The following entry is written by Donald Jones. The piece is particularly timely as it comes during a week when KMPG released their estimated costs on low carbon electricity generation possibilities, showing the lowest cost option as 70% nuclear.
There
is a widely held belief that commercial nuclear-electric plants are
only capable of baseload operation when in fact they can be more flexible than a
natural gas-fired generating station. This belief has led the Ontario
government to restrict nuclear generation to 50 percent of total demand, in its
Long-Term Energy Plan, to avoid more surplus baseload generation
(SBG). It may also have provided some of the rationale
for the expansion of wind/gas generation. In France nuclear meets nearly 80
percent of the electricity demand so the output of nuclear units has to be
changed throughout the day to match the load on the grid, load-following. In
Ontario the nuclear units operate baseload but units at Bruce B can be
held at reduced output overnight when demand on the grid is low,
load-cycling.
The
Independent Electricity System Operator (IESO) has stated that in
general coal-fired units can be dispatched down to 20 percent
of full output, and combined cycle gas turbine (CCGT) units down to 70
percent even though they can operate at lower power outputs. Generating
units are dispatched by the IESO, that is, sent instructions to raise or lower
electrical output, at five minute intervals day and night. If units are
operating below their dispatchable power range they will not be able to respond
to the dispatch instruction in the time allowed. This means that a hot
coal-fired unit is more flexible than a CCGT unit in meeting a variable
demand on the grid. Hydro is technically very flexible but suffers from
water management regulatory restrictions. New nuclear build in Ontario will
be highly manoeuvrable with a dispatchable power range wider than gas or coal
and could even have dispatching preference over hydro. See
Appendix which describes the operation of the Ontario grid.
In order to be available to help restore the grid after a grid
blackout or get back on line after a loss of load all CANDUs
(except Bruce A) are capable of quickly reducing reactor power to 60
percent of full power, holding at reduced power, and then returning more
slowly to full power using their adjuster rods. The unit electrical
output would be held to around 6 percent full power, just
enough to supply the plant's auxiliary services load, with the reactor
held at around 60% full power and steam bypassed around the turbine to the
condenser. Pickering A and B do not have steam bypass to the condenser but
bypass steam to atmosphere. The reactors using bypass to condenser can
remain at 60 percent full power indefinitely until the grid or load
are re-established. In this so called "poison prevent" mode the already hot
turbine can then be quickly brought up to 60 percent power to feed the
grid causing the bypass valves to close and the slower return to
100 percent power output can then begin. During the 2003
August blackout in Ontario and the north-eastern U.S. some units at
Bruce B and Darlington were put in this mode. For various
reasons, Bruce A and Pickering A and B units are shutdown after a grid
blackout.
All the Ontario CANDUs were designed for baseload operation.
Darlington and Bruce B also included the capability for
some load-cycling using reactor power changes, without
using turbine steam bypass. They were not designed for
load-following. In the past some domestic units and off-shore units did
accumulate considerable good experience with load-cycling, with some deep power
reductions, but not on a continuous daily basis. For example back in the
1980s several of the Bruce B units experienced nine months of load-cycling
including deep (down to 60 percent full power, or lower) and shallow
reactor power reductions. Analytical studies based on results of
in-reactor testing at the Chalk River Laboratories showed that the reactor fuel
could withstand daily and weekly load-cycling. Since then, for various
reasons, the Bruce and Darlington units have been restricted to
baseload operation and are not allowed to vary reactor power for load following
or for load cycling although Bruce B is allowed to reduce unit
electrical output by bypassing steam that would otherwise go through the
turbine. Slow reactor power changes can be made as part of normal
operation. Reactor power reductions to around 60 percent of full power
combined with steam bypass, poison prevent mode, is still allowed at
Bruce B and Darlington for unanticipated events such as a loss of load
or grid blackout. For the way that Ontario's nuclear units interact with
the grid see Reference 1.
