Physicochemical Characteristics of Accumulated Dust on Mirror Surface and Its Influence on Fresnel High Magnification Focused Photovoltaic/Thermal System
Objective Fresnel high magnification focused photovoltaic/thermal system has the advantages of a high concentration ratio,small footprint,low lens processing cost,etc.,and it has a broad application prospect in the field of solar electric and thermal utilization,but the accumulation of dust particles on its mirrors leads to the reduction of the system's thermal and electrical efficiency.The current study focuses on two points.One is the effect of the settling law of the accumulated dust particles and their particle size distribution on the concentrating effect,and the other is the effect of the dust density on the mirror surface on the electrical and thermal output characteristics of Fresnel high magnification focused photovoltaic/thermal system.Accumulated dust affects the Fresnel lens transmittance,which in turn weakens and disperses the distribution of concentrating solar energy flow on the surface of photovoltaic cells and reduces their power output.Therefore,it is necessary to effectively remove the accumulated dust.Specific strategies should be adopted for the physical and chemical properties of the accumulated dust,such as the inclination angle and wind speed of the wind blade in wind power dedusting,as well as the configuration and selection of cleaning agents in water jet cleaning.There are many different types of dust on the mirror surface,and it is necessary to prioritize the removal of the accumulated dust that has the greatest impact on the electrical energy output.In view of the geographical characteristics and the influence of anthropogenic activities and the composition of naturally accumulated dust particles at the test site,in this study,six kinds of particle samples such as calcite,albite,marble,loess,coke,and coal gangue are selected as the research objects,so as to analyze the correlation of the influence law of the physicochemical nature of the accumulated dust particles on the Fresnel high magnification focused photovoltaic/thermal system,provide a theoretical basis for the prediction of the law of dust accumulation in a specific region and the electrothermal output characteristics of the Fresnel high magnification focused photovoltaic/thermal system after dust accumulation,and direct the cleaning of the dust on the mirror surface of the Fresnel concentrator.Methods The research is mainly carried out by experimental methods.First,an X-ray diffraction analyzer,energy spectrometer,and scanning electron microscope are used to study the particle morphology,material,and elemental composition.Second,the powder is uniformly arranged on the surface of the lens by means of artificial dusting,and the density of accumulated dust is 1,3,and 5 g/m2.The output characteristic test of the Fresnel high magnification focused photovoltaic/thermal system is carried out to analyze the influence of different accumulated dusts on the electrothermal output.Finally,the grey correlation method is used to process the experimental data and analyze the influence of the physicochemical properties of the accumulated dust on the comprehensive performance of the Fresnel high magnification focused photovoltaic/thermal system.Results and Discussions From the material composition,the main component of coal gangue is ferrous oxide,with a content of 56.30％.The main component of albite is calcareous albite,with a content of 42.38％.The main components of calcite and marble are magnesium calcium carbonate,and the contents are 94.55％and 89.12％respectively.The main component of coke is silicate,with a content of 46.08％.The main component of loess is silica,with a content of 51.48％(Fig.4).In view of the element composition,calcite and marble are the same,mainly composed of oxygen elements.Albite is mainly composed of oxygen and silicon.Coal gangue is mainly composed of oxygen,calcium,and silicon.The carbon content in coke is the highest,about 80％.The main components of loess are oxygen and silicon(Fig.5).Coal gangue and coke accumulation of dust particles have a great influence on the electric power of Fresnel high magnification focused photovoltaic/thermal system.When the dust accumulation density is 1,3,and 5 g/m2,the corresponding electrical efficiency of the coal gangue decreases by about 26.7,30.1,and 33.9 percentage points,and that of the coke decreases by about 29.3,30.1,and 33.1 percentage points compared with the clean state(Fig.7).Compared with the cleaning system,the thermal efficiency of coal gangue and loess dust particles decreases significantly.When the dust density is 1,3,and 5 g/m2,the corresponding thermal efficiency of coal gangue decreases by about 17.0,30.6,and 42.2 percentage points,while that of loess decreases by about 19.9,30.1,and 42.4 percentage points(Fig.9).The comprehensive correlation degree of CaMg(CO3)2,CaCO3,SiO2,and Fe2O3 with electrical power exceeds 0.68,and thermal power exceeds 0.62,showing a strong correlation(Fig.10).The correlation degrees of particle elements O,Al,Ca,and Mg with the electrical and thermal power of the Fresnel high magnification focused photovoltaic/thermal system all exceed 0.71,showing a strong correlation(Fig.11).Conclusions From the perspectives of particle morphological features,material composition,element composition,and proportion,the physicochemical properties of six typical dust particles on the concentrator mirror are investigated,and it is found that the shapes of different types of dust particles are different;the material composition of the dust particles is complex,and the types of element composition are more varied.The rocky dust particles have more dolomite,albite,and calcium carbonate.The highest content of elemental O is found in the particles,except for the coke.Mg,Si,and Ca elements also appear more frequently.A comparison test of Fresnel high magnification focused photovoltaic/thermal system shows that all kinds of accumulated dust have different impacts on the thermoelectric output,in which the coal gangue in the mirror surface dust increases by 1 g/m2,and the gangue dust particles corresponding to the comprehensive performance of the Fresnel high magnification focused photovoltaic/thermal system decreases by 15％.All kinds of accumulated dust have the most serious impacts on the system's electro-thermal performance.The composition of the material affects the electrical power(E)and thermal efficiency(T)of the Fresnel high magnification focused photovoltaic/thermal system in the following order:ESiO2＞EFe2O3＞ECaCO3＞ECaMg(CO3)2and TSiO2＞TCaCO3＞TFe2O3＞TCaMg(CO3)2,and the elemental composition has the same effect on the electrical and thermal outputs(P),with the following order:PO＞PAl＞PCa＞PMg＞PFe＞PSi＞PC.The results of the study provide a reference for predicting the electrothermal performance of the Fresnel high magnification focused photovoltaic/thermal system in a specific area of dust accumulation and provide a basis for the de-dusting of the Fresnel condenser mirror surface.
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