Evaluation of a 0.7 kW Suspension-Type Dehumidifier Module in a Closed Chamber and in a Small Greenhouse

Abstract

Controlling humidity inside greenhouses is crucial for optimum plant growth and controlling physiological disorders and diseases. The humidity response and uniformity depend extensively on the evaluation of the dehumidifier. The objective of this research was to evaluate a low-powered suspension-type dehumidifier module in terms of humidity changes and spatial and vertical variability in a closed chamber and in a small greenhouse. A wireless sensor network including 27 sensor nodes was used to collect the data during the humidity changes from 80% to 70% and 90% to 70%. The humidity response results showed that the times required for dehumidification from 80% to 70% and 90% to 70% were 13.75 and 21.51 min, respectively, for the closed-chamber operation. Similarly, for the small greenhouse, 18 and 35 min were required to reduce the humidity levels from 80% to 70% and 90% to 70%, respectively. The spatial and variability results indicated that the changes in humidity at the rear and bottom layers were slower than those in the other layers of both experimental areas. The findings of this study would aid in the development of dehumidification strategies and sustainable agriculture for monitoring and controlling humidity in greenhouses using low-powered dehumidifiers.

1. Introduction

Humidity is one of the essential factors of the greenhouse microclimate that should be maintained for optimum plant growth. Excessive humidity sometimes combines with the appearance of free water (condensation) to create favorable conditions for the development of fungal diseases and enhance leaf necrosis, as well as promoting the formation of soft and thin leaves [1]. For these reasons, controlling the humidity inside greenhouses is a fundamental concern for better crop cultivation. The conventional method used to control the humidity level within a considerable range consumes significant energy. The primary concern of consuming significant energy is maintaining the low-cost dehumidifier, ventilation, and air conditioning system [2,3,4]. Many energy-saving humidity control systems are being developed to reduce greenhouse energy utilization [5,6]. However, it is very challenging to achieve uniform humidity inside a greenhouse because it is directly related to the operational time and HVAC system [2].

With respect to the control and uniformity of the inside greenhouse humidity level, the primary goal is to either “keep the humidity in” or “keep the humidity out” based on the plant growth threshold level [5]. Research on the development of humidity-control techniques for greenhouses began in the 1940s [7]. Researchers have attempted to establish the optimum humidity range for crop growth and define sustainable humidity ranges for the prevention of crop damage. Researchers have also reported several types of dehumidification systems (e.g., liquid desiccant dehumidification systems [8,9], solar desalination systems [10,11,12], and compressor-type dehumidification systems [13,14]) to maintain a healthy relationship between crops and the inside greenhouse environment. In maximum greenhouses, stand-alone high-energy-consuming dehumidifiers are used to create the desired humidity level inside [15]. However, a group of small dehumidifiers can provide a more uniform humidity distribution. Recently, for large greenhouses, some multizone independent control techniques with dehumidifiers have been introduced to control different zone environments using small independent control systems [16]. A detailed experiment including humidity response and variability is needed to know the performance of the small dehumidifiers. Therefore, in this research, two different mediums (a closed chamber and a small greenhouse) were used to evaluate the performance of a single unit dehumidifier that could provide useful key outcomes for the management of advanced conditions such as large greenhouses with multizone dehumidifiers.

The humidity response and variability in greenhouses vitally influence the measurement of humidity uniformity in crop growth [17]. Boulard et al. [18] clarified that humidity usually drops on the greenhouse surface and in areas just above the floor and below the roof. Several researchers have focused on achieving uniform humidity distribution with various techniques. Considerable work has been conducted to improve the efficiency of natural ventilation in greenhouses by using the variability method to measure the uniformity level [17,19,20,21]. Kempkes and Van de Braak [22] and Kempkes et al. [23] checked the environmental uniformity to evaluate the effect of different component positions. Ahmed et al. [24] and Al-Helal et al. [25] investigated the spatial distribution of temperature and relative humidity by studying several greenhouses covers. The humidity response plays a crucial role in evaluating the humidity distribution over time. Different layers and planes’ humidity levels are needed to find the humidity distribution at different conditions. Therefore, humidity response and variability were studied using the humidity condition for the dehumidifier operation, which is typically used to analyze different layers on spatial and vertical planes.

