How is capacity factor for battery storage connection calculated?

[hbshrestha]

In the [documentation], it is stated that the

connection entity cf parameter - [per unit] average capacity factor of the flow, i.e. the utilization rate of the connection where flows in both directions are considered as utilization. Average of the absolute flow [MWh/h] divided by the capacity of the connection.

I took the average of the absolute flow available in group_flows_t sheet and divided it by my battery capacity, but I did not get the same capacity factor that is in unit_cf_node_flow sheet. I might have missed something. Could you clarify what is the mathematical equation used for this calculation?

[jkiviluo]

What is in the group from which you are taking the group_flows_t? Have you compared the with the flows in the connection? Also, you mention unit capacity factor ('unit_cf_node_flow'), but I believe your storage uses connection instead of charge/discharge units?

In principle, this is how cf for a connection is calculated:
sum( abs(r_connection_dt[c, d, t])
/ complete_hours_in_period[d]
/ entity_all_capacity[c, d])

where r_connection_dt =
r_process__source__sink_Flow__dt (flow to the battery after losses)

• r_process__sink__source_Flow__dt (flow to the electricity node after losses)

[hbshrestha]

I got the connection capacity factor from connection_cf sheet instead of unit_cf_node_flow sheet. I stand corrected in this.

Thanks for sharing the mathematical equation, Juha! From the equation, I think the statement in the documentation should be "sum" rather than "average" of absolute flow divided by the capacity of the connection. Isn't it?

I have included a sample of FlexTool output. group_flows_t has the hourly battery flows for a battery technology in Algeria.
I have calculated its capacity factor in Calculation sheet. Could you verify it is correct way of calculating battery CF?
algeria_battery_flows.xlsx

[ArttuTupala]

I think that the problem here is that the group_flows_t sheet is made for grouping the results in the point of view of the balance nodes. The flows there are the ones leaving or arriving to the node. The difference is efficiency. In the capacity factor calculations, the flow is the outgoing flow for both directions meaning the efficiency losses are taken away. This is because the capacity limits the flow after the losses.

In the group_flow_t sheet, the flow arriving back from the battery is correct, but the flow going to the battery does not have the efficiency losses, because from the point of view of the node it loses both the connection losses and the flow arriving to the battery.

If you want to calculate the capacity factor from the timeseries, it is better to use the connection_flow_t sheet or just multiply the charging flow with the efficiency in the group_flows_t

Also, it is an average as the flows are divided by the hours.

[hbshrestha]

Thanks for the clarification, Arttu!

I still used the value from group_flows_t sheet in my calculation. From your explanation, I inferred the formula for calculating annual capacity factor for battery connection as follows. I got the same CF value that was present in the connection_cf sheet.

Annual CF for battery connection = (absolute value of charging flows in a year in MWh * efficiency + absolute value of discharging flows in a year in MWh) / (battery installed capacity in MW * 8760 hours)

I have corrected my calculations in the attached file.

algeria_battery_flows.xlsx

[jkiviluo]

All right, great. I have done a small clarification to the docs (push pending). It's time series average, but of the absolute values. I will move this to discussion for reference.