Energy Cascade Utilization And Flexibility Enhancement Of The Combined Heat And Power Unit

As one of the most effective way of the utilization of fossil fuel,combined heat and power generation(CHP)unit can notably improves the comprehensive energy utilization efficiency.The pollutant emission during the heat supply process is centralized controlled with the help of the co-generation of heat and power.However,the extraction steam is usually adopted to heat the circulating water in the traditional extraction condensing(EC)CHP plants,leading to a huge waste of the high-grade heat.On the other hand,the coupled power and heat productions of CHP plants are restricted by the feasible operation region,namely "heat-led" mode,which lower the peak-load regulation flexibility and limit the renewable energy accommodation.Thus,it is of importance in both academic and engineering applications to develop an energy-saving and flexible operation mode for large-scale CHP units under both designed and off-designed conditions.This paper gives detailed research on the heat and power co-generation process of the coal-fired CHP units,mainly from the following aspects:Fistly,through the fuel specific consumption analysis based on the second law of thermodynamics,the energy consumption distributions of CHP unit under different heating mode are revealed,providing an energy-saving trend in theoretical.After that,a district-level muti-energy heating system based on CHP units is constructed to improve its peak shaving capacity.Finally,the electric heat pump or thermal energy storage are integrated with the CHP units respectively.From these perspectives,research on both the energy cascade utilization and flexibility enhancement of CHP units are conducted.The lowest desirable unit consumption is obtained with the application of the fuel specific consumption analysis method to heating systems consisting of CHP units and district heating(DH)network.The additional fuel specific consumption of each subsystem during the entire heating season is further analyzed based on a practical CHP unit.It is concluded the heat transfer temperature difference is the main reason which induces the additional fuel specific consumption.A high back-pressure(HBP)cascade heating scheme for CHP plant is developed to reduce the irreversible temperature difference of the heating system through recovering the exhaust steam of the steam turbine for heating.The thermal performance of the cascade heating system,as well as the additional fuel specific consumption of each subsystem,are analyzed in detail under different operation conditions varying with the ambient temperature.Revealing the energy-saving mechanism of different heating modes from the perspective of energy conservation.Under the specific heating situation of the case region,the unit heating consumption of the cascade heating system ranges from 6.38 kg/GJ to 1 5.53 kg/GJ,which is 65%lower than the EC-CHP heating system,providing a theoretical foundation for on-site heating retrofit.In view of the typical power supply structure of the district heating area in northern China,a multi-energy complementary cogeneration and heating system from district level is constructed,which contains thermal power units,wind turbines and CHP units.Three kinds of heating-supply modes are introduced which are electrical-heating mode(EH),CHP unit with high back-pressure turbine heating mode(HBP-CHP)and auxiliary heat source heating mode(AHS),respectively.Then an optimized dispatch model of the heating system is established to coordinate heat and electricity in production and consumption with the optimization objective of reducing the total production costs and realizing the efficient accommodation of the wind power.Through the optimization of the scheduling system,the energy consumption and electro-thermal characteristics of the heating system under three kinds of heating-supply modes are obtained and compared.On this basis,the combined heating mode by combining the electric heat pump with the HBP-CHP heating mode is further proposed for the optimal system coal-saving effect and the better ability to accommodate the wind power.On the basis of the above-mentioned work,a novel HBP-CHP cascade heating system coupled with an electric heat pump is proposed to meet the demand of flexible peak-load regulation.The thermoelectric load characteristics of a typical HBP-CHP heating system and a novel HBP-CHP heating system integrated with an electric heat pump under different conditions are obtained.The technical and economic evaluation of the new system is carried out with reference to the actual heating demand.And the operation strategy of the novel system to minimize the wind abandonment is explored.The results show that the HBP-CHP heating system coupled with electric heat pump can reduce heating energy consumption and expand the peak-load adjustment range of the heating unit.The application of thermal energy storage is focused in the final part to further maintain a stable heat-supply.The possibility of decoupling heat and power generation of CHP units through thermal energy storage integrated with the DH network in a serial way is analyzed firstly with numerical study.The variation characteristics of the temperature and the inclined thermosphere in the single storage tank are obtained under the simultaneous storage and discharge operating condition.The dynamic thermal performance of the inclined layer heat storage tank is analyzed under two different operating modes,i.e.,single storage/releasing and simultaneous storage/releasing operation.The effects of different operating parameters,such as inlet flow rate and inlet temperature range,on the temperature distribution of the heat storage tank and the thickness of the slanted thermosphere are analyzed.The relative results can provide a reference for the application of inclined thermosphere thermal storage technology in CHP units.