Battery Storage Terminology

The "depth" of the cycle. For example, if the depth of the cycle has been degraded of 40% on a 50MWh battery, then the current full cycle of the battery is only delivering a charge and discharge of power 50*(1-0.4)=30MWh and not the full (and initial) 50MWh. Such degradation of 40% typically happens after 10,000 cycles for large-scale stationary batteries. By taking a linear assumption this corresponds to a cycle degradation of 0.004% per cycle.

According to the cycle period, the cycle limit sets a maximum number of full cycles the battery is allowed to charge/discharge along this period.

The periodicity of the cycle limit, either daily or yearly.

Depth of discharge (DoD) is the percentage of a battery’s capacity used during a discharge (100% = full discharge).

Users have the ability to prevent the energy storage asset from receiving any charge from the grid by selecting Grid Charging ‘No’. This feature is meant to accommodate analysis of hybrid projects in markets where the energy storage asset is only permitted to charge from the renewable energy resource. In either case, the value of the energy being charged into the energy storage asset is referenced to the spot market price for that particular hour.

Round trip efficiency (RTE), or "AC/AC efficiency", is the ratio between the energy supplied to the storage system (MWh) and the energy retrieved from it (MWh). This efficiency is expressed as a percentage. It includes all the systems of conversion of the electricity between the grid, the renewable asset, and the battery.

The ‘Storage Duration’ of the energy storage asset refers to how much battery capacity (in MWh) the system is capable of charging or discharging. Thus a 100%, 2hr system refers to an energy storage project with a power ratio equal to the max AC power of the solar project and the ability to discharge at that power level for 2 hours. We also assume the charge and discharge times at max power are the same, which in this example would be 2 hours. We also assume the project is sized to accommodate efficiency losses such that the project is capable of delivering the specified quantity of energy at rated power to the point-of-interconnection (POI) after efficiency losses are taken into account. Therefore, our model requires a quantity of charging energy commensurate with that which is needed to overcome the efficiency losses.

When referring to a hybrid solar-plus-storage project, the ‘Storage Power Ratio’ is the size of the battery inverter capacity in relation to the size of the maximum solar inverter capacity or wind turbine capacity. Thus a 100% power ratio hybrid project represents an energy storage asset with import or export capabilities equal to the full AC power of the solar project.