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In this study, we tested local heating using hung plastic air ducts (0.5 m or 1.5 m height from ground surface) and measured air and soil temperatures and soil heat flux in order to confirm that the local heating reduces energy consumption compared with conventional heating by air ducts laid on pathways. On greenhouse tomato production in winter, heating using a higher arrangement of plastic ducts consumed less energy owing to larger low temperature areas below the air ducts. Soil temperature and soil heat flux were significantly higher under the laid air ducts in conventional heating than in other positions and air duct arrangements, whereas the decrease was more drastic the further away from plastic ducts. Heating using hung air ducts hardly affected soil temperature and soil heat flux and there was no difference between the heights of 0.5 m and 1.5 m. Estimation of soil heat flux in the entire greenhouse revealed that a quarter of the energy reduction was derived owing to the hung air ducts. Furthermore, the higher arrangement of plastic ducts tended to reduce the contribution of soil heat flux to energy reduction. Since the equipment of greenhouse in this study was simple (single layer plastic film, no thermal screens), adding a thermal retention system can increase the contribution of soil heat flux. Adversely, lower outer temperature would decrease that contribution.

Evaluation of Air Duct Arrangements of Local Heating on Greenhouse Tomato Production Based on Fuel Consumption and Soil Heat Flux