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Amain challenge in reconciling the
growth of the world’s energy needs
with the drastic reduction of the
Greenhouse Gas Emissions (GHG)
cannot be achieved by the sole
massive use of renewables. It can
only be met sustaining renewables with other
complementary strategies such as, the use of low
carbon fuels, increasing efficiency of fossil-fuel-
based energy conversion systems, implementation
of carbon capture and storage technologies,
emission reduction on large scale transport,
waste heat recovery. According to IEA’s “Energy
Technology Perspectives” 2015 report, among the
technologies required to achieve the “2 Degrees
Scenario”, renewables will be responsible for 30%
1Co3fa%tphte,uarCenOda2nnrduecdCleuOacrt2ioeSnnteoinrrgatyghefeo(rpC8oC%wSe,)rotsfeetchchetnocor ulbomygiue2lsa0t5ifvo0er, and reliability problems when used in this way. As
reductions (data elaborated from [1]).
The increasing share of renewable sources of
energy [2], unpredictable and intermittent for
their own nature (especially wind turbines, since
wind day-ahead predictions are less reliable than
solar ones), will produce more power fluctuations
in the electricity grid. In such a scenario, fossil
fuel power plants will play an important role as
electrical grid back-up systems by compensating
those fluctuations [3]. Thus, Gas Turbine (GT)
back-up power plants will be required to stably
perform loading and unloading phases quicker
than the current ones, still maintaining low pollutant
emissions. At the same time they will be required
to be “parked” at their minimum environmental
load (with an increased turndown ratio), or even
shut down, when their power is not needed. For
example, in 2013 Spain and Germany experienced
13 and 25 GW unpredictable daily load variations,
respectively. National load ramps more than 5
GW/h are expected to be common. The required
ramp rate will be approximately three times what is
achieved today.
Critical changes in managing some power plants
are already experienced by users. As an example,
Fig. 1 - Typical weekly trend
of a combined cycle power
plant in Italy (courtesy of Enel).
Power plant operation in
2012, with respect to 2005,
shows an increased number
of start-up / shutdown cycles,
reduced operation at base
load, and operation at mini-
mum environmental load dur-
ing night-time
differences in power plant operation with respect
to the past are shown in figure 1 for a combined
cycle power plant in Italy. As a matter of fact,
the best available fossil-fuel technologies, i.e.,
Ultra-SuperCritical Pulverised Coal (U-SCPC),
Integrated Gasification Combined Cycle (IGCC)
and Combined Cycle Gas Turbine (CCGT), can be
flexible, but have been found to suffer operational
a consequence, in 2013, 14% of the European
installed capacity (corresponding to 24.7 GW) was
idled, closed or at risk; in 2015/16, up to 50 GW
may be closed [4].
Furthermore, the intermittent operating mode of
GT plants in Europe increased ethmeisirsCioOn2p, eCrOMaWndh
remarkable to note
NOx emissions, e.g., the CItOis2
increased by 8% in 2014.
that in the same year 2014, the global emission of
CduOe2 did not rise for the first time in 40 years (mainly
to a lower energy consumption in China, and a
major shift from coal to natural gas both in US and
Asia) although the global economy grew by 3% [5].
It is also observed that the demand to quickly
ramp up and down in load may enhance so called
“combustion dynamics” (i.e., thermo-acoustic or
operational instabilities) thus increasing the risk
of failures, damages and unexpected overhauls.
These issues lead to the need for more load-flexible
gas turbines without penalties in performance and
emissions.
As already stated, CCS technologies will be
responsible for 13% of the “C2OD2 ergerdeuecstiSocneinnartihoe”
power sector to achieve the
[1]. However, it is observed that CCS deployment
is occurring too slowly due to high costs and a
lack of political and financial commitment. CCS
technologies are technically feasible but expensive
and negatively impacting load-flexibility; Open-
Cycle Gas Turbines (OCGT) have higher load-
flexibility than other energy conversion systems, but
the low concentration of CuOn2atotrnacetxivhea;usCtosmmbainkeeds
the CCS implementation
Cycle Gas Turbines have the highest efficiency,
making competitive the electricity cost also in the
Impiantistica Italiana - Settembre-Ottobre 2015 41