Effects Of Root Zone Ventilation Technique On Microclimate Regulation And Lettuce Growth In A Plant Factory

With the continuous maturity of related technologies in plant factory with artificial lights（PFAL）and the continuous improvement of consumers’demand for high-quality vegetables,more newly established PFALs have diverted from experimental and demonstration into production.In order to improve land use efficiency and reduce production costs,these PFALs shown a trend of large-scale and intensive cultivation.However,the environmental parameters closed to the plants were significantly affected by the physiological activity of leaves and boundary layer resistance of the canopy,leading to suboptimal microclimate deviated from the set values,poor ventilation uniformity and inefficiency of air conditioning,especially when the PFAL is restricted by the building structure and the internal space is irregular in shape,the drawbacks of the conventional environmental control method are more prominent.In order to solve this problem,this study proposed the theory of precise micro environment regulation for each cultivation area,and suggested the solution of using the interlayer between the cultivation plate and nutrient solution as the airflow channel to transport the appropriate parameter air directly to the vegetables.In order to further elaborate the characteristics of air distribution under the conventional mode,CFD（Computational Fluid Dynamics）simulation was carried out under the airflow organization form of lateral ventilation in the canopy of conventional environmental control（CEC）,and the characteristics of airflow in the upper part of vegetable canopy were described.Based on the proposed principle of rhizosphere ventilation（RV）,this innovative air distribution form was simulated by CFD,and the theoretical feasibility of RV was proved.In order to verify the accuracy of the simulation,a RV system was developed.By adjusting the fan speeds and their running modes,low rotation rate continuous root zone ventilation（LCRV）and high rotation rate intermittent root zone ventilation（HIRV）treatments were proposed.The effects of these treatments on microclimate change of butterhead lettuce（Lactuca sativa var.capitata）20 days after transplanting were examined,and the performances on environmental regulation were evaluated by comparison with conventional environment control（CEC）under identical room environment conditions at the same time.Based on the research above,the water cooled heat exchangers were used to control the temperature of the air introduced into the interlayer of the hydroponic system.Interlayer cool airflow（ILCA）was proposed to introduce room air into the internal canopy through reserved vent holes in cultivation boards and interlayer between cultivation boards and nutrient solution surface.The key operating parameters of ILCA were optimized to acquire precise microclimate control with minimal energy consumption.A quadric universal rotational combination design method was used to investigate the effects of operating parameters on the microclimate and to obtain the optimal combination.After those researches,the following conclusions are obtained:1)By using CFD simulation,the air distribution characteristics of lateral ventilation and RV were further clarified,which showed that the RV could provide the cultivation area with uniform and stable airflow,there is not much difference in velocity between ventilation units at different positions in the cultivation area;as the fan rotation rate increases within the test range,the average velocity of each ventilation unit was from 0.051to 0.11 m/s,and each ventilation unit is evenly ventilated under various rotation rate,which was better than the CEC.2)The RV could greatly reduce the temperature,inhibit the humidity increasing of the interlayer and the internal canopy,as well as increase the CO₂ concentration of the internal canopy.In the interlayer,LCRV earned the lowest temperature which was 2.67 and 4.07℃ lower than CEC and the room value,respectively.RH of CEC was the highest,which was100%,while was 90.27%in HIRV.In the internal canopy,the temperature of LCRV was the lowest,which was 1.77 and3.47℃ lower than CEC and room value,respectively.The RH increasing was most obvious in CEC,which was 27.56%higher than room value,and about 7%higher than LCRV and HIRV.LCRV earned the highest CO₂ concentration about 617×10^-6,which was 139×10^-6higher than CEC.3)The RV could reduce the temperature of nutrient solution and slow down the downward trend of dissolved oxygen（DO）.In NS,its temperature in LCRV was 4.03℃ lower than CEC.Meanwhile,HIRV slowed down