Meaning
Performance ratios compare the total energy recovered from a storage system to the energy used to charge it. A high round-trip energy efficiency indicates that minimal power is lost to heat or internal resistance during the cycle. This figure determines the economic feasibility of large-scale energy storage projects.
Loss Factor
Energy is consumed by the chemical reactions, the internal resistance of the cells, and the power electronics. The round-trip energy efficiency accounts for these losses in both directions of current flow. Cooling systems also draw power, which further reduces the net efficiency of the entire system.
Thermal Dissipation
Heat generated during the charge and discharge phases is the primary source of energy loss. Low round-trip energy efficiency requires larger cooling units to manage the temperature of the battery. This adds cost and complexity to the pack design and reduces the available space for active materials.
Laboratory tests measure the voltage drop at various current levels to calculate the resistive heating effect. These figures are then used to model the energy balance of the system under different operating profiles.
System Valuation
Project owners use this metric to calculate the cost of stored energy over the lifetime of the asset. A small improvement in round-trip energy efficiency can lead to large savings in electricity costs over twenty years of operation. It is a primary factor in the selection of technology for grid scale applications.
