During the hot summer, greenhouse cooling is essential for the production of high-quality crops, which require cooler ambient temperatures. Although many cooling options exist, they all depend on the availability of affordable resources, such as electricity and groundwater (Kumar et al., 2009; Sethi and Sharma, 2007). Conventional greenhouse cooling techniques typically use fog or pad and fans in conjunction with natural or mechanical ventilation and shade screens (Ahemd et al., 2016). Although individual components of used systems, such as fogging nozzles, fans, and shade screens, are affordable, high-quality water supply is required to generate the fog, and electricity is necessary to power high-pressure pumps or ventilation fans. To ensure occurrence of the adiabatic evaporation, the air temperature cannot be allowed to decrease below the wet bulb temperature (Alkhedhair et al., 2016). Consequently, evaporative cooling systems are incapable of significantly decreasing high temperatures that prevail during the hot and humid summer season. However, some crops, such as strawberry, require cooler temperatures (Hidaka et al., 2017; Ledesma et al., 2008; Tanino and Wang, 2008; Wang and Camp, 2000) compared with those that can be achieved by the previously mentioned cooling systems.
Heat pumps can also be used in greenhouse cooling systems. Although they offer a wide range of temperature control, their efficiency is largely dependent on the pump’s power consumption and type of refrigerant used (Sagia and Rakopoulos, 2016). In addition, high costs associated with the installation and operation of heat pumps have caused them to become less popular, despite their versatility in terms of performing sustained heating and cooling operations during cold and hot seasons, respectively. In this study, a GSHP system was developed using a heat pump converted from an air source (a commercial air conditioner) to a groundwater source by means of a simple heat exchanger (Moritani et al., 2017b). One important factor necessary for efficient cooling of a greenhouse is minimization of the volume that must be cooled. Spot cooling, which is used primarily in bench cultivation practices, has recently become popular in Japan as a means of increasing energy efficiency (Ikeda et al., 2007; Yamasaki, 2013). This method involves installation of a cooling tube in the soil, because root-zone temperature affects the quality and yield of strawberry fruit. Geater et al. (1997) reported that a constant, high root-zone temperature on the order of 35 °C in a hydroponic system reduces the root dry weight whereas the greatest fresh weight is yielded under conditions corresponding to a constant temperature of 23 °C. Biela et al. (1999) concluded that optimal temperatures of the root zone are 17, 23, and 29 °C, which correspond to attainment of the greatest transpiration rate, leaf area, and fruit dry weight, versus 11 and 35 °C, with the poorest results obtained at 35 °C. Sakamoto et al. (2016) demonstrated that roots exposed to cooler temperatures of the order of 10 to 20 °C had greater fruit weight, but roots maintained at a temperature of 30 °C resulted in the development of irregularly shaped fruit. Utagawa et al. (1989) observed that relatively cooler temperatures, in the range of 13 to 23 °C in terms of root media, not only increased the root weight but also reduced sugar content in the berries. Although experimental conditions, such as the strawberry cultivar, type of substrate, and ambient temperature, differed among studies just cited, soil temperatures on the order of ≈20 °C invariably resulted in better strawberry growth and quality. Moritani et al. (2017b) previously reported methods for cooling soil and strawberry crowns using GSHPs during the summer. They observed that the temperature distribution along the soil bed over a length of 20 m remained unaffected by the direction of water flow within two parallel tubes installed on the soil bed, be it in the same or opposite directions.
The soil bed for bench cultivation is usually placed within a container made of polystyrene foam to facilitate easy maintenance of cooler soil temperatures. However, heat flux at the soil surface and that through drainage of excess irrigation water tends to reduce the cooling effect. As a result of the greater cooling demand during the middle hours of the day, it is essential that the soil cooling system be able to reduce the temperature rapidly in response to heat inflow into the soil. In this study, two types of soil containers using two different cooling methods were compared to determine the optimum container design for maintaining cooler soil temperatures through use of the proposed GSHP.
