The information gathered from testing and experience has resulted in improved long-term performance and reduced cost for parabolic trough systems. A quarter century after first being commissioned, annual solar energy production of the first SEGS plants is higher today than when the plants were first fired up, due to increased and optimized operating procedures and component upgrades. In addition, the long operating history of parabolic trough plants means that operation and maintenance costs are predictable and well understood.
Building on the proven performance of parabolic trough technology while tapping into the growing body of knowledge about trough design and performance, SkyFuel addressed and advanced certain key features such as replacing the heavy, fragile and expensive glass mirrors with light, shatter-proof reflector panels made with ReflecTech® mirror film and using a light, tubular aluminum space frame to boost performance and reliability while lowering cost.
Modular and Scalable
Parabolic trough plants are inherently modular and scalable. Modularity is important for achieving low cost through high volume production of components and subsystems. The ability to simply add more rows of troughs to scale up to produce hundreds of megawatts further makes parabolic trough power plants the premier choice for utility-grade solar power. Relative to glass-based troughs, the SkyTrough can be deployed economically in very small solar fields, due to the simple assembly process.
Validation and verification at the modular unit level mitigates risk as the system is built up. The design requirements of a parabolic trough can be verified at the level of an individual module. Once the performance of a module has been vetted, the performance of an entire solar field can be reliably predicted. For the SkyTrough, the basic unit of modularity is the solar collector assembly (SCA), which is comprised of eight (8) identical parabolic modules; four on either side of a central drive and control unit.
The basic layout of a parabolic trough plant involves connecting 4, 6 or 8 solar collector assemblies to form a “loop” and several loops are combined into the solar field heat transfer fluid circuit. Header pipes carry the heat transfer fluid from the loops to and from a steam generator to drive a steam turbine, and produce electricity. Typically, the output of a parabolic trough solar thermal plant is increased by adding more loops in parallel.
Smooth and Predictable Output
A parabolic trough solar field also has inherent “free” energy storage that, depending on the size of the plant, allows electricity production to continue well after the sun has been blocked by a cloud. Depending on the plant’s size, such thermal “momentum” can last 15 minutes or more. It means that the electrical output of the plant can remain constant (or change very gradually) during periods of partial cloud cover or as the sun sets giving utilities plenty of time to shift to other generating sources. Such inherent thermal energy storage is the result of the heat that has been collected and absorbed into the heat transfer fluid circuit. Other energy technologies that convert a renewable resource directly into electricity, such as wind and photovoltaics, can have dramatic swings in output on a minute-to-minute or even second-to-second basis during partly cloudy days, and this can present serious challenges to the transmission system. The constant and predictable output of a parabolic trough solar plant is an important benefit when considering the reliability of the power grid.
The ability to store hours of output in a dedicated thermal energy storage system, as demonstrated by projects in Spain, allows around-the-clock generation of firm power.
Finally, all of the parabolic trough system’s attributes make it most attractive to the banking community, which is particularly risk averse following the global financial crisis. Parabolic trough is the only proven CSP technology that has been continuously operating for over 25 years with performance and reliability characteristics completely understood. It is known to work; risk is mitigated; and parabolic trough is, therefore, the most attractive CSP option for banks to finance. Parabolic trough is the only currently “bankable” CSP technology.