Molten salt is gaining popularity as an alternative P2X technology
SOLAR LESSONS
Molten salt technology used in concentrated solar plants (CSP) is being used as the basis for long-term storage processes
COAL REPLACEMENT
Salt storage projects could be used at retired coal plants in order to reduce infrastructure costs
KEY QUOTE
Salt storage, and thermal storage in general, is underrated
Focus on Power-to-X (PtX) technologies—using renewable electricity to produce another energy carrier or fuel—has increased massively in the last couple of years. Politicians, companies and experts believe PtX technologies can play a significant role in the energy transition, particularly for hard-to-abate sectors such as heavy-duty transport. Investment in PtX projects is increasing and several countries have presented ambitious approaches: the Danish government proposed a strategy earmarking €168 million for the development of green hydrogen, aiming for 4-6 gigawatts (GW) of electrolyser capacity by 2030. But PtX is not only about green fuels and the “X” does not only mean green hydrogen or ammonia. One of the PtX solutions garnering attention uses the energy from wind and solar to heat salt, which is then used as an energy source in high-temperature industrial processes, PtX processes or in district heating. Kurt Engelbrecht at the Technological University of Denmark (DTU), says salt storage could become an important solution in securing a balanced and stable energy system based on renewables. “Salt storage, and thermal storage in general, is underrated. It is a proven technology, it can be implemented and scaled quickly, and it is not costly to do so,” says Engelbrecht. A prerequisite to this is that large scale salt storage projects are due to be established within the next few years. “Things are beginning to move forward, but there has been a bit of a chicken and the egg situation. We still lack large investments and demonstration projects, which are necessary for salt storage to really take off as a potential future energy solution,” he says. 
CSP LESSONS Danish company Aalborg CSP is one of the companies that has chosen to pursue salt storage drawing on its experience within concentrated solar power (CSP) technology where salt is already used. With a molten salt storage system, wind or solar energy can be stored as hot salt which can then be channelled to produce steam to drive a steam turbine for electricity or used in combined heat and power (CHP) projects feeding into district heating. Aalborg CSP has partnered with Norwegian thermal battery firm Kyoto Group to set up the first molten salt storage plant in Denmark, which will be provided electricity from wind turbines. The pilot project will be used for district heating and will be operational in 2022. The idea is to, “Bring the technology to scale and show that salt storage systems can easily be integrated with the existing CHP or steam systems,” says Svante Bundgaard of Aalborg CSP.
ACCESSIBLE AND SCALABLE Many in the energy sector believe large scale storage facilities will be required to balance the increased share of renewables in the energy mix. Storage would, according to the International Renewable Energy Agency (IRENA) help make energy systems more stable, flexible, and cheaper to build and operate. Batteries have increasingly proved their worth in terms of delivering power over a short period of time, but there is a lack of cost-effective solutions that can store and provide large amounts of energy for longer periods. “We need large-scale storage that can store energy when there is no sun and wind and if we look a few years ahead, demand for this will only increase,” says Brian Vad Mathiesen at Aalborg University. He adds that salt storage could be a relevant option in several areas. John Hald of DTU agrees: “We know the technology works and it is in operation at different CSP facilities around the world. You can buy it today build it where you want and add several tanks to make it as big as you want—and it is relatively efficient.” Looking at the efficiency of salt storage it is currently possible to extract 35-45% of the energy stored in salt as electricity. The remaining energy can be extracted as heat. This is a lower return than using hydropower where you can extract about 75-80% again as electricity. “But unlike hydropower, salt storage is not geographically limited to places with mountains or water and can be set up everywhere,” says Hald. The round trip efficiency of energy storage in batteries is between 80-95% but can, to-date, only supply electricity over a shorter duration.
INCREASED EFFICIENCY Vad Mathiessen says the current efficiency level could limit the popularity of salt storage systems. “The efficiency affects the costs and could potentially make it difficult to compete with other storage solutions especially in areas where there are other options for instance district heating,” he says. Consequently, a lot of the research is focused on improving the efficiency of salt storage. Danish firm Hyme, a spin-off from Copenhagen-based nuclear startup Seaborg Technologies, developed a salt storage solution based on sodium hydroxide that can contain more heat per salt unit compared to sodium nitrate, which is the most commonly used salt in CSP and other salt storage projects. Hyme’s preliminary trials show a conversion efficiency of 50-65% with sodium hydroxide. This salt is also about 90% cheaper than the cost of nitrate salts. “So far our tests with sodium hydroxide show that we can increase efficiency and reduce the amount of salt needed significantly. This would make salt storage a much more competitive and attractive alternative to other storage solutions,” says Ask Emil Løvschall-Jensen of Hyme. The company expects to be able to keep the cost for installed energy in salt of about €17 per kilowatt-hour (/kWh) and the total material and equipment costs of entire plants with ten hours of storage below €100/kWh. The corresponding cost for lithium batteries with the same capacity and ten hours of storage would be €166/kWh, according to data from the United States government-backed National Renewable Energy Laboratory (NREL). 
