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Experts join forces to deliver extra efficiency for CSP plants via molten salt

The most important sources of renewable energy, the sun and the wind, are infinitely available – but they do depend on meteorological conditions, the time of day and the time of year. In the future, storage methods that smooth out these fluctuations will therefore play a decisive role in round-the-clock, dependable and efficient supply of energy.

Concentrated solar power (CSP) solar-power plants make it possible to charge thermal molten-salt storage tanks with solar energy at rational cost during the daytime and generate power when needed after dusk. Thanks to this “as-needed” power supply, which is independent of the available sunlight, these systems are a key element in the energy turnaround. MAN Energy Solutions and Dillinger are working jointly on finding answers to the core challenge of the energy turnaround: that of generating large quantities of renewable energy and integrating it intelligently into a reliable electricity and/or steam production system for use in industrial processes.

MAN Energy Solutions is a leading mechanical and plant engineering company. The New Technologies department, under the management of Dr Christian Schuhbauer, devotes itself to the important topics of the generation and storage of electrical energy. A key part is played by steam turbines of up to 180MW output and molten-salt reactor systems. For around four years now, the experience gathered in the field of heat management has also been incorporated into the development of molten-salt receivers for CSP solar-power plants. The most recent project started is Solar Tower II of the German Aerospace Center (DLR) in Jülich.

Dillinger is an integrated iron and steel producer. It is a leader in the field of tailored-to-application heavy-plate products. Long years of cooperation in pressure-vessel engineering link MAN Energy Solutions and Dillinger. For more than 10 years, the steel producer has been active in the solar-power plant sector, supplying large tonnages of heavy plate and shell courses distinguished by their large dimensions and high individual weights for projects in this field.

These products are used in steam generators, vessels, reactors and other pressure-bearing equipment of all types. Projects supplied with steel from Dillingen include, inter alia, the CSP solar-power plants of KHI Solar One, in South Africa, Andasol I, in Spain, and GTL Pearl, in Qatar.

Green electricity at night thanks to solar energy

The global output of commercially operated CSP power plants amounted in 2021 to just 6GW. The necessity of providing enormous quantities of renewable and, at the same time, storable energy for the decarbonisation of industry has in recent years resulted in exponential growth of this type of energy generation.

CSP power plants can be operated cost-effectively only in high-sunlight regions with a high level of direct solar insolation, such as the sunbelt, located close to the equator, however. Parabolic-trough power plants have been tested for the longest period of time and are also the most widely used type: their trough-shaped mirror collimates the incident sunlight on to an absorber tube at their caustic curve. Here, the concentrated solar radiation is converted to heat which is then yielded to the thermal oil flowing as the working fluid through the absorber tube. Molten salt is used to store the heat. CSP solar towers are surrounded by a large array of mirrors (known as “heliostats”), which automatically track the position of the sun. These heliostats reflect the sunlight on to a central receiver located at the top in the tower.

Unlike the arrangement in the parabolic-trough system, in a CSP solar tower molten salt flows through the receiver, with the result that salt is, simultaneously, the heat-transfer fluid and the storage medium. The use of molten salt as a working fluid permits higher temperatures, with the consequence that solar towers have a higher efficiency, and thus a higher cost-effectiveness, than the parabolic-trough plant. In both system types, the molten salt heated up by the concentrated solar energy is routed from the heat-storage tank to a steam-generating system for the production of electricity. Here, superheated steam is produced at high pressure and used to drive steam turbines which, via a generator, produce electricity in the power-plant unit.

The solar heat not required during daytime is stored using the molten salt in the heat-storage tank and converted to electricity only in the evening or during the night.

700MWth receiver for reference plant

The central components of a CSP power plant take the form of the collector system – in other words, the mirrors, as the energy-collecting system – and the receiver (in the case of a tower) or the absorber (in the case of a parabolic trough) as the energy-conversion system. The energy-collecting system charges the storage unit, while it discharges the energy-conversion system either simultaneously or after a delay. Also necessary are thermal oil or molten salt, as the working fluid, molten salt as the heat-storage fluid, a hot-salt tank, a cold-salt tank, salt pumps and the steam generators and steam turbines required as further components of the energy-conversion system.