Recently, different dehumidifying techniques have been used, including vapor compression [26] and desiccant dehumidifier [27]. The conventional type of dehumidifier is located in one or two locations in a greenhouse. This type of dehumidifier needs a high electric load (5–20 kW), space, and operational cost. On the other hand, this study used a condensation system. The main advantages of this approach include compact, safe, low power (0.5 kW), and noiseless technology that can have a significant impact on dehumidifying the greenhouse as well as on sustainable agriculture. Previously, the low-powered dehumidifier was mainly used to generate freshwater from ambient air and an irrigation system in a greenhouse [28]. We developed and evaluated the performance of the small-scale suspension-type dehumidifier to control the status of individual actuating components [6] in the greenhouse. The basic performance of the suspension-type dehumidifier with a condensation cooling system was evaluated in greenhouses with regard to the remote monitoring and control of the components, the required time to achieve the target humidity, and the spatial and vertical variability of humidity to assist the dehumidifying process for the greenhouse.

Based on the previous research, this type of dehumidifier was designed to maintain low power, but the major challenge is keeping the humidity uniform due to the low efficiency [29]. Before placing the dehumidifier modules in a commercial greenhouse, evaluation was necessary to investigate the dehumidifying behavior. The process of the low-powered suspension type is to install the units at multiple heights and locations in greenhouses to improve the response time and uniformity of the humidity control. Therefore, the objective of this study was to evaluate a low-powered suspension-type dehumidifier module in terms of humidity response and variability in a closed chamber without interaction with the outside, and a small greenhouse with interaction with the outside, to determine suitable conditions for various working environments.

2. Materials and Methods

2.1. Structure and Working Principle of the Suspension-Type Dehumidifier Module

The prototype low-powered suspension-type dehumidifier consists of a condensation cooling system. The condensation cooling system is used to condense the steam and lower the water content in the air. At the center of the dehumidification system, a compressor compresses the humid air and pumps the refrigerant, which circulates throughout the entire system. The higher the pressure is, the higher will be the temperature. A pipe conveys the compressed refrigerant to the condenser to remove the relative humidity from the room air by cooling it below the dew point, which causes the out-dropping of moisture (condensation) [13]. The major elements of the low-powered suspension-type dehumidifier include an extractor fan (0.1 kW), a compressor (0.6 kW), two heat exchangers (a condenser and an evaporator), and an expanding element, as shown in Figure 1. The condenser and evaporator gather humidity in the dehumidifier tank and remove it from the outside. After passing through the evaporator, the cooled and dried air flows through the condenser, where it is heated, and dry and dehumidified air from the condenser is sent back to the room. The extractor fan forces the flow of humid air through the heat exchangers. The size and diameter of the dehumidifier were considered small due to adjusting the low-powered compressor. To use this dehumidifier in agrivoltaic systems, the low-powered compressor would be an effective tool.

Theoretical equations were derived to determine the dehumidification capacity and the amount of water condensation generated by the dehumidifier. The moisture levels in greenhouses include condensation on the greenhouse roof, cover and plant leaves, air exchange by ventilation and infiltration, and the performance of the dehumidifier module used for moisture removal.

Table 1. Variable notation, definitions, and measurement units.

Notation	Definition, Units
$A_g$	Area, m2
${\rho }_i$	Indoor air density, 1.205 kg m−3
$V_g$	The volume of air, m3
$t$	Time, s
$E_p\left(t\right)$	Evapotranspiration, kg m−2 h−1
$E_{add}\left(t\right)$	Moisture added to or extracted from greenhouse air, kg m−2 h−1
$E_c\left(t\right)$	Condensation rate on inner cover surface, kg m−2 h−1
$E_V\left(t\right)$	Moisture exchange for ventilation of infiltration, kg m−2 h−1
$E_{dh}\left(t\right)$	Moisture removal from inside air by dehumidification, kg m−2 h−1
$h_{ci}$	Convective heat transfer coefficient for greenhouse covers inner surface, W m−2 K−1
$e_i$	Air–water vapor partial pressure, kPa
$e_{sc}$	Saturated air–water vapor pressure on greenhouse covers inner surface, kPa
$C_p$	Specific heat, 1005 J kg−1 K−1
$P$	Atmospheric pressure, 0.1013 kPa
$\Delta T$	Temperature difference between the indoor air and the near cover, K
$p_i$	Indoor air density, 1.205 kg m−3
$q_v$	Air flow rate, m3 h−1
$M_{water}$	Amount of water removed, kg

summarizes the variable notation, definitions, and measurement units used in this study.

