The method traditionally used to store energy produced from renewable sources is based on the use of batteries, typically lithium–ion (Li–on) cells. This article will present an innovative technique for temporary energy storage that promises to overcome the limitations of Li–ion batteries, offering longer lifespan and reduced costs.

Technologies used for the production of renewable energies are becoming increasingly important, motivated both by their competitiveness in terms of costs, and because of their low environmental impact and contribution to solving the climate issue. Renewable energies have some limitations, however, including being (by their nature) an intermittent source of energy.

This requires the introduction of appropriate systems capable of storing the energy produced (for example, the solar energy captured during the day), and then releasing it into the grid during the night, or whenever there is a need to compensate for any peaks in energy demand.

Energy Dome, a startup founded in 2019 and headquartered in Milan, Italy, has developed a novel technology which aims to drastically reduce greenhouse gas emissions into the atmosphere, thus helping to solve the environmental issue. This new technology, named CO2 Battery, is actually a long–duration energy storage which allows renewable energy production to be more affordable and dispatchable.

The new technology developed by Energy Dome is essentially a form of electrical energy storage with a sweet spot that lasts from 4 to 24 hours. The principle is to store renewable energy (such as solar or wind energy) when it is available in large quantities, and then re–enter it into the grid when there is greater demand and less availability of energy.

“Today, the leading technology for energy storage is represented by lithium–ion batteries, which however are suitable for applications with duration of two to four hours and characterized by a high number of charging/discharging,” said Claudio Spadacini, CEO at Energy Dome. “When you need to manage storage with longer lifespan, the price of Li–ion batteries becomes uncompetitive.”

The technology patented by Energy Dome is based on a thermodynamic principle, more precisely on the compression of a gas (carbon dioxide), starting from the initial values of atmospheric pressure and temperature up to the liquefaction of CO2 at high pressure.

As such, the new technology differs significantly from the electricity storage system used by Li–ion batteries, which is a chemical process. Furthermore, the system developed by Energy Dome can benefit from a high energy density without requiring high pressures for its operation.

“A relevant benefit of our system is that it is built using only steel, water, and CO2 — that is, components which can be found anywhere in the world. CO2, which operates as a working fluid and can be obtained industrially or as products of other processes, is not consumed as the system is closed,” Spadacini said.

Any type of electricity storage system consists of two phases: a charging phase in which the produced electrical energy is stored in another form of energy (chemical, electrothermal, mechanical, or other), and a discharging phase during which the previously stored energy is fed into the grid to produce electricity. The system, whose two charging and discharging phases are shown in Figure 1, uses electricity to compress CO2 and exploits the expansion of the same, in a subsequent phase, to generate electricity.

The CO2 is stored inside a tank (“dome”) at atmospheric pressure and temperature, to then be compressed in the “COMP” stages during the charging phase. The fluid at high pressure and temperature is then cooled (releasing heat) and condensed. Liquid CO2 is then stored in a series of pressure vessels.

In the discharging phase, the high pressure fluid is first evaporated and subsequently heated using the heat developed in the charging phase. The high temperature and high pressure fluid then expands in the turbine producing electricity, then reaching again the dome at a pressure comparable to the atmospheric one.

Since the energy introduced into the system during the charging phase is higher than that produced in the discharging phase, to respect the energy balance it is necessary to release some thermal energy into the environment through an air–cooled heat exchanger.

Since the process involves only two thermodynamic transformations (compression and expansion), the losses are reduced and it is possible to obtain a round–trip efficiency (RTE) higher than 75% (77% ± 2%), a higher value than similar systems which use compressed air or liquid air as working fluid.

Although Li–ion batteries are able to provide a nominal RTE of approximately 95%, the real RTE is between 70% and 80% due to the degradation of performance caused by wearing and aging. Also, when used with reduced depth of discharge, the life of Li–ion batteries is reduced to 7–10 years.

“The efficiency of CO2 Battery is comparable to that of Li–ion batteries, while the CAPEX is 40% lower at the beginning, and then drops significantly with economies of scale,” Spadacini said. “Furthermore, the duration is three times higher and there is no degradation over time.”

Despite being a young company, Energy Dome has already started a commercial demonstration plant in Sardinia, Italy (Figure 2). In addition to this plant, which has recently been successfully launched, two to three full–scale plants with scalable power capacity of 20 MW and 200 MWh of energy capacity will be launched, and the commercialization phase will begin.

The company has recently announced a new round of funding ($11 million) that will help them to accelerate the development of energy storage facilities in Europe, the U.S., and worldwide using their CO2 Battery technology. The new funding will enable Energy Dome to order turbomachinery and other equipment needed to build their 20 MW/200 MWh/10 hour duration energy storage facilities.

“Our challenge is to create an alternative storage system to lithium–ion batteries which costs half, has a three times longer duration, and the same performance,” Spadacini said.

Maurizio Di Paolo Emilio has a Ph.D. in Physics and is a Telecommunications Engineer. He has worked on various international projects in the field of gravitational waves research designing a thermal compensation system, x-ray microbeams, and space technologies for communications and motor control. Since 2007, he has collaborated with several Italian and English blogs and magazines as a technical writer, specializing in electronics and technology. From 2015 to 2018, he was the editor-in-chief of Firmware and Elettronica Open Source. Maurizio enjoys writing and telling stories about Power Electronics, Wide Bandgap Semiconductors, Automotive, IoT, Digital, Energy, and Quantum. Maurizio is currently editor-in-chief of Power Electronics News and EEWeb, and European Correspondent of EE Times. He is the host of PowerUP, a podcast about power electronics. He has contributed to a number of technical and scientific articles as well as a couple of Springer books on energy harvesting and data acquisition and control systems.

How is the 'subsequently heated' achieved??? There is a long time between charging and discharging so the waste heat from charging presumably will have already dissipated before the discharge cycle starts. For example, solar panels charging during the day and then the battery discharging at night. "In the discharging phase, the high pressure fluid is first evaporated and subsequently heated using the heat developed in the charging phase"

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