Long relegated to mere tech demonstrations and research and development facilities, solar thermal installations built around a central tower are now making the leap to full-scale production sites.
Leading the way is Gemasolar in the south of Spain.
In order to ensure precise and reliable operation of the plant´s mobile heliostats that focus the sunlight, the vast array comprising 2,650 mirrors is equipped with 5,300 robust geared motors supplied by NORD Drivesystems.
The Gemasolar plant’s basic set-up consists of a central tower structure with a receptor area in its top segment, a liquid circulation cycle including storage tanks and heat exchanger facilities for power generation via an adjacent turbine, and an array of mirror units that focus the rays of the sun onto the receiver.
These mirrors are designed to turn and tilt in order to ensure that as much sunlight a possible is reflected onto the designated area on the tower – from dawn till dusk, as long as the sun remains in the line of sight throughout the day.
NORD Drivesystems has supplied 5,300 NORDBLOC.1 design geared motors for the Gemasolar heliostats. In each of the 2,650 flat mirror units, two such geared motors enable highly accurate movements for two axes to track the path of the sun.
Keeping the heat in the can
The Gemasolar site extends over an area of 185 hectares to accommodate the vast field of heliostats.
Operated by Torresol, a joint venture between Spanish engineering giant SENER Ingeniería y Sistemas and Masdar, Abu Dhabi’s state-owned future energy enterprise, Gemasolar is the first ever commercial-scale CSP (concentrated solar power) plant with central tower technology that implements a heat storage system based on molten salts.
Liquefied nitrate salts are pumped up from a storage tank, run through the receiver section, and absorb the heat impact of the highly concentrated solar radiation in that tower segment. The temperature of the liquid that has passed through it usually exceeds 500 ºC.
Once they leave the receptor, the molten salts flow through a heat exchanger where they cool down again, with the resulting water vapor driving a steam turbine that feeds a generator. The generated energy is supplied into the electrical grid. Most notably, though, the molten salts cycle at Gemasolar incorporates an innovative storage option.
Whenever there is more heat energy available than the turbine is able to convert, the extra energy is stored by diverting some of the flow of molten salts before the liquid reaches the heat exchanger. Kept in a special tank, the hot medium can be used at a later time when insufficient solar radiation – or none at all – is available for standard operation of the plant.
This solution enables the system to generate power from stored heat for up to 15 hours, i.e. throughout long periods of cloudy skies or even darkness. The resulting total of 6,500 hours of productive operation per year makes this plant much more efficient than more conventional renewable energy facilities that are totally dependent on changing conditions.
Always geared towards the sun
The sunlight concentration is achieved by 2,650 flat mirrors all continually pointing at the same receiver region on the installation’s one central tower.
Given the size, weight, and shape of these mirror units, each of them depends on a powerful, sturdy, and robust drive solution to ensure reliable tracking of the sun.
Obviously, this is an application exposed to very high environmental temperatures, so all equipment used here must be manufactured to withstand such conditions. Moreover, each heliostat has a flat surface of about 120 m², which makes them markedly susceptible to strong, not to mention gale-force winds that naturally occur from time to time.
Still, proper operation of the heliostats and the power plant as a whole is ensured in all but the most extreme weather conditions. The geared motors working throughout the array of mirrors play an instrumental part in that. Torresol’s heliostats are equipped with size 5 NORDBLOC.1 series geared motors.
Compared to same size previous generations of one-piece housing solutions, these systems are suitable for much greater forces than before. Mounting options are particularly user-friendly, allowing for either cost-efficient, direct motor mounting, or an attachment of very short, space-saving lightweight IEC adapters. Ventilation is ensured in all mounting positions.
The gears’ aluminium housings provide robust, natural corrosion protection out of the box – without the need for a paint finish. Based on FEM-optimised designs, these models are not only more robust, but also considerably lighter than their predecessors.
For gears up to size 6, the UNICASE design enables the mounting of larger bearings – the units therefore withstand higher overhung loads, or last longer under a given load. For even more demanding conditions than high temperatures and adverse environmental conditions, ATEX versions of all types can also be supplied.