By Simran Khokha
With increasing technological advances, renewable energy sources - like wind, solar and hydroelectric power - are becoming more accessible to the public and to industry for the benefit of our planet. These are great times, but not without their own challenges. Energy once generated, needs to be stored, and for the past 3 decades, lithium-ion storage batteries have been extensively used for this purpose.
The complete transition from fossil fuels to renewable energy sources requires a fully developed flow from energy generation to storage. Consequently, much work is being done in terms of generating energy using renewable resources: from electric cars, to complete power grids utilising only solar energy. Companies like Tesla and billionaires like Bill Gates have come forward to try to solve the issue of climate change, with many investing in these technologies. This ensures that the whole process can be environmentally friendly and carbon emission-free, which would not only reduce the emission of greenhouse gases but also reduce our carbon footprint. While we do harness electricity using wind, solar and hydroelectric energy, the majority of electricity still comes from the power plants that run on fossil fuels. Furthermore, even though current renewable technologies have made it cheaper for us to use renewable energies for transportation, power grids and industry, there is still a need for storing this energy to be used as a backup during windless days, cloudy weather and nighttime.
Malta Inc. a Google X project started at moonshot factory has taken up this challenge and is building a system that stores energy as heat (in the form of molten salt) and as cold (in a chilled liquid), based on well-established principles in thermodynamics. The concept was being investigated for nearly 2 years before Malta was created, and because of its future potential to provide clean energy as well as its commercial viability, the project was identified as a product that could meet global energy demands. Using molten salt is currently the most common method to store heat in large power plants and provide a constant on-demand supply of solar power without the need for fossil fuel backup systems. Companies such as German-based Frenell, formerly Novatec, offer a turnkey solution combining a solar field and a proprietary direct molten salt technology. “We have made improvements to our systems, which uses molten salt directly as the heat transfer fluid,” said Max Mertins, CTO of Frenell. He also stated that “they allow for a large temperature difference, around 280 K, and lead to heat storage costs ranging from 15 to 25 EUR/kWh” whereas energy storage using Li-Ion batteries costs around 175 USD/kWh as of 2018. Moreover, molten salts offer exceptional heat transfer, chemical stability, and thermal energy storage (TES) capability.
How do they do that?
The process includes generating electrical energy from wind, solar or any other forms of renewable source, and then converting it into thermal energy. As explained by Malta, the electricity drives a heat pump, which converts electrical energy into thermal energy by creating a temperature difference. Later, this thermal energy can be converted back to electricity to be redistributed on the electrical grid.
In charging mode, the system operates by taking electricity from the grid to drive a heat pump that creates streams of hot and cold air, which are directed to tanks of molten salt (heated to 565°C) and anti-freeze (cooled to -65°C). The heat is then stored in the molten salt, while the cold is stored in the anti-freeze. The reason why anti-freeze is used is because the process of thermal storage approach always uses the molten salt mixture in liquid rather than solid state. Avoiding salt freezing is an essential element in thermal storage, as this can reduce conversion efficiency, while freeze-thaw cycling within pressure-retaining systems can lead to maintenance problems. This thermal energy can be stored for a long duration (days to weeks) due to effective thermal insulation. Then, when electric energy is required, the above charging process is effectively reversed using this heat engine — basically a compressor and a turbine — which drives a generator to produce electricity.
This discharge process is similar to the way a jet engine works. Jet engines draw in cold air, compress it in a compressor, then the jet fuel is combusted to heat the air, and the hot air is expanded through the turbine, sending it spinning at high speeds. Malta uses the hot molten salt to heat the air (instead of jet fuel), while the cold anti-freeze liquid cools the air that is normally rejected to the atmosphere by a jet engine. In this way electricity can be sent back to the grid when it is needed.
Professor Suresh Garimella, the president of the University of Vermont and distinguished professor emeritus as well as his team at Cooling Technologies Research Centre (CTRC) at Purdue University have been working on cutting edge thermal technologies alongside their industrial partners. On our question to Professor Garimella about the future prospects of energy storage systems, he wrote “Energy storage is a critical enabler as the world transitions to a renewable energy economy that is often characterized by intermittent sources. Solar thermal energy storage is a viable technology at scale, and molten salt – especially when supplemented by the much cheaper rock materials used in thermocline storage – hold promise, as our work has shown. Storage and transmission are the two major issues that deserve significant attention for our future power needs.”
Companies like Protarget have made this research a reality and are commercially installing solar thermal systems in various parts of Europe. Their solar thermal system, designed and commissioned in early 2020, uses molten salt technology and since then they have had major cost-cutting benefits in terms of reducing fossil fuel and carbon footprint. Hence, molten salt technology as TES offers significant cost reduction potential but still requires further research in the area of material, storage components and system integration for full commercial applicability.
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