66 TRIBUNE LIBRE | REVUE TECHNIQUE LUXEMBOURGEOISE 4 | 2016
_Fig. 6 Sankey diagram of the energy in the P2G chain
The working capacity of the gas grid in the EU-28 was around 108 755m m3 (excluding the volume of pipes and pipelines) in 2014 [35]. Assuming an energy density of 0.0364MJ/L (atmospheric pressure) of the natural gas , the theoretical energy capacity can be calculated, which equals to 1100TWhCH4. This enormous energy can be converted into electricity in modern combined cycle power plants (CCPP) with an efficiency rate of 60%, which would equal to 660TWhel. That is 21.3% of the electricity consumption in the EU-28 in 2013 [36]. This means that the natural gas grid alone could supply the EU-28 for 2,5 months in case of zero electricity production. In a realistic case scenario, the latter should never be the case though and the present gas grid should already be able to cover the seasonal fluctuations with ease. Apart from the reconversion into electricity, the SNG can also be used in stationary applications like heating in households, commerce and services or the chemical industry. Furthermore, it can also find application in natural gas cars. As for storage possibilities, methane can use the same compressed gas storages like hydrogen including the pore storages. 4.5. SNG production costs In 2015, average gas prices in the EU-28 amounted to 3.4ct/ kWh for the industry and 7.1ct/kWh for households [37]. The SNG production costs depend highly on the setting (e.g. electricity price, full load hours‌) and have to be able to compete with the natural gas prices. In the literature, different scenarios have been analysed with only a few of them currently seeing a chance to compete with the natural gas price. Vandewalle et al. [38] calculated a price of 4-8 ct/ kWh assuming 3000 full load hours (FLH) of the plant per year, an electricity price of 0-5ct/kWh and a selling price of 10₏/t of the supplemental oxygen produced during the electrolysis. On the other hand, Schaaf et al. [39] assume a price of 13.5 – 17ct/kWh with 3000FLH, 5ct/kWh electricity and a large scale plant with 110MW SNG output. 5. Pilot plants A broad overview of current pilot projects can be found for example in Gahleitner [14]. 95% of these plants analysed there were located in Europe and North America as the interest for renewable energies and P2G is the strongest here. Concerning the power sources for the electrolysis, 76% of the pilot plants obtained the power directly from renewable energy sources or from programmable devices,
which simulate a fluctuating electricity supply. Overall, the focus is shifting from independent renewable sources to a public grid connection. Many plants used a combination of renewable energy sources, which offer some significant advantages as for instance solar and wind energy compensate each other relatively good with their production peak in summer and winter respectively. To better compensate the fluctuations, smooth out the input of the electrolyser and increase the full load hours and therefore the economic efficiency of the P2G plant, 53% of the projects used a battery. 67% of the plants used alkaline electrolysers with efficiencies ranging from 54% up to 85%. The rest used PEM electrolysers where the efficiency rates range from 52% to 79%. 2 plants used both systems. Concerning the further hydrogen usage, 83% of the pilot plants use it to generate electricity in fuel cells. The rest uses an internal combustion engine, a cogeneration plant, produces methane or places it at the disposal at filling stations. Mentionable of the current pilot plants are especially the Audi e-gas project, operating since June 2013 in Lower Saxony in Germany, which produces methane using a nearby biomethane plant as CO2 source. With an input power of 6 MW from the public grid and an H2production of 1300 m3/h using 3 AEL electrolysers, it is currently one of the biggest P2G plants worldwide with an efficiency of 54% [40] [41]. Also worth mentioning is the German Energiepark Mainz, operating since 2015 with an electrical input power of also 6 MW. Unlike the Audi e-gas project it only produces hydrogen using 3 PEM electrolysers resulting in an higher efficiency of 75% [42] [43]. 6. Conclusion Overall, there is a clearly visible trend towards P2G plants with higher capacities. The numerous projects underpin the strong interest in the P2G concept. Nevertheless, compared to conventional power plants with a few hundreds or thousands of MW, the sizes are still very small and a few decades of further research and development are necessary to fully integrate P2G into the existing infrastructure. The present plants were also built for demonstration purposes with no or few regard to profitability. The latter, however, is crucial for a further integration and can only follow with rising prices of fossil