| The gradual depletion of traditional energy and environmental pollution have forced greenhouse energy utilization to shift towards green and energy-saving.As a form of shallow geothermal energy utilization,an earth-air heat exchanger(EAHE)has been gradually applied in greenhouses due to its low cost and high energy efficiency.However,the structural design method,heat exchange characteristics,operation mode,and influence mechanism of the main parameters of the greenhouse EAHE system are still unclear and need to be optimized urgently.To address the above issues,this paper adopts the methods of theoretical analysis,experimental measurement,numerical simulation,and application verification to carry out the research.The main results are as follows:(1)A structural design method for the greenhouse EAHE system is proposed,the system parameters are determined,and the performance study of different operation modes is carried out.The method determines reasonable parameters of the EAHE system based on the greenhouse cooling and heating loads,the system efficiency-heat transfer unit number method,and energy-saving and economic indexes,and is verified by measured data.The results showed that the horizontal meandering EAHE system constructed based on this design method can enable the greenhouse to reach the target temperature.In the study of different operation modes,it was found that the use of intermittent operation modes was favorable to improving the effectiveness of the EAHE system,and the average coefficient of performance(COP)of the system in the heating condition in intermittent operation was 58.25% higher than that in continuous operation.In winter heating and summer cooling conditions,the average outlet temperature of the EAHE system was 9.26 °C and 10.55 °C higher than the inlet,respectively,and the average daily heat exchanges were 18.86 MJ and 18.78 MJ,with average daily COP of 22.49 and 23.52,respectively.(2)The theoretical heat transfer model of the EAHE system and the numerical simulation model of Computational Fluid Dynamics(CFD)were constructed and combined with the measured data to investigate the influence of each parameter on the heat transfer performance of the EAHE system.The results showed that the maximum Mean Absolute Percentage Error(MAPE),Mean Bias Error(MBE)and Root Mean Squared Error(RMSE)of the predicted and measured values of the mathematical model and the CFD simulation model for the outlet temperature of the system in the heating condition are 8.94%,-0.95 ℃,0.96 ℃,and 6.81%,-0.80 ℃,and 0.85 ℃,respectively;and in the cooling condition,the maximum MAPE,MBE and RMSE of the CFD simulation model for the predicted and measured values of the system outlet temperature are 6.73%,-1.95 ℃,1.72 ℃ and 8.61%,-2.78 ℃,2.47 ℃,respectively,which indicates that the two models have good consistency with the measured values,and they can be used for predicting the change of the air temperature in the tubes and for parameter analysis.The results of the parameter analysis show that the soil in the vertical direction in the summer cooling condition will have a negative effect on the air inside the outlet pipe of the EAHE system.Increasing the pipe length can improve the heat exchange efficiency,but it is appropriate to design the pipe length with the system heat exchange efficiency around 80%.Increasing the air velocity,pipe diameter,and running time will reduce the heat exchange efficiency,but increasing the air velocity and pipe diameter will significantly improve the heat exchange capacity.The natural recovery time of the soil temperature around the pipeline in winter heating condition and summer cooling condition is 0.93～1.11 times and 1.92～2.10 times of the system operation time,respectively.(3)Constructing a mathematical model of the greenhouse thermal environment with an integrated EAHE system.A mathematical model of the greenhouse thermal environment of the integrated EAHE system was constructed according to the principle of heat transfer and based on the energy balance of the air module inside the greenhouse,and the model was validated in greenhouses with and without planted crops,respectively.The results showed that the MAPE,MBE,and RMSE of the mathematical model for the predicted and measured values of greenhouse air temperature with and without planted crops were 11.47%,0.86 °C,and 1.22 °C,and 5.71%,0.80 °C,and 1.32 °C,respectively,which indicated that the model could be used to predict the indoor air temperature in the greenhouse of the integrated EAHE system.(4)The effects of the EAHE system on the greenhouse thermal environment,tomato growth,and yield quality were investigated.The results showed that operating the EAHE system during high temperature periods in the spring and summer sunny days could reduce the indoor air temperature of the greenhouse by 1.64 ℃,and the air temperature in the crop canopy at 28 cm and 100 cm from the ventilation zone by 2.94 ℃ and 1.48 ℃,respectively.During the low temperature periods in the fall and winter days,running the EAHE system could increase the indoor air temperature of the greenhouse by 1.31 ℃,and the air temperature in the crop canopy at 28 cm and 100 cm from the ventilation zone by 1.90 ℃ and 1.36 ℃,respectively.During cloudy days in fall and winter,the EAHE system increased the air temperature inside the greenhouse by 1.20 ℃,and the air temperature inside the crop canopy at 28 cm and 100 cm from the ventilation zone by 1.65 ℃ and 1.25 ℃,respectively.In addition,the use of the EAHE system promoted tomato growth,significantly increased yield and quality,and advanced the harvesting of fall and winter tomatoes.The payback periods calculated based on both energy efficiency and the input-output ratio of the EAHE system were 8.79 and 8.21 years,respectively,which were within the service life. |
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