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Crushed hot rocks to store wind energy

The bulk storage of electricity as a flexibility tool will become economically viable with the growing availability of large volumes of low-cost renewable energy. Crushed hot rocks could provide a cheap storage solution

One big pot, five tonnes of crushed rocks and a giant hairdryer may sound like ingredients from a fairy tale, but a Danish energy company believes they could be a solution for storing electricity as heat when supplies of wind energy are in excess of immediate needs. As its pilot project comes to an end, SEAS-NVE says it is ready to scale-up operations

Flexibility:
Balancing the ups and downs of supply and demand in a power system with a high proportion of variable input from renewable energy will require flexible management of both, helped by digitally enabled electricity trade and injections of stored power as required

Latest solution:
Denmark is exploring the idea of storing energy from wind power by using it to heat packed layers of crushed rocks

Advantages:
Relatively cheap, uses readily available materials, low maintenance costs, durability, ability to provide electricity and heat

Disadvantages:
Needs a fair bit of space and the conversion to heat and back to electricity does not make for an efficient process

Key quote: “
Stone storage technology represents coupling between the power sector and the heating sector, which is a gigantic advantage if we are to be 100% renewable”

Altering the volume of electricity being used at a given moment has, until now, been a more cost-effective way of balancing supply and demand than storing excess renewable energy and releasing it when needed. The use of demand side management as a balancing tool will only grow as digital tools increase the ability to flexibly manage power systems. But research also indicates that the bulk storage of electricity as a flexibility tool will become economically viable with the growing availability of large volumes of low-cost renewable energy. At the end of a two-and-a-half-year pilot project, experts in Denmark are increasingly confident that storage solutions based on capturing energy as heat in crushed rocks will play a role in future systems. “We don’t really need storage today,” but it will have an increasing role in balancing energy systems with the move towards 100% renewables, says Ole Alm, head of development at SEAS-NVE, the Danish energy group leading the hot rock trials. Denmark’s goal is to be 100% fossil free by 2050. Wind power that is not needed domestically is currently sold to neighbouring countries such as Norway, Sweden and Germany when it makes economic sense. But as these countries increasingly have their own significant supplies of energy from renewable sources, they will be less interested in buying from Denmark, says Alm. DNV GL, a Norway-based energy consulting and services firm, forecasts that once renewables reach 40% to 50% of any power market, the need for storage takes off exponentially to balance markets. In 2017, 43.4% of power produced in Denmark came solely from wind turbines.

How it works

One storage solution being explored by SEAS-NVE is so-called High Temperature Thermal Energy Storage (HT-TES). This consists of around five tonnes of crushed rock being placed in large insulated bowls. Wind energy heats the crushed rocks to 600°C. A few days later, when there is no wind, air is blown over the rocks with a large ventilator. Once the cold air has been heated to 600°C, a heat exchanger extracts steam from the bowl. The steam is then passed through a generator, producing electricity. “The aim is to take excess renewable electricity, store it in a cheap way and produce electricity on days when there is no wind,” says Alm. While the hot rock storage system sounds simple, questions remain over its efficiency. Preliminary forecasts indicate that storing wind power in this way and turning it into electricity again will have an efficiency rate of approximately 40%. Advocates insist the remaining 60% is not entirely lost, with just over half of this expected to be fed into Denmark’s district heating networks, leading to a final energy loss of around 20%. Alm acknowledges the need to “optimise processes to get as much power out as possible”, but he also believes the system has advantages compared to other storage solutions in the market.

Alternative solutions

“A battery storage system would produce almost 100% electricity, but would also cost at least ten times, perhaps even 20 times, as much,” he says, adding that a hot rock storage system does not lose capacity over time as batteries do. The hot rock storage system is also cheap to build, maintenance costs are low and, once in place, the rocks can be heated and cooled continuously for decades.“In hot rocks you can store large amounts of power to cover several days of consumption much cheaper than in batteries,” says Alm. “The good thing is that we do not have to invent something new and we do not need rare earth minerals or other ingredients that may be in shortage in the future,” he adds. The system also compares favourably to another storage system being tested, namely hydropower, where water is stored underground in reservoirs or compressed air energy storage (CAES), says SEAS-NVE. Gorm Bruun Andresen, associate professor at Aarhus University, concurs. “First, rocks are not a scarce resource and the technology is environmentally friendly,” he says. “Second, it is cheap and scalable and, unlike hydropower, it does not require a specific location.” And while the hot rock storage system is today mostly viewed as a power-to-power system with residual heat merely considered as a free bonus, this may change in the future when renewable electricity is more commonly used to produce heat, primarily through heat pumps, says Bruun Andresen. “The hot rock storage technology would fit well into a system that is already producing heat from electricity” where heat is no longer a waste product, he adds. “The yield is not as high as with heat pumps, but the hot rock storage system is flexible. I could imagine that in future most heat will come from heat pumps as it is the most efficient technology, but the hot rock storage system can provide flexibility in terms of producing heat and electricity.” The system is also much cheaper than heat pumps, he adds. Bruun Andresen sees a future in which heat pumps provide a stable level of heat, while hot rock storage systems help balance supply and demand in the electricity and heat systems and provide electricity and heat to those who need it, such as factories or neighbouring countries. “We will then have a much more efficient energy system because the hot rock storage technology represents coupling between the power sector and the heating sector, which is a gigantic advantage if we are to be 100% renewable,” he concludes.

Business case

Another potential problem is the fact the hot rock storage system takes up a fair amount of space. An area the size of a football pitch would be needed for six full-scale storage units able to provide storage capacity to power Denmark for an hour. But Alm suggests this should not block progress since: “We will only make money if we sell the stored electricity, and so the idea is to get rid of it as fast as possible, perhaps just store it for a day or a few days.” The total budget for the pilot phase, which finishes in May 2019, is DKK 8.5 million (€0.9 million). In addition to examining storage potential, the project is also analysing the challenges and prerequisites for adding high-capacity energy storage to an existing grid, an existing market model and an existing tax system. “I hope that within the next six months we will have sufficient results to make a decision on whether to proceed with the project,” says Alm. The next leg of the project would run from 2020 to 2024 and involve, hopes SEAS-NVE, the testing of a full-scale storage system. Two of the project partners, Danish Energy, an industry lobby organisation, and Energinet, the national grid operator, have created a model to predict when it is economically feasible to use the hot rock storage system. It uses a variety of parameters, including the energy mix, future renewable energy projects, international grid connections, expected electricity consumption for electric vehicles and heat pumps, and the price of the storage system. “Today, the rock storage system would be picked for one or two hours a year, which is not viable,” says Alm. “But the more wind power we get and the more thermal power stations are shut down, the more often the storage system will be chosen. In five to ten years it looks like we have a business case.” A final report from Danish Energy and Energinet is currently being completed and will help inform the decision on whether to proceed with the project after 2019.

Writer: Karin Jensen