Power-To-Gas

With this selection, the Nominal fuel demand related to the lower heating value, the Supplied fuel power, and the Consumed electric power are calculated and specified in the Technical output data.

The share of the electrical energy that does not flow into the production of gas is converted into waste heat. This waste heat is calculated as difference from the electrical capacity\( \begin{equation} \begin{aligned} P_{el\ nominal} \end{aligned}\end{equation}\)and the fuel demand\( \begin{equation} \begin{aligned} \dot{m}_{nominal} \cdot H_{i} \end{aligned}\end{equation}\)using the following formula:

\( \begin{equation} \begin{aligned} \dot{Q} = P_{el\ nominal} -\dot{m}_{nominal} \cdot H_{i} \end{aligned}\end{equation}\).

If only methane is to be produced, hydrogen is also required, which must be produced in the first step. The methanation process requires carbon dioxide.

Part Load Behavior

As with all energy converters, the part load behavior can be determined by a Constant efficiency or characteristic curves.

If Absolute characteristic is selected, the relationship between the electrical Power consumption\( \begin{equation} \begin{aligned} P_{el\ nominal} \end{aligned}\end{equation}\)and Fuel power\( \begin{equation} \begin{aligned} \dot{m} \cdot H_{i} \end{aligned}\end{equation}\)is established via a characteristic curve\( \begin{equation} \begin{aligned} f_{el\ abs}\end{aligned}\end{equation}\)with the following formula:

\( \begin{equation} \begin{aligned} P_{el\ nominal} = f_{el\ abs} (\dot{m} \cdot H_{i}) \end{aligned}\end{equation}\).

When selecting Relative characteristic, the relationship between the fuel power\( \begin{equation} \begin{aligned} \dot{m} \cdot H_{i} \end{aligned}\end{equation}\)and the electrical power\( \begin{equation} \begin{aligned} P_{el\ nominal} \end{aligned}\end{equation}\)is established via a characteristic curve\( f_{el\ rel}\). In addition to the values in the table, enter the design variables as for the Constant efficiency (see above).

The following formula is used to calculate the part load behavior:

\( \begin{equation} \begin{aligned} f_{el\ rel} \left ( \frac{P_{el}}{P_{el\ nominal}} \right) = \frac{\dot{m}}{\dot{m}_{nominal}} \end{aligned}\end{equation}\).

Flow Temperature

When Modeling with external waste heat (see above) and simulating with the material property Temperature-dependent, enter the Flow temperature (outlet temperature) for the water circuit as in all components of the heating networks (see following figure) in order to calculate the mass balance and the Inlet temperature (return temperature) after optimization of the energy balance.

You can read more about this in the article about the special features of the thermal components.

Economic Data

In addition to the Resource costs and revenues and any Reinvestment costs, the Investment costs and the special Costs and Revenues of the power-to-gas plant are taken into account in the economic evaluation.

Carbon dioxide is used in the methanation process. Revenues from the use of carbon dioxide are generated by the sale of CO₂ emission certificates (EUA for CO₂ emission rights). EUA stands for European Union Allowance. Specify the EUA price for CO₂ emission rights in the Economic input data.

In the process of water electrolysis, water is used as a raw material and oxygen is produced. If the Water price and the Oxygen price are stored in the Economic input data (see following figure), these Costs and Revenues, as well as the EUA price for CO2 emission rights, are shown as Resource costs and revenues in the Economic output data of the component and taken into account in the economic balance in the Variant analysis as Operating costs.

In the Variant analysis form Economy, the Water costs are listed under Operating costs, and the revenues from the sale of the produced oxygen and the EUA for CO₂ emission are listed under Operating revenues (see following figure).

The Investment costs you can specify as Fixed investment costs or as Investment cost function. The standard value in the component template is the investment cost function of a water electrolysis plant with its peripheral equipment. For a plant with a reactor for methanation, the costs are significantly higher.

The following diagram and table provide orientation values for entering investment costs. To transfer these approximate values to TOP-Energy, first copy the table to Excel. From there, you can use the clipboard to insert individual columns into the table in TOP-Energy.

Technical Output Data

Under the Technical output data (see following figure) the calculated Consumed electric power is indicated. It includes not only gas production, but also, if specified in the Input data, the extraction of CO₂ from the air and the compression of the produced gas. For this reason, the total Consumed electric power may be greater than the Maximum power consumption of the pure power-to-gas plant, which relates solely to water electrolysis and methanation and is given in the Input data.

Balances of the CO₂ Emissions and Primary Energy

The article How Life Cycle Assessment (LCA) Works describes with two examples how the savings of CO₂ and primary energy are balanced when using a Power-to-Gas component.