QMT Features: July 2011
Measurement for energy
Thousands of giant heliostat mirrors are precisely positioned  by a laser radar systems at the world;s first thermosolar electrical generation plant in Andulsia, Spain.

Gemasolar, in the  Andalusian region of Spain, is the world’s first commercial-scale plant to apply central tower receiver and molten salt heat storage technology, opening up the way for new thermosolar electrical generation technology, potentially delivering a much higher yield than other solar technologies. Starting in 2011, Gemasolar will supply clean and safe energy to 25,000 homes and reduce CO2 emissions by more than 30,000 tons a year.

Gemasolar is a milestone project from Torresol Energy, a joint venture between SENER and financial partner Masdar. SENER – a large engineering, construction and systems integration company.  Torresol Energy was set up to build the world’s first CSP plant featuring tower technology with a molten salts receiver. Its thermal storage concept allows Gemasolar to continue producing electricity even when a cloud passes over. With thousands of 11 x 12 metre heliostat mirrors targeting the sunlight receiver, the salt substances heat up and descend to the hot salts tank where they are stored at more than 500 degrees Celsius. From here, the salts are transferred to heat exchangers and subsequently to the turbine and electrical transformer, before adding electricity to the net grid. Although the use of a tower surrounded by flat-mirror heliostats is a less mature technology than parabolic trough technology, it potentially offers a higher energy yield.

A Laser Radar measurement system  helps achieve the slightly parabolic reflective shape that is different for each of the 2,650 heliostats which surround the tower. These are moved around two rotation axes to reflect sunlight towards the  tower receiver. The Laser Radar instrument, from Nikon Metrology, has been selected to ensure accurate panel positioning during heliostat manufacturing. This ensures that reflected sun beams precisely point to the sunlight receiver in the tower up to 1 kilometre away. Accurate heliostat mirror positioning allows Gemasolar to capture as much heat as possible and distribute it evenly across the outer receiver surface to avoid damage due to overheating.

Automated mirror panel inspection
Measuring heliostats with a Laser Radar is faster and more practical than any other  measuring system, such as a laser tracker.  The first step in building a Gemasolar heliostat is the automatic laser inspection that takes place after finishing heliostat construction. To measure heliostat mirror surfaces from above, the Laser Radar instrument has been tilted and mounted on a post. A rotary table has been integrated to easily switch measurement between four transportable heliostat assembly decks. It takes roughly 5 minutes to measure a single heliostat in Nikon’s Laser Radar Enhanced Metrology mode. This is much shorter time compared to a laser tracker system - which was considered for purchase before the Laser Radar system was selected. Laser Radar technology is also much less cumbersome as a laser tracker requires a crane to precisely position a large gauge with spring-loaded targets on top of the heliostat’s reflective surface.

With the Laser radar , mirror points can be  measuring without having to manually touch the points. The control system, developed by SENER, runs measurement routines that have been programmed off-line so that the Laser Radar can be used to automatically inspect one heliostat after the other.

When an operator starts the inspection routine, the Laser Radar system sequentially measures the required points. All produced heliostats are inspected in Enhanced Metrology mode. For every point of a heliostat, the Laser Radar scans a 5 by 5 millimetre region, searching for the precise position to take the measurement.

All heliostats are designed to exhibit a different slightly parabolic reflective shape, depending on the position of the heliostat in relation to the tower. Immediately after completing inspection, the measurement report rolls out of the printer and is automatically saved on the network and sent to SENER for verification. The implemented control system decides whether the required curvature for each heliostat mirror array is achieved within specification. Based on the mirror deviation values specified in the inspection report, assembly workers properly modify the orientation of the mirror panels. After tuning the mirrors, the Laser Radar runs final inspection verification to confirm accuracy before turning out a new heliostat.

Non-contact inspection performed in a fraction of the time has been the main driver for SENER to opt for the Laser Radar inspection system. The Laser Radar is a key component in realizing SENER’s objective to precisely assemble the solar field of heliostats, and as such generate electricity with greater efficiency and higher thermal storage capacity than parabolic trough technology. Gemasolar is an international reference and a starting point in the cost-cutting strategy of the thermosolar power industry, paving the way for similar plants around the world.l

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