Environmental Engineering Reference
In-Depth Information
Figure 3.1
Electricity generation in the United States from 1950 to 2011 showing the
percent of total electricity generated from different sources including coal, petroleum,
natural gas, nuclear, and hydroelectric. The insert provides a closer view of the
smaller-scale electricity generation sources. Data used in figure from EIA
2013
Electricity is the ultimate “real-time product” and due to limited storage options, it must
be produced to meet demand. Electricity storage is currently limited and expensive, so
to ensure reliable electricity service, generating capacity needs to be sufficiently flexible
to meet intense and infrequent peak demand. The electricity generation capacity of a
power plant/unit is the maximum level of electricity supply possible. Capacity should
at all times meet base load (the amount of power required to meet minimum electricity
demands) and must be sufficient to meet peak load (the amount of power required to meet
maximum electricity demand) when it occurs. While base load is generally predictable
and stable, the system is managed to meet fluctuations in peak demand, which varies by
time of day, day of the week, month of the year, and through larger economic cycles.
For example, electricity demand spikes on hot summer afternoons when people turn on
air conditioning units to cool their homes. Traditionally, large coal and nuclear power
plants with high capital costs but low operating costs are operated at a constant level, with
minimal shutdown (only for maintenance), to meet base load. These power plants have
long ramp-up times (it takes time to turn them on/off and to adjust generation), so they are
unable to be particularly responsive in meeting peak demand. Natural gas plants, which
are less expensive to build but were, before the shale gas boom, more expensive to run,
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