【New Energy Applications】The Crescent Dunes Power Plant, the world’s largest molten salt tower power plant, has been shut down temporarily due to a leakage of molten salt from its storage tanks

But in fact, this minor molten salt leak occurred a few days after Kevin Smith, CEO of SolarReserve, Elizabeth Sherwood-Randall, deputy secretary of the U.S. Department of Energy, and other local officials from Nevada announced on October 11 at the Crescent Dunes power plant their plans to develop a 2GW Sandstone solar thermal power project. If December is included in its regular maintenance period, then chronologically, this power plant experienced unplanned shutdowns from late October to November due to this accident. Salt leakage from the welds of molten salt tanks is one of the common problems associated with such tanks, and it occurs frequently, especially in large molten salt tanks used in large-scale solar thermal power plants. According to industry experts, the problems with the welds in the molten salt tank of the Xinyue Sand Dune Power Plant are likely due to the large size of the tank; the larger the molten salt tank, the higher the technical requirements for welding.

The Crescent Dunes power plant has an installed capacity of 110 MW and a heat storage capacity of 10 hours. In accordance with the American Petroleum Institute API-650 standard, the volume of the storage tanks should, in principle, not be too large. This is why the Solana trough-type power plant, with an installed capacity of 280 MW and a heat storage capacity of 6 hours, is equipped with 6 sets, totaling 12 storage tanks. In terms of heat storage capacity, Solana requires more molten salt than Crescent Dunes; yet Crescent Dunes only has one set of storage tanks. As a result, the volume of each storage tank in Crescent Dunes is about 1/3 larger than that in Solana’s tanks. An increase in tank volume necessitates a greater wall thickness, which in turn raises the difficulties of welding and imposes stricter requirements on the manufacturing process.

Regarding the construction of solar thermal demonstration power plants in China, since the vast majority of these projects will employ molten salt heat storage technology, great attention should be paid to the manufacturing of the storage tanks, particularly with regard to welding, in order to avoid repeating past mistakes. Although from a technical perspective, repairing salt leakage in tank welds is relatively simple, it will cause the power plant to be shut down for a certain period of time, taking a long time and affecting power generation revenues. Grikas also said that the New Moon Dunes Power Plant has been gradually increasing its annual designed power generation capacity to 500 million kWh over the past year or so – since it began trial operation connected to the grid in November 2015, with official confirmation of its full-scale grid connection in February 2016. ” Based on an annual power generation of 500 million kWh, the average monthly generation is 41,667 MWh. According to the most recent available data, the plant’s monthly generation in September this year reached 30,514 MWh, setting a new high since it was connected to the grid; this figure represents 73.2% of the average monthly generation, representing an increase of about 8% compared to August’s 28,267 MWh.

The Crescent Dunes Power Plant is the world’s first 100 MW-class molten salt tower solar thermal power plant, and its construction took longer than initially estimated by the authorities. As for the time required to reach the designed power generation capacity during this transition period, Solar Reserve previously said it would take about a year; however, it seems that this estimate might be a bit optimistic. In any case, there was no prior successful experience in developing 100 MW-class molten salt tower power plants before the Crescent Dunes project; it is therefore normal for certain issues to arise during their construction and operation. For a high-tech-intensive project like CSP power plants, sufficient room should be provided for trial and error and the accumulation of experience. We do not avoid the various problems that arise during development; rather, our goal is to gather as much practical experience as possible so as to ensure a more stable progress in the industry’s development.

Analysis of the seven causes of the molten salt leakage incident at the New Moon Dunes solar power plant December 7, 2016, Global New Energy Network. The New Moon Dunes plant, which has the largest installed capacity among existing molten salt tower power plants in the world, recently experienced a minor leakage of molten salt from its storage tanks. As a result, the plant has been out of service since late October of this year, and it is expected to resume normal operation only in January next year. Excluding the planned maintenance period in December as claimed by SolarReserve, this power plant was unable to generate electricity for at least an entire month in November due to this accident. In addition to the economic losses resulting from the maintenance of the heat storage tanks, the greater loss is the decrease in electricity sales revenue caused by the shutdown of the power plant. According to the latest data, the power plant generated 30,514 MWh of electricity in September this year. The PPA for this project stipulates a price of 0.135 dollars/kWh; based on this, it can be roughly estimated that the loss in revenue from electricity sales resulting from one month of operation shutdown is around 4 million dollars. From this perspective, the economic losses caused by molten salt leaks in operating power plants cannot be underestimated. It is understood that salt leakage accidents are more likely to occur in concentrated solar power projects, especially in molten salt tower power plants; the world’s first commercial molten salt tower power plant, Gemasolar, is said to have experienced similar incidents as well. So, for the solar thermal demonstration projects that are set to be launched in China, how can such accidents be avoided to the greatest extent possible? First, we need to figure out what causes salt leakage accidents in storage tanks. During discussions on related topics, several industry enthusiasts shared their views on the causes of this accident. According to industry experts, there could be as many as seven reasons for salt leakage in storage tanks, including issues with temperature and quality control during welding, stress on the tank structure, heat treatment carried out after welding to relieve stress, corrosion of the tank’s steel material, a low rate of non-destructive testing of welds, and oversized dimensions of the salt tanks, among others. Since the developer of the Crescent Dune Power Station has not disclosed specific details regarding the leakage, domestic industry experts have only analyzed and interpreted this issue from their professional perspectives, for reference only.

