Energy: First ENEA prototype for solar storage using green sulphur

Storing solar energy using sulphur is the goal of the SULPHURREAL project, funded with nearly 4 million euro by the European Union, as part of which ENEA has built an experimental prototype in its laboratories at the Energy Technologies and Renewable Sources Department at the Casaccia Research Center (Rome).

The basic concept of the project is to use the energy produced by concentrating solar power to cyclically activate chemical reactions based on sulphuric acid and sulphur and/or sulphur-based raw materials, which may also come from large-scale industrial processes.

"The project stems from the need to store energy, thermal or electrical, using so-called solar fuels, of which hydrogen is an example. Hydrogen offers many advantages when used as a carrier for decarbonisation, but presents some logistical challenges in its storage and transportation " explained Salvatore Sau, researcher at the Energy and Thermal Storage Laboratory. "Sulphur, on the other hand, is solid and does not present transport and storage problems. However, its combustion produces a toxic and polluting gas, sulphur dioxide," he said.

The prototype (Figure 1), evaporates sulfuric acid and then sequentially decomposes it into sulfur dioxide and oxygen using heat from a concentrating solar irradiation source. Then the sulfur dioxide obtained, which is not released into the atmosphere, reacts with water to produce sulfuric acid and elemental sulfur. The sulphur, in turn, stores a significant portion of the solar energy used to decompose the sulfuric acid. The sulfur thus obtained can be burned later to release the stored solar energy.

"The Sulphurreal project involves the construction of a burner -which is being studied at ENEA- to contain sulphuric acid and decompose it into sulphur and oxygen, which is being studied at ENEA. The solution with the greatest advantages," he continued, "seems to be an electrolyzer inside which, with a voltage of less than 1 volt to produce sulphur at the cathode and a sulphuric acid solution at the anode, the sulphur can be separated by filtration," concluded Sau.

This initial experimental prototype will be followed by a laboratory-scale plant, to turn the process developed under the Sulphurreal project into a method for thermal energy storage from discontinuous renewable sources.