Preface:
In the future, renewable energy will be the main power generation, but the randomness, intermittence and volatility of renewable energy itself make the security and stability of power grid and power supply system tested. Gas turbine has the advantages of small land occupation, good reliability, high efficiency, quick start and stop, etc., and will play a vital role in the transition stage of power grid.
At present, the combined cycle mode of gas turbine can increase the efficiency by about 20% compared with the simple cycle configuration. By adopting higher combustion temperature and improving component performance, the power generation efficiency of gas turbine can be significantly improved. Aiming at the development direction of gas turbine technology in the world, China's aviation gas turbine has conducted in-depth research to further improve the power generation efficiency of gas turbine, and compiled this paper, in order to trigger more beneficial thinking in the industry.
Thermodynamic cycle
Nowadays, most gas turbines in the world operate in a simple cycle, and only a few gas turbines use compressors for intermediate cooling, reheating (sequential combustion) or internal heat recovery through a regenerator. Simple cycle requires higher inlet temperature and higher component performance, and the efficiency of each generation of gas turbine will be slightly improved. However, to improve the efficiency, it is necessary to constantly improve the technology. In view of the high demand of thermodynamics and materials science, the improvement of these efficiency needs to be discussed economically. Based on this, there have been some voices calling in the industry-other cycles, such as hot Brayton cycle, will be considered for future combined cycle gas turbines to achieve similar efficiency without reaching such high temperature. It is necessary to further explore the influence of rated and off-design conditions on cycle performance, burner operation, cooling flow management, turbine thermal management and important flexibility.
Gas pipeline design
At present, the gas path design of compressor and turbine in gas turbine has achieved the refinement level of core isentropic flow (the flow away from annulus). The highly three-dimensional blades obtained by multi-objective optimization are now widely used, resulting in unprecedented aerodynamic efficiency. Further improvement of internal efficiency will be moderate, especially in the compressor.
Nevertheless, some secondary flows can further improve the efficiency. Turbine clearance control is one of them. It is estimated that it is possible to achieve a combined cycle efficiency gain of 0.25% through active clearance control system. Active clearance control system can be used to upgrade new gas turbines and existing devices; Some of them depend on the axial displacement of the rotor, while others work radially. No matter what method is adopted, it is faced with the challenge of engine transient temperature distribution. These challenges are affected by load changes, and start or stop more frequently. The future system needs to be able to further reduce the leakage flow at the compressor end and avoid the physical contact between the rotating parts and the stationary parts.
Cooling system
About 20% of the compressor flow is discharged from the gas path to cool and seal the (high pressure) hot part of the engine. Most of them are used to cool the first-stage turbine blades. At the root of the gas path in the high pressure section of the turbine, inhaling hot gas may also lead to mechanical failure and aerodynamic loss. When the high-stress parts of gas turbine (such as rotor disk) are overheated by the hot gas sucked from the gas path, mechanical failure may be triggered. The rim seal usually exhausts cooling/sealing air from the compressor together with the internal seal to prevent it from flowing into the cavity, but this will also reduce the efficiency of the gas turbine. The air discharged from the compressor will reduce the thermal efficiency, and more importantly, the interaction between the outlet and the core flow in the gas path will lead to further power loss. These phenomena are also affected by transient operation, because it will change the pressure and temperature distribution of all relevant flows, as well as the tolerance of sealing elements.
Therefore, the improved design of secondary gas path, multi-objective topology optimization and active control of cooling flow are areas that need further research to further improve the performance of existing and new gas turbines.
Bottom circulation
Modern heat recovery steam generator generates steam at pressure level and combines it. At present, it can recover as much energy as possible when it is technically feasible. Limited by the lowest flue temperature, this will cause condensation in flue gas flow. From the perspective of the second law (i.e., exergy damage), supercritical high-pressure evaporator can reduce this irreversibility, but the related cost may not be compensated by marginal performance enhancement (it is estimated that the combined cycle efficiency point of the most advanced technology is 0.5 percentage point).
Multi-pressure heat recovery steam generator (HRSG) attracts the most attention at the low exhaust temperature of gas turbine. When the pressure decreases from single pressure to multi-pressure, the performance increases with the temperature, and when the hot gas temperature (HRSG inlet) is about 700℃, the difference between the two layouts disappears. With the increase of gas turbine exhaust temperature (now over 650℃) and the economic cost of combined cycle power plants that need to operate with lower capacity coefficient, it may be possible to adopt subcritical single-pressure reheat bottom cycle.
Additional information
In the next few decades, the efficient power generation of combined cycle power plants will depend on the partial load efficiency rather than the rated operating efficiency. Therefore, improving part-load performance and transient response (transitioning to higher load as quickly as possible to reduce running time under low load) will be crucial to the flexibility of operation and the interests of power grid. The same applies to reducing the minimum environmental load of gas turbine, thus reducing the number of start-ups and shut-downs, and reducing the related fuel consumption, prolonging the service life and reducing emissions. All this is related to improving the overall efficiency of combined cycle and simple cycle gas turbines.
