Innovation in Global Power - Parsons Brinckerhoff

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Thermal – Achieving New Efficiencies, Reducing Carbon Emissions http://www.pbworld.com/news_events/publications/network/ The NuGas TM Concept: Combining a Nuclear Power Plant with a Gas-Fired Plant By Paul Willson, Manchester, UK, 44 161 200 5210 willsonPa@pbworld.com; and Alistair Smith, 44 161 200 5114, smithAlistair@pbworld.com Nuclear power is experiencing renewed interest around the world because of its low carbon emissions and affordability. As with other thermal generation technologies, however, its thermal efficiency is limited. PB has developed a new concept that combines current nuclear technology with combined cycle gas turbine technology to achieve unprecedented levels of thermal efficiency. The authors explain how it works and how it can be implemented in new installations or in retrofitting existing nuclear stations. PB’s power specialists in the UK have developed and patented a completely new concept for high-efficiency electricity generation. This ground-breaking development, called NuGas TM , combines the advantages of nuclear power generation with a smaller combined cycle gas turbine (CCGT)-based power technology to create a low-cost, highly reliable hybrid system that: • Increases output and thermal efficiencies to levels that are far higher than even the most ambitious forecasts • Achieves a simple, safe and effective interface between the cycles. Improved performance comes from better use of heat in the steam cycles of the CCGT and nuclear plant where currently unavoidable large temperature differences prevent the maximum work being obtained from the heat. By linking a CCGT with the low-temperature steam cycle typical of a nuclear power plant, these temperature differences can be reduced significantly, releasing additional power output without going outside conventional design conditions. Because NuGas TM enhances thermodynamic cycle design rather than changing operating conditions to improve efficiency, it introduces no new technology risks in its implementation. This is a significant advantage over the more complex and unproven technologies being introduced for new gas turbine designs as engineers pursue ever higher temperature operation. NuGas TM can be used either for retrofitting existing nuclear stations or for new-build installations. While the new-build design allows for maximum optimization, 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. Improving Thermal Efficiency When analyzing 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 the latest pressurized water reactors (PWRs) is just 37 percent. Even if there were no losses in the system, the maximum Ideal Efficiency would still be well below 100 percent. For a PWR operating at an upper steam temperature of 540°F (280°C), the maximum possible efficiency would be just 45 percent. The way to push the Ideal Efficiency up is to increase the upper temperature in the cycle, which is why gas-cooled high temperature reactors are again being considered. Temperatures have been pushed up also in fossil fuelled power plants, and the most modern coal-fired super-critical boilers can achieve a thermal efficiency of 44 percent. This level is now being exceeded when two cycles are combined, such as the CCGT, where temperatures are around 2300°F (1200°C) and a thermal efficiency of 57 percent can now be achieved. The desire to raise system efficiencies beyond current levels has proven challenging, however. Despite considerable investment in research and development, it appears that significant incremental improvements are becoming more expensive and harder to achieve without sacrificing reliability. By combining current nuclear and CCGT technologies. NuGas TM raises thermal efficiencies to unprecedented levels. Under this concept, the two separate power generation systems can operate in tandem as a single combined unit on the same site. In the case of breakdown or planned maintenance, either the nuclear plant or the gas turbine-powered unit can revert to independent operation, thereby maximizing availability of power and minimizing upset to the power networks. PB Network #68 / August 2008 8
Thermal – Achieving New Efficiencies, Reducing Carbon Emissions http://www.pbworld.com/news_events/publications/network/ The basic concept would allow a large nuclear power plant with a typical output of 800 to 1700 MWe to be combined with a 300 MW CCGT generating unit. Linking the steamcycles of the two plants enables them to operate as an integrated power production unit and reduces losses of potential output, increasing total efficiency. Cycle efficiency gains enable the CCGT to contribute an increased output for no additional fuel, with the efficiency of converting the energy in the gas to electricity increased to about 62 percent. How NuGas TM Works Although a CCGT system has a high thermal efficiency, it relies on using the heat from the exhaust gases of the gas turbine to boil water to produce steam that drives the turbine. As the exhaust gases cool, water is evaporated in the boiler tubes but temperature differences of up to 400ºF (200ºC) arise in the boiler due to the large amount of heat needed to evaporate the water. These temperature differences limit the potential work that can be extracted from the steam, reducing the output of the steam cycle. The NuGas TM cycle overcomes this limitation by ‘borrowing’ a small proportion (typically 10 percent) of the steam from the nuclear steam cycle (point ‘A’ on Figure 1). 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 rather than boiling water enables a much lower temperature difference to be maintained in the heat recovery system, maximizing the value of the energy recovered. The heat in the gas turbine exhaust flow between about 570ºF and 320ºF (300ºC and 160ºC) is recovered via a high temperature economizer (‘C’) to generate high temperature Figure 1: Schematic Combination of the Steam Cycles. feedwater, which is returned to the nuclear cycle (‘D’), ensuring that the inlet temperature to the steam generator is maintained close to the design value. The heat in the gas turbine exhaust below about 320ºF (160ºC) (‘E’) is used to heat part of the condensate from the high temperature steam turbine (‘F’) before it is deaerated and returned to the nuclear cycle feed pumps (‘G’). The remaining condensate from the high temperature steam turbine is returned to the nuclear cycle condensate system (‘H’). The flows of energy around the cycle differ somewhat to those in a conventional CCGT. Figure 2 shows a simplified Sankey diagram for the NuGas TM cycle, including the energy exchanges between the CCGT and PWR cycles shown along the lower edge of the diagram. Identifying the separate performance of the CCGT cycle when it is linked to the PWR cycle requires that the design PWR energy balance be maintained. Thus, the CCGT returns power to the PWR to compensate for the reduction in output due to the ‘borrowed’ steam and returns rejected heat in the CCGT cooling water to the PWR to account for the reduced heat rejection from the nuclear turbine condenser. The diagram therefore shows the additional energy input, the additional losses and the additional power generated by the cycle, demonstrating its high efficiency. Safety Considerations Downstream failure is limited. 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 percent step change in flow to the main steam turbine without exceeding the appropriate limits for a frequent operating condition. It is likely, nevertheless, Figure 2: Simplified Sankey Diagram for NuGas Cycle. 9 PB Network #68 / August 2008
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