Maximise renewable resources with thermal energy storage

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.

Raymond C Decorvet
MAN Energy Solutions

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Article Summary

On 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.

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 tested turbo-compressor with integrated expander at the heart of ETES is comparable to a conventional domestic fridge but represents a real advance in city-scale energy system management. Furthermore, while the ETES process not only allows heating and cooling to be distributed according to demand – as well as the option of converting this resource back into electricity – the process can be powered by renewable electricity such as wind or solar. This raises the possibility of using renewables and other carbon-free energy sources to provide the heat needed for previously tough-to-reach sectors.

Recognising the advantages of this integrated energy management approach, DIN Forsyning – the Danish multi-utility company that operates the district heating network in the port city of Esbjerg, nearby Varde and part of the island of Fanø – is installing two large-scale ETES heat pump units.

Using renewable electricity for heat was one of the big selling points for the ETES approach. “What we see from the major global trend is that renewable electrical power will be the basis of the whole energy system in some way. We see ETES as part of a transition,” said Claus Nielsen, Business Development Director at DIN Forsyning.

“If we are to bring more renewable energy into the district energy system in Esbjerg then we have to find a smart way to integrate the network with the electricity system. The best method we have seen is the electrically driven heat pump,” he added.

MAN Energy Solutions is developing a two-unit heat pump for the city with a total capacity of more than 50 MWth and is supplying the entire system, including the heat exchangers and all the associated electrical infrastructure. The turnkey project will supply around 235,000 MWh of heat annually and will largely replace an existing coal-fired thermal plant scheduled for decommissioning in 2023.

Once completed in April 2023, the ETES project will form the backbone of a network of smaller and more sustainable heat sources for the city as part of its plans to become carbon neutral by 2030. It will be the largest CO₂-based heat-pump plant deployed to date.

Energy to power the system will be supplied from nearby wind farms. However, as any heat source can be recovered as usable energy through the ETES system, the Bay of Ho (Ho Bugt) in the Wadden Sea, a UNESCO World Heritage Site, will be employed as an energy resource. Nielsen explained: “Because we are close to the coast, we have a big heat sink. It’s at a low temperature, but it’s stable, and with eight million cubic metres of new water each tide, there is a huge and stable heat source accessible from the ocean.”

The ETES system will extract heat energy from the bay by fractionally cooling the water. This heat will then be supplied to 100,000 of the city’s residents connected to the network as well as the company’s commercial and industrial customers. DIN Forsyning currently delivers around 1 million MWh to its district heating network.

System-wide benefits of flexibility
While the ETES system enables the use of renewable energy in normally unreachable heating and cooling applications, it also offers a number of associated benefits. The flexibility ETES provides allows energy to be stored or delivered depending on supply and demand. For example, when demand for electricity is high, ETES can convert energy in the heat store into usable power. When there is excess renewable generation, this can be stored as heat and cold.

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