TWEST™ – repurposing coal fired plants as thermal energy storage facilities

12 January 2022

The phase out of hundreds of GW of coal plants globally is creating an immediate challenge: what should be done with these valuable assets? E2S Power’s innovative idea is to replace the boilers with thermal energy storage using its TWEST™ (Travelling Wave Energy Storage Technology) concept. This repurposes potentially stranded assets while addressing the urgent need for increased energy storage capacity worldwide. By Fabrizio De Candia COO, E2S Power

As we strive for a carbon-free world, the share of global power generation from intermittent renewable sources is increasing rapidly. This growth will drive the need to address intermittency, dependable capacity, and grid stability with energy storage solutions. It has been estimated that the large-scale storage market will increase by ten times in the coming years.

At the same time, at COP 26, more than 40 countries agreed to phase out their use of coal-fired power and several nations have already committed to phase out coal power plants by 2030 and earlier. This means that hundreds of GW of coal-fired power plants will be decommissioned, raising the following questions: how do we ensure dependable capacity? how are we going to repurpose these power plants? how do we transition economies where these plants are located to a carbon free future?

E2S Power is aiming to address these challenges with its thermal energy storage solution. The technology is based on the concept of reusing most of the fossil fuelled power plant’s equipment and infrastructure and turning it into a clean energy storage plant.

For this purpose, E2S Power has developed a simple and compact system that converts surplus electrical energy from wind farms or solar power plants into heat, stores the heat using innovative high energy density materials, and, when required, generates superheated steam at constant temperature to produce electricity using the existing coal plant steam turbines.  

A novel energy storage system, TWEST™ (Travelling Wave Energy Storage Technology) – simple, compact and self-contained – is at the heart of the E2S power plant conversion concept.

TWEST (Figure 1) consists of three key components: 1 – electric radiant heaters; 2 – MGA storage blocks; and 3 – steam generators in an insulated enclosure. The electric heaters are designed for temperatures higher than 1000°C and facilitate heat transfer to the MGA storage blocks during charging.

The MGA (Miscibility Gap Alloy) blocks (Figure 2) are a high energy density storage medium developed by partner company MGA Thermal. They are made of graphite and aluminium arranged in blocks similar to building blocks or bricks. One component (aluminium) has a lower melting point (660°C) than the other (graphite) with the phase change latent heat providing high performance energy storage. Graphite, with a higher melting point, remains solid and acts as a matrix, keeping the block in a solid form and embedding the melting particles. Thanks to the phase change, better energy storage density is achieved compared with other thermal energy storage technologies.

The steam generators, employing advanced high-temperature-resistant alloys, are mounted in the MGA storage blocks. E2S Power’s proprietary “Travelling Wave” technology ensures that a constant steam temperature is delivered to the host plant.

The system is packaged in a standard “container” module that allows stacking up as required to provide the storage capacity wanted and the plant footprint needed to adapt to a variety of power plant sizes and configurations (Figure 3). The superior energy density means that the installation is compact.

TWEST is designed to easily integrate into an existing power plant (Figure 4), with three main interfaces: electricity supply for charging the system; feedwater input; and steam supply into the existing steam cycle.

How it works

The E2S Power concept converts existing coal fired power plants into energy storage facilities by substituting the E2S thermal energy storage system for the boiler and integrating with existing infrastructure, thus eliminating carbon dioxide emissions while utilising an otherwise stranded asset.

During the charging process, electricity from renewable sources powers the electric heaters, which heat up the MGA storage blocks to a temperature near 700°C.

During the discharge process, heat stored in the MGA blocks is transferred to feedwater via the embedded steam generators, which then supply steam with properties matching the requirements of existing steam turbines to generate electricity.

The system is designed to make maximum use of existing plant infrastructure to minimise costs, including high voltage switchyard, existing steam turbine and associated balance of plant equipment (condensers, cooling towers, generators, transformers, etc).

With E2S Power’s “Travelling Wave” technology (see Figure 5) one set of blocks is heated to about 700°C, while a second set of blocks is heated to a temperature near the host plant steam turbine requirements. During discharge, the steam transfers heat from one set of blocks to the other set, maintaining a constant temperature at the exit.

Compact, simple and efficient

E2S Power offers a cost-effective and easy-to-integrate solution for transforming fossil fuel power stations into thermal storage systems. Some of the key advantages include compact and simple system design – with minimum disruption to existing plant.

The components employed and the design ensure a long life, estimated at 30 years, with negligible performance degradation, while the MGA thermal storage materials are abundant and recyclable. In addition, by reusing the existing synchronous generators of the host plant, the technology is better able to provide voltage and frequency support than batteries.

The TWEST system can generate steam in minutes and can potentially provide heat to keep the steam turbine on hot standby for faster startup.

The storage efficiency (conversion of electrical energy into heat) is higher than 99% as there is good heat transfer via radiation and therefore almost no losses in this process. The roundtrip efficiency, which depends on the existing steam cycle efficiency, is typically around 40%.

Tapping into a large market

After successfully validating the thermal energy storage concept in their demonstration facility in Belgrade, E2S Power is currently developing three utility scale pilot projects with major utilities in Europe and North America. At the same time, E2S Power is pursuing full commercial projects with power companies worldwide.

Just recently the company signed a memorandum of understanding with SNC-Lavalin to explore opportunities with North American power companies.

This collaboration aims to tap into the huge market represented by over 200 GW of coal fired capacity in North America that may be decommissioned in the coming years by combining E2S Power’s technology with SNC-Lavalin’s EPC and plant integration expertise.  


Figure 1. TWEST™ (Travelling Wave Energy Storage Technology). 1 – electric radiant heater; 2 – MGA storage blocks; and 3 – steam generator
Figure 2. MGA (Miscibility Gap Alloy) blocks (photo: MGA Thermal)
Figure 3. How modules are stacked together, 18 x 50 MWh example
Figure 4. TWEST™ is designed to be readily integrated into an existing coal plant, essentially replacing the boiler
Figure 5. The Travelling Wave principle. One set of blocks (TWESB1) is heated to about 700°C, while a second set of blocks (TWESB2) is heated to a temperature near steam turbine requirements. During discharge, steam transfers heat from one set of blocks to the other, maintaining a constant temperature at the exit

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