Introduction

In Korea, summer temperatures inside greenhouses rise excessively, making it unfavorable for crop growth. To produce high-quality ornamental crops, cooling methods are essential during the summer season. Cooling methods include ventilation, shading paint, and roof sprinkler systems (Kim et al. 1997;Woo et al. 1994). A cooling using air conditioners is costly in terms of installation and maintenance, making it economically unfeasible. Thus, it is only partially used at night or limited to cultivating high-value crops (Kim et al. 2001;Lee et al. 1999;Nam et al. 2012). The efficient greenhouse cooling methods include evaporative cooling systems, such as fan-and-pad and fog systems. Among these, the fog system lowers temperatures by spraying fine water droplets into the air. However, this system consumes a significant amount of water, which is a concern given the global intensification of water scarcity. For sustainable agricultural production, excessive water use must be avoided. Moreover, excessive moisture supplied to plants and soil can lead to serious pest and disease outbreaks (Park et al. 2020). In contrast, the fan-and-pad system reduces greenhouse temperatures by drawing external air through wet pads, humidifying and cooling the air before it is circulated and expelled through the greenhouse. The fan and pad system has been reported to have approximately 1.4 times higher cooling efficiency than the fog system, although its initial installation cost is higher (Nam 2005).

However, the pads used in fan-and-pad systems are often made of paper (corrugated cellulose), which is not durable and requires regular maintenance. Additionally, to maintain cooling efficiency, the pads must remain consistently wet, necessitating a continuous flow of water, leading to significant water consumption. Corrugated cellulose pads, when soaked in water for extended periods, can facilitate microbial growth and decay, resulting in durability issues. Nanocomposite hydrogel is a functional material with a polymer structure composed of organic and inorganic components, capable of absorbing water and retaining moisture through swelling (Kim et al. 2022). Previous studies have shown that hygroscopic hydrogels made using LiCl demonstrate excellent moisture absorption capabilities (Graeber et al. 2024). Therefore, this study aims to explore the feasibility of replacing the paper pads currently used in fan-and-pad systems with nanocomposite hydrogel pads.

Materials and Methods

Preparation of Nanocomposite Hydrogel

In this study, Acrylamide (suitable for electrophoresis, ≥ 99%, Sigma-Aldrich, USA) was used as the monomer. N,N′-Methylenebisacrylamide (powder, for molecular biology, suitable for electrophoresis, ≥ 99.5%, Sigma-Aldrich, USA), Ammonium persulfate (for molecular biology, suitable for electrophoresis, ≥ 98%, Sigma-Aldrich, USA), and N,N,N′,N′-Tetramethylethylenediamine (BioReagent, suitable for electrophoresis, - 99%,, Sigma-Aldrich, USA) were used to synthesize the hydrogel. Lithium chloride (ACS reagent, ≥ 99%, Sigma-Aldrich, USA) was incorporated into the hydrogel to impart hygroscopic properties. To prepare the hydrogel, 8.36 g of acrylamide was completely dissolved in deionized water, followed by the sequential addition of 5 mg of N,N′-Methylenebisacrylamide, 14.2 mg of ammonium persulfate, and 12 μL of N,N,N′,N′ -Tetramethylethylenediamine. The mixture was left overnight to synthesize the hydrogel. To create a flat, large-surface hydrogel, the mixture was cured between a glass plate and silicone for one day. After curing, the hydrogel was peeled off from the glass plate, fully swollen in deionized water to remove unreacted monomers, and measured to be 200 × 100 mm in size with a thickness of approximately 2 mm. The prepared hydrogel was then fully dried and swollen in LiCl solutions with various saturation levels (% sat), ranging from 0% sat (pure deionized water) to 25% sat, 50% sat, 75% sat, and 100% sat. This process resulted in hygroscopic hydrogels with different LiCl solution saturation levels. The swelling ratios of 0.5 g samples of hygroscopic hydrogels prepared in 25% sat, 50% sat, 75% sat, and 100% sat LiCl solutions were measured at 25°C and 100% relative humidity using a temperature and humidity chamber (Lab House, temperature & humidity chamber) after 0, 3, and 24 hours.