Condensation on the plant leaves was considered negligible because no plants were considered for this study. The time required to reduce the humidity level to a certain level was calculated using the energy–moisture balance Equation (1) [15,30].

$$\frac{0.62{\rho }_i{V}_g}{A_{g}P}\frac{d{e}_i}{dt}=E_p\left(t\right)+E_{add}\left(t\right)-E_c\left(t\right)-E_V\left(t\right)-E_{dh}\left(t\right)$$

(1)

The amount of condensate is proportional to the difference between the inside air vapor pressure and the saturation vapor pressure at the inner surface of the cover. It is calculated using Equation (2) [31].

$$E_c=\frac{0.67h_{ci}\left(e_i-e_{sc}\right)}{C_{p}P}$$

(2)

For the polyethylene plastic film greenhouse, the convective heat transfer coefficient was calculated using Equation (3).

$$h_{ci}=0.74+0.43\Delta T$$

(3)

The moisture removed from the greenhouse by air exchange, including ventilation and infiltration, was calculated using Equation (4).

$$E_{V=}\frac{0.62p_i{q}_v\left(e_i-e_0\right)}{A_{g}P}$$

(4)

If any artificial dehumidification system is applied in the greenhouse, such as a mechanical refrigeration dehumidifier, then the moisture removal rate by the dehumidification system was calculated using Equation (5).

$$E_{dh}=\frac{M_{water}}{A_g\Delta t}$$

(5)

The indoor relative humidity was calculated for a given inside temperature and humidity ratio using Equation (6).

$$RH=\frac{e_i}{e_s}\times 100$$

(6)

2.2. Experimental and Analytical Procedures

2.2.1. Time Response to Dehumidify

The sensor nodes were fabricated using a microprocessor (Arduino Uno, Arduino, Torino, Italy), a temperature and humidity sensor (AM2315, Adafruit, New York City, NY, USA), a 433 MHz wireless communication module (SX1278 LoRa, Chengdu Ebyte Electronics Technology Co., Ltd., Chengdu, Sichuan, China), and a 9-V alkaline battery for the power supply. All sensor nodes were connected to the master node by a wireless sensor network, and data from all sensors were collected in a notebook at a frequency of 4 Hz [32]. A program was built to collect promptly all the sensor data on a computer using LabVIEW program (ver. 2014; National Instruments, Austin, TX, USA). A universal asynchronous receiver/transmitter (UART) module (E15-USB-T2, Chengdu Ebyte Electronics Technology Co., Ltd., Chengdu, Sichuan, China) was used to hold the RF receiver module and connect it to a computer through serial communication.

To maintain low energy consumption, from 80 to 85% range relative humidity is commonly used to control the humidity for large greenhouses [33]. In this study, for a closed chamber and a small greenhouse, the humidity levels were reduced from 80% to 70% and from 90% to 70% to evaluate the performance of the dehumidifier module. Each experiment was repeated three times, and the required time and condensed amount of water in the closed chamber were recorded. Each time, the inside humidity was artificially increased up to the target level, and after it reached an equilibrium stage, the experiment was started. A control logic was set in the program to operate the module until the average humidity reached the desired level, and a relay was used in the controller node as a switch to turn on or off the module. Figure 2 shows the components of the monitoring and controlling unit sensor node used in the experiment.

2.2.2. Humidity Variability during Dehumidifier Module Operation

The uniformity of the humidity inside the closed chamber was also verified by vertical and spatial distribution analyses. The spatial and vertical humidity variability during the performance period was evaluated to estimate the humidity distribution. For the measurement period, the inside and outside humidity values were measured, and the entire greenhouse was divided into 27 equal segments. Each minute of the humidity dataset was classified into one of the segments according to the time of measurement. This system has a structure of receiving and storing greenhouse environmental information from each sensor node in real-time, enabling the humidity change to be estimated. The remote-site database stores information collected from each sensor node [34]. The humidity data were interpolated using the ArcGIS software package (ver: 10.8.1; Esri, Redlands, CA, USA) [35,36] and inverse distance weighting according to the planes (Figure 3) to determine the humidity uniformity. Figure 3 shows the sensor placement diagram and photographs of both experimental areas.

