STE/CSP Technologies > Parabolic Trough

[image_with_text title_tag=”h2″ image=”354″ title=”Parabolic Trough – Palma del Río II”]

Photo Credit: Acciona

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Parabolic troughs (PT) 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 absorber tubes, achieving operating temperatures of 350oC to 550oC. A heat transfer fluid pumped through the absorber tube transfers the thermal energy to a conventional steam turbine power cycle. Most plants use synthetic thermal oil to transfer heat. The hot thermal oil is used to produce slightly superheated steam at high pressure, to feed a steam turbine connected to a generator to produce electricity. Thermal oil has a top temperature   of   about   400oC, which limits the conversion efficiency of the turbine cycle. Researchers and the industry have developed alternatives. One example is the direct generation of steam in the absorber tubes, another using molten salt as the HTF. Prototype plants of both types are currently being built.

Currently, parabolic troughs are the most widely used technology around the world, particularly in Spain and the United States where the installed capacity of operating plants is over 2,370 MW and 1,836 MW, respectively. Plants range in size from 5 to 280 MW. Parabolic troughs are considered a “mature” technology. For example, in terms of supply chain, a number of manufacturers currently fabricate this technology. What’s more, there is good experience in engineering procurement and construction (EPC) and 20 years of operating experience allows for good confidence on the operation. Therefore, projects using parabolic trough technology can be considered low-risk.

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.

Summary

Parabolic Trough plants collect the direct solar radiation in collectors consisting of parabolic trough mirror concentrators with a linear absorber tube. The heat transfer fluid is normally synthetic oil which circulates through the tubes at inlet outlet/temperatures of 300/400 ºC. Higher outlet temperatures would increase the overall efficiency of the plants but they would degrade the oil. That is why other HTF like water with direct steam generation and molten salts are being considered. The typical size in Spain is 50 MW due to regulatory restrictions, nevertheless this is a very flexible concept that can fit with few MW or go up to several hundred MW. Typical storage capacity in Spain is 7 hours at nominal power.

In Europe, the installed capacity of parabolic trough plants in operation reaches 1650 MW and 623 MW are under construction. The parabolic trough technology is the most mature available on the market with a track record since the 1980’s.

  • Uses parabolic mirrors to concentrate solar radiation on linear tube receiver
  • Provides heat storage capabilities
  • Temperature of the working fluid: 400ºC for synthetic oils. Molten salts or direct steam generation are not commercially proven.
  • First plants came on line in the 80s and are still working
  • Has operational experience, modularity and a large number of components providers
  • Size: 20 to 300 MW
  • Solar energy to electric power performance: Design point 16%. Annual 14%.
  • Prices of PPA that will make projects feasible: between 15 and 20 c€/kWh depending on the level of solar radiation, size of the plant, capacity factor, financial conditions, etc.
  • Proven utility scale technology
Case Study

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

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

Solana is a 280 MW solar thermal power plant with six hours of molten-salt thermal energy storage. Located near Phoenix, in the Arizona desert, Solana covers 777 ha of land. It generates enough electricity to supply approximately 71,000 homes and avoids 427,000 tonnes of CO2 every year. Solana solar thermal created about 15,000 construction jobs with the plant employs 85 full-time workers.

Source: Abengoa.