Research On Solar Power Tower Plant Performance Based On The Dual-receiver

Recently,with the rapid development and promotion of renewable energy technology,direct steam generation(DSG)solar thermal power tower generation technology attracts attention in our country owing to its low cost and high efficiency.To solve the problems of complex heliostat field concentrating strategy and long start-up time of DSG solar power tower plant,a dual-receiver that is composed of a external receiver(boiling section)and a cavi ty receiver(superheating section)is proposed based on the heat transfer characteristics of boiling and superheating process.Researches on plant system integration and thermal economic simulation are conducted based on the novel dual-receiver.Based on the structure of the dual-receiver,concentrating models of the heliostat field are built to analyze its performance.The optimal heliostat field design scheme of the 10 MWe DSG solar power tower plant in Sevilla,Spain is obtained by distribution and iterative calculation of the concentrating heat flux.The global heliostat field efficiency is 72.17% and the number of heliostats that focus on the boiling section and the superheating section are respectively 442 and 182.The allocation scheme of the heliostat field can not only ensure the dual-receiver surface heat flux not exceed the safety range,but also provide the required heat of the boiling and superheating section.According to the calculation of the annual operation conditions,the heliostat field efficiency does not completely increase with solar altitude,but the maximum surface heat flux of the dual-receiver shall occur at summer solstice 12:00.Therefore,during the process of heliostat field and dual-receiver design,the dual-receiver surface heat flux of this time should be verified to ensure engineering safety.Thermodynamic models of the dual-receiver are built to reveal its operation performance.At design point,the dual-receiver thermal efficiency of the 10 MWe DSG solar power tower plant is 86.55%.When compared with the dual-external receiver,the dual-receiver thermal efficiency increases 3.2% and the corresponding power plant efficiency increases 0.88%.When varying the operation time,although the external receiver is used for the boiling section,the thermal efficiency of the boiling section is 5%-9% higher than the superheating section because of lower surface temperature.The dual-receiver outlet steam temperature remains by dynamic allocation of the number of heliostats that respectively focus on the boiling section and the superheating section.When varying the tube outer diameter,the thermal efficiency of the boiling section increases little by decreasing its tube outer diameter,because the main heat resistance is from the wall.However,the thermal efficiency of the superheating section increases largely by decreasing its tube outer diameter,because the major heat resistance comes from the internal convection.The tube outer diameter of the superheating section exists a optimal value by considering both the internal convection and parasitic loss.Based on the dual-receiver,system integration models of the DSG solar power tower plant are built for thermal economic assessment.In case that the rated electric power is fixed of 50 MWe and the land area is changeable,the lowest levelised cost of electric energy(LCOE)is 21.4c/k Whe when the solar multiple and storage hour are respectively 2.7 and 9h.If the site is altered and the annual direct normal irradiance(DNI)increases 55%,the annual electricity production increases 20.4%,the lowest LCOE decreases 30.1% and the optimal solar multiple decreases to 2.0.If the sub-system cost is altered,the sensitivity analysis results show that the influences of the heliostat field and dual-receiver costs are the largest.The optimal solar multiple and storage hour increase with decreasing costs of the heliostat field and thermal storage system.In case that the land area is fixed of 4.8km2 and the rated electric power is changeable,the lowest LCOE is 21.77c/k Whe when the solar multiple and storage hour are respectively 1.7 and 3h.If the site or the sub-system cost is altered,the conclusions are similar to the case of fixed rated electric power.Therefore,related conclusions of site and cost sensitivity analysis are general and affected little by restricted domain.If the fixed land area increases from 2.15km2 to 8.11km2,the lowest LCOE decreases from 24.53c/k Whe to 20.92c/k Whe,while the decreasing rate slows down.The optimal solar multiple and storage hour remains because of relatively stable trendency of annual electricity production and total investment cost.In order to decrease the DSG solar power tower plant storage system cost,a composite storage material of nano-salt with higher specific heat and suitable for large-scale engineering application is prepared by one-step method.When compared with the pure binary salt,the maximum enhancement of specific heat occurred at a Cu O nanoparticle concentration of 0.5wt%.For this case,the increments for the liquid phase is up to 11.48% and the melting point increases 3?.Therefore,the nano-salt is suitable to be the sensible storage material and should remain higher working temperature than melting point in avoid of freezing security issues.Based on the images of scanning electron microscope(SEM),it deduces that the specific heat enhancement shall be related to the needle-like semi-solid layer on the Cu O nanoparticle surface.The specific heat of the nano-salt with low concentration Cu O nanoparticles could be predicted exactly by assuming the semi-solid layer with a specific heat value and adopting the modified mixing model.