Policy:
Germany will close its coal and lignite power stations within the next 20 years and is now working out how to go about it without disrupting electricity supply *

Experiment:*
One solution being explored is to replace lignite or coal boilers with molten salt based thermal storage heated by surplus electricity from renewable sources

Aims:
Lower the costs and improve the efficiency of molten salt technology

In January 2019, Germany agreed a roadmap for the step-by-step closure of its coal and lignite power station fleet by 2038. The federal government is now drafting legislation for implementation. Various solutions are being explored to help create a steady and secure electricity supply in a post-fossil fuel era. One is to replace lignite or coal boilers with molten salt based thermal storage heated by surplus electricity from renewable sources. DLR, a German aerospace research centre, is leading investigations into molten salt thermal storage technology. The first thermal storage electricity generation plant is foreseen at a site in the Rheinisch lignite region in western Germany where around 10 gigawatts (GW) of lignite power stations will close in the next 20 years. Molten salt thermal storage is already used with concentrating solar power (CSP) plants, such as solar towers, parabolic trough and parabolic dish plants. In sunny countries like Spain, Morocco and Egypt, CSP plants generate electricity during the day, with some solar energy going to heat a stored salt solution. The stored heat is released for electricity generation at night, cooling the salt making it ready to be heated again the next day. The International Energy Agency forecasts an increasing role for CSP with thermal storage after 2030 as technology advances and mass deployment lead to cost reductions. It predicts thermal storage capacity of CSP globally will grow from 13 gigawatt hours (GWh) in 2017 to 34 GWh in 2023.

Cheap and available

Until around 2014, solar thermal power plants used thermal oils as the heat transfer fluid and salt as the storage medium. Since then, molten salt is increasingly used instead, being cheap and available worldwide, and more stable over long periods. It also enables higher process temperatures of about 550°C compared with thermal oil temperature limits of about 390°C. The salt is permanently maintained in a molten, liquid phase so it can be used as a storage medium and a heat transfer fluid for passage through a heat exchanger to create steam for electricity generation. The properties of salt are well known. It is easy to pump in a molten state and even at high temperatures no pressure has to be applied in the system. This is in contrast to working with water or steam where high pressure is needed for systems with high temperatures. The solar salt mixture usually comprises sodium nitrate and potassium nitrate which, says SolarReserve, a solar thermal energy storage technology company, can be used as high grade fertiliser when power stations are eventually decommissioned.

From south to north

DLR is aiming to demonstrate the potential of molten salt storage technology in cooler climates, like north-west Europe. Storage can ease the challenges of keeping variations in electricity supply and demand in balance, adding an extra flexibility option for power system management. “We need high-capacity, highly efficient energy storage that can be built anywhere at reasonable cost,” says André Thess, director of the DLR technical thermodynamics institute. He believes thermal storage has the potential to become an “ideal, gigawatt-hour scale energy store”. A gigawatt hour of electricity produced from a molten salt heat storage plant would be sufficient to supply a medium-sized town of 100,000 households with electricity for 24 hours, says Henner Kerskes from Stuttgart university. The extra cost of the store and supplying electricity from it can make sense when the marginal cost of the energy generated to fill the store is zero or close to zero. DLR, with government support, has been operating an industry-scale test facility aimed at improving CSP thermal storage systems in Cologne since September 2017. A hundred tonnes of liquid salt circulate in the test system, the temperature raised and cooled between 250°C and 560°C. Instead of separate hot and cold storage tanks used in CSP systems, the project is testing a single vertical tank with hotter material at the top and cooler material at the bottom. Cheaper filler material such as ceramics or stone can then replace much of the liquid salt. Industry partners are also testing if system components can withstand contact with the liquid salt, and the controls technology. Lessons learned will be applied in the Rheinisch project.

Efficiency improvements

Round-trip efficiency, the ratio of electricity output to electricity input, of the technology is currently at 35%, with one GW of electricity produced for every three GWh of energy put into the molten salt heat store. Scientists believe efficiency improvements may be achieved if low temperature heat pumps or electricity immersion coil methods for heating molten salt are replaced with high temperature heat pumps operating beyond current maximum temperatures of around 120°C. These are not yet commercially available. “This is where we want to get to in the short term,” says Kerskes. An EU research project dubbed HP-MOSES performed a technical and economic feasibility analysis in 2017 of a large-scale (50 MW to 1000 MW) molten salt energy storage system based on high temperature heat pumps. The round-trip-efficiency was 55-60% for the most suitable system compared with 70-85% for a conventional hydro pumped storage plant. The project summary predicted the technology can be “a cost-effective and commercially viable large-scale site-independent energy storage system”. DLR’s molten salt feasibility study will take about a year. If results are favourable, construction of a demonstration project in the Rheinisch region could begin in 2021.

Writer: Sara Knight