Use of surplus wind electricity in Ireland to produce compressed renewable gaseous transport fuel through biological power to gas systems

Power to gas (P2G) may be used to store curtailed electricity whilst converting the energy vector to gas. To be economically viable these systems require cheap electricity and a cheap concentrated source of CO2. Biogas produced from anaerobic digestion typically comprises of 60% methane and 40% CO2. The P2G system substitutes for the conventional upgrading system by using hydrogen (derived from surplus wind electricity) to react with CO2 and increases the methane output. The potential CO2 production from biogas in Ireland associated with typical wet substrates is assessed as more than 4 times greater than that required by the potential level of H2 from curtailed electricity. Wind energy curtailment in 2020 in Ireland is assessed conservatively at 2175GWeh/a. Thus P2G is limited by levels of curtailment of electricity rather than biogas systems. It is shown that 1 GWeh of electricity used to produce H2 for upgrading biogas in a P2G system can affect a savings of 97 tonnes CO2. The cost of hydrogen is assessed at €0.96/m3 renewable methane when the price of electricity is €c5/kWeh. This leads to a cost of compressed renewable gas from grass of €1.8/m3. This drops to €1.1/m3 when electricity is purchased at €c0.2/kWeh.