By John Kemp
LONDON Nov 19 Integrating an increasing supply
of wind, solar and other variable sources of power onto the grid
will require conventional plants to become more flexible. The
question is whether coal-fired generators are up to the
On a grid with more renewables, conventional gas, coal and
nuclear generators have to be able ramp their output up and down
at short notice to offset short-term changes in wind speed and
Plants will need to be capable of "two-phase operation":
generating power for a few hours to meet morning and evening
peak demand and then going offline, with the cycle repeated once
or even twice in 24 hours.
"Fossil power plants with a highly versatile operating
response are the key to integrating renewables,"
power-engineering firm Siemens wrote in a technical paper issued
For the industry and policymakers, the core issue is what
sort of conventional generation provides the best combination of
flexibility, cost and reliability to work alongside renewables
on the emerging 21st century power grid.
FLEXIBLE GAS VERSUS COAL
The obvious choice is to rely on open-cycle gas turbines
(OCGT) to back up variable resources. Resembling the jet engines
employed on aircraft, OCGTs are already used to provide
fast-response emergency power supplies at times of peak demand.
Because these turbines do not have to heat large volumes of
water to raise steam, they can ramp up quickly. OCGTs can ramp
up to maximum output in just 10-15 minutes, compared with the
four to eight hours that it takes a large coal power plant to
reach full output even from a warm start.
But OCGTs are also inefficient and expensive. Peaking plants
that use OCGT technology charge very high prices to supply extra
power for 100 hours a year or less. They also generate lots of
So the power industry is searching for ways to make the rest
of the conventional generation portfolio more flexible too. Coal
producers and generators are understandably keen to talk up the
ability of coal-fired units to step into the gap.
Until recently, the debate largely ignored coal. Coal is
neither as clean as nuclear nor as flexible as OCGT. The
assumption was that nuclear would run as baseload and gas would
provide flexible response. But the power industry has begun to
take another look at how coal can be run more flexibly in
HIGHER OPERATING COSTS
The operational and financial challenges of operating
conventional plants in a more flexible mode are enormous.
Large-scale power plants take hours to warm up to operating
temperature and synchronise their turbines with the grid.
"(Grid operators) may have to cycle resources on and off
more than once a day," the North American Electric Reliability
Corporation explained in a recent report on integrated
renewables in California.
"At times this may not be an option because the down time
between shutdown and start-up of a resource may be too long,
which would prevent the resource from being restarted in time
for system peak," NERC concluded.
While they are warming up, conventional plants waste huge
amounts of fuel without producing useful output.
And repeatedly heating up and cooling down the boilers,
economisers, pipework, turbines and other components shortens
their life-span and requires more expensive maintenance.
On top of all this, cycling power plants must recover their
more expensive operating costs as well as the expense of
building them, while getting paid for fewer hours of generation
INCREASED WEAR AND TEAR
"Increased rates of component life consumption due to
thermal fatigue, mechanical fatigue and wear caused by
differential thermal expansion comprise the bulking of cycling
(costs)," according to the International Energy Agency's Coal
Industry Advisory Board.
"Every time a power plant is turned on and off, the boiler,
steam lines, turbine and auxiliary components go through
unavoidably large thermal and pressure stresses," the National
Renewable Energy Laboratory (NREL) wrote in a report on cycling
costs published in April 2012.
"While cycling-related increases in failure rates may not be
noted immediately, critical components will eventually start to
fail," NREL observed.
"Shorter component life expectancies will result in higher
plant equivalent forced outage rates and/or higher capital and
maintenance costs to replace components at or near the end of
their service lives."
Increased wear and tear also heightens the risk generating
plants will be unavailable when they are needed most, reducing
CAN COAL BE MADE FLEXIBLE?
The 2013 annual report of the IEA's Coal Industry Advisory
Board includes a case study of how coal-fired power plants in
Germany have responded to the cycling requirements imposed by
the installation of large quantities of wind and solar on its
In Germany, combined load and generation adjustments have
risen as high as 50 gigawatts in an eight-to-10-hour period -
equivalent to more than 60 percent of the country's peak power
In response, production from coal plants has been
successfully turned down to just 20-60 percent of normal output.
The advisory board claims part-loaded coal-fired plants have
been able to ramp down by as much as 3 percentage points per
The board's optimism is spoiled, however, by the financial
woes of Germany's big coal-fired generators. RWE this
month announced yet more job cuts and said that 2014 would be a
"valley of tears" for conventional power producers.
Rivals E.ON and GDF Suez have also
warned of a prolonged crisis in the European power industry.
Enormous renewable generation, subsidised through feed-in
tariffs, coupled with low wholesale power prices and the low
number of hours in which coal and gas-fired power plants are
able to operate mean that revenue is inadequate for conventional
REVENUE ADEQUACY AND DESIGN
In principle, coal-fired plants could still have a role in
meeting residual demand if they can be made sufficiently
flexible and appropriately compensated.
But they will need to achieve increased turndowns, faster
and less damaging start-ups, faster load changes and reserve
shutdowns at minimal cost, as the coal industry board
Much of the current flexibility has come from older, smaller
and less-efficient subcritical plants.
More modern coal plants are designed to maximise efficiency
by making them larger and employing supercritical and
ultra-supercritical steam cycle systems. But that has also made
them harder to run in a flexible mode.
The proposed new generation of super-efficient and
greenhouse-friendly integrated gasification and combined cycle
(IGCC) and oxy-combustion coal plants fitted with carbon capture
and storage (CCS) are likely to be even less flexible, if they
are ever built.
In IGCC and oxy-combustion plants, gasifiers, boilers,
turbines, air separation units and the carbon capture system
must all be made to work smoothly together. It may not be
possible to turn down one sub-system without disrupting the
efficient operation of the others.
New coal plants may need changes, starting early in the
design phase, to enable them to operate in a more flexible mode.
The requirement for flexible operation currently conflicts
with other trends such as the need for increased efficiency and
for carbon capture to reduce emissions.
Power plant designers and operators need to focus on
flexibility for coal-fired generation to have a future in a
power grid with a large share of renewables.