Potential of thermally integrated high-temperature electrolysis and methanation for the storage of energy by Power-to-Gas

摘要

Natural gas is one of the most important primary energy sources with a well developed infrastructure, especially in Europe, and among fossil fuels, natural gas has the lowest CO_2 emissions. It is used in almost all thermal applications in the industry and in domestic, commercial as well as mobile applications. In addition to the fossil primary energy sources, the share of renewable energies in primary energy consumption is steadily increasing. Amongst others, the share of renewable energy in gross final consumption was 13.0% in 2011 compared with 7.9% in 2004 for the EU27 [1]. However, besides all advantages, this results in a problem caused by the strongly fluctuating electricity supply during the generation by wind power and photovoltaics. An exemplary scenario: If the power supply exceeds significantly the demand, wind power stations have to be switched off and the potential of the systems is not completely utilized. Storage and energy transport technologies are needed providing necessary storage capacity, storage time and transport capacity. Indeed, chemical storages are the only possibility to provide capacities for the case of long-term or seasonal storage. Especially in Europe, such energy storage is the well developed natural gas infrastructure. The Power-to-Gas-Technology (PtG) is a promising way to transfer electrical excess energy into chemical bond energy like hydrogen or Synthetic Natural Gas (SNG), which can be fed directly into the natural gas grid providing that the gas quality is in compliance with the limit values. The generation of SNG is realized by coupling electrolysis and methanation. The high-temperature electrolysis offers advantages based on its thermodynamic characteristics compared to conventional electrolysis working at low temperatures. In this way a significant increasing efficiency can be achieved for the reconversion of SNG into heat and electrical power. An example: The efficiency of reconversion SNG generated by PtG with low-temperature electrolysis by Combined Heat and Power (CHP, η≈85%) is 47%. In case of using PtG with high-temperature electrolysis the efficiency of reconversion increases up to 68%. High-temperature electrolysis is based on the Solid Oxide Electrolysis Cell (SOEC) technology, which is still at a rather early development state, but shows, due to its thermodynamic characteristics at high temperatures major advantages. Different variations of coupling HT-electrolysis (700-1000°C) and methanation (200-400°C) are conceivable due to their temperature levels. Amongst others, the heat of reaction of the exothermic methanation can be completely used for the evaporation of the process water for the electrolysis. Co-electrolysis of water and carbon dioxide and recirculation of process flows are further opportunities. Different possible process schemes for coupling high-temperature electrolysis and methanation are shown in the presentation and are compared with the low-temperature electrolysis case.