Study On The Cooling Effect Of Water Circulating Substrate Cooling System In Tomato Root Zone In Solar Greenhouse

Currently,most of the common cooling techniques for greenhouses are aimed at reducing the temperature of the greenhouse air.Some of these cooling techniques are mainly suitable for large-scale greenhouses and are difficult to apply or not cost-effective in traditional Chinese solar greenhouses.There is relatively little research on active cooling techniques for substrates in soilless cultivation systems in Chinese solar greenhouses.As a movable refrigeration equipment,the application effect of a chiller has not been reported for greenhouse cooling.To explore and implement active cooling technology for soilless cultivation in Chinese solar greenhouse production,to improve production capacity under high temperature conditions and achieve year-round continuous production,this study proposes a water circulating substrate cooling system（WCSCS）using a chiller as a cooling source,foam boxes as cultivation containers,and polyethylene raised temperature resistance（PE-RTⅡ）pipes laid in the foam boxes as circulation pipelines to remove excess heat from the substrate through water circulation.The experimental area was divided into TS₁（single PE-RTⅡpipe）,TS₂（double PE-RTⅡpipes）,and the control group CK（no cooling）to investigate the feasibility of the cooling system,its effects on tomato growth and yield,and its cooling performance in combination with shading.The main results are as follows:（1）Under the outdoor average temperature conditions of 17～34℃in the Yangling area of Shaanxi province in summer,WCSCS can reduce the substrate temperature by6.5～8.5℃and keep the root zone temperature of tomatoes below 33℃,the highest tolerable temperature for tomatoes.Compared to the control area without WCSCS,the use of WCSCS significantly（P<0.05）increased tomato plant height,stem thickness,dry weight,fresh weight,leaf area,net photosynthetic rate,total root length,and total root projection area,with no significant difference between TS₁and TS₂.The growth rate of TS₁and TS₂increased by 60.1%and 81.6%compared to the control group,and the light energy utilization rate increased by 56.25%and 81.25%,respectively.The refrigeration energy consumption per unit area of the system was 35.2～67.5 W·m^-2,and the coefficient of performance（COP）was 5.3～8.7.（2）Based on the previous experimental results,the system was optimized in four aspects:the length of the circulating pipeline,the cover material above the substrate foam box,the circulating water flow rate,and the operating strategy.The connection method of the circulating pipeline was changed from parallel to series,and the pipeline length was increased four times.The pipeline length in the TS₁and TS₂experimental areas was approximately 18 m and 35 m,respectively,further tapping the cooling potential of the system.The substrate temperature distribution was even along the water flow direction,and the maximum temperature difference of the substrate average temperature in the TS₁and TS₂experimental areas was 0.4℃and 0.9℃,respectively.The transparent plastic film cover material was changed to a black and white film,and the average substrate temperature decreased by 4.8℃.Under the configuration conditions of this experimental system,a circulating water flow rate of 0.2 m³·h^-1had a better cooling effect.The addition of a nighttime operating strategy for WCSCS alleviated the cooling pressure during the daytime.（3）Under the conditions of shading nets and black and white film covering the substrate in both experimental areas and control areas,the optimization of WCSCS resulted in an average temperature reduction of 6.1 to 8.0℃in the substrate.The daily effective accumulated temperature of the substrate in the TS₁and TS₂experimental areas increased by 92.7%and 130.0%,respectively,compared to the control group for 40 consecutive days,and TS₂increased by 15.7%compared to TS₁.TS₂showed significant improvement compared to the control group in net photosynthetic rate,dry weight,fresh weight,and yield,with increased of 62.6%,295.8%,344.1%,and 626.9%,respectively.TS₂also showed improvements of 3.6%,66.5%,63.9%,and 63.7%compared to TS₁.The energy consumption per unit area of the cooling system in TS₁and TS₂was 53.3～67.7 W·m^-2and56.5～69.2 W·m^-2,respectively,and the COP of the cooling system in TS₁and TS₂was5.0～5.9 and 6.2～7.3,respectively.Without considering tomato production costs such as seeds and fertilizers,the benefits of tomato production in the control area CK and the TS₁and TS₂experimental areas were-0.4 RMB·m^-2,-8.7 RMB·m^-2,and 1.5 RMB·m^-2.（4）A software for analyzing water cycle substrate cooling system has been developed based on the analysis of heat transfer processes using substrate cooling and the development of fitting equations for substrate temperature.The water cycle substrate cooling system（WCSCS）is feasible for substrate cooling in Chinese solar greenhouses in hot summer regions,and has good cooling capacity.The system’s cooling source is provided by a chiller,which has a low cost,small size,is movable,and the cooling temperature can be set.Combined with a water storage tank,it saves water resources during use and is not limited by the water resources and weather conditions of the usage area.In addition,the system can be combined with other cooling methods to provide a more suitable growth environment for tomato growth,improve the production capacity of the Chinese solar greenhouse,and help achieve year-round crop production.