energy storage
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New technology is offering an economic approach to largescale energy storage. An electro thermal energy storage (ETES) breakthrough does more than address bulk power storage though.

By coupling electricity, heat and cooling ETES represents an opportunity to break the energy system from reliance on fossil fuels.

This article was originally published in Smart Energy International 5-2019. Read the full digimag here or subscribe to receive a print copy here.

There’s a compelling need to increase utility-scale energy storage capacity in response to the dramatic growth in intermittent renewable generating capacity like wind and solar. Although current technologies such as chemical batteries are rapidly improving, to date bulk power storage has been dominated by pumped storage hydropower. While pumped storage is low cost, efficient and typically offers huge capacities it is also strictly limited by geography.

Now though, new technology has emerged that offers bulk power storage at scale and with the potential for far lower costs than existing battery chemistries, such as lithium-ion. Perhaps even more significantly, Electro-Thermal Energy Storage (ETES) connects heating, cooling and electricity storage together. As a result, the system can meet multiple energy storage and supply needs simultaneously.

The ETES thermodynamic cycle

Based on a novel and reversible thermodynamic cycle, ETES is a scalable and efficient technology that supports sector coupling between the distinct energy needs of heating, cooling and electricity. With ETES, heating needed for food processing and district heating can meet cooling for applications like data centres, warehousing and large commercial buildings, as well as electricity storage capabilities to support grid balancing and renewable energy optimisation – all in a single system.

By allowing industrial, commercial and domestic sectors to combine their needs, this offers a comprehensive and efficient solution to a host of energy system challenges while keeping capital and operational expenditures to a minimum.

Currently the only solution available that is capable of using, storing and distributing heat, cold and electricity simultaneously, the patented tri-generation energy-management system is based on the use of CO2 (R744) as the working fluid. At its core ETES allows the conversion of electrical energy into thermal energy in the form of hot water and ice and vice versa. The energy is stored in a series of thermally insulated water tanks, making the system low risk and very robust with high resilience. Similar to a domestic refrigeration unit, in ETES the closed CO2 cycle sees the working fluid compressed or expanded through turbo-machinery to store or extract energy. Depending on specific demands, energy stored as either heat or cold may be directly distributed or efficiently reconverted back to electrical energy as required.

During the charging cycle, electrical energy from any source – such as excess renewable energy for example – is used to power a turbo-compressor. The C02 working fluid is compressed to around 140 bar and 150°C or more by this turbo-compressor. Passing through a heat exchanger, heat from the compressed CO2 is transferred to the hot storage tanks. There may be as many as four such tanks, for example three at atmospheric and one pressurised, depending on demand and application. Each tank is maintained at a separate temperature. The cooler but still pressurised CO2 then passes into an expander where the pressure drops, and it condenses to a liquid and cools down further. At this stage, the second set of heat exchangers chills the cold storage tank to produce ice.

In the reverse process, gaseous CO2 passes through the heat exchangers on the cold side. It condenses to a liquid while the temperature of the cold tank is increased. The now liquid CO2 has its pressure increased by a pump before being evaporated back to gas in the hot side heat exchangers. Now at supercritical conditions, the heated and pressurised CO2 passes through an expansion turbine where an attached generator is used to produce electricity as required.

Using water in simple insulated tanks, some of which may only require minimal insulation, along with standard turbomachinery equipment means the system has a low environmental impact and is reliant on well-proven and extensively deployed systems. As a result, overall capex is expected to be competitive with other commercially available energy storage systems.

Tried and tested technology

Scalable and site-independent, commercial development of ETES sprang from a cooperation agreement between MAN Energy Solutions Switzerland and ABB Switzerland that was announced in March 2018.

Among the core components of the ETES system is MAN’s hermetically sealed HOFIM turbo-compressor. Built for rugged extremes and used, for example, in subsea compression station applications, these units are multi-stage radial compressors. With casings designed for 220 bar, HOFIM compressors feature a 7-axes active magnetic bearing system and are arranged in a single shaft configuration together with a high-speed electric motor.

The turbo-compressor has no oil or sealing systems, which reduces complexity. Compared with traditional compressor designs HOFIM designs have a 60% smaller footprint and 30% less mass. The HOFIM compressor family, designed for subsea applications where reliability and service longevity are paramount, is currently available in a power range of 4 to 16MWe.

With a process based on solid, well-proven engineering and thermodynamic principles, ETES is essentially a simple reconfiguration of tried and tested technologies to produce efficient and sustainable energy storage and conversion system.

Market opportunities for heating, cooling and electricity storage

Modular and easily scalable, ETES features multiple thermal storage tanks, each at a different temperature. This quality makes it an ideal energy storage solution for mid- to large-scale thermal and electrical consumers and process industries. Hot side temperatures range from 20 to 120°C while the cold side goes down to zero. This is well suited to many process industries as well as functions such as sterilisation, heating and cooling for large public buildings and chilling increasingly prevalent data centres.

Furthermore, although conceived with renewables in mind, any source of electricity may be used to energise ETES. Consequently, margins may be found in some markets by ‘time-shifting’ energy from low-cost nighttime power to peak load daytime markets, for example. Using ETES there are clearly multiple opportunities available from peaking markets and other power system functions such as grid balancing.

Uniquely tailored to each application, a typical system could feature an input of 5MWe, a six-hour charging cycle and storage capacities of 30MWhe, 110MWhth heat and 80MWhth of cold. Depending on factors such as usage mode as well as electricity purchase and sale prices, based on these figures a return on investment period of just five years or less is credible.

Certainly, there is no question over the huge surge in global demand for energy storage. A report from IHS Markit notes that a total of 4.3GWh of grid-connected energy storage is expected to be installed worldwide in 2019 alone. By 2025, annual installations are expected to reach more than 10GW.

Heating and cooling is a sector where renewable energies have made limited inroads, yet it accounts for about half of all global energy demand. By efficiently coupling heating, cooling and electricity, ETES offers an opportunity to maximise the renewable energy contribution to the energy sector as a whole and transform the bulk energy storage market at a stroke.

About the authors

Raymond C. Decorvet is responsible for business development for the MAN ETES technology at MAN Energy Solutions Switzerland Ltd. Based in Zu rich, he holds a degree in economics.

Mario Restelli is responsible for sales for the MAN ETES technology at MAN Energy Solutions Switzerland Ltd. Based in Zurich, he holds a degree in mechanical engineering.