| The use of Organic Rankine Cycle(ORC)for low to medium temperature of solar thermal power generation technology has advantages in small-scale solar thermal power stations.However,its application are limited due to intermittent solar radiation and low efficiency of ORC.Based on these issues,this study has focused on improving the steadiness and generation efficiency of solar thermal power generation system.The research work carried out in this study includes the following: 1)Study on heat storage tank: a research study on CSP project in Pu'er,Yunnan,designed a heat storage system(heating power 100 kW,supply/return temperature 200°C/100°C,heating time 6h),and used FLUENT as the simulation platform,effect of thickness on insulation board and insulation layer are analyzed;2)Study on thermal efficiency of ORC: For ORC system,working fluid is selected and mathematical model for system is established,EES is used for simulation study of conventional ORC system,extraction regenerative ORC systems,and internal regenerative ORC systems.Effect of various parameters on the generation efficiency of the ORC system are analyzed for different conditions of cycle.The main results of this paper are as follows: 1.Design of Heat storage tank: Based on a comparison of single-tank and double-tank technologies,this study has proposed a single-tank dual-chamber heat storage system using a cylindrical single tank in vertical position with insulation board.The heat storage tank use T55 oil,with diameter of 2.6 m and height 3.6 m,insulation board using foam glass of thickness 0.15 m,the gap between insulation board and tank should be less than 2.6mm.2.Performance study of Heat storage tank: According to the results obtained form simulation,temperature difference within fluid is less than 2°C in the space above 0.2 m in the process of heat dissipation,and inlet for suction of hot fluid shall be placed in the upper 1/3 area.In addition,the thickness of the insulation board has minor effect on the performance of the heat storage tank while insulation layer on sidewall has greater influence.The recommended thickness of the heat storage tank for this design is 0.2 m.3.Study of conventional ORC system: organic working fluid R123 is taken,the degree of superheat increased from 0°C to 35°C at the evaporator outlet,and the generation efficiency only increased by 1.6%.However,the specific volume increased by 32.8%,so it is suggested that degree of superheat shall be minimized and evaporation pressure shall be increased in order to improve the system efficiency,i.e.,evaporation pressure increased from 1.5 MPa to 3.5 MPa,and the generation efficiency increased by 19.1%.4.Study of Extraction regenerative ORC system: The generation efficiency is related to the extraction pressure,so when the extraction pressure is kept constant,both the extraction coefficient and efficiency reaches its critical value.Increasing extraction pressure,the efficiency will increase first and then decrease.For example,at the evaporating pressure of 2.8 MPa,the efficiency increases first and then decreases with increasing of extraction pressure,like extraction pressure is 0.7 MPa,the generation efficiency reaches the maximum,which is increased by 13.5% compared to the conventional cycle,and then gradually decreases,when it reaches to 2.0MPa the efficiency is less than the conventional cycle.5.Study of Internal regenerative ORC system: For dry working fluid in ORC system,the generation efficiency increases with the increase of regenerative coefficient;similarly overheating can also improve generation efficiency,as the regenerative coefficient is proportional to effect of overheating.For example,under the evaporating pressure of 2.8 MPa,the regenerative coefficient changes from 0 to 1,the efficiency increases by 15.1%.When the regenerative coefficient is at 0.6,the degree of superheat changes from 0°C to 35°C,the efficiency increases by 10.4%.This study has proposed and simulated a single-tank dual-chamber heat storage tank design;discovered that extraction regenerative system has the potential to increase generation efficiency by 13.5% and internal regenerative heat has the potential to increase generation efficiency by 18.0%.The research in this study has provided a guidance and significance for the design of small solar thermal power stations. |
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