Abu-hamdeh, N.H. & Reeder, R.C. 2000 Soil thermal conductivity: Effects of density, moisture, salt concentration, and organic matter Soil Sci. Soc. Amer. J. 64 1285 1290
Ahemd, H.A., Al-Faraj, A.A. & Abdel-Ghany, A.M. 2016 Shading greenhouses to improve the microclimate, energy and water saving in hot regions: A review Scientia Hort. 20 36 45
Alkhedhair, A., Jahn, I., Gurgenci, H., Guan, Z. & He, S. 2016 Parametric study on spray cooling system for optimising nozzle design with pre-cooling application in natural draft dry cooling towers Intl. J. Therm. Sci. 104 448 460
Barrett, G.E., Alexander, P.D., Robinson, J.S. & Bragg, N.C. 2016 Achieving environmentally sustainable growing media for soilless plant cultivation systems: A review Scientia Hort. 212 220 234
Bartzanas, T., Boulard, T. & Kittas, C. 2002 Numerical simulation of the airflow and temperature distribution in a tunnel greenhouse equipped with insect-proof screen in the openings Comput. Electron. Agr. 34 207 221
Biela, M.M., Nonnecke, G.R., Graves, W.R. & Horner, H.T. 1999 326 High root-zone temperature inhibits strawberry reproductive and vegetative growth development HortScience 34 499 (abstr.)
Cho, H.-Y. & Lee, K.-H. 2012 Development of an air–water temperature relationship model to predict climate-induced future water temperature in estuaries J. Environ. Eng. 138 570 577
Gao, Z., Horton, R. & Liu, H.P. 2010 Impact of wave phase difference between soil surface heat flux and soil surface temperature on soil surface energy balance closure J. Geophys. Res. Atmos. 115 D16 D16112
Geater, C.A., Nonnecke, G.R., Graves, W.R., Aiello, A.S. & Dilley, C.A. 1997 High root-zone temperatures inhibit growth and development of Fragaria species Fruit Var. J. 51 94 101
Hidaka, K., Dan, K., Imamura, H. & Takayama, T. 2017 Crown-cooling treatment induces earlier flower bud differentiation of strawberry under high air temperatures Environ. Control Biol. 55 21 27
Ikeda, T., Yamazaki, K., Kumakura, H. & Hamamoto, H. 2007 Effect of cooling of medium on fruit set in high-bench strawberry culture HortScience 42 88 90
Kumar, K.S., Tiwari, K.N. & Madan, K. 2009 Design and technology for greenhouse cooling in tropical and subtropical regions: A review Energy Build. 41 1269 1275
Ledesma, N.A., Nakata, M. & Sugiyama, N. 2008 Effect of high temperature stress on the reproductive growth of strawberry cvs. “Nyoho” and “Toyonoka.” Scientia Hort. 116 186 193
Michel, J.C. 2010 The physical properties of peat: A key factor for modern growing media. 1 May 2018. <https://hal-agrocampus-ouest.archives-ouvertes.fr/hal-00729716/document>
Moritani, S., Araki, S. & Nanjo, H. 2017a Estimation of heating performance of heat pump in greenhouse. Proc. Annu. Mtg. Irr. Drainage Rural Eng. 140–141 (in Japanese).
Moritani, S., Nanjo, H. & Itou, A. 2017b Partial cooling of strawberry plant by water tube utilizing geo-thermal heat pump J. Appl. Hort. 19 186 190
Sagia, Z. & Rakopoulos, C. 2016 Alternative refrigerants for the heat pump of a ground source heat pump system Appl. Therm. Eng. 100 768 774
Sakamoto, M., Uenishi, M., Miyamoto, K. & Suzuki, T. 2016 Effect of root-zone temperature on the growth and fruit quality of hydroponically grown strawberry plants J. Agr. Sci. 8 122 131
Sethi, V.P. & Sharma, S.K. 2007 Experimental and economic study of a greenhouse thermal control system using aquifer water Energy Convers. Mgt. 48 306 319
Song, J., Wang, Z.H., Myint, S.W. & Wang, C. 2017 The hysteresis effect on surface–air temperature relationship and its implications to urban planning: An examination in Phoenix, Arizona, USA Landsc. Urban Plan. 167 198 211
Tanino, K.K. & Wang, R. 2008 Modeling chilling requirement and diurnal temperature differences on flowering and yield performance in strawberry crown production HortScience 43 2060 2065
Utagawa, Y., Ito, T. & Gomi, K. 1989 Effects of root temperature on some physiological and ecological characteristics of strawberry plants Reiko grown in nutrient solution J. Jpn. Soc. Hort. Sci. 58 627 633 (in Japanese)
Xu, X., Wu, Z., Chen, Y., Huang, G. & Zhu, T. 2014 Plant root spatial distribution measurements based on the Hough transformation Neurocomputing 145 209 220