Tech Insights

Enel, Brenmiller Open Rock-based Energy Storage Facility in Italy

November 21, 2022 by Darshil Patel

The Enel Group has partnered with Brenmiller Energy to launch a thermal energy storage facility in Italy’s Tuscany region that stores energy in crushed rocks.

Most renewable energy sources depend on some natural energy source and are seasonal. Therefore, storage systems are essential to reduce high energy demands and strain on the power grid. One method for managing such requirements is thermal energy storage (TES), or heating the material and extracting energy from that heat when needed. Thermal storage can reduce carbon dioxide emissions and costs while increasing overall efficiency.


Brenmiller TES system deployed at an Enel project site.

Brenmiller TES system deployed at an Enel project site. Image used courtesy of Omri Cohen/Brenmiller via Business Wire


Thermal storage works with widely different technologies, and the two principal types are water and rock-pile storage. Water has high thermal capacities, while rocks or concrete can be heated at much higher temperatures and provide a high overall volumetric capacity. To put into perspective, an insulating material of around 2.8 cubic meters would provide sufficient energy for a single home to meet 50 percent of its heating demand.

Energy storage in rocks, therefore, is a potential and emerging technology to store surplus wind or solar energy. These systems have received international attention, with two concrete thermal storage systems under development in Germany and one in Denmark.

The Enel Group and Brenmiller Energy recently inaugurated a TES system in Santa Barbara, located in Italy’s Tuscany region, that stores energy in crushed rocks. It can store up to 24 megawatt-hours (MWh) of electricity at a temperature of about 550 degrees Celsius for five hours.



Enel's thermal energy storage facility.

Enel’s thermal energy storage facility. Image used courtesy of Business Wire

Heat Transfer Between Rocks and Air Proves Beneficial

Heat storage in rocks uses a hot air system. Heated air circulates from the collectors through the rock bed to store heat. There is a good heat transfer between rocks and air, minimizing the temperature differences from air to rocks. Additionally, the rock pile has low conductivity when airflow is present, which reduces heat loss. 

The rocks are not heated uniformly, but by layers up to the temperature of the air. This effect is beneficial since cold air returns to the collector to increase the system's efficiency. While heating from the storage, the air is hot at nearly the same temperature delivered from the collector during charging.

When excess energy is available, the storage system is charged using heat pump systems that move heat from one set of tanks to another. The heat can be stored for a long time, and when the need for energy arises from the power grid, the heat is returned from hot tanks to cold tanks using a turbine, which generates electricity.


Thermal Energy Storage Provides Resiliency to Power Plant

The sustainable TES system inaugurated by Enel and Brenmiller uses a two-stage charge and discharge process. During charging, the steam produced by the facility passes through pipes to heat crushed rocks. Upon discharging, the stored heat warms the pressurized water and generates steam for electricity. The system provides critical resiliency to the power plant.

This project is a part of a collaboration between Italian and Israeli entities. The Israeli Innovation Authority supported Brenmiller with one million euros in financing. The cooperation aims to accelerate partnerships between Israeli companies and large Italian industries.