Frequency regulation ensures grid stability by injecting or absorbing
active power to keep the frequency inside its limits (Figure 5). In doing
so, the ESS helps the grid accommodate more renewable sources.
The operational profile of the ESS will depend on the number and
amplitude of grid frequency deviations. Typically, deviations are of short
duration and only infrequently at full amplitude. Operational characteristics of the ESS can vary considerably depending on the specific application and location, especially the distribution of discharge cycles at each
Frequently, operators request that the ESS associated with wind or
solar plants also provide frequency regulation services on top of its initial
smoothing or shaping function. Furthermore, fast reacting energy storage
enables renewable plants to effectively contribute to frequency regulation
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.
Smoothing aims to keep production within a
given forecast window. The ESS compensates for
power sags and, like ramp rate control, it will
experience many small to medium charge and
discharge cycles with a cumulated energy throughput equivalent to several full charge and discharge
cycles (Figure 2).
Power shaping uses an ESS to shape the power
output of a plant to deliver steady and predictable
power like baseline generation (Figure 3).
An ESS used for a typical PV farm in this mode
will deliver a large discharge in the morning, before
charging up during peak daylight hours in the middle of the day and discharging again later in the day.
A typical example for shaping of a 10 MW solar power
plant would be an ESS providing 5 MW power and 10
MWh energy. The average DOD would be 35 percent,
with a daily energy throughput of 7 MWh, or 0.7C.
Peak shaving is intended to reduce congestion on the
grid at peak times (Figure 4). It is mostly used to reduce
load in periods of high consumption. For example,
the ESS discharges to supply consumption peaks, thus
relieving the grid from supplying peak power.
The technique is also possible on the supply
side. For example, the ESS charges (absorbs energy)
when the PV or wind plant’s power exceeds a set
limit. It releases energy into the grid later in the day
once the peak has subsided. It thereby ensures that
the output of a plant never goes beyond an agreed
limit and avoids revenue loss through curtailment.
In both cases, peak shaving avoids or defers
investments in grid infrastructure that would otherwise be necessary to cope with peaks in consumption or generation.
FIGURE 5: Frequency regulation allows ESS to inject or absorb active power
to or from the grid