the DO depletion to a big extent.At the end of the experiment,DO in HIRV was still maintained at 3.8 mg·L^-1,while the other treatments were only 2.8 mg·L^-1.4)The regression equations in terms of coded factors between experimental factors and the lowest air temperature（Y_A）,balance time（Y_B）and cool down time（Y_C）at the 0.05significance level are listed below:Y_A=24.51+2.19X₁+0.77X₂-0.13X₃+0.12X₄+0.54X₁²+0.44X₂²+0.11X₃²+0.048X₄²+0.079X₁X₂-0.10X₁X₃+0.051X₁X₄-0.062X₂X₃+0.55X₂X₄-0.24X₃X₄ Y_B=3.04-0.30X₁-0.30X₂-0.47X₃-0.59X₄+0.040X₁²-0.31X₂²+0.39X₃²-0.14X₄²-0.97X₁X₂-0.12X₁X₃+0.078X₁X₄+0.38X₂X₃-0.17X₂X₄+0.62X₃X₄ Y_C=18.02+7.14X₁-7.73X₂+0.96X₃-1.19X₄+0.79X₁²-0.62X₂²+0.26X₃²-0.97X₄²-1.44X₁X₂-0.75X₁X₃-2.23X₁X₄+0.25X₂X₃+3.64X₂X₄-0.50X₃X₄The above regression equations are significant and the lack of fit was not significant.The above equations fit the experiments well.Through the analysis of the above equations,room temperature had the greatest effect on the lowest air temperature,followed by cool water temperature.For balance time,the most remarkable factor was the airflow rate,followed by the cool water flow rate and then the cool water temperature.The cool water temperature had a maximal influence on the cool down time,followed by the room temperature.Took the outlet air temperature of 24℃ as the target,to shorten the operation time and lower the energy consumption of the system as a principle,Design Expert software was used to optimize the combination.The optimal combination of ILCA was determined as room temperature 28℃,cold water temperature 18.5℃,cold water flow rate 0.63 L·min^-1,airflow rate 1.2 cm³·s^-1,balance time 4.8 minutes,cooling time 5.0minutes.With this combination of parameters,the minimum outlet temperature can reach24℃.5)Lettuce cultivation experiments were carried out under the optimized environmental parameters.By comparing with CEC,it was found that ILCA can significantly improve the microclimate parameters of the plant:reduced the interlayer temperature by 2.5℃ and the temperature inside the canopy by 1.1℃ and 2.1℃;maintaining the NS temperature in21.5±0.3℃,which was 2.3℃ lower than the control;alleviated the decreasing trend of DO concentration in NS,result in a slowly decreasing from 5.8 mg·L^-1 to 5.4 mg·L^-1 during the three-day monitoring period.6)Under the influence of the microclimate improvement,the survival rate of the old leaves increased and the root growth can be restricted without affecting the yield.The results showed that the fresh weight of roots was significantly reduced by 41.7%and the root length was shortened by 23%.The total root length,root surface area,root projected area,root volume and average root diameter were also significantly lower than the control by 39.1%,38.2%,37.9%,48.1%and 12.1%,respectively.Significantly reduce the nutrient consumption of non-edible root,conducive to the substantial increase of nutrient utilization efficiency,and plays a positive role in prolonging the use time of NS,cutting the production cost of hydroponic fertilizer.7)The total power consumption of CEC and ILCA were 627.6 k Wh and 294.4 k Wh,respectively.Combined with the yields data,the energy-biomass conversion efficiency（EBCE）for both treatments were 29.4g·k Wh^-1 and 59.5g·k Wh^-1,respectively.The EBCE in ILCA decreased by 50.6%compared with CEC.In summary,RV technique overcomes the airflow occlusion and boundary layer resistance of canopy effectively.Benefit from it,uniformity,efficiency of ventilation and environment control were improved.There were obvious differences between the microclimate around canopy in CEC and plant factory room environment,showed the importance of using microenvironment parameters around the plants as the temperature control targets,instead of the room value.RV played a positive role in lowering internal canopy temperature.It was feasible to cut down energy consumption by increasing room setting temperature while reducing refrigeration capacity and start-up frequency of air condition system.The ILCA could effectively improve the plant microclimate,significantly reduce the root weight and nutrient consumption without affecting the yield of lettuce and achieve energy saving of 50.6%compared with CEC.When the atmosphere temperature is lower than 28℃,the CEC with the optimized operating parameters could replace the conventional environmental control method to regulate the environment inside the plant factory.In future technological renewal of plant factories,this kind of precise microclimate control technology for each cultivation units will replace existing whole space ventilation mode and applied in medium and large plant factories.