Economic dimensioning of a large-scale battery storage system – FfE project with Qair Deutschland GmbH

As part of the project “Economic assessment of large-scale battery storage systems size”, FfE analyzed the economic integration of battery storage systems into existing energy concepts. The aim of the project was to develop a flexible tool with which the optimum dimensioning of a large-scale battery storage system can be determined. The storage size is determined as a function of the technical characteristics of existing systems and economic threshold values for various future years.

Methodology

The analysis was based on an existing site in Germany where Qair Deutschland GmbH is planning wind and PV systems. In order to fully utilize the available transformer capacity of the local substation and thus the grid access of the site, the construction of a battery storage system is being considered.

The primary goal of the project was a data-based estimate of the maximum capacity of the battery storage system that can be implemented economically at the site. In the first step, 50 site-specific generation time series each were generated for wind and PV, based on five target years (2025-2045, in 5-year increments) and ten historical weather years (2010-2019). Together with the nominal output of the substations, these generation time series were used to determine the capacity gap at the substations which a battery storage system could utilize.

To simulate a market-based operation mode of the battery storage system, forecasts of the Germany-wide load and generation of renewable energy sources were created for the five target years using FfE’s own ISAaR energy system model. It was assumed that the storage system charges at times of high residual load (high load and low generation of renewables), while it discharges at times of low residual load, and since the residual load correlates strongly with the electricity price, a market-based mode of operation was thereby modeled.

In the next step, the resulting battery operation mode could be matched with the hourly capacity gap of the substation and the system could be optimized accordingly. The target requirement was to dimension the storage system in such a way that as much energy as possible can be traded and the cut-off of the storage system due to insufficient transformer power is minimized to a predefined threshold value. Various threshold values for a maximum acceptable storage cut-off were compared with respect to the eventual battery storage capacity.

Results

Due to the large unused transformer capacity of the substations at the site, a possible battery storage system can be dimensioned generously. The variance in renewable energy generation between the different calculated weather years is around 10% and has an influence on the maximum possible battery storage size.  Oversizing the storage system by increasing the accepted cut-off thresholds enables a larger dimensioning of the storage system with only a marginal increase in the probability of cut-off. Figure 1 shows that this advantage of increasing the storage capacity decreases as the threshold values increase, as can be seen from the curve flattening towards the right.

The project enables an initial estimate of possible storage sizes and a preselection of suitable battery storage systems. Further simulations and profitability analyses can then be carried out for those systems.