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Shigeoki Moritani, Hirotada Nanjo, Atsushi Itou and Teruki Imai

Ground-source heat pumps (GSHPs) have been used to chill water to facilitate cooling of ‘Natsuakari’ strawberry (Fragaria ×ananassa) grown within containers during the summer. Two types of soil containers and cooling systems have been considered. In one system, cold-water tubes were placed under as well as over the top of the soil, whereas the other cooling system used cold water passing through tubes placed under the soil and within the irrigation channel to facilitate bottom irrigation. The cooling efficiency of each system was evaluated by observing temperature relationships between greenhouse air and soil. The relationship was represented by means of an elliptic curve, the geometric center and tilt angle of which indicated representative daily soil temperatures and degree of temperature stability, respectively. Both values were observed to be lower for the bottom irrigation system during the two plant growth periods considered in this study, thereby indicating that colder and relatively constant soil temperatures can be maintained via greater heat convection. This greater cooling method was facilitated by rapid transfer of cold water through the bottom irrigation channel into the root zone, resulting from reduction in soil moisture content induced by plant transpiration in addition to heat conduction from the soil to the cooling tube. Measured soil temperatures for the buried-tube system were observed to be coldest when the tube was chilled considerably (9.4 °C). Although the setup of the considered bottom watering system was rather sensitive in that the system required maintenance of a constant water level throughout the container, both systems effectively produced cooler soil temperatures compared with the case in which no GSHP was used.