For these scenarios to become a dynamic market reality, a number of barriers have to be overcome. The main barriers are economic. Therefore, any means of reducing capital expenditure (capex) and operating expenditure (opex) will contribute to greater BESS deployment. Then there is the issue of uncertain ageing of the batteries, as well as the tendency of battery manufacturers to oversize their products to ensure they comply with contractual specifications. “The energy storage management system (eSMS) proposed by Alstom (one of the few such software solutions on the market), together with its complementary SISTOS sizing tool co-developed with the CEA, is designed to mitigate these constraints,” Boyra explains.
“The eSMS is a collection of algorithms – that rely on embedded battery models – integrated into the substation control system of a multi-MW BESS. Its three main optimisation criteria are efficiency, availability and ageing.”
The ratio between energy retrievable from and energy charged to the BESS defines its energy efficiency. Boyra points out that “by modelling influences on efficiency and keeping track of state of charge and temperature, charge and discharge efficiency maps can be determined. Using such maps, the eSMS can find optimal power setpoints that minimise system energy loss and ultimately increase system efficiency.”
Battery availability depends on the state of charge and the temperature of the battery. Often (depending on the battery technology), the battery management system limits the maximum discharge current when the state of charge of the battery reaches low levels (near zero) and the maximum charge current when the state of charge reaches high levels (near unity). The eSMS can distribute the power through the battery fleet so that charge/discharge current limiting is avoided.
A battery’s performance is not constant over its service life but decreases with time and usage. This battery ageing has two origins – cycle ageing (loss of capacity due to the effects of charge/discharge cycles) and calendar ageing – and two effects: a reduction of available capacity, and an increase in conversion losses. “The eSMS can optimise the BESS ageing by considering the ageing factors when distributing the overall power setpoint among battery units.”