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[June 2007] Paul Willson, PB Power
Introduction
A completely new concept, developed by UK-based PB Power for future high efficiency electricity generation, combines the advantages of nuclear power generation with combined cycle gas turbine-based power technology. The patented, ground-breaking development has been designed to create a low-cost, high reliability, high efficiency hybrid system from existing proven technologies, for either retrofitting to existing nuclear stations or for new-build installations.
Thermal efficiencies
When analysing a nuclear power station design, the question often asked by non-engineers or scientists is ‘why can’t you convert all the heat generated in the reactor into electricity’. For example, the thermal efficiency of a typical PWR is just 32%. However, the second law of thermodynamics states that in a closed cycle not all the supplied heat can be used to do work, therefore even if there was no friction or other imperfection in the system, the maximum Ideal Efficiency would still be well below 100%. For a PWR operating at an upper steam temperature of 280C the maximum possible efficiency would be just 45%. By increasing the upper steam temperature it is possible to push up both the ideal and practical efficiency, which is why gas-cooled and high temperature reactors deliver more power from their thermal rating.
In fossil fuelled power plants temperatures have also been pushed up and the most modern coal-fired super-critical boilers can achieve a thermal efficiency of up to 44%. However, the way to significantly exceed this level is to combine two cycles such as the Combined Cycle Gas Turbine (CCGT) where gas turbine inlet temperatures are around 1200ºC. By generating power in the gas turbine and using the exhaust gases at around 600ºC to drive a steam turbine cycle, a thermal efficiency of over 57% can now be achieved.
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Efficiency approaches to the Ideal Efficiency limit |
This technology, fuelled on low cost and easily handled natural gas, has gained increasing popularity over nearly three decades because of its high efficiency and low capital cost. Since its initial introduction, CCGT power generation technology has undergone continuous intensive development, ramping the overall cycle efficiencies from around 50% in 1990. Representing billions of dollars of R&D funding, progressive improvements to heat recovery systems and steam turbines coupled with the key development of advanced gas turbine technology have resulted in achieving this significant increase in performance and overall efficiency.
However, despite the rising cost of gas providing a strong incentive to raise system efficiencies, even modest incremental improvements are now proving difficult and almost prohibitively expensive to achieve. Figure 1 shows the efficiency development in nuclear, coal and CCGT plants compared to the Ideal Efficiency limit. It can be seen that although CCGT efficiency has continued to increase it is approaching a plateau and is not reducing the gap with the Ideal Efficiency line.
A Combined Nuclear and Gas Plant
The renaissance of interest in nuclear power as a safe, reliable, low-carbon source of energy has provided the impetus for a fundamental reconsideration of available power generation systems. This has included special emphasis on the possible synergies derived from the combination of current nuclear and CCGT technologies. Following extensive research, engineers at UK-based PB Power have developed and patented a low cost, low-risk engineering solution called NuGas™, enabling the two separate power generation systems to operate in tandem as a single combined unit. The innovative concept also allows either the nuclear plant or the gas turbine-powered unit to operate independently, maximising availability of power during routine shutdowns.
Until now, gas-fired CCGT and large-scale nuclear power generation have been regarded as two completely disparate and rival technologies. However, the new concept enables a conventional CCGT plant to be integrated with a large nuclear station, either as a retrofit or a total new-build power plant, by simply linking their respective power-generating steam systems. This not only provides a rapid and cost-effective ‘bolt-on’ means of increasing the total generating capacity, but also enables the CCGT unit to operate at unprecedented levels of efficiency, using proven systems and equipment at conventional operating conditions.
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The NuGasTM Cycle |
The basic concept would allow a large nuclear power plant with a typical output of 1200MWe, to be combined with a 400MW CCGT generating unit. Linking the steam-cycles of the two plants not only enables them to operate as an integrated power production unit, but reduces thermodynamic losses simultaneously in both systems, increasing total efficiency. Thermal efficiency gains provide an additional 43MW of power without additional fuel or operating cost, boosting the potential output of the combined power plant to a total of almost 1650MW, without exceeding conventional design conditions.
How Does it Work?
Although a CCGT system has a high thermal efficiency, it uses the heat from the exhaust gases of the gas turbine, to raise steam that drives the turbine. Due to the large amount of heat needed to boil water, large temperature differences occur in the boiler which limits the potential cycle efficiency. The NuGas™ cycle overcomes this by ‘borrowing’ a small proportion (typically 10-15%) of the steam from the nuclear steam cycle (point ‘A’ on Figure 2). The dry saturated steam is superheated using the exhaust heat of the gas turbine. The high temperature steam (‘B’) is then used to drive a separate conventional condensing steam turbine to provide additional output from the plant. Superheating steam results in much reduced temperature differences than boiling water so significantly more power is generated in the steam turbine than for a conventional CCGT.
