Power-to-Gas: renewable energy for methane production

In recent years, the challenge to reduce climate-changing emissions has driven a continuous increase of power generation from renewable energy sources (RES). This growth is expected to continue as stated in Italy’s Climate and Energy Plan.  However, the considerable presence of non-programmable sources such as solar and wind power creates problems related to the stability of the electricity supply.
It is rarely possible to consume RES energy when it is produced and the increased balancing needs makes grid management more complex.
Moreover, the energy required for "thermal" purposes, i.e. for heating, has inherent seasonality with  demand peaks in the winter months against RES production concentrated during the springtime for wind and summer for solar (Figure 1).

Figure 1 - Thermal demand compared to non-programmable production from renewable energy sources. (Source: SGI processing on PNIEC data)

Energy storage technologies and strategies, especially when integrated with generation from non-programmable renewable sources, are currently one of the most important topics of both basic and applied research.

The Italian Regulatory Authority for Energy, Networks and the Environment (ARERA) started a process for the definition of innovative initiatives in the use of gas transport networks (DCO 420/2018/R/gas The general objectives of the Authority's intervention stems from a growing attention to the development and widespread application of innovative technologies to support decarbonisation, the integration of renewable sources in the existing infrastructure as well as between the electricity and gas sectors.

Among the possible storage strategies currently under study, Power-to-Gas (PtG) is one of the key research topics of the global Green energy innovation policy. PtG aims to integrate the direct exploitation of renewable resources, both programmable (biomass of plant and/or animal origin) and non-programmable (wind and photovoltaic), with a process of hydrogen production on the one hand (by water electrolysis) and synthetic methane production on the other (by methanation, based on the so-called Sabatier reaction).

Hydrogen, thanks to its calorific value, is an important energy vector already widely used in industry. Although it is the most widespread element on Earth, it is not available in nature in its pure state: 96% of hydrogen nowadays is produced from the transformation of fossil hydrocarbons that contribute significantly to the emissions of CO₂.  This hydrogen is hence called grey hydrogen.

Hydrogen can also be produced entirely carbon free by electrolysis, i.e. the splitting of the molecule of water into oxygen and hydrogen in the gaseous phase. This process requires an energy input from outside: it is, in fact, the electricity available from RES to power the electrolyser that allows the decomposition of the molecule of H₂O into its constituent elements. The hydrogen produced in this way is called green hydrogen.

The further passage of methanation is a chemical-physical process that allows to obtain methane starting from the reaction of gases such as hydrogen and carbon dioxide (recovered from biogas upgrading processes in biomethane, and that would normally be vented in the atmosphere). From a thermodynamic point of view, these are exothermic reactions favoured by the simultaneous presence of low temperatures and high pressures. The final aim of this process, which can reach efficiencies close to 80%, is to produce synthetic natural gas with specifications suitable for injection into the gas transmission network, thus allowing this green gas to access to all the additional functionalities of the national gas system (storage and distribution).

In the context of PtG SGI, in partnership with ENEA and qualified industrial partners, has started the Pegasus Project.