A project consortium consisting of German industrial companies – including MAN Energy Solutions – and the DLR has developed in a case study a CSP reference power plant of the current maximum technologically conceivable size. This maximum size is determined by the quantity of radiated heat which actually reaches a receiver of a diameter of 20m from mirrors located at a distance of 1km. The reference power plant takes the form of a 250m high solar tower with a 700MWth receiver, a heat-storage tank and a cold-storage tank each filled with 53,000 tonnes of molten salt, and a 200MWe power-plant unit.

The molten-salt receiver designed for this purpose by MAN Energy Solutions is equivalent in its conception to a prototype already tested at Jülich. The reference plant with its 700MWth receiver permits full-load operation for 12 hours from sundown to sunrise. Two 200MWe power-plant units are planned for a “peaker” variant, i.e. for demand-orientated use at times of peak loads.

Jülich solar energy prototype has higher salt temperatures

MAN Energy Solutions has developed a molten-salt receiver suitable for temperature ranges up to 620°C for Solar Tower II at the DLR’s Institute of Solar Research in Jülich. On a surface area of some 10 hectares, 2,000 mirrors collimate the sunlight in Jülich and direct it on to the external receiver located at a height of 55m. Conventional molten-salt receivers operate at only 565°C.

MAN Energy Solutions is planning to use the innovative high-temperature receiver to test new temperature ranges and the limitations of the use of molten salt. This will also open up new potentials for everyday business using molten-salt reactors in, for example, the chemicals industry. For this reason, the control system of the test facility at Jülich has been designed by MAN Energy Solutions in such a way that it is as comparable as possible to classical power plants and will permit scaling-up of this prototype to industrial dimensions.

Only the otherwise customary turbine has been omitted, since the test facility will not generate any electricity. Analogously to a classical power plant, the receiver features an inlet tank and an outlet tank. The inlet tank also functions as a receiver vessel, to permit rapid reaction in case of any problems. Molten salt cools the receiver by one MWth per m2. Failure of the flow of molten-salt would cause immediate and permanent damage to the receiver. In case of operational problems, the inlet tank for this reason continues to feed salt into the receiver until the mirrors (heliostats) are aligned in such a way that no further solar energy strikes the receiver.

High demands made on the steels used

The attention of the MAN Energy Solutions designers in the case of CSP solar-power plants of this type is focused not only on optimisation of output and efficiency, but also on the materials needed for the plant’s highly stressed components. “The steels needed for the construction of vessels of 50m diameter and 60mm plate thickness are exposed to high thermal, mechanical and corrosive loads”, explains departmental manager, Christian Schuhbauer. The use of AISI 347H (1.4961) stainless steel is routine practice for the heat-storage tanks of solar-tower power plants.

Thermal oil, the working fluid of a parabolic-trough plant, is normally used at up to 400°C, since it is degraded at higher temperatures. For this reason, the molten salt used in this type of system also has only an analogous temperature. Dillinger supplies for such tanks the carbon steel SA 204 C in plate thicknesses of 100mm, a width of 5,200mm and an optimised weldability. This high-temperature fine-grained structural steel has a high yield strength at elevated temperatures and can be used up to service temperatures of 450°C.

The cold-salt tanks, in which the salt cooled down to 290 to 300°C in the steam generator is stored, are fabricated in carbon steel SA 204 B. This high-temperature steel is characterised by a slightly higher yield strength compared to SA 204 C. The high-temperature molybdenum-alloyed structural steel 16Mo3 has been selected for the steam generators and heat exchangers, in wall thicknesses of up to 110mm, for this reference power plant. A significantly higher-performance alternative is provided by Dillinger in the form of the high-strength quenched and tempered fine-grained SA533 B2 steel, a material already tried and proven in many comparable generators at thicknesses of 20 to 200mm. Its mechanical strength is, at a yield strength of 485 MPa, significantly higher than that of 16Mo3 (275 MPa). This permits lower component wall thicknesses and thus allows designers a lighter overall structure.

