Study On Combined Heat And Power Economic Dispatch For Natural Gas Combined Cycle Plants

The natural gas combined cycle(NGCC)plants operated in combined heat and power(CHP)mode have the advantages of high efficiency,flexibility,reliability,etc.These features qualify NGCC plants to make up for the shortcomings of intermittent renewable energy,with the result that NGCC plants have played an important role in the current and future world energy market.However,NGCC plants also bring significant challenges to combined heat and power economic dispatch(CHPED)problems in the view of their complicated operational characteristics,which limits the full utilization of their advantages.In this thesis,the study on CHPED models for NGCC plants has been performed with aims of improving the accuracy and feasibility of calculation results.A CHP generation station composed of a 1×1 and a 2×1 NGCC plants is taken as the specific study object.The main contents of this thesis are as follows:(1)Thermodynamic models of 1×1 and 2×1 NGCC plants have been established with consideration on the variation of the characteristic parameters of facilities under off-design conditions.On the basis of the thermodynamic models,the simulation of the CHP operation process of NGCC plants is carried out.The off-design performance of NGCC plants under various modes is studied comprehensively and systematically.Besides,the formation mechanism of the plants’ performance variation is also analyzed in depth.Calculation results show that the off-design performances of the 1×1 plant and the 2×1 plant show a similar variation pattern.In terms of the feasible operating zone of the NGCC plants,with the increment of the ambient temperature,the maximum power load and the maximum heat load both rise firstly and then decline.The maximum power load reaches its highest value at the ambient temperature of 5°C approximately.The maximum heat load reaches its highest value at the ambient temperature of 10°C approximately.In terms of the energy utilization efficiency of the NGCC plants,as the power load increases,the efficiency rises at low heat load,while declines at high heat load.As the heat load increases,the efficiency rises continuously.Under the condition of 100% gas turbine output,the efficiency rises firstly and then declines with the increment of the ambient temperature,and reaches the maximum at the ambient temperature of15°C approximately.Under the condition of the same power load and heat load,the efficiency rises continuously with the increment of the ambient temperature.(2)A calculation strategy for the fuel consumption of NGCC plants applicable to CHPED models has been presented.Then an integrated fuel consumption model of NGCC plants is developed based on the actual operational status showed from field data and the off-design performance variation characteristics calculated from thermodynamic models.With the help of the static CHPED model,the applicability of each operating mode portfolio of the two NGCC plants is studied,from the view of feasible operating zone and fuel consumption of the whole CHP generation station.There are 4 operating mode portfolios for the two NGCC plants,when the 2×1 plant is operated with both gas turbines.Calculation results show that the portfolio of both plants operated in extraction-condensing mode is applicable for the demand scenarios with large fluctuations,and the demand scenarios with low heat demands of the station.At the demand scenarios with high heat demands,the 1×1 plant can be switched to backpressure mode.In that portfolio less fuel consumption of the station is achieved with good loads variation capacity.The station loads variation capacity of the other 2 operating mode portfolios is relatively small,which leads to poor applicability to the energy market with large amounts of intermittent renewable energy connected to power grid.(3)A combined heat and power real-time economic dispatch(CHPRED)model has been proposed,which aims to perform optimization for the real-time operation process of NGCC plants.The CHPRED model is improved from the static CHPED model by taking into account the requirement on load regulation quality from the power grid.Based on the power and heat loads control logic in the field operation process,the loads variation process of NGCC plants is modelled.The calculation method of the regulation time of power demands under the influence of the heat load variation process is further developed.Then,the regulation time model of each plant is integrated into the CHPRED model to improve the feasibility of the economic dispatch calculation results.Case studies prove that the CHPRED model can enhance the economic performance of NGCC plants,on the premise of guaranteeing the feasibility of calculation results,which overcomes the defect of the static CHPED model.NGCC plants are simulated to operate with the demands calculated from the CHPRED model.All the power demands are balanced with actual loads within the regulation time limits,and the actual heat loads follow the change of heat demands in time.There is no unfeasible economic dispatch calculation result.In comparison with the dispatch strategy according to nominal loads,the CHPRED model saves the fuel consumption by up to 0.38% on a single demand,and 0.15% totally during the test period.(4)A novel combined heat and power optimal scheduling(CHPOS)model applicable to NGCC plants has been proposed.In the CHPOS model the operating mode switch rules and loads variation capacity constraints of NGCC plants are considered in detail.The start-up rules of gas turbines and the heating mode switch rules of steam turbines are formulated.The calculation method of the loads variation capacity of NGCC plants under various operating mode is deduced.Moreover,a CHPOS model for the NGCC station equipped with thermal energy storage(TES)facilities is developed.The model aims to optimally schedule the demands of NGCC plants and TES facilities in the future energy market with large amounts of intermittent renewable energy connected to power grid.Case studies prove that the CHPOS model can properly schedule the operating mode switch and heat and power demands of NGCC plants.The CHPOS model can provide advice on operating behavior for the station operators,on the premise of satisfying constraints of operational characteristics of NGCC plants and demands balance of the station.The CHPOS model saves the fuel consumption by 2.28% over the plants operated in single operating mode portfolio.Based on the operational characteristics of TES facilities,the CHPOS model can properly schedule the demands of NGCC plants and TES facilities,so that the conflict between heat and power generation process of the NGCC station in the future energy market is resolved.The CHPOS model achieves the improvement on operational flexibility of NGCC plants while guarantees the operational safety and economic performance.(5)A combined heat and power performance optimization system(CHPPOS)that can be applied in the field operation process of NGCC plants is developed.The system structure,functions design and field deployment process of the CHPPOS are introduced.The CHPPOS field implementation case on the basis of the existing supervisory information system of the station is presented.