Since the steam bypass system in the present nuclear units was
not designed for the frequent use necessary to alleviate SBG this
system should be made more robust as part of the upcoming refurbishment of Bruce
and Darlington. Such a system could then provide a degree of load following
as well as load cycling, automatic generation control (AGC- see
Appendix) and a dispatchable power range better than a CCGT, depending
on the design of the steam bypass system. Steam bypass system design and
its advantages for units undergoing refurbishment is described in Reference
2. If all the present Ontario units were refurbished to have the same, or
better, steam bypass capability as Bruce B, and if many new manoeuvrable
units were built, this would go a long way to reducing Ontario's dependence
on precarious gas-fired generation that is subject to future gas price
escalation and availability concerns - see Reference
3.
Bruce B units have frequently dropped around 300 MW
overnight, using steam bypass, to alleviate periods of SBG. This means
each unit can provide 300 MW of dispatchable power with
electrical output held at 63 percent of full power. On occasion units
have dropped over 440 MW to operate at 46 percent of full electrical output. The
power down, and later power up, takes up to two hours using a steam bypass
system that was not originally designed for this kind of use. Reactor power
is kept constant at full power, around 822 MW. Under these circumstances this is
better than the 70 percent dispatchable limit of the CCGTs. However, for
operational reasons, Bruce Power presently prefers to make one big
power move rather than a series of smaller, say 80 MW, power
reductions during any SBG period, which restricts dispatchability
somewhat in comparison with CCGTs. SBG is exacerbated by self-scheduling
wind generation and since the existing wind generation projects have priority
access to the grid it means that nuclear has to be powered down or even
shutdown to accommodate wind if hydro and gas generation have been
already reduced to must-run power levels. There will be around 8,000
nameplate MW of wind on the grid by 2018, in the belief that it will
reduce the greenhouse gas emissions from the gas-fired generation that is
replacing coal. Significant reductions are unlikely - see Reference 4. Although
it can be done, dispatching clean low cost nuclear, and hydro,
to integrate wind makes no technical, environmental or
economic sense.
For new CANDU build, whether ACR-1000 or EC6, up to 100 percent steam
bypass combined with a reactor power that can be varied if
necessary, anywhere between 100 percent and 60 percent full
power, would be used to vary unit electrical output down to zero
if required, at high up and down load ramping rates. This will provide
dispatchable load-following, load-cycling, and AGC capability,
with a dispatchable power range much greater than that of CCGTs and
coal. Overnight load-cycling would be done by varying reactor power with
little if any steam bypass. Although the energy in the bypassed steam is being
wasted, at least at present, CANDU fuel costs are very low. Even so,
operating the plant regularly at less than full power, whether by reactor
power changes or by steam bypass, will reduce the capacity factor and
increase the unit cost of electricity generated.
The loading rate of a CCGT unit is set by temperature
transients in the thick walled components of the heat recovery steam
generator and the rest of the steam side, typically for today's
plants up to 5 percent full power per minute. The loading rate of a CANDU
unit using steam bypass would be set by turbine metal temperatures,
typically up to 10 percent full power per
minute with relatively low temperature nuclear steam. This
is also better than the maximum 5 percent per minute load ramping
rate that the EPR and AP1000 can achieve, and this not over all of
their fuel cycle. The hydro stations are extremely flexible and
can load at high ramp rates when available. However
there can be restrictions on the operation of stored water hydro units due
to water management regulations, environmental concerns, and from
public safety concerns around the dams because of sudden variations
in water levels. All this could reduce the flexibility of some of
the hydro generation to respond to dispatches at high ramp rates, so
in some circumstances dispatching
nuclear units using steam bypass could be a much better option for the
grid operator.
France provides a precedent for load-following and
load-cycling in Ontario. France has been producing nearly 80
percent of its electricity from its nuclear fleet for many years with
the balance coming from hydro and fossil fuels in about equal amounts. France
has 58 pressurized light water reactor units on line so the national grid
controller can select units that have been recently refueled and have high
reserve reactivity so have the flexibility to provide
dispatchable load-following, load-cycling, and AGC. Power is varied by so called "grey"
control rods and boron use is minimized. Steam bypass is not used for these
operations. When units are around 65 percent through their 18 to 24
month fuel cycle they play a diminishing part in load-following and when 90
percent through their fuel cycle they are restricted to baseload operation.
