IEEE Power & Energy Magazine - November/December 2017 - 36

Energy Shifting Example
18
Peak
15
Storage
Hours
12
Recharge
Storage
9 Wind Power
Discharge
6
3
0
-3
-6

Regulation and Load-Following Example

Estimated from 2 April 2014 Wind Data
0

4

8

12

Midnight

16

Noon
Time of Day (h)
(a)

20

and load-following services as generation from wind and
solar fluctuate along with system load. Grid-scale energy storage also provides a fast response to rapidly inject or withdraw
power to maintain grid stability following severe disruptions
and emergency events. Figure 5 compares battery energy storage used for energy shifting and regulation operation.
Over time, increased renewable deployments will change
the market opportunities for storage in at least three ways. First,
increasing solar photovoltaic (PV) penetrations will change the
shape of the net demand profile, making loads "peakier," and
increase opportunities for shorter (fewer than 8-h) duration
storage to meet peak demand requirements. As discussed previously, the majority of storage deployed in the United States
and internationally is pumped storage. These power plants typically have 8+ h of storage to provide reliable peaking capacity.
Shorter peaks created by PV will allow six, four, or even fewer
hours of stored energy capacity to provide similar levels of reliability, reducing the cost to implement battery energy storage
as a peak capacity resource.
Second, both PV and wind tend to suppress prices during
periods of high output, with less impact to on-peak prices,
thus creating an opportunity for arbitrage. This occurs even
without significant curtailed renewable energy.
Finally, hybrid renewable and storage projects are being
deployed to share common components to reduce costs. For
example, storage can be deployed on the dc side of a PV system's interconnection, allowing shared use of the inverter,
transformer, and system interconnection. This is not meant
to imply that storage should be coupled to specific renewable
projects; while it may be convenient to think about adding storage to an individual renewable project to create a "firm" renewable generator, such applications would be highly inefficient,
as they would eliminate much of the value associated with
value stacking-specifically, the ability of storage to charge
from multiple sources to ensure that stored energy is available
whenever needed by the grid.

24

Plant Output Power (MW)

Plant Output Power (MW)

load. Other studies have found that there are likely more costeffective methods to address high ramp rates and the inevitable curtailment of renewable energy that will occur during
periods of high supply and low demand (at least at current
storage costs).
Many regions have integrated substantial levels of wind
and solar generation without the need for battery energy storage. For example, ERCOT generated over 15% of its annual
load from renewable sources in 2016, reaching 50% instantaneous penetration. The Southwest Power Pool exceeded 17%
annual and 52% instantaneous. Ireland has exceeded 20%
annual and 60% instantaneous. Therefore, the industry has
proven that today's levels of renewable integration are possible
without the need for battery energy storage.
However, as wind and solar penetration continue to
increase, there will likely be a point where incorporating battery energy storage will be the most cost-effective
option for further integration of renewables. This inflection point is highly system specific and depends on the
cost of storage, the region's generation mix, fuel prices,
and the need for ancillary services. Examining the impact
of renewables on the grid, it is possible to conclude that the
potential exists for significant synergies between renewables and storage that will increase the cost-competitiveness of both technologies.
Some people look to energy storage as the natural complement to the resource's inherent variability. Energy storage can
charge when the wind is blowing and the sun is shining, and
discharge can occur during periods of calm winds or cloudy
days; nevertheless, industry experience has shown that most
power grids do not yet require energy shifting of renewable
resources from one time period to another.
Today, the area in which most battery energy storage provides value to renewable integration is in the management
of short-term variability on the power grid. Short-duration
charging and discharging can provide frequency regulation

4
Storage
Discharge

3

Storage
Recharge

2
1

Ramp Limit
±0.5 MW/min

0
5:30

5:35

Actual
Wind
Power
5:40

5:45

5:50

5:55

6:00

Midnight
Time of Day (h:mm)
(b)

figure 5. A comparison of (a) energy shifting and (b) regulation to facilitate wind integration. (Source: GE Energy Consulting.)
36

ieee power & energy magazine

november/december 2017



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