Turning coal plants into storage assets

How E2S Power is giving otherwise stranded assets a second life in the renewable energy future. By Carlos Hàƒ¤rtel.

Global energy markets are at the onset of one of their most significant transformations since the invention of electricity.

Today, still more than two-thirds of all electric power is generated from fossil fuels, most of it by plants running on some sort of coal.

Conventional power stations, however, face a very certain future of retirements.

Estimates about the total capacity of thermal power plants to be retired over the next 10 to 20 years vary, but are well in the range of thousands of gigawatts.

Concurrently, renewables keep growing at an undiminished pace. Due to their intermittent and mostly non-dispatchable nature, on the other hand, wind and solar power need energy storage systems enabling them to cope with short- and long-term load variations.

Consequently, their continued expansion is triggering a rapid growth of storage capacity realised by both greenfield and brownfield projects.

At E2S Power, we’re developing a storage solution which in time can convert existing coal-fired plants into thermal batteries.

This not only allows reusing existing infrastructure ” it also helps to protect local employment, which is a point of major political concern in many regions worldwide.

The extensive installed base of thermal power plants offers an enormous market opportunity for those who develop conversion solutions. The sheer scale of the required ramp-up of storage capacity will necessitate all storage options on the table to contribute to the challenge ” not one technology or solution will be able to shoulder it alone.

In regions where a large number of coal plants are still in operation, converting those can be a key contributor to providing the storage capacity required.

Giving otherwise stranded assets a second life in the renewable energy future not only has financial benefits to the owners or operators: the continued use of valuable infrastructure also helps to minimise future CO2 emissions associated with the massive build-up of energy storage capacity, where green-field projects may come with a significant carbon footprint.

How it works

E2S Power’s solution basically consists of substituting the boiler with a thermal energy storage system while reusing all of the remaining infrastructure (see Figure 1).

During off-peak hours, the thermal battery is charged with surplus electricity from renewable sources, which is taken from the grid using the existing step-up transformers.

One core technology our solution is built on is a novel composite material called MGA, which was developed and is now manufactured by our Australian partner MGA Thermal Pty Ltd.

Figure 2 illustrates the basic architecture of the E2S storage module ” which we call the ‘Hamster’ ” which contains an assembly of slab-like components consisting of the MGA storage blocks, steam generator plates, and electric radiating heaters.

All components are arranged in an enclosure, which is thermally insulated against the environment and filled with nitrogen for oxidation protection.

During charging, electrical energy powers the radiating heaters, which raise the temperature of the MGA storage blocks to the required level (which may be plant specific).

During the discharge process, heat stored in the MGA blocks is then transferred to the water running through the steam generators, which create steam of just the right properties so that existing steam turbines can use it to generate electricity.

The conversion of heat to electricity thus happens in about the same fashion as if the plant was still powered by coal.

For each individual brownfield case, the system can be tailored so that it makes use of all existing plant infrastructure, which minimizes costs.

It can be operated at the same voltage and current levels as already present at the generators to feed the electric heaters, use the same high-voltage switchyard, the existing steam turbine, and the accompanying balance of plant like condensers, cooling towers, heat sinks, generators, or transformers.

The steam generators of the E2S Hamster are made of advanced high temperature resistant alloys, which can operate at 700à‚°C and are mounted in between the MGA storage blocks.

Electrical heaters are also specially designed to resist temperatures higher than 1000à‚°C to facilitate the heat transfer to the MGA blocks using thermal radiation.

The MGA elements used for storing the thermal energy are special composites made of graphite and aluminium. The metallic component, i.e. the aluminium, has a melting point of around 660à‚°C, which is lower than the maximum system temperature during the charge-discharge cycle.

The latent heat associated with phase change of the metal during operation is the main reason for the very high energy density our storage technology can achieve.

On the other hand, the melting point of graphite, which forms the matrix and which contains the embedded and finely dispersed metal particles, is significantly above the maximum temperatures in the enclosure at all times.

The matrix therefore remains solid throughout operation, keeping the whole MGA slabs in solid form. From a practical standpoint, this feature of MGAs is a key advantage.

In a sense, the MGA technology allows the utilisation of a metallic phase-change material for heat storage at the simplicity and robustness of systems using e.g. steel or concrete as storage elements.

At the same time, it uses a lot less valuable space than those storage media. Steel and concrete would be three times and 20 times, respectively, the volume of an MGA system at the same temperature level and energy content.

A final yet important point is the fact that the use of the abundantly available and non-toxic base materials graphite and aluminium can help alleviate some of the criticism often raised concerning environmental impact, tight supply chains, and recyclability of the materials built into energy storage devices today.

Complementary not competitive

The high melting point of aluminium enables to perfectly tailor the E2S Hamster to existing thermal plants’ infrastructure.

An innovative design of the electric heaters ensures that all existing power electronics equipment in the thermal plant can be utilised without any costly modifications.

Converting an existing thermal plant has modest capital requirements, and projects can be executed swiftly thanks to our modular design, which allows prefabrication of components.

First experiments indicate that the system’s storage capacity will experience very little degradation over time, which minimises the need for major outages over the lifetime of the plant.

After a productive supply chain has been built up, adding hundreds of MWh of capacity per month already seems feasible in the mid-term.

It’s worth pointing out that our solution is largely complementary to ” and not in competition with ” Li-ion batteries. The target application for a converted thermal plant is balancing variations in load and supply over the diurnal cycle or a period of several days.

Given the large spinning masses of steamturbine and generator trains, supporting grid stability is possible too, although during the discharge phase only.

Currently, our first technology demonstrator of approximately 500 kWh thermal storage capacity is being tested in Belgrade, Serbia, with one single E2S Hamster cell of MGA coupled with an electric heater and a steam generator.

The system produces steam of up to 700à‚°C, which subsequently is discharged to a condenser. After completion of the test campaign and any design or process refinements it may suggest, we expect the system to be mature for a mid-scale field prototype.

Discussions are under way with power plant operators, who have a need for augmenting their fossil power plant with a thermal storage system for greater flexibility. Commissioning of the prototype could be as early as 2022, preceding the product rollout for the Hamster in 2023.

ABOUT THE AUTHOR

Dr Carlos Hàƒ¤rtel is director of Marketing & Business Development at E2S Power.