Optimization Of Gradient Light Changing Environment In Microalgae Columnar Photobioreactor Based On Numerical Simulation

The carbon dioxide emitted by the large consumption of fossil fuels aggravates the global greenhouse effect,and the fossil energy crisis is also constantly affecting the economic development of countries.Therefore,it is urgent to seek alternative energy sources with renewable low-carbon emissions.As the most promising third-generation biomass resource,microalgae can play a key role in regulating biological carbon sequestration and mitigating the greenhouse effect.Setting artificial light source in photobioreactor for large-scale cultivation of microalgae is an effective way to obtain high concentration microalgae biomass.Optimizing the light environment of the reactor and reasonably regulating the light energy input during the culture process can promote microalgae biomass accumulation and reduce energy consumption.Previous studies have shown that incremental lighting,that is,changing the light environment by dividing the growth stage,can make microalgae culture more energy-saving and efficient,but there is still a lack of systematic research on how to set up variable illumination environment.In this paper,the process of cultivating Chlorella vulgaris in a cylindrical photobioreactor was optimized by numerical simulation,involving reactor structure optimization and system setting incremental lighting.The optimization of reactor structure is carried out by adding optical cavity includes cylindrical single optical cavity,multi-optical cavity and multi-layer cylindrical optical cavity.In the case of biomass concentration of 1 g/L,the light setting of the reactor was optimized.The optimized three reactors can achieve better light environment,and save light energy input by 23.46%,19.89%and 38.74%,respectively.This shows that the optical cavity bioreactor is more energy-efficient in the setting of the light environment.In this paper,the growth process of Chlorella vulgaris was divided into five growth stages.According to the numerical simulation results,gradient lighting was set in three kinds of optical cavity bioreactors.In the single-cavity bioreactor,the incident light intensity was changed with the growth stage,and the selected internal incident light intensities were 6,7.5,15,22.5 and 30 W/m²,respectively.At this time,the light distribution can be improved during the whole culture process,and 17.33%of the light energy input can be saved.However,there are problems such as excessive volume of the optical cavity and limited improvement of the light environment of the reactor in the later stage of culture.Gradient lighting settings were performed in two multi-cavity bioreactors by changing the incident area.The number of optical cavities in the cylindrical multi-cavity bioreactor at each growth stage was 6,9,12,15 and 19,respectively.The light source spacing of the multi-layer cylindrical cavity bioreactor in each growth stage was 22.5,22.5,11.7,7.5 and 5 mm,and the incident light intensity was 5,10,7.5,10 and 10 W/m²,respectively.After setting gradient lighting,the two multi-cavity bioreactors effectively optimized the light environment in the later stage of culture.Compared with the control group,the light area volume increased by 44.63%and 53.32%,the radial velocity increased by 22.86%and82.90%,and the light energy input decreased by 18.55%and 18.03%,respectively.This shows that changing the gradient of the incident area is more energy-efficient than only changing the incident light intensity.The performance of the two multi-cavity bioreactors was compared by simulating the light-dark cycle of microalgae cells.The results showed that the proportion of light area time of microalgae in the two reactors was not much different,but the light-dark cycle frequency of microalgae in the multi-cylinder light-cavity bioreactor was 0.80 s^-1,which was 67.51%higher than that in the cylindrical multi-cavity bioreactor.Therefore,the multi-layer cylindrical optical cavity is the best light source structure for setting gradient variable light in the cylindrical reactor,which indicates that the combination of light guidance and diversion can effectively improve the light-dark cycle frequency of microalgae cells.Based on the numerical simulation results,this study systematically sets gradient lighting in the three proposed cavity bioreactors,which is of great significance for the development of gradient light setting to cultivate microalgae energy-efficiently and develop microalgae industry.