2.2.3. Experimental Sites and Test Conditions

The experiment was performed in a closed chamber (L = 5.70 m; W = 2.90 m; H = 2.50 m) and in a small greenhouse (L = 7.00 m; W = 5.00 m; H = 3.10 m) to validate the theoretical results. Two experiments were performed, one in a closed chamber at the Agricultural Production Machinery and Precision Agriculture Lab, Chungnam National University, Daejeon, Republic of Korea, and the other in a small plastic greenhouse at the Agricultural Research Field, Chungnam National University, Daejeon, Republic of Korea (36°22′12.66″ N, 127°21′10.40″ E). The sensor locations were divided into three layers (top, middle, and bottom) and three sections (front, center, and rear). The closed chamber was covered with polyisocyanurate insulation, and no ventilation system was designed in the closed chamber. On the other hand, the small greenhouse is designed for temperature control during the daytime when the indoor temperature is high; it also helps to remove moisture from the greenhouse. Different environmental conditions such as inside greenhouse water vapor transfer including air exchange processes through vents or leakage, condensation, and evaporation need to be considered.

In each section, nine sensors (for measuring temperature and humidity) were placed, as shown in Figure 3. In the closed-chamber experiment, the bottom layer was situated 0.50 m above the ground surface. The middle and top layers were placed 1.25 and 2.00 m above the ground surface, respectively. There was a 0.50 m clearance between the top layer and the roof. The front and rear sections were situated 0.50 m from the sidewalls. The center section was placed at the midpoint of the other two sections.

In the same manner, sensors were placed inside a small greenhouse. The bottom layer was situated 0.50 m above the ground level. The middle and top layers were placed 1.40 and 2.30 m above ground level, respectively. The front and rear sections were 1.00 m from the front and rear walls, respectively. In total, 27 sensor nodes were placed throughout the greenhouse, with nine sensor nodes in each section. These sensors are used to determine the vertical and spatial differences in humidity.

Table 2. Description of the experimental sites.

Type	Size (m3)	Cover Material	The Average Inside Temperature during Operation (°C)
			90% to 70%	80% to 70%
Closed chamber	41.30	Polyisocyanurate insulation	16.50 ± 0.14	13.78 ± 0.08
Small greenhouse	108.50	Plastic	21.35 ± 1.45	19.98 ± 0.25

described the site and condition of this experiment.

3. Results

3.1. Performance of the Suspension-Type Dehumidifier Module

3.1.1. Closed Chamber

Theoretically, the times required to reduce the humidity from 80% to 70% and from 90% to 70% were calculated to be 13.75 and 21.51 min, respectively. In the experiment, the times required were 15.03, 15.90, and 16.47 min, and 22.00, 22.37, and 22.74 min for three trials with 0.071 ± 0.08 and 0.136 ± 0.05 kg of water removal, respectively. Figure 4 shows the theoretical and practical decrease in humidity with respect to time for the closed-chamber experiment. The theoretical and measured values of the relative humidity exhibited the same variation pattern. A relatively better agreement between the theoretical and measured values was found at the experiment ending time at other positions. For 80% to 70% conditions, the measured time range was lower than that for the 90% to 70% condition. At the end of the experiment, both the theoretical and measured values were very close to each other, which means that the considered theoretical variables were applicable to calculate the dehumidifier performance.

