Although pioneering vast numbers of new advances, the renewables industry is also finding great value in reappropriating existing technologies from other power generation sectors. This is particularly true of the back-end operations of the solar power generation industry, a prime example of which can be seen in its embracing of molten salt pumps.
Molten salts have been used in various industrial applications for almost 70 years, so to describe them as ‘established’ is something of an understatement. When deployed in solar applications, molten salt is pumped to solar collectors (tower or parabolic mirror fields) where its temperature increases from around 300°C to up to 600°C. The high-temperature salt is then stored in a tank and transferred to a heat exchanger where the thermal energy is used to create steam that will generate power using a turbine/generator. This circuit acts as a thermal battery, which enables solar plants to deliver power round-the-clock in good solar conditions.
Power engineering giant Sulzer is a well-known expert in molten salt pumps for various applications. Cécile Hortelan, who works in technical sales, says that the firm “has long been involved in the design and manufacture” of the pumps that transfer the high-temperature fluids around the cooling circuits. “In fact, we were involved in pioneering the technology for use in the solar industry more than 20 years ago, enabling heat energy to be stored overnight.”
24-HOUR SOLAR POWER
When it comes to solar applications, Hortelan says that the advantages of using molten salt pumps are clear. “Without some form of storage system, solar installations can only deliver power during daylight hours, which severely restricts their usefulness to the local grid,” she explains. “Storage options are limited. Batteries are very expensive – both in CAPEX and maintenance. Molten salts offer a cost-effective and efficient method of storing and releasing energy uniformly over a 24-hour period.”
REFLECTING THE SITUATION
Proof of the pumps’ popularity can be found in a flurry of recent contracts for Sulzer, including a recently announced, first-of-its-kind project in China. “A large concentrated solar power tower (CSP) project in China has selected Sulzer to supply both the hot and the cold molten salts pumps for the installation,” explains a proud Hortelan.
The planned 100MW site is part of a major programme of renewable energy, driven by the increasing demand for sustainable energy sources in China and around the world. “Sulzer has a long track record of delivering hot and cold molten salt pumps to solar installations around the world, including recent projects in China,” explains Hortelan. “The 100MW CSP tower project is the first of its type to use Sulzer pumps for both the hot and the cold circuits. The three cold molten salt pumps will be fitted with 2.6MW motors and will extend 18m below the base plate into the cold molten salt tank. These will be matched with two hot pumps, with all the pumps being manufactured in our Suzhou plant.”
According to Hortelan, the most challenging element of this project was the increased operating pressure required in the molten salt circuits. “Sites with smaller outputs normally operate at around 50 bar, but this project needed a 100-bar supply pressure from the pumps,” she explains.
Each pump is built to order – and this project had to be engineered to account for the increased pressure and the slightly larger power requirement. Hortelan says: “Compared with the more common, lower pressure pumps, there are several crucial points, such as the throttle bushing and bush bearing design, that need to be carefully assessed, properly engineered and tested before the manufacturing process can begin.
“Also, in this case, the end user wanted to achieve certain objectives in the delivery of this project,” she adds. “It wanted international brands to supply the key equipment, which ideally would be manufactured in China and cost-effective compared with the competition.”
So, did Sulzer rise to the challenge? “We were able to meet all these objectives,” confirms Hortelan. “The engineering of the bare-shaft pump will be completed in Europe and the manufacturing will be a close collaboration between the facilities in Belgium and China. In addition, we will be able to offer a comprehensive support package for the products that will be delivered via our local service centre.”
Hortelan says the project is expected to be operational in the next few months, adding that: “As part of the contract our local engineers will be on hand to support the installation and commissioning of the equipment. Due to their length, the pumps will be assembled on site before installation in the storage tanks.”
This installation follows several other successful projects in China for Sulzer including three hot pumps for another 100MW CSP as well as six hot and five cold pumps for a 42MW parabolic trough installation. And Hortelan believes that it’s her firm’s bespoke approach that’s proving especially attractive to solar customers: “We engineer every pump to the customer’s specific application. We also operate a process of continuous improvement, where designers work to widen the operational envelope and create greater efficiencies,” she states.
HERE COMES THE SUN
After pioneering many advances in pump technology, Hortelan says that Sulzer’s next focus is on improving operational efficiencies. “In the molten salts arena, we have recently developed pumps that can operate at 750°C as opposed to the more ‘normal’ conditions of 600°C,” she reveals. “Due to such high temperatures, we had to tackle many challenges for the materials used for the components and seals that comprise the pump. However, once complete, this will deliver an additional level of efficiency for the operators.”