The heat in the gas turbine exhaust gases below about 300ºC (570ºF) is recovered to provide feed heating for a large part of the high temperature steam turbine condensate flow, reducing power losses from steam extraction in the main cycle.
‘Borrowing’ steam from the main steam cycle reduces the steam turbine power output but the additional generation in the high temperature steam turbine more than compensates for this reduction. The calculations of efficiency of the NuGas™ cycle recognise this necessary compensatory generation.
Safety Considerations
The extraction of steam from the main steam system has the potential to disturb reactor operating conditions. However, the PWR system is designed to allow for a 10% step change in flow to the main steam turbine without exceeding the appropriate limits for a frequent operating condition. Despite the limited impact of any disturbance from the small flow to the added cycle, a suitably qualified shut-off valve and protection logic would be installed to reduce the potential risk from any downstream failure in the NuGas™ cycle to very low levels.
The interconnection design minimises installation risks and ensures that the main plant is unaffected by operation or maintenance of the NuGas™ plant. This also ensures that the integrity of the nuclear safety case for the plant is preserved and that CCGT operation can continue independently during reactor outages. This is fundamental, as significant costs would be charged by the grid operator for increasing the loss of generation resulting from a single fault. In addition, the economics of operation of the NuGas™ cycle would be adversely affected if the availability of either plant was to be degraded by the interconnections.
Since NuGas™ raises overall efficiency by enhancing the thermodynamic cycle rather than changing operating conditions, it is not only inexpensive, but introduces no new technology risks in its implementation.
Efficiency Improvement
For a combined cycle plant using F-class industrial gas turbine technology, capable of operating at a typical efficiency of 57 percent, use of the NuGas™ cycle increases efficiency to over 63 percent, far higher than even the most ambitious forecasts for advanced gas turbine-based developments. The use of a higher performance gas turbine will further increase the NuGas™ efficiency since its advantage lies in the enhanced thermodynamic performance of the steam cycle.
NuGas™ can be applied either to a new-build project or to an existing nuclear power plant. While the new-build design allows for maximum optimisation, the retrofitted option will enable rapid return on investment, with minimal impact on normal day-to-day operation of the existing nuclear plant during construction of the CCGT unit.
While the description above applies to a PWR, the NuGas™ technology includes similar cycle concepts for the BWR and CANDU reactors, each offering similar advantages in performance and low impact of interconnection.
New Build
Currently the two leading candidate Pressurised Water Reactor (PWR) designs for new nuclear construction are the EPR reactor with a nominal power rating of 1600MWe from Areva and the Westinghouse AP1000 reactor with an output of around 1140MWe. Either the EPR or AP1000 could be integrated with a NuGas™ cycle to offer extra capacity with the highest possible efficiency for fossil fuel conversion, without significantly increasing the loss of output in the event of a reactor trip.
The NuGas™ concept can be applied at a capacity cost similar to that for a CCGT, typically in the range £400-500 per kW. This compares favourably with the cost of nuclear capacity and even with the current high cost of gas, financial analysis indicates that unit costs will be competitive, even at higher than usual rates of return on investment.
The other advantage of adding NuGas™ to a new build plant is that it offers flexibility of despatch for a significant part of the capacity, making it more suitable for the modern power trading environment.
Backfit
The renaissance of interest in new nuclear power plants will mean that by 2015 and beyond more nuclear plants will be brought on-line. Meanwhile many utilities waiting for their new nuclear plants to be licensed and built will be faced with a generating capacity gap during the next 8 years. Many are therefore considering building interim plants with a low capital cost and rapid construction times, characteristics of the CCGT. Building a CCGT and combining it with an existing nuclear power plant can provide a rapid method for increasing power generation capacity with exceptionally high thermal efficiency making it far more profitable than stand-alone CCGTs. The necessary connections to the nuclear steam cycle could be made during the statutory outages on the nuclear plant so minimising any disruption and cost.
Margin for Capacity Growth
A further key advantage for the NuGas™ concept arises when enhancements of fuel design and improved confidence in the plant design increase operating margins such that the reactor thermal capacity can be raised. Up to a limited ceiling this extra output can be handled by the existing plant, but beyond this limit costly power generation upgrades become necessary. Since the NuGas™ cycle increases steam utilisation capability by at least 10 percent, the additional reactor output can be converted to power without expenditure on costly upgrades, making the inclusion of the NuGas™ cycle, either as a new build or retrofit, even more attractive financially.
Conclusion
By recognising the fundamentals of cycle efficiency to drive down carbon emissions, it has been possible to develop a novel concept which brings together the best aspects of both nuclear and gas-fired power generating technologies. The concept is now being developed with utilities and plant vendors and is targeted to go into service before 2011.
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