Decarbonisation Technology August 2022 issue

A new approach to energy management and storage, electro-thermal energy storage is bringing balance to some of the hardest to tackle clean energy challenges Maximise renewable resources with thermal energy storage

Raymond C Decorvet MAN Energy Solutions

O n the road to net zero, there are some pretty steep hills to climb. Hard-to- reach energy-intensive sectors, along with heating and cooling of existing building stocks, have hitherto resisted the siren call of sustainability. However, a new approach to energy storage and management now opens up even the toughest nut to clean energy. The breakthrough is centred on tried and tested technology, but its novel deployment offers a real opportunity for corporate, communities, and even whole cities to reach their net-zero ambitions. Sector coupling – in which heat demand is connected with sources of so-called ‘waste heat’ and demand for cooling – is a remarkably efficient approach to meeting system-wide energy consumption. In developing electro- thermal energy storage (ETES), MAN Energy Solutions has created a bulk energy storage system that links electricity, heating, and cooling in a high-efficiency reversible process. In a first for a major city, Esbjerg in Denmark is putting the ETES system at the heart of its plans to become entirely carbon-free by 2030. Reversible conversion of electricity into thermal energy The ETES storage system is built around a multi- stage radial turbo-compressor. Developed for highly efficient gas compression and proven in the harshest subsea environments, the HOFIM compressor allows the reversible conversion of electricity into thermal energy stored in simple and scalable insulated water tanks. Incredibly robust, oil-free, and hermetically sealed, the high-speed motor-compressor runs

on magnetic bearings and uses environmentally safe and non-toxic CO₂ as the working fluid. This refrigerant (R744) is successively compressed or expanded in a closed cycle with the outputs of hot and chilled water or electricity as desired. Initially, the CO₂ is compressed to around 140 bar and 150°C. It then passes through a heat exchanger and the hot store. The hot side may include up to four storage tanks at different temperatures or a direct supply to consumers. Once pressurised, the CO₂ is then expanded where it condenses and cools. A second heat exchange process uses the now liquid CO₂ to produce ice or cold water for the cold storage tank. To reverse the process and produce electricity, gaseous CO₂ is passed through the cold side heat exchangers, where it condenses while raising the temperature of the cold tank water. Once liquefied, the CO₂ is passed through the compressor, where the pressure is increased. It then goes through the hot side heat exchangers, increasing the temperature and pressure still further. Heated and pressurised, the CO₂ passes through an expansion turbine. A coupled conventional generator produces electricity. The current round-trip efficiency of ETES is 45%, but continued development is expected to see around 60% achievable in the near future. However, unlike chemical batteries, which degrade during each charge and discharge cycle and have a lifespan of only 10-12 years, the impressive efficiency figures from the ETES system remain constant throughout its more than 35-year design life. ETES and the carbon-neutral city Simple, reliable, and efficient, the tried and