As we strive towards a zero-carbon power sector, the high-level penetration of renewable energy sources such as wind and solar into the grid becomes crucial. However, these energy sources are variable and intermittent, requiring the use of long-duration energy storage systems to control the temporal mismatch between supply and demand.

Long-duration energy storage technologies include thermal, mechanical, and chemical energy storage systems, among others.

Thermal energy storage

Thermal energy storage (TES) systems store thermal energy in various types of materials for later use in heating, cooling, or power generation applications. There are three different types of TES: sensible heat storage, latent heat storage, and thermochemical energy storage.

Sensible heat storage is a relatively mature technology that has been applied worldwide in a variety of applications. In Finland, several borehole thermal energy storage systems, energy piles and cavern thermal energy storage systems have already been installed. Additionally, numerous above-ground tank thermal energy systems are currently connected to district heating networks. However, research on sensible heat storage is still needed, particularly on materials and system design.

The Finnish company Polar Night Energy has developed an innovative sand-based heat storage system that utilizes surplus wind and solar power to heat sand up to 600 – 1 000 °C. This stored heat can then be used to generate steam or heat water for various industrial and heating applications. Their first commercial “sand battery”, with a heating power of 100 kW and an energy capacity of 8 MWh, was installed at Vatajankoski power plant area providing district heating to the city of Kankaanpää in Western Finland. Additionally, a 3 MWh demonstration pilot is being tested in Tampere.

PICTURE 1. Thermochemical energy storage reactor at the Energy Research Center in Varkaus (Image: Markku Huhtinen)

On the other hand, latent heat storage and thermochemical storage systems are in the early stages of development with several ongoing demonstration projects worldwide, including in Finland. For instance, the Energy Research Centre of Savonia University of Applied Sciences located in Varkaus features a thermochemical reactor (Picture 1) designed to test different chemical reactions. The reactor is air cooled, and the released energy is determined by measuring the temperature difference of cooling air. The reactor was designed and built within the project Heat Circulation Innovation Platform.

Mechanical energy storage

Mechanical energy storage systems store potential or kinetic energy for future use. Pumped hydro storage (PHS) is currently the dominant technology for large-scale mechanical energy storage, accounting for approximately 95% of the global energy storage capacity. According to the International Hydropower Association (IHA), the total installed PHS capacity was estimated at 165 GW in 2020, with about 157 GW of additional installations in the pipeline worldwide. Meanwhile, the International Renewable Energy Agency (IRENA) models a 2050 Net Zero scenario with 400 GW of pumped hydro storage.

New versions of pumped hydro as well as other types of mechanical energy storage systems are being developed to overcome the disadvantages of traditional PHS such as dependence on geographical conditions (for example, elevation difference and water availability), environmental impacts, and high investment costs.

Alternative well-known mechanical energy storage technologies include flywheels and compressed air energy storage (CAES). Additionally, novel systems in development include liquid air energy storage (LAES), pumped heat energy storage (PHES), and gravity-based energy storage (GES). In Finland, there is potential to repurpose decommissioned mines for various energy storage methods, including novel pumped hydro, CAES, and GES.

Chemical energy storage

Chemical energy storage systems store energy in the bonds of chemical compounds such as hydrogen, ammonia, and methanol. These systems are considered promising emerging technologies that can contribute to the green energy transition and provide seasonal storage of renewable energy. In Finland, several ongoing clean hydrogen projects are underway, reflecting the belief that hydrogen could become a new export industry and pillar of the Finnish economy.

More information on different energy storage technologies can be found in the following state-of-the-art report: https://urn.fi/URN:NBN:fi-fe20231220156226.

Raquel Mier González

Research Engineer

raquel.miergonzalez@savonia.fi

School of Engineering and Technology

Savonia UAS, Varkaus Campus