CANDU flexibility is not affected by fuel burn-up limitations since it is
refueled on-line.
Nuclear is not a one trick pony.
Appendix - How the Ontario power grid works
As of mid 2011 the Ontario grid consisted of 11,446 MW of nuclear with 1,500 MW more refurbished generation to come on line in 2012, 4,484 MW of coal-fired generation, 9,549 MW of gas and oil-fired generation mostly combined cycle gas turbine (CCGT) but includes the rarely used 2,140 MW oil/gas-fired Lennox thermal units, 7,947 MW of hydro-electric base, intermediate and peak generation, and 1,334 nameplate MW of wind generation. The
grid consists of many generating stations located throughout the province feeding consumers through a network of high voltage transmission lines, transformers, switchgear, and low voltage distribution lines to major consumers including local utilities. Electricity cannot be stored in large amounts so generation and demand has to be kept in balance at all times. If demand exceeds supply all the generators on the grid slow down and the normal grid frequency of 60 Hertz (reversals per second of alternating current) will drop. All electric motors working off the grid would similarly slow down. If supply exceeds demand the frequency will increase. It is the job of the Independent Electricity System Operator (IESO) to ensure that these frequency swings keep within very tight tolerances. It does this by dispatching hydro, coal and CCGT (hardly any simple cycle gas generation) at five minute intervals, not necessarily the same generator, to move power up or down. In the morning the power moves would generally be in an upward direction and in the evening in a downward direction but there can also be small reversals in the general trend. This is called load-following (load-cycling refers to powering down units overnight when demand is low). This brings the grid into a rough balance. In order to bring the frequency into its narrow operating band around 60 Hertz the IESO automatically controls the output of a very small number of selected generators that have the capability to continuously and rapidly vary their output over a seconds to minutes time scale. These are some hydro units at Niagara Falls and, in the past, some coal-fired units. This is called Automatic Generation Control (AGC).
The second to minutes supply/demand variations on the grid, including the erratic fluctuations of wind, are smoothed out by the rotational kinetic energy of the many generators on the grid, by the hydro and fossil turbine-generators on the grid changing their output by normal speed governor action over a limited range (called primary frequency control), and by AGC (called secondary frequency control, normally automatic but can also be done manually). Primary control limits the frequency deviation caused by changes in supply and demand, and secondary control restores the frequency to normal by removing the frequency deviation, or offset, by changing the setpoint of the speed governor of the generating unit(s) on AGC. Nuclear units presently do not take part in frequency control. The current AGC regulation service requirement from the IESO is for at least plus or minus 100 megawatts at a ramp rate of 50 megawatts per minute but this may be changed to allow other generators to supply this service. The designated unit(s) that is on AGC service is kept in its desired operating range by dispatching hydro, coal and combined cycle gas generation at five minute intervals. This dispatching allows for the normal daily demand changes (load-following), including the intermittency of wind. Since valuable hydro is fully committed, gas or coal generation is used to cater for wind intermittency. As well as frequency, voltage levels at points on the grid also have to be maintained but that will not be discussed here.
Reference
1, "IESO - less dispatching of nuclear if you please", Don Jones
Reference
2, "Ontario Electrical Grid and Project Requirements for Nuclear Plants",
2011 March 8 report from the Ontario Society of Professional Engineers to
Ontario's Minister of Energy, http://www.ospe.on.ca/
Reference
3, "An alternative Long-Term Energy Plan for Ontario - Greenhouse gas-free
electricity by 2045", Don Jones, http://coldaircurrents.
Reference 4, "IESO - will Ontario's wind turbine power plants reduce greenhouse gas emissions?", Don Jones
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