SA533 B2 is thus also an interesting option for the salt collector of the receiver panels up to a temperature of 450°C. This collector connects the pipes through which the salt flows back and forth in a meander pattern for heating up. Each of these panels in a 700 MWth CSP receiver will be up to 25m high and have a width of 3.5m. Up to 16 panels featuring up to 50 tubes are required in such a plant and form a receiver diameter of around 40m. For the tanks, too, the unusually large, wide and thick plates supplied by Dillinger provide additional savings potential: on wall thicknesses, and thus on weight, but also on welds. Thanks to the plate lengths that the steel expert can supply, the steadily rising tank diameters of 40m and more can be fabricated from just a single plate, thus again reducing the number of welds necessary. And in the case of longitudinally welded shell courses with internal diameters of up to 3,800mm, the large width and length of the suppliable plates also help to significantly reduce the number of circumferential welds.

Holistic capabilities ensure greater efficiency and safety

If required, Dillinger can produce shell courses of up to 4.3m in length with wall thicknesses of more than 200mm, depending on mechanical strength, diameter and plate width. This is a capability of which MAN Energy Solutions has already made use, since it can itself only form plates of up to 3m. Another service boosts the efficiency of solar-power plant engineering: cutting of heavy plates precisely to length and width, including weld-edge preparation on ultra-modern edge-milling machines. This machining process takes place in a single operation; it is applied to all four edges simultaneously, with no intermediate turning. Plates of up to 160mm in thickness, up to 25m in length and up to 5m in width can thus be machined to extremely tight tolerances. This not only relieves the fabricator of the machining preparation of the plates supplied, it also significantly shortens and simplifies the welding process.

Safety and reliability thanks to metallurgical forecasting models

The minimums for materials properties for use in pressure-bearing equipment are strictly stipulated in codes and specifications. The real challenge can, however, be found in the additional requirements demanded by customers, fabrication companies and specialist welding engineers. To assure a service life of three decades, including multiple repair cycles, the steels must exhibit the minimum specified mechanical properties without any losses even after repeated welding operations and subsequent stress-relieving annealing. Intensive discussions involving all those participating are a routine occurrence until the end customer has approved the specification of the steels for the plant or facility he is planning. In this context, Dillinger provides suppliers such as MAN Energy Solutions with the assurance that the mechanical properties will meet the relevant specifications contained in the standards even after repeated repair welding. This is made possible by especially reliable metallurgical forecasting models, using which Dillinger checks customers’ specifications for their feasibility. This is of exceptionally great importance in the case of special steels used in great thicknesses, such as can be required for fabrication of pressure-bearing equipment.

These smart models are continuously trained on a neurone-network basis, with the result that the experts are able, even before the start of production, to submit a reliable prediction of the implementability of the required properties and a corresponding risk evaluation. After production of the steel, the data determined are checked again and confirmed on the certificate.

In this way, companies such as MAN Energy Solutions are able to demonstrate to their customers that steel products exhibit the stipulated mechanical properties even after multiple heat-treatment operations and that the system can thus be operated without danger for three or even more decades. At the same time, they have, in working with the heavy-plate producer, the certainty of needs-orientated scalability of delivery quantities even in the case of very large engineering projects.

Solar electricity for green hydrogen and solar fuel

MAN Energy Solutions’ view of its cooperation with Dillinger is correspondingly positive: in decades of cooperation, the Saarland steel suppliers have proven their quality again and again, as Christian Schuhbauer notes: “Our experience is that Dillinger is an entirely dependable partner with an extremely high delivery reliability.”

He adds: “There are, in any case, only a few suppliers for plates of 120mm and above. So such a dependable supplier located not too far away is especially appreciated.”

That also applies, in his estimation, to the role that will also be potentially played by solar-electricity-based generation of hydrogen and its derivatives for the production of synthetic kerosenes for the aviation industry. For this application, correspondingly large and thick plates will be needed not only for the components of the solar-power plant but also for the synthesis reactors in downstream processes (such as Fischer-Tropsch) for production of kerosene. An extremely successful cooperation between Dillinger and MAN Energy Solutions took place using this process12 years ago on a large-scale Shell GTL-Pearl project in Qatar. There are also good prospects of the anticipated rise in demand for CSP solar-power plants and the pressure on demand for synthetic kerosene from the aviation industry generating interesting starting points for even more successful cooperation.

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