Parabolic Trough

Parabolic troughs are the most mature of the concentrating solar power technologies and they are commercially proven. The first systems were installed in 1912 near Cairo in Egypt to generate steam for a pump which delivered water for irrigation. At the time, this plant was competitive with coal-fired installations in regions where coal was expensive.

In the trough system, sunlight is concentrated by about 70–100 times on the absorber tubes, achieving operating temperatures of 350 to 550°C. A heat transfer fluid (HTF) pumped through the absorber tube transfers the thermal energy to a conventional steam turbine power cycle. Most plants use synthetic thermal oil for the job of transferring heat. The hot thermal oil is used to produce slightly superheated steam at high pressure which then feeds a steam turbine connected to a generator to produce electricity. The thermal oil has a top temperature of about 400° C which limits the conversion efficiency of the turbine cycle, so researchers and industry have developed advanced HTFs. One example is direct generation of steam in the absorber tubes, another using molten salt as HTF. Prototype plants of both types are currently being built.

Currently parabolic trough is the most widely used technology around the world, particularly in Spain and in the United States where plants in operation generate over 1000 MW (tbc) and 500 MW, respectively.

Around the world, parabolic trough projects in operation are between 5 and 280 MWe in size, and existing plants are producing well over 3900 MW of electrical capacity. In California nine plants were developed and grid-connected in southern California in the 1980s, forming about 2 million m2 of mirror area, named solar electricity generating systems (SEGS). After an industry hiatus, commercial construction of parabolic trough plants has resumed with the 64 MW project called Nevada One, owned by Acciona, which will produce 130 GWh of electricity annually. In Spain, the Andasol and Solnova projects in construction will together provide 250 MW of capacity, and more than 14 more projects of their type are proposed since the introduction of a sufficient feed-in tariff. The largest single parabolic trough installation yet proposed is called Solana, and is planned for a site in Nevada.

Current power plants in Spain are limited to 50 MW per plant by the Special Regime. About 60% of the plants which have been registered for the FiT include a molten salt two-tank system providing 7.5 hours of storage. In the United States, power plants are built with much larger turbines (>100 MW), taking advantage of the fact that, in this technology, while energy collection performance is practically unaffected by size, costs of generation are lowered considerably.


In terms of the maturity of the technology, Parabolic Trough can be considered “mature”, since a number of manufacturers are available for erecting entire plants or subsystems.  There is good experience in engineering procurement and construction (EPC) and 20-year operating experience allows for good confidence on the operation. Therefore, these can be considered low-risk projects.

A new generation of parabolic trough plants aims to reach a higher HTF temperature, allowing the full integration of the solar field and the storage system. This “second generation” should provide significant improvements in the average conversion efficiency and further reduction of costs. Although a demonstration plant has already been built, adequate operating experience is still needed and components with enhanced performance and durability are being studied and developed.


While SEGS in the U.S. and the Solnova projects in Spain use a synthetic oil for heat transfer, other developers are building plants that direct steam generation within the absorber tubes. Using direct steam eliminates the need for a heat transfer medium, and can reduce costs and enhance efficiency by 15–20%. The Andasol plant, the first Parabolic trough power plant in Europe,  uses direct steam; it is also a first-of-its-kind, utility-scale demonstration of the EuroTrough design and thermal storage using molten salt technology

Nevada Solar Parabolic Trough Plant

The SEGS and Solnova plants use a system where the plant can operate also burn natural gas on days when sunlight is weak. Parabolic trough systems are suited to a hybrid operation called Integrated Solar Combined Cycle (ISCC), where the steam generated by solar is fed into a thermal plant which also uses fossil-fuel generated steam, generally from natural gas. Currently 25 MV Hassi R’mel in Algeria, 20 MW ISCC Al Kuraymat in Egypt, and 20 MW Ain Beni Mathar Plant ISCC in Morocco are the examples of the operational ISCC CSP plants, forming an interim step towards complete solar generation in the energy mix.

Case Study: SEGS – Pioneering the technology

Nine plants were constructed in the US Mojave desert by Israeli/American company Luz between 1984 and 1991, the first only 14 MWe, and the final two were 80 MWe, known collectively as Solar Energy Generating System (SEGS). They use solar-generated steam and also gas back-up, but the gas component is limited to 25% of the total heat input. They have more than 2 million square metres of parabolic trough mirrors. They were built with US 1.2 billion, in private risk capital and institutional investors. However, Luz had early difficulties making a profit because of market issues of energy price fluctuations and tax status. However, the technology is proven and shows that CSP plants have a potentially long operating life. Today, just the three plants at Kramer Junction are delivering 800–900 million kWh of electricity to the Californian grid every year, reaching today a total accumulated solar electricity production of almost 9 billion kWh, which is roughly half of the solar electricity generated worldwide to date. Since construction of the SEGS plants, they have reduced operation and maintenance costs by at least one third. Trough component manufacturing companies have made significant advances in improving absorber tubes, process know-how and system integration. The annual plant availability constantly exceeds 99% and anecdotally, the plant performance level has dropped only about 3% in around 20 years of operation.

Source: SolarPACES

Case Study: Andasol Solar Power Plants – Using thermal storage

The Andasol solar power plants located near Andalusia (Spain) is a 150 MW CSP station and Europe’s first commercial plant to use parabolic troughs. The Andasol plant uses tanks of molten salt as thermal energy storage. The Andasol consists of 3 plants: Andasol 1 (completed in 2008), Andasol 2 (completed in 2009) and Andasol 3 (completed in 2011). With a gross electricity output of around 525 GWh per year and a collector surface area of over 510, 000 square meters – equal to 70 soccer pitches.
Each power plant has an electricity output of 50 megawatts and operates with thermal storage. The plant is designed to optimise heat exchange between the heat transfer fluid circulating in the solar field and the molten salt storage medium and the water/steam cycle. With a full thermal reservoir the turbines can run for about 7.5 hours at full-load even if it rains or long after the sun has set. The heat reservoirs are two tanks 14 m in high and 36 m in diameter and contain liquid salt. Each provides 28,500 tons of storage medium. Each plant supplies up to 200,000 people with electricity and saves about 149,000 tons of carbon dioxide per year compared with a modern coal power plant.

Source: ACS Cobra

Andalusien / Spanien
Pressereise zum Kraftwerk Andasol 3 in Andalusien.
Anwesend: Prof. Dr. Fritz Vahrenholt CEO RWE Innogy und Dr. Frank Dinter, Head of Solar RWE Innogy
Copyright by RWE Innogy
Gildehofstrasse 1
45127 Essen

Case Study: Solana – Largest CSP plant using parabolic trough

Up-to-date, Solana is a 280-MW unitility-scale CSP plant with six hours of molten-salt thermal energy storage near Phoenix, Arizona. Solana delivers enough electricity to supply approx. 70,000 homes while avoiding 475,000 tons of CO2 every year. The CSP plant covers an area of 780 ha and created about 15000 construction jobs with the plant employing 85 full-time workers.

Source: Abengoa