A power-to-gas system integrating co-electrolysis and methanation

Researchers in Italy extensively analyzed four different configurations of their proposed system to assess its potential, including under non-optimal, ‘off-design’ conditions and concluded that, for now, subsidies would still be required for the commercial production of synthetic natural gas.

Researchers from the University of Bologna, in Italy, have analyzed the performance of a power-to-gas system which couples a high-temperature co-electrolyzer based on solid oxide electrolyzer cell (SOEC) technology, to an experimental reactor.

The researchers were aiming to forecast the ‘off-design’ performance of the proposed system – how it would perform other than when receiving an optimal amount of electricity input, for example from intermittent renewables generation.

The conclusion reached was that such a system could become economically viable, but only with the help of incentives, given the current price level of the synthetic natural gas such a facility would produce.

The Bologna team developed a numerical model for their proposed system and calculated sub-models to simulate the behavior of its SOEC technology and heat recovery components.

Modeling started with a real wind production profile – although the researchers stressed any electric source could be used, including PV – and four power-to-gas system configurations were analyzed, including the annual operating time to be expected of each.

Pressurization

The analysis considered parameters such as operating temperature and pressure. The researchers said pressurization was evaluated to be able to maximize SOEC power density, improve the quality of the synthetic natural gas generated and reduce the size of auxiliary components.

“Furthermore, different temperature settings have been evaluated in order to explore possible thermal synergies among the sub-sections; the thermal synergy can be achieved operating both the co-electrolyzer and the methanation section within relatively high temperature ranges,” the Bologna group said. The system would operate at temperatures of 450-600 degrees Celsius.

The researchers found that under off-design conditions, the values of temperatures and of mass flows of streams in the heat recovery stations varied from those observed under standard conditions, mainly due to a differing electric power input and outlet temperature in the SOEC part of the system.

Of the four set-ups studied, the researchers said the one operating at ambient pressure was the most effective. In that configuration, the co-electrolyzer and high-temperature methanation sections operated at the same temperature and could theoretically produce 184 metric tons of methane per year at an overall efficiency of around 75%.

Feasibility

The competitiveness of the power-to-gas (P2G) system would be strongly dependent on the cost of such a plant, the researchers acknowledged. “At the current technology readiness level, the economic competitiveness of the process is strongly affected by the synthetic natural gas [selling] price,” the Bologna group said. “Starting from an SNG [synthetic natural gas] cost of about €70/MWh, the P2G investment cost became positive coming up to a value of about €1,500/MW for an SNG cost of €100/MWh.”

The economic feasibility of the system was evaluated using cashflow analysis based on net present value – a method used to determine the current value of future cashflows generated by a project.

According to the Bologna group, today’s prices would require state intervention in the SNG price to make such a system viable.

The findings of the analysis are presented in the study Numerical prediction of off-design performance for a Power-to-Gas system coupled with renewables, published in Energy Conversion and Management and on the ScienceDirect website.

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