Salt is gaining ground as a contender for thermal energy storage. In the molten form it is now commercially available for the storage of heat from concentrated solar power on timescales of days. In the dry form on the other hand, the technology is still emerging, with the potential to open the way for storage on timescales of months and to support the integration of high levels of renewables in addition to providing an alternative to other storage systems such as batteries.
Swedish company SaltX Technology is a pioneer of this ‘salt hydrate’ technology and is piloting a large-scale plant with Vattenfall in Germany.
The principle of the technology is that energy is stored chemically by separating salt from water and then released by combining them again. SaltX’s secret sauce is its now patented nanocoating of the salt, which is claimed to offer several advantages. One is that it prevents the salt from becoming sticky and thus to retain its original single crystal form, which in turn increases the number of charge-discharge cycles it can undergo. Another is that the salt is non-corrosive and also it is non-toxic and recyclable.
Energy storage pilot
The pilot plant with a 0.5MW power output/input and total storage capacity of 10MWh was commissioned at Vattenfall’s Reuter combined heat and power plant in Spandau, Berlin earlier this year.
The plant is utilising a calcium oxide (quicklime) salt and comprises a pair of insulated carbon steel silos for storing the dry salt (CaO) and hydrated salt (Ca(OH)2). These are connected by a dehydration screw that separates the hydrated salt into steam and CaO and a hydrating screw that rehydrates the CaO with steam along with the production of heat up to about 500oC. This heat is then being fed into the local district heating network.
The objective is to demonstrate the large-scale feasibility of the technology in a research and development setting, explains Markus Witt, who is responsible for the project at Vattenfall Wärme Berlin AG.
“We have demonstrated that the technology and process work intrinsically but we need to optimise the various process parameters,” he says. “Questions are what volumes of salt can be used with different input energies, how quickly the storage medium reacts and how the process can be controlled.”
Besides providing a storage mechanism for excess renewable supply, the technology also is envisioned for grid management applications, Witt adds.
Another key issue is the business case around of the technology. “We are looking at that but at this stage it’s difficult to say as we are still in the R&D phase and there may be changes to the technology. But the salt is very cheap and the nanocoating is straightforward and key factors will be the input energy costs, which in Germany are very volatile, and the value that is extracted,” Witt says. “There is a lot more work to be done but we see a definite potential. Energy storage is so important it makes sense to put our efforts into this.”
For Vattenfall the project also forms part of its long-term rebuilding of the Reuter power plant. In the current year, the company intends to start operating Europe's largest power-to-heat facility, an energy store based on storage of hot water. During 2020, the Reuter C coal power plant will be taken out of service as a step in the plan to phase out all coal as a fuel in the company's heating operations in Berlin by 2030.
With tests on the pilot still under way, detailed results, and a decision on how Vattenfall will proceed with the technology, are expected only towards the end of the year.
But SaltX is looking ahead with two prospects in view. One is to modularise the solution which should simplify scaling and delivery to the market.
The second and more broadly is to commercialise the technology and SaltX has entered into agreements with Steinmuller Engineering in Germany and Inerco Ingeniería, Tecnología y Consultoría in Spain to build initial pre-commercial units during 2020. The Steinmuller agreement is focussed primarily on the German market, while initial target markets identified in the Inerco alliance include Spain, Portugal and south and central America, including Mexico, according to company statements.
“The pilot at Vattenfall in Berlin was an important step that took us in the right direction. The next step is to learn from that pilot and build pre-commercial pilots together with strategic partners on key markets,” says Carl-Johan Linér, CEO of SaltX Technology.