A megawatt-scale lithium-ion (Li-ion) energy storage system (ESS) can be vital in successful grid integration of a large wind or solar plant by addressing the intermittency and unpredictability inherent in renewable energy. The challenge, however, is sizing the ESS for maximum operational and financial benefit. This is because an ESS can have several distinct roles, and only by understanding its role and the specifics of its site can engineers specify the right ESS for the job.
Ramp Rate Control
Grid operators often must limit the rate of change at which power is injected into the grid-the ramp rate. The output of a photovoltaic (PV) array of several megawatts can drop by 70 to 80 percent in about a minute. The ESS, therefore, must discharge in a way that ramps the net facility output down smoothly over seven or eight minutes (Figure 1). The ESS can absorb or release energy when a sudden shift in wind or passing cloud causes a step change in output. Ramp rate control ensures that the facility ramps at a rate that is compatible with the power system. This is particularly true for island grids, because they lack the inertia of mainland networks and are susceptible to disruption, which could be caused by simultaneous uncontrolled ramping of several renewable facilities.
The ESS will experience many small charge and discharge cycles. Over the day, the cumulative energy charged and discharged in 24 hours, known as throughput, can amount to around two to three multiples of the capacity of the ESS (2C to 3C).
Typically, a 10 MW solar farm would be combined with an ESS capable of delivering 5 MW of power and storing 1.3 MWh of energy. The facility would operate at an average depth of discharge (DOD) of 6 percent and a cumulated daily energy throughput of 2.5 MWh, which is equivalent to 1.9 times the capacity (1.9C).
In contrast, wind generation generally varies at lower amplitudes so a typical 10 MW wind farm could be equipped with a 2.5 MW ESS, delivering 0.58 MWh energy storage. It would operate at an average DOD of 4 percent with a cumulated daily energy throughput of 1.9 MWh, or 3.2C.
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