Research On Integration Mechanism And Operation Characteristics Of Solar Tower And Parabolic Trough Aided Coal-fired Power Generation System

In the current energy structure of our country,coal-fired power generation is still the main source of electricity,which leads to problems such as pollution and greenhouse gas emissions.The target of reaching the peak of carbon emissions in 2030 puts forward new requirements for China's clean,low-carbon,safe,efficient utilization of energy and further optimization of energy structure.At present,China's coal-fired generating units are mainly large-capacity and high-parameter units,with various indicators at the world's advanced level.The emission level of pollutants has been relatively low after ultra-low emission transformation.The potential is limited to achieve energy conservation and emission reduction from the viewpoint of inside the units.It is an effective way to further reduce coal consumption,carbon emissions and increase the proportion of renewable energy by coupling solar thermal resources with coal-fired generating units.At the same time,the cost of solar energy utilization can be reduced and the efficiency of solar power generation can be improved by sharing high-efficiency power generation equipment.In this paper,the solar tower and parabolic trough aided coal-fired power generation system are taken as the research object and the coupling mechanism and operation characteristics of the integrated system are explored.The main research progress and conclusions are as follows:Firstly,the models of the heliostat field,the tower and the receiver,parabolic troughs,thermal energy storage system and power block are established according to the composition of a typical solar tower and parabolic trough aided coal-fired power generation system.The calculation of heliostat field layout and efficiency are verified according to the parameters in the literature,as the basis for the subsequent research.Secondly,the thermodynamic characteristics of solar-thermal conversion and transfer process in four different coupling methods of solar tower and parabolic trough aided coal-fired power generation system are analyzed.Compared to the alone tower or parabolic trough aided coal-fired power generation,the absorbed solar energy volume and solar energy share under the safe operation constraints can be significantly improved by reasonably coupling solar tower and parabolic troughs together with coal-fired power generation system.The exergic loss caused by temperature difference in heat exchange process can be reduced by using parabolic troughs and solar tower in series to collect solar thermal energy,and the energy utilization can be improved by utilizing the energy at different temperature grades.At the same investment cost for solar field,in the coupling scheme of parabolic trough and solar tower system collecting solar heat energy in series,heating molten salt in different sections,and high-temperature molten salt exothermic at different temperatures,the coal consumption rate is reduced by 1.58?4.21g/kWh and the solar exergy available for the system is improved by 6.67?10.62%compared to the single solar tower or parabolic trough aided coal-fired power generation system.Thirdly,the synchronous influence mechanism of two introduced solar energy on the thermodynamic process of the coupled system is studied.The maximum coupling of two strands of solar energy under different load conditions and the thermodynamic properties such as system efficiency,coal consumption rate and solar power efficiency under different load conditions are investigated.The daily operation characteristics of the integrated system under different irradiation and load conditions are observed.Taking a 600MWe supercritical unit as an example,with the feedwater extraction percentage from the first-stage high-pressure heater increasing from 0 to 100%,the maximum proportion of reheated steam extraction in the safe range increases from 35.0%to 42.4%,from 33.6%to 38.3%,and from 34.7%to 38.0%under 100%,75%and 50%loads,respectively.The maximum solar energy absorbed by reheated steam is increased by 24.2MWth(28.5%),11.4MWth(19.9%)and 5.5MWth(14.1%),respectively.As the result,the standard coal saving rate of the system is increased by 13.2g/kWh,10.6g/kWh and 9.0g/kWh,respectively.Then,the model predictive control method is established to study the optimal operation control method and operation characteristics of the integrated system under the influence of solar radiation and electrical load fluctuation.The optimal operation control of the integrated system in the typical day and consecutive 10 days are explored taking the minimum cumulative coal consumption as the objective function.The predicted weather information and the power load demand combined with the safety constraints of operation parameters are considered in the optimization process.The advantages and realization methods of the predictive control method are analyzed compared with the conventional standard control method.The influences of the prediction error,thermal energy storage capacity and solar field scale on the applicability of the control method are analyzed.Compared with the original coal-fired power station,the average coal consumption rate of the integrated system in typical day is reduced by 13.4g/kWh using the standard control method,saving 156.6 tons of coal,The average coal consumption rate is further reduced by 1.8g/kWh(13.6%)under the model predictive control method compared with the standard control method,with 21.3 tons of coal further saved.During 10 consecutive days of operation,the average coal consumption rate of integrated system is reduced by 14.8g/kWh under model predictive control method,with 1 898.4 tons of coal saved,which is 20.3%higher than that under the standard control method.Through the observation of optimized behavior,the model predictive control method is found to improve the global performance of the coupled systems mainly in the following aspects compared with standard control:(?)In the sufficient solar irradiation condition.the mass flow of middle-temperature molten salt in the parabolic troughs is restricted to reserve some cold salt for high-temperature cycle,rather than maximizing the solar energy input at each moment.(?)Instead of releasing the most solar energy into the coupled system at each moment,the stored heat energy is release gradually at the appropriate time.Considering the prediction error of 5%-20%,the coal saving of the coupled system decreases by 1.07%-1.93%in 10 days with model predictive control method applicated.This study provides a reference for the operation of the integrated energy system with multiple energy sourcesFinally,the generalized model,integration method and the thermodynamic performance distribution of the multi-heat source integrated system are studied.The internal and external irreversibility of the system are considered with finite-time constraints.The dimensionless parameter is used to express the integrated method of multi-heat source coupled system at the maximum output power.The distribution map of output power and thermal efficiency of the integrated system with two heat sources are investigated.Results show that the power and efficiency of the system show different trends in the regions around the maximum power output point.The study on the influences of coupled heat source temperature shows that the efficiency of the case system decreases by a maximum of 2.25%and the share of energy from the first heat source decreases by more than 10.40%with the introduction of coupled heat source and its temperature rising.With the increase of external irreversibility,the maximum output power of the system decreases and the energy demand for the first heat source increases.In the case of higher temperature of the second heat source,the system performance is more sensitive to external irreversibility,while there is little difference in the sensitive to internal irreversibility.The higher internal irreversibility results in the increase of the second heat absorption temperature at the maximum power and the narrowing of the optimal coupling range of heat source temperature,with the decreasing of the output power and efficiency of the system.Four specific cases study of solar aided coal-fired power generation system show that the coupling temperature of solar energy at maximum power is 265.56? and 253.76? in the two solar tower aided coal-fired power generation systems,respectively,meaning that the coupling temperature is above the highest-pressure feedwater preheater considering both output power and efficiency.The coupling temperature of solar energy is 221.80? and 221.70? at maximum power in the two parabolic trough aided coal-fired power generation systems,respectively,meaning that the coupling temperature is above the second high-pressure feedwater preheater considering both output power and efficiency.This study provides a reference for the integration of complex energy systems with multiple heat sources.