A350 MWh battery storage facility somewhere in Europe saw revenues fall short of expectations. Subsequent analysis revealed that the situation could have been far worse.
Similar to series-connected photovoltaic modules, the weakest component in a battery storage system ultimately determines overall performance. “Battery analysis showed that in this system, the most fully charged cell was at 100%, while the least charged cell was only at 75%,” says Lutz Morawietz, head of algorithm development at Volytica.
In a series configuration, a cell that has not yet reached full charge cannot be charged further once the most charged cell hits its limit. The same constraint applies during discharge: the least charged cell empties first, prompting the battery management system to terminate the process – leaving unused energy in the remaining cells.
The case study found a daily energy gap of between 15 and 40 MWh, meaning that up to 11% of the installed capacity could not be offered on the market.
A second issue compounded the problem. Determining the state of charge is particularly challenging in lithium iron phosphate (LFP) systems. Using its manufacturer-independent monitoring platform, Volytica identified errors of up to 50% in state-of-charge estimates for individual racks. As a result, the system reported more available energy than it could actually deliver. If this overestimated capacity is traded but cannot be supplied, balancing costs are incurred.
In this project, the marketer estimated that the energy gap translated into a weekly risk of €25,000 to €110,000. In the German energy market, such impacts are typically reflected in balancing energy costs and can erode several percentage points of total revenue.
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