Carbon Intensity calculations

The carbon intensity of each country is measured from the perspective of a consumer. It represents the greenhouse gas footprint of 1 kWh consumed inside a given country. The footprint is measured in gCO₂eq (grams CO₂ equivalent), meaning each greenhouse gas is converted to its CO₂ equivalent in terms of global warming potential over 100 year (for instance, 1 gram of methane emitted has the same global warming impact during 100 years as ~34 grams of CO₂ over the same period).

The carbon intensity of each type of power plant takes into account emissions arising from the whole life cycle of the plant (construction, fuel production, operational emissions, and decommissioning). The carbon intensity factors used in is further explained here.

Each country has a CO₂ mass flow that depends on neighbouring countries. In order to determine the carbon footprint of each country, the set of coupled CO₂ mass flow balance equations of each countries must be solved simultaneously. This is done by solving the linear system of equations defining the network of greenhouse gas exchanges. Take a look at this blog post for a deeper explanation. We also published our methodology here.

Storage

Some zones have storage capacities installed to balance their grids. Storage today mostly comes in the form of utility scale batteries and pumped hydro, which are the only two supported types of storage on Electricity Maps.

Storage and discharges affect the carbon intensity of the electricity grid. To understand why, imagine a grid with a coal power plant, a wind turbine, a utility scale battery and a consumer whose demand equals the production plus the discharge minus the storage on the grid.

Figure 1: Schematic grid at hour h₀

Imagine that at an hour h₀, the wind turbine produces 10 units of energy, of which 7 are consumed and 3 are stored. That's what is represented in Fig. 1. The intensity of the consumed electricity is thus equal to the intensity of the electricity produced by the wind turbine.

Figure 2: Schematic grid at hour h₁

Now imagine that at a subsequent hour h₁, there is no more wind, and that the consumer still requires 7 units of electricity. That's the situation represented on Fig. 2. The coal power plant can be turned on, providing 4 units of electricity, and the rest can be supplied by discharging the battery with 3 units.

To compute the carbon intensity of the electricity on the grid, one must now make a weighted average of the carbon intensity of the electricity coming from the coal power plant and of the discharged electricity. The discharged electricity is only made up of wind, so we end up with a carbon intensity that is 4/7th that of the coal electricity and 3/7th of the wind.

That's how the storage affects the carbon intensity of the electricity on the grid.

In practice, with real world electricity systems, it is much harder to track the exact content, and hence, carbon intensity, of the stored and discharged electricity.

At Electricity Maps, we therefore use an approximation. We compute the average carbon intensity of the electricity at times when storage systems were charged, weighted by the amount of electricity stored to obtain the average carbon intensity of the discharged electricity. The carbon intensity of discharged electricity per zone is accessible in its respective configuration file located here.

Figure 3: Analogy of discharged electricity as an import with a constant carbon intensity

Let's take the example of Switzerland, as in Fig. 3, to illustrate in practice what this means. When storage systems (hydro dams) are discharged, the carbon intensity of the Swiss grid is mixed with the carbon intensity of the discharged electricity, 92 gCO₂eq/kWh in that case, as if it was an import. The carbon intensity of the discharged electricity being the weighted average intensity detailed above.