(John Kemp is a Reuters market analyst. The views expressed are
By John Kemp
LONDON, April 18 Forcing coal and gas-fired
power plants to ramp up and down more frequently to compensate
for the growing number of wind farms connected to the power grid
will reduce their efficiency and increase maintenance costs.
The adoption of renewable energy laws and renewable
portfolio standards (RPS), requiring power purchasers to give
priority to clean generation from intermittent sources such as
wind, will require fossil fuel generators to operate more
flexibly, according to a recent study funded by the U.S.
Department of Energy.
Plants originally designed to provide baseload will in
future have to operate on a load-following or cycling basis,
ramping up and down at short notice. The actual costs of using
baseload plants to follow load are poorly understood, but likely
to be substantial.
"Utilities know that thermal cycling does damage plant
components, but the total cost impact is rarely fully understood
in terms of increased forced outages and increased operation and
maintenance costs," according to a report on the "Impact of load
following on power plant cost and performance" published by the
National Energy Technology Laboratory (NETL) in October 2012.
Every shutdown and cold start cycle at one small (340
megawatt) coal-fired power unit in Texas cost an extra $157,000
- including $120,000 for additional maintenance and almost
$16,000 in wasted fuel - according to one engineering study
conducted by consultants Intertek.
The problem is that fossil fuel plants, especially older
varieties, were not designed to be repeatedly powered up and
Major components at Britain's oldest coal-fired power
plants, for example, were designed to last for 5,000 hot starts,
1,000 warm starts and 200 cold starts ("Renewable electricity
and the grid: the challenge of variability" 2007).
As different parts of major components heat up and cool down
faster than others, differential rates of expansion cause high
stresses. The resulting metal fatigue and creep causes cracking,
significantly increasingly the maintenance budget and raising
the risk of failure and unplanned outages.
Pipework, boilers and blades on steam turbines in coal-fired
power stations are all expensive components at significant risk
from repeated restarts and shutdowns, and a source of extra
Simple gas-fired turbines handle ramping up and ramping down
better. But many modern gas-fired plants employ combined cycle
systems (one gas turbine followed by a steam turbine in
sequence) because it is more efficient. Unfortunately, the steam
turbine component has all the same cycling problems as a
Some CCGT plants can de-couple the two turbines, which gives
more flexibility but at the cost of reduced efficiency.
The secondary steam turbine can be decoupled from the gas
turbine when the plant is operating in load-following mode, in
which case the efficiency benefits of running a combined cycle
plant are lost. Or the two turbines can continue to operate
together, in which case the slow ramping up or down of the steam
turbine limits the flexibility of the overall system.
Efforts to make fossil-fuel generation cleaner by building
coal plants employing integrated gasification and combined cycle
(IGCC) with carbon capture and storage (CCS) systems, or CCGT
plants with CCS, would likely make matters even worse.
These plants have so many expensive and energy intensive
processes that they would not be able to adjust power output up
and down quickly without wasting enormous amounts of energy in
the process, slashing their efficiency and raising emissions per
unit of power massively.
It takes 150-250 minutes for a natural gas plant with
combined cycle to start up from cold, 12-15 hours for a
conventional coal-fired power plant, but an estimated 24 hours
to start up an IGCC plant, according to NETL.
Most power plants have some flexibility to vary their power
production in the short term by operating at less than their
full output. But even here flexibility is limited by operating
constraints, which may be getting worse for the newest and most
energy efficient plants.
Conventional coal-fired plants can turn down their output by
a maximum of about 50 percent. Any lower and their efficiency
drops to the point where they violate air quality controls.
Combined cycle gas plants can achieve turn-down rates of about
40 percent. IGCC plants can only achieve turn down of 20
But as power generation plants get more efficient, they also
become more complex and costly to construct, and increasingly
uneconomic to run at anything other than full output. CCGT and
IGCC (if it were ever widely built) need to operate as base
Shutting down and restarting CCGT and IGCC systems also
wastes enormous amounts of fuel, as systems have to be heated to
operating temperatures. In the event of an unexpected ramp down,
any unusable gases have to be flared off or run inefficiently
through a single-cycle turbine.
NEW GRID TOPOLOGY
Relatively inefficient single cycle gas turbines are likely
to be the only practical option for load-following on the grid.
Demand for them seems set to rise as the grid becomes less
flexible in other ways, with increased numbers of wind turbines,
as well as CCGT units and perhaps IGCC and supercritical
In the meantime, the large fleet of coal-fired power
stations that will continue to provide significant power in the
United States (and other countries) through 2030 will operate in
an increasingly inefficient and costly fashion, with increased
carbon emissions per unit of power generated and significantly
higher maintenance charges.
"Boiler and turbine manufacturers have recognised that the
power plant of the future will be required to follow load and
cycle much more than in the past," according to NETL.
"Manufacturers are designing systems and components to
better survive the cycling environment and developing controls
and operating procedures to accommodate rapid load changes," the
But the cost of equipment and maintenance seems set to rise
significantly as more durable equipment is rolled out and old
plants are retrofitted.
While the case for more wind generation will remain
compelling for policy makers, the extra emissions from less
efficient operation of back-up fossil fuel plants must be set
against the savings from wind power.
(Editing by Anthony Barker)