| The increased penetration of intermittent renewable energy,especially wind power and solar photovoltaic,challenges the grid stability and requires an enhanced grid operating flexibility.Due to the uncontrollable nature of renewable power and the shortage of electricity storage facilities and flexible energy sources,the supply-demand balance of the grid is hard to keep in real time and significant curtailment of renewable power has happened,leading to large-scale energy waste and financial loss.In the countries treating coal as the main energy source,like China and Germany,the operational flexibility of the coal-based power generation is required to be improved for the increasing demand for renewable power accommodation.This paper focuses on the system reconfiguration toward flexible operation and its effect on the energy performance of the power generation system.Based on the advanced exergy analysis,it is revealed that the thermodynamic mechanism of the effect on the system performance attributed to the generalized system reconfiguration toward improving operational flexibility.On this basis,the steam regulation on the turbine side and the combustion enhancement on the boiler side are chosen as the main approaches for flexibility enhancement.And several high-efficient energy storage schemes with a low efficient penalty and the operational strategies thereof are proposed and evaluated.Considering the technical constraint of flexible operation,all the technical approaches for flexibility enhancement can be generalized into two series:(1)the retrofit on the components constraining flexible operation,and(2)energy storage.Based on the advanced exergy analysis,the energy interaction between these system reconfigurations and the power plant system is revealed,and the main sub-procedures in the reconfiguration that affects the system performance are located.This research will support the conceptual design of specific technical routes for flexibility enhancement.On this basis,the system reconfiguration approaches taking advantage of the thermal inertia of the district heating network,the thermal energy storage potential of fuel,and combustion enhancement are comprehensively investigated.The energy transfer and conversion characteristics of the integrated combined heat and power plant and district heating system is investigated with a comprehensive thermodynamic model.The thermal capacity of the heat user system provides inertia in the dynamic balance between the heat supply from the plant and the heat dissipation to the environment,which is the reason why the district heating network can act as a thermal energy storage facility.The sensitivity distinction to steam regulation between power generation and district heating is revealed quantificationally,which is the decisive issue for district heating networks using for operational flexibility enhancement.The results in the case study indicated that the steam regulation can significantly improve the ramp rate of the power plant(4.5 MW/min of the conventional power plant to up to 10.9 MW/min)almost without negative influence on the indoor temperature of the heat users.Setting the variation range of steam extraction properly and improving the speed of steam extraction is beneficial to further improving the ramp rate of the power plant.An energy level upgraded thermal energy storage concept based on low-rank coal drying(LD-TES)is proposed for reducing the minimum load of power plants fueled by low-rank coal.A simple experiment was employed to verify the feasibility of energy storage through LRC drying.Compared with traditional low-temperature TES,the energy level upgrading nature of LD-TES leads to significant thermodynamic advantage.The results show that the resorption in the storage process will not negatively influence the stored thermal energy.For the case 300 MW power plant,10 MW level of electricity storage can be achieved by LD-TES with high round-trip efficiency(92.8%).The carbon emission rate of the power plant can also be significantly reduced by LD-TES(128 t and 77 t of daily carbon emission reduced compared with the original plant and the hot water tank scheme)due to the energy conservation effect.The analysis of carbon emission also shows that,accommodated renewable power should not be the only criterion for improving operational flexibility of power plants due to the non-negligible impact of the system efficiency.A concept of coupling solid-oxide cell stack based power-to-gas with coal power plants is proposed to allow for dual functions of(1)storing excess renewable electricity and(2)reducing the minimum load of coal power plants by combustion stabilization with oxygen-rich air from power-to-gas.The performance and operating strategy of the integrated concept are evaluated with detailed off-design characteristics of the considered case.The results show that the integration of power-to-gas affects the distribution of the heat absorbed by radiative and convective heat exchangers in the boiler,stabilizes coal combustion,and reduces the superheat degree of live/reheated steam.It allows the power plant for operating at a significantly low load of down to 22%of the nominal load,compared with 40%before the coupling;meanwhile,a very limited penalty is caused with the plant efficiency reduced from 34.4%down to 34.1%(with 13%of the normalized power-to-gas capacity).Minimizing the power-to-gas contribution to the accommodated renewable power is advantageous for a minimal carbon emission;nevertheless,maximizing the power-to-gas contribution with the coal power plant at high load allows for a maximal system efficiency. |
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