Molten salt energy storage or how AES Bulgaria sees the future

Recently, apart from the usual electricity prices peaks, we also witnessed negative prices on the energy exchange. In this context, the excess electricity generation, especially from RES, highlighted the need for energy storage systems. This topic is an integral part of the broader EU-wide debate about decarbonization of the economy and overall electricity generation changeover in order to reduce CO2 footprints and to fight climate change. The energy storage systems concept is for them to operate as a giant battery to store energy in periods of excess supply (when prices are low or negative) and release this power for consumption when there is a shortage and the prices are very high.

AES' purpose is to accelerate the future of energy, together. Since 2016, AES has transformed its global portfolio and has become a pioneer and leader in responsible energy transitions, leading the development of new low- and zero-carbon technologies like energy storage in the markets where the company operates. Indeed, AES is a frontrunner and leader in the development of utility-scale energy storage solutions since 2007. However, the pace of adoption of energy storage systems has dramatically picked up globally over the last few years given their ability to enable the deployment of large-scale renewables over the world. Such system is among the future plans of AES Bulgaria - the largest investor in Bulgaria’s energy sector in the past 30 years. The company which operates TPP AES Galabovo, the most modern and the newest coal-fired TPP in the country, and St. Nikola, the largest wind farm, develops various ideas for its future operations to align with Bulgaria’s ambitious decarbonization targets and to prolong the lifespan of its power plant. This would help TPP AES Galabovo preserving jobs and keep providing ESO with strategic system services such as primary regulation, frequency regulation, etc. in order to safeguard national grid stability.

AES Bulgaria is considering two main projects, with the aim of converting the existing assets of TPP AES Galabovo to different fuel or usage.  These projects would include a conversion of one of the units of TPP AES Galabovo to an energy storage facility and conversion of the other unit to waste-derived fuel (Solid Recovered Fuel - SRF) and/or biomass, which could be implemented in parallel or separately. Biomass for energy would be produced, processed and used locally in a sustainable and efficient way in full compliance with the EU Renewable Energy Directive.

The project for energy storage through molten salt would involve retrofitting one of the units of TPP AES Galabovo by replacing the main boiler with the so-called ‘power-to-salt’ Carnot battery with a steam generating system. The battery relies on molten salt as a medium for storing thermal energy given that molten salt has excellent heat retention properties. When energy demand increases, the stored heat can be extracted from the molten salt and converted into energy through steam generation which is then used for energy production.

This sounds quite innovative, isn’t it? ‘The energy storage facility project would incorporate a large storage capacity of 1,725 GWh into the energy system (i.e., 345 MW per hour which can be used for 5 hours) – the first of its kind – to support Bulgaria in times of demand peaks. This would ultimately improve the stability of the grid and allow Bulgaria to accelerate the integration or renewables’, commented Ivan Tzankov, Managing Director of AES Bulgaria.

The second project considered by the company is conversion of the other unit of the TPP to SRF and/or biomass. SRF is produced from non-recyclable ‘end-of-life’ waste including predominantly paper and plastics which cannot be recycled, and therefore otherwise would end up in a landfill. Biomass is typically a combination of wheat straw, corn straw and miscanthus. The company believes that applying alternative fuels can significantly reduce carbon emissions and the operational costs. ‘This project would preserve 345 MW of TPP AES Galabovo’s capacity for energy production, allowing for a gradual replacement of lignite generation with more renewable sources’, Ivan Tzankov added.

The company calculates that the two projects would lead to avoiding approximately 2.5 million tons of emitted carbon per year.

They would allow AES Bulgaria to reuse the core of the most valuable assets of TPP AES Galabovo - turbine and generator - allowing Bulgaria to continue benefiting from AES Bulgaria’s modern assets. Each of the two projects would also have a positive effect on workforce employment and offer opportunities for further development of new skills and competencies. at TPP AES Galabovo.

AES Bulgaria has so far conducted comprehensive studies aimed at assessing both the technical feasibility and the commercial aspects of these projects. In particular, AES Bulgaria has completed a conceptual design study for the conversion to an energy storage facility and is currently working on a more detailed technical study with the aim of finalizing the design of the future storage. As regards the conversion to SRF and/or biomass, AES Bulgaria has so far completed a similar study and is currently advancing the preparation of co-firing test of SRF and lignite to be performed in Q1 2025. Biomass supply pre-feasibility study has also been performed which shows that there is good biomass supply potential in Bulgaria. The studies completed thus far have shown that both options are technically feasible, and that the expected completion date for both projects could be in 2027, assuming that the decision to pursue these projects is made soon.

AES Bulgaria has preliminarily estimated that the approximate investment for the conversion to an energy storage facility would be around €400 million, while the investment for the conversion to SRF and/or biomass is preliminarily estimated to be around €460 million.

For the alternative conversion options discussed above, AES is currently exploring opportunities to access various sources of EU funding, such as the Innovation Fund, Modernization Fund, and Just Transition Fund, and promoting the development of capacity mechanisms in Bulgaria.

The last developments in Bulgaria’s energy sector have proved the urgent need of both energy storage systems and flexible balancing capacities. The plan for transforming TPP AES Galabovo addresses effectively both issues.