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.
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