Research On Photothermal Water Evaporation Based On Biomass Carbon Materials

Currently,the freshwater resources of the whole world are in short supply,and a large amount of seawater resources and sewage have become possible sources of clean water in the future.However,conventional desalination and sewage treatment require a large amount of electricity costs,and these treatment methods may also cause secondary pollution.Based on this,solar steam generation based on photothermal materials has become a research hotspot at home and abroad.The core of this technology is solar thermal conversion devices.At present,most high-efficiency solar thermal conversion devices have problems such as complicated preparation process,expensive raw materials and difficult expansion,which restricts further exploration of this technology and future development in water treatment industry.Therefore,the biomass carbon material,which has the advantages of abundant sources,low price,simple preparation and easy expansion,high photothermal efficiency,and reusability,has become one of the most promising solar thermal conversion devices.In this paper,inspired by the transpiration of natural plants,biomass carbon materials are used as solar thermal conversion devices.First,the conventional tube furnace inert atmosphere carbonization method was used to prepare lotus root biochar at different carbonization temperatures.It was found that water evaporation rate driven by lotus biochar increased with the increase of carbonization temperature,but even at a high temperature of 900°C,the light-steam conversion efficiency of lotus root biomass carbon reached only 61.3%under 1 sun(1000W?m^-2),and the water evaporation rate was 1.2 kg?m^-2?h^-1,which was not much higher than that of other biomass carbon materials.When increasing the height of the carbonized lotus root,because the macroscopic pores of the lotus root greatly increase its side surface area,its dark evaporation surface area increases,which not only increases the total water evaporation rate,but also reduces the overall surface temperature because the dark evaporation takes away a part of the heat.Therefore,the solar-to-steam efficiency of the 7 cm-lotus carbon is 11%higher than that of the two-dimensional photothermal conversion device.However,the preparation process of lotus root biochar requires freeze-drying pretreatment,so it is difficult to obtain practical applications.Afterwards,considering that the price of corncobs is lower and its hydrophilicity is strong,the agricultural waste corncobs were used as raw materials,and biomass carbon materials at different carbonization temperatures were prepared under the same carbonization method as lotus root.Under 1 sun,the corncob carbonized at 800°C exhibits a high light-steam conversion efficiency of 93%,far exceeding most of the explored biomass carbon materials,and the carbonization process of the corncob does not require freeze-drying pretreatment,so it is more expected to be prepared and applied on a large scale.However,although the conventional tube furnace inert atmosphere carbonization method can be used to prepare carbon materials from different biomass sources,it has the problems of high energy consumption,large pollution and many by-products.Therefore,in the end,we use crop cotton as raw material,by the method of flat heating and contact carbonization to prepare biomass carbon.By characterization methods of the material,the effects of different carbonization methods on the efficiency of solar thermal conversion devices were compared and analyzed.It was found that the bio-carbon devices prepared by two carbonization methods had similar performance of light-to-steam.However,the plate heating carbonization method not only has a shorter preparation time,but also requires less power consumption.At present,most of the structure of solar steam generation system based on biomass carbon materials is a traditional double-layer structure floating type,which has large heat loss,limiting its practical application.In order to further increase WER of the system and the biomass carbon material is used as a solar thermal device for solar steam generation system to have a wider application range,this article first uses cotton on the basis of preparing carbonized corncob STCD（solar thermal conversion device）.As a one-dimensional water channel,the cotton core improves the thermal management of the DSSG（Direct solar steam generation）at low cost,making the overall light-to-steam conversion efficiency of the DSSG higher.Among them,the one-dimensional cotton core（diameter of which is 8 mm）as water channel can increase the evaporation rate to 1.57 times of the traditional floating type.Then,on the basis of the research of carbonized cotton as STCD,cylindrical cotton is used as the support material and water transportation channel of the system,and the height of the cotton column is increased to increase the dark evaporation surface area to improve the efficiency of the system.When the height is 6cm,the cotton column water channel device system has introduced a cold evaporation surface to increase the water evaporation efficiency to 2 kg?m^-2?h^-1.In addition,on the basis of the existing"spray evaporation"technology,this work introduced the carbonized biomass material prepared in this article to replace the original hot air separation chamber,and designed a more compact,simple and efficient DSSG system with carbonize cotton as STCD,the system achieves a water evaporation rate of 2.05 kg?m^-2?h^-1 under 1 sun.