Environmental Engineering Reference
In-Depth Information
In the “Twenty Percent by 2030” cases, the results indicated that with a
fixed amount of wind generation, storage can lower electricity prices. Two
factors were reported to influence the drop of electricity prices: (1) a reduc-
tion in the amount of traditional generation capacity to be built and (2) the
ability to store off-peak wind, enabling some wind farms lacking storage to
become highly desirable when storage becomes available. The development
scenarios forecast by this work showed that storage was brought online only
when wind supplied 15% of the nation's energy.
Analyses commissioned by Xcel Energy's Public Service Company of
Colorado subsidiary and modeling by DOE's National Renewable Energy
Laboratory (NREL) revealed a greater need for energy storage as penetration
of wind energy increases. The Energy Storage Research Group at University
of Colorado identified two fundamental challenges to renewable wind inte-
gration that storage can help address: (1) ramp rate challenges and (2) capacity,
further classified as short time scale (under 1 hour) and longer time scale
(over 1 hour) challenges. Both types can be addressed with PHES or CAES.
RampRateChallenges
The ramp rate is the short time scale challenge of wind integration. Methods
for developing ramp rate curves can be accessed through Levine and Barnes'
paper from the 2009 Renewable Power Generation Conference
10
or by con-
tacting the authors directly. Wind generation is not the only variability
driver on the grid today; the electric load is variable as well. This implies
that (1) utility operators are accustomed to managing variability and (2) load
variability plus wind variability represents the load profile that dispatchable
generation must meet. In all cases modeled by the authors, the variability of
the wind increased the variability of the net load when combined with the
load. The curves in Figure 1.2 show the variabilities of 2006 Ft. Collins loads
compared with wind energy generation data from typical Colorado wind
development locations from 2007 onward, modeled using standard energy
generation mathematics.
Table 1.2 quantitatively describes the ramp rates that Xcel/Public Service
will need to manage both the current wind system (1 GW) and the proposed
system (1.5 GW). The table and figure reflect the same information and indi-
cate that the current system will incur fewer extreme ramping events in a
1-year period than the planned system. To accommodate the increased need
for ramping, additional ramping resources should be added to the system's
mix (Figure 1.3). Note that the current and proposed systems must ramp both
up and down. The ramp rates depend on the generators in the system and
can vary from a few minutes for gas-fired turbines and pumped hydroelectric
generators to hours or even days to start up coal-fired plants from a cold start.
