Enhancing the collectivity of water vapor in a plastic film tunnel using an improved underground condensation system

We improved an earth-air heat exchange water distillation system proposed by Ishikawa et al. (J Jpn Soc Irrig Drain Reclam Eng 64:225–230, 1996), a technology that can be used to obtain freshwater from saline or agricultural drainage water, to enhance the collectivity of water vapor in a plastic film tunnel. Shifting the heat exchange pipe from the inside to the outside of the tunnel enhanced the water vapor collection ratio by 2.6-fold. Moreover, change in the pipe structure from one PVC pipe with a diameter of 100 mm to four PVC pipes with diameters of 50 mm doubled the collection ratio. Water vapor condensation in the pipes mainly occurred during the daytime, whereas that condensed on the plastic film continuously occurred throughout the day. In the case of the improved system with four φ50 pipes embedded outside the tunnel, the total volume of water collected by both the pipes and plastic film was 46% of that of evaporated water in the tunnel. The collectivity was 1.3-fold higher than that of the control, corresponding to the model of Ishikawa et al. (1996). Considering the shifting of pipes, the difference in the average air temperature between the tunnel and pipe during fan operation was the primary determinant of the water volume collected by the pipes. Considering the change in pipe structure, the average air temperature in the tunnel during fan operation was the primary factor of the water volume collected by the pipes, followed by the difference in average temperature between the tunnel air and inner pipe wall during fan operation.