Environmental Engineering Reference
In-Depth Information
first is a need to smooth the rapid variations that may occur as small clouds
pass overhead (short term variability) and the second is the continuing need
for power at nights and on cloudy days (long term variability). These chal-
lenges in variability become more significant if the variable (including wind
and solar) power of the system exceeds more than 10 to 15% of the total power.
When the power fluctuations are less than 10% of the total power of the grid,
they may be covered by spinning reserves. However, when a change in load
requires additional generation or the supply of renewable energy is more
than a system can absorb, changes in system operation are required.
Figure 1.2 clearly shows periods when peak wind power may be avail-
able during low loads and little wind energy may be available during high
demand. Simply stated, the supply of renewable energy is only partially cor-
related with the demand for electrical power. During periods of low renew-
able energy and high demand, the multiple approaches to solving the problem
include energy storage, demand response, and bringing gas-fired generators
on line. The most common solutions are operating gas-fired generators on
line and reducing demand by turning off loads that under agreement may be
controlled in this fashion. If a system has hydroelectric power and batteries
available, they may also be utilized to meet variations in loads.
During periods when a system has more renewable energy available than
it can absorb, the renewable energy may be disconnected from the grid.
The details of management of these systems depend on the kinds of power
sources available, the loads, and grid structures. Note that power from both
coal-fired and nuclear power plants cannot be reduced below a given level
without causing serious maintenance, operating, and reliability problems. It
may take hours or even days to start coal-fired and nuclear generators after
they are shut down. Coal generation may also be limited by allowable ramp-
ing rates and tripping off line. Gas-fired generators are susceptible to supply
shortfalls and price fluctuations.
Transmission from generation sites to loads may be improved by the capa-
bility of dispatching dispatch power from storage on demand. The extent to
which energy storage becomes economical to use and its location depend
on the system to which it will be integrated. For example, it may be desir-
able to locate some storage near a wind or solar farm so as to supply peak
loads when the wind is not blowing or absorb power when it is not needed.
It also may be desirable to locate storage near loads to minimize transmis-
sion losses or delay the need to build new transmission lines in areas with
growing demands.
As the world embarks on a new era of distributed and at times nondispatch-
able electrical energy systems, the ability to manage increased levels of variable
generation will be critical to each operating region. To manage the increased
variability, future electric grids will require flexibility in load and generation
management. This variability represents the cost for decreased emissions and
increased fossil fuel savings. How utility companies and other energy provid-
ers manage the increased variability will depend on the resources available