I. Process quality issues in welding? “I believe the weld leakage this time may be related to the fact that the contractor did not strictly control the temperature during the welding process in accordance with the welding procedures. Different materials require different welding process temperatures, and this temperature depends on the material of the tank, the flux used, the groove shape, and so on. “said Yu Xiaohua, Product Sales Manager for Greater China at Bintel Valve Fluid Control Company. In addition to the welding temperature, an industry expert from Shandong Electric Power Construction Third Engineering Company suggested that it could also be due to the thick steel plates, non-standard groove dimensions, and incomplete welding during the welding process. Additionally, wind protection measures may not have been properly in place during the use of the automatic welding machine. Wang Jun from Yantai Ourun Mechanical and Electrical Technology Services Co., Ltd. added, “Problems related to welding also include inadequate cleaning of the weld area before welding, poor use of welding gas for protection, insufficient stress relief after welding, and inadequate flaw detection after welding.” ”

II. Do huge internal stresses in the tank cause leaks? Many industry experts believe that the main cause is stress concentration resulting from large temperature differences inside the heat storage tank. Possible causes of stress problems inside the tank include fluctuations in the temperature of the molten salt, which lead to stress cracks; under certain cycles of heating and cooling, these cracks can further propagate. “The weld is the weakest part of the tank, the area most prone to stress concentration. Moreover, due to the large size of the tank, thermal analysis and temperature monitoring points should be installed to ensure uniform heating, thereby preventing stress from concentrating in certain areas. ”Yu Xiaohua said.

III. Is the corrosion resistance of the flux material insufficient? The issue with the welds of the storage tank this time is also related to the corrosion resistance of the flux material, which is another factor that needs to be taken into consideration. Therefore, to prevent similar problems from occurring, some experts suggest raising the corrosion resistance standards of the solder or adding corrosion-resistant components. However, Wang Jun believes that this statement does not apply to the Crescent Dune Power Station. According to the developer, the scale of this leakage is very small; if the corrosion resistance of the solder were insufficient, there should have been widespread cracking. Therefore, in the long run, it is advisable to carry out pickling and passivation treatment on the surface of the tank, especially at the weld areas.

IV. Does molten salt corrosion of the tank steel cause salt leakage? Tong Guanghua from Beijing Baifuli Technology Co., Ltd. pointed out that the thermal expansion and contraction of molten salt exert pressure on the materials, thereby breaking the crystal lattice of the steel and causing corrosion in the storage tanks. He Yuanzhi, however, speculated that the weld leakage here was not caused by corrosion. “For large-diameter molten salt tanks, what concerns me most is the issue of tank expansion under high temperatures and large diameter differences. The bottom of the tank expands significantly as it slides on the foundation; if there are local restrictions in the structure, such as the components like heaters expanding in a direction different from that of the tank bottom, or if the stress calculations for the piping do not take into account the expansion amount of the tank diameter, this could lead to weld cracks. (The large pipes of the molten salt tank are at the top; pipe stress may not be the cause this time.)”

V. Was post-weld stress relief (heat treatment) not carried out properly? Yu Xiaohua believes that for such large containers, the thicker steel plates used should undergo post-weld heat treatment to eliminate stress. Furthermore, other equipment and pipelines welded to the tank should also undergo post-weld heat treatment. Therefore, it is speculated that this salt leakage is likely related to this. However, He Yuanzhi from Bihaizhou (Beijing) Energy-Saving and Environmental Protection Equipment Co., Ltd. holds a different view. He states that large storage tanks have the material in/out ports, instrument connections, and maintenance manholes at the bottom of the tank all concentrated on several tank wall panels. The wall plates of large oil tanks are made of low-alloy steel plates. Those wall plates that have pipe openings/maintenance holes require post-weld stress-relief heat treatment, which is carried out in a heat treatment furnace in the factory; usually, no defects arise as a result of this heat treatment. For low-alloy tanks, the tank dimensions are adjusted during design so that the designed wall thickness is less than the thickness required for post-weld heat treatment. Austenitic stainless steels generally do not undergo post-weld stress-relief heat treatment. Furthermore, given the size of the molten salt tank and the on-site construction conditions, it is also not possible to carry out solution treatment or stabilization treatment on the welds on site. The operating temperature of the molten salt tank reaches up to 560°C, which is sufficient to meet the temperature requirements for post-weld stress relief heat treatment under reduced heating conditions; thus, welding stresses are naturally relieved during operation. Therefore, He Yuanzhi believes it is not a problem of inadequate post-weld stress relief (heat treatment).