Cooling Effects of Nanocomposite Hydrogel

The hygroscopic hydrogel prepared with 100% sat LiCl solution, which exhibited the highest hygroscopic efficiency in previous experiments, was used for further testing. This experiment aimed to evaluate the feasibility of applying nanocomposite hydrogel to the fan-and-pad system. A small-scale wind tunnel device was constructed in the laboratory to assess cooling effects under various temperatures and fan speeds. The experiments were conducted in a controlled incubator room with adjustable temperatures. The wind tunnel device, fabricated using a 3D printer, had dimensions of 12.6 × 12.6 × 38 cm (Fig. 1). A fan was installed on one side of the wind tunnel to circulate air, while the temperature on the opposite side was measured using a temperature sensor (E52, OMRON, Japan). The recorded data were collected using a data logger (GL240, GRAPHTEC, Japan). Airflow speed was measured and adjusted using a digital variac and an anemometer (WT87A, WINTACT, China). Six airflow speed treatments were tested: 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 m･s^-1. The incubator room temperature was controlled using a heater and set to 25°C and 35°C for each treatment.

Results and Discussion

Preparation of Nanocomposite Hydrogel

The nanocomposite hydrogel was immersed in LiCl solutions with saturation levels of 25% sat, 50% sat, 75% sat, and 100% sat to observe changes in its weight. The results showed that as the concentration of LiCl solution increased, the weight of the hydrogel decreased (Fig. 2). The swelling ratio of the hydrogel over time was measured for each LiCl concentration (25% sat, 50% sat, 75% sat, and 100% sat) to evaluate its hygroscopic properties (Fig. 3). The swelling ratio increased over time and was proportional to the amount of LiCl in the solution. The results indicate that the hygroscopic properties of the nanocomposite hydrogel improved as the concentration of LiCl increased, showing a time-dependent increase in swelling. These findings are consistent with previous studies, which demonstrated that the swelling ratio of polyacrylamide hydrogels increases with higher LiCl concentrations (Graeber et al. 2024). Based on these results, the hydrogel with the highest swelling ratio and water retention capacity (100% sat) was selected for subsequent experiments.

Cooling Effects of Nanocomposite Hydrogel

At a temperature of 25°C and a wind speed of 1.0 m･s^-1, the temperature differences over time (0, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 minutes) were observed as -6.1, -6.0, -5.8, -5.7, -4.9, -4.7, -3.5, -2.6, -2.0, -1.8, -1.6, -1.3, -1.3, -1.1, -1.1, -1.2, -1.1, -1.0, and -0.8°C, respectively (Fig. 4A). The maximum cooling effect was achieved at 0 minute (immediately after operating the fan), with a temperature reduction of -6.1°C (Fig. 4B). While the cooling effect gradually diminished, it was sustained for up to 18 minutes, with an average temperature reduction of -3°C maintained for the first 7 minutes. At a wind speed of 3.5 m･s^-1, the temperature differences over time (0, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 minutes) were -2.0, -1.8, -1.7, -1.4, -1.3, -0.9, -0.9, -0.9, -0.8, -0.7, and -0.6°C, respectively. The cooling effect lasted up to 11 minutes but was the least effective compared to other treatments. This result suggests that higher wind speeds lead to increased evaporation of moisture from the hydrogel, thereby reducing its cooling efficiency. At 25°C, the best cooling performance and duration were observed at a wind speed of 1.0 m･s^-1.

At a temperature of 35°C and a wind speed of 1.5 m･s^-1, the temperature differences over time (0, 2, 4, 6, 8, 10, 12, 14, and 16 minutes) were -10.0, -9.2, -8.0, -7.4, -5.8, -1.4, -0.9, -0.7, -0.7, -0.7, -0.6, -0.6, -0.5, and -0.4°C, respectively (Fig. 5A). The maximum cooling effect was observed at 0 minute (immediately after operating the fan), with a reduction of -10.0°C (Fig. 5B). The cooling effect was sustained for up to 13 minutes, with an average temperature reduction of -5°C during the first 5 minutes.

The results indicated that the optimal wind speeds for cooling at 25°C and 35°C were 1.0 m･s^-1 and 1.5 m･s^-1, respectively, with better performance observed at relatively lower wind speeds. These findings align with previous studies where polyacrylamide/polyvinyl alcohol hydrogels demonstrated cooling effects of approximately -6.2°C and -3.6°C, respectively, under outdoor temperatures of around 35°C (Xu et al. 2023). The cooling effect was maximized due to water retention during swelling, while evaporation gradually reduced the cooling efficiency.

This study is significant as it highlights the potential of replacing conventional paper with nanocomposite hydrogel in fan-and-pad systems. However, further research is needed in the following areas: