Calculation and analysis of efficiencies and annual performances of Power-to-Gas systems

This paper describes a generic and systematic method to calculate the efficiency and the annual performance for Power-to-Gas (PtG) systems. This approach gives the basis to analytically compare different PtG systems using different technologies under different boundary conditions. To have a comparable basis for efficiency calculations, a structured break down of the PtG system is done. Until now, there has not been a universal approach for efficiency calculations. This has resulted in a wide variety of efficiency calculations used in feasibility studies and for business-case calculations. For this, the PtG system is divided in two sub-systems: the electrolysis and the methanation. Each of the two sub-systems consists of several subsystem boundary levels. Staring from the main unit, i.e. the electrolysis stack and/or methanation reactor, further units that are required to operate complete PtG system are considered with their respective subsystem boundary conditions. The paper provides formulas how the efficiency of each level can be calculated and how efficiency deviations can be integrated which are caused by the extended energy flow calculations to and from energy users and thermal losses. By this, a sensitivity analysis of the sub-systems can be gained and comprehensive goal functions for optimizations can be defined. In a second step the annual performance of the system is calculated as the ratio of useable output and energetic input over one year. The input is the integral of the annual need of electrical and thermal energy of a PtG system, depending on the different operation states of the plant. The output is the higher heating value of the produced gas and – if applicable – heat flows that are used externally. The annual performance not only evaluates the steady-state operating efficiency under full load, but also other states of the system such as cold standby or service intervals. It is shown that for a full system operation assessment and further system concept development, the annual performance is of much higher importance than the steady-state system efficiency which is usually referred to. In a final step load profiles are defined and the annual performance is calculated for a specific system configuration. Using this example, different operation strategies are compared.

This paper describes a generic and systematic method to calculate the efficiency and the annual performance for Power-to-Gas (PtG) systems. This approach gives the basis to analytically compare different PtG systems using different technologies under different boundary conditions. To have a comparable basis for efficiency calculations, a structured break down of the PtG system is done. Until now, there has not been a universal approach for efficiency calculations. This has resulted in a wide variety of efficiency calculations used in feasibility studies and for business-case calculations. For this, the PtG system is divided in two sub-systems: the electrolysis and the methanation. Each of the two sub-systems consists of several subsystem boundary levels. Staring from the main unit, i.e. the electrolysis stack and/or methanation reactor, further units that are required to operate complete PtG system are considered with their respective subsystem boundary conditions.
The paper provides formulas how the efficiency of each level can be calculated and how efficiency deviations can be integrated which are caused by the extended energy flow calculations to and from energy users and thermal losses. By this, a sensitivity analysis of the sub-systems can be gained and comprehensive goal functions for optimizations can be defined.
In a second step the annual performance of the system is calculated as the ratio of useable output and energetic input over one year. The input is the integral of the annual need of electrical and thermal energy of a PtG system, depending on the different operation states of the plant. The output is the higher heating value of the produced gas andif applicableheat flows that are used externally.
The annual performance not only evaluates the steady-state operating efficiency under full load, but also other states of the system such as cold standby or service intervals. It is shown that for a full system operation assessment and further system concept development, the annual performance is of much higher importance than the steady-state system efficiency which is usually referred to.
In a final step load profiles are defined and the annual performance is calculated for a specific system configuration. Using this example, different operation strategies are compared.

Introduction
Power-to-Gas (PtG) systems use electric energy to produce hydrogen or methane. The hydrogen is generated in a first step by electrolysis. In an optional second step which is usually referred to as "methanation", the hydrogen is mixed with carbon dioxide and converted into methane. If the latter is synthesized as described, it is also referred to as synthetic natural gas (SNG).
With PtG systems, seasonal storage of renewable electrical energy can be achieved. Boer et al. In addition to the effect of seasonal storage, PtG provides flexibility and stability in the electricity grid due to providing secondary control reserve [4], using surplus electricity [5][6][7] or due to coupling with energy production facilities directly, as investigated in [8,9]. PtG is also described in literature as an economic alternative to network expansion [10]. All contributions cited so far are based on an average efficiency for the performance of the PtG systems.
A view on techno-economic analysis of different PtG concepts are done by [11,12]. The studies of [13][14][15], complemented the technoeconomic analysis with a life cycle assessment. The key messages of [16,17] are the feasibility of improving the efficiency and reduction of CO 2 emissions with PtG in the electrochemical and steel industry.
Increasing the hydrogen content in the injected gas increases the efficiency of a PtG plant, as more of the gas does not undergo the methanation process with its associated losses. PtG allows to increase the hydrogen contend of the natural gas. Hydrogen-rich natural gas reduces emissions of carbon monoxide, nitrogen oxides and unburned hydrocarbons [18][19][20]. The implication of different gas qualities on end user devices has been investigated by [21,22]. A decreasing energy duty is one negative aspect of hydrogen-rich gases.
Focusing on different PtG applications and different aspects of PtG, the results and conclusions of the currently available publications and studies are difficult to compare with each other. When calculating the efficiency of a PtG system or the amount of gas produced, some publications use values from own equilibrium simulations, e.g., [23], others rely on literature studies and select values from other publications, e.g. [5,13,15,24], which are mostly not deduced from scientific analysis but e.g. specific field experience. Also the description of plant operation are difficult to compare since deviating measuring points and process

PtG
Power-to-Gas SBL