Repurpose
sites Installing salt storage to retired coal plants would help cut costs
RETROFIT COAL SITES According to Hald salt storage systems could already, “Replace the coal plants and it wouldn’t be that difficult,” he says. “Salt storage systems could be integrated with coal plants and make use of existing hardware and technology, and not demand new infrastructure. Similar [conversions] have already been done at most of the plants in Denmark, where coal has been replaced with straw or wood pellets. You could do the same with energy stored in salt with the energy stemming from renewable sources,” Hald adds. Aalborg CSP’s Svante Bundgaard says its salt storage system can be, “Integrated with and reuse the existing energy infrastructure at coal plants. It would in fact be quite easy to reuse hardware and infrastructure such as turbines, condensing systems and grid connection.” Cost estimates for salt storage coal-replacement—where the turbine is recycled—is approximately €22/kWh including charging, storage, discharging. Coal plants and district heating is also an area where Løvschall-Jensen sees opportunities for salt storage. “A country such as Poland, there is a great need to reduce dependence on burning coal. But at the same time, they have a lot of hardware and an infrastructure that could run for many years to come and which would cost a lot to replace. Here salt storage would be ideal because it reduces costs and enables you to build on the existing infrastructure.” Brian Vad Mathiesen, however, is more sceptical about whether salt storage will prove to be profitable when it comes to coal plants: “Storage in salt will be difficult in relation to electricity supply because it would be competing against solutions that are cheaper for example biomass and biogas.”
HIGH-TEMPERATURE PROCESSES The area where both experts and companies see the biggest potential for salt storage is in high-temperature manufacturing processes. Most of this heat is currently generated using natural gas, coal or other fossil sources, which gives these processes a high carbon footprint. For high-temperature processes up to 500-700°C, salt storage could replace fossil sources. Hyme expects its salt storage operating temperature will be able to go up to 700°C, while Aalborg CSP says that its technology can reach approx 560°C. Looking at industry steam turbines in Denmark they are currently operating at a maximum temperature of 600°C, which means that it would be possible to shift from gas and other fossil sources to renewable energy stored in salt. “If we take industrial steam processes based on steam turbines, salt storage energy systems could be implemented now and would not demand huge investments or new technological breakthroughs or hardware—the energy stored in salt can be used directly to boil steam that would drive the turbine,” DTU’s Engelbrecht says. One example is in dairy production which uses approximately 150°C temperatures for pasteurisation. Today, this process is primarily based on gas, “But it would be possible to replace this gas with heat from energy generated by wind turbines and stored in salt,” says DTU’s Engelbrecht. 
Dairy production Lower heat processes that currently depend on fossil fuels could switch to salt storage technology
SCALE NEEDED Despite the need for storage solutions and the possibilities salt storage systems entails, it is still a technology that has been known for years but has yet to reach commercialisation or scale. Today there are only a few small plants and pilot projects. John Hald says price and volume are the primary reason for this. “There isn’t yet a value chain around salt storage with a clear demand, which can make it attractive and economically profitable,” he says. Engelbrecht adds: “We can solve the technological barriers, but we lack the industry, the value chains and the investments.” But he is optimistic because of the increasing activity in the area. “The EU and countries such as Japan are starting to set up more and larger demonstration projects with several actors onboard, which enables a greater scale of production and a stronger value chain,” says Engelbrecht. New salt chemistries that are cheaper are also being developed, further advancing the business case for salt storage projects. “There has been a need to improve the technology itself and to find more cost-effective solutions. A big challenge has been to control different types of salts, which are often very aggressive towards the containers and therefore often become too expensive in material costs,” adds Engelbrecht. Hyme’s Løvschall-Jensen says they have so far been able to control the corrosive tendencies with new salt chemistries in laboratory tests, but the company now needs to bring its results to market and show that it can build a large-scale salt storage plant with all the essential components. “This is a necessary step if we are to bring the technology to commercialisation,” he says. Hyme is planning to build a plant in Esbjerg, where surplus electricity from wind turbines can be stored for up to 14 days. They have raised about €10 million from backers, including Anders Holch Povlsen who owns the international retail clothing chain Bestseller and is Denmark’s richest man. They have also just recently received €3.36 million from Energy Technology Development and Demonstration Program (EUDP) to fund the project. •
TEXT Anna Fenger