3.1.2. Small Greenhouse

Figure 5 shows the theoretical and practical decrease in humidity over time for the small greenhouse experiment. During the experiment, to reduce the humidity level from 80% to 70%, the dehumidifier required 18.00, 19.08, and 20.20 min for three trials and condensed 0.19 ± 0.12 kg of water. To reduce the humidity from 90% to 70%, the dehumidifier required 35.00, 35.84, and 37.00 min for three trials and condensed 0.369 ± 0.15 kg of water. The theoretical and measured values of the relative humidity had the same variation pattern. A relatively better agreement between the theoretical and measured values was found at the experimental ending time at other positions. For the 80% to 70% condition, the measured time range was lower than that for the 90% to 70% condition. During starting of the experiment, the deviation between theoretical and measured values was become more prominent due to the greenhouse high moisture exchange for ventilation of the infiltration and condensation rate on the inner cover surface. On the other hand, at the end of the experiment, the theoretical and measured values were very close to each other, which means that the considered theoretical variables were applicable to calculate the dehumidifier performance.

Table 3. Comparison of theoretical and experimental errors based on the dehumidifying period.

Operation	Error of Period (%)
	80% to 70%			90% to 70%
	Trial 1	Trial 2	Trial 3	Trial 1	Trial 2	Trial 3
Closed chamber	9.22	15.52	19.72	2.26	4.00	5.71
Small greenhouse	9.31	15.85	22.66	1.72	4.18	7.53

shows the comparison of theoretical and experimental errors based on the dehumidifying period. The highest error was found as 22.6% for the small greenhouse in the 80% to 70% condition. On the other hand, the lowest was recorded as 1.72 % for the small greenhouse in the 90% to 70% condition. For both closed chamber and small greenhouse conditions, the theoretical and experimental error was limited to the 90% to 70% reduction.

3.2. Spatial and Vertical Variability of Humidity

3.2.1. Closed Chamber

After the humidity level stabilized to 80% under equilibrium conditions, the dehumidifying operation was started inside the closed chamber. During the initial period, the average humidity was 79.7%~79.75% for the bottom layer and 80.80%~81.80% for the top layer. During the middle period (after 7.5 min of operation), at the front side, the maximum humidity was found in the top and bottom layers (72%), and the minimum humidity was recorded near the middle layer (64.5%). The humidity level of the air released through the module decreased drastically. In front of the dehumidifier, the humidity levels were reduced by 29.55% and 35.70% for the 80% to 70% and 90% to 70% conditions, respectively.

Similarly, in the middle and rear sides, the humidity level varied from 58% to 78% and from 65.5% to 78%, respectively. During the final period (after 15 min of operation), the average humidity inside the closed chamber was 70%. The humidity levels on the front, middle, and rear sides varied from 55.2% to 67.5%, 48.8% to 70%, and 61.1% to 71.5%, respectively (Figure 6). After 15 min of operation, the overall humidity level was 70%. However, the bottom layer of the closed chamber had a humidity level of >70%.

Table 4. Spatial variability of temperature and humidity inside the closed chamber when the humidity level was reduced from 80% to 70%.

Layer	Temperature (°C)		Humidity (%)
	Starting	After 15 min	Starting	After 7.5 min	After 15 min
Top (9 locations)	13.85 ± 0.05 a	13.58 ± 0.05 a	80.02 ± 0.03 b	67.09 ± 1.64 c	60.11 ± 3.52 d
Front	14.45 ± 0.02 a	14.21 ± 0.03 a	80.01 ± 0.03 b	65.01 ± 1.65 c	59.63 ± 2.65 d
Center	13.88 ± 0.02 a	13.54 ± 0.04 a	80.23 ± 0.03 b	67.55 ± 0.56 c	60.14 ± 3.36 d
Rear	13.22 ± 0.05 a	13.01 ± 0.03 a	80.02 ± 0.03 b	69.05 ± 1.85 c	61.18 ± 3.52 d
Middle (9 locations)	13.89 ± 0.05 a	13.94 ± 0.10 a	80.73 ± 0.66 b	70.39 ± 3.43 c	62.11 ± 2.46 d
Front	14.21 ± 0.02 a	14.31 ± 0.05 a	80.79 ± 0.60 b	68.68 ± 3.52 c	60.54 ± 3.65 d
Center	13.89 ± 0.03 a	13.89 ± 0.01 a	80.90 ± 0.56 b	70.89 ± 2.12 c	60.25 ± 1.25 d
Rear	13.58 ± 0.04 a	13.62 ± 0.06 a	80.44 ± 0.72 b	72.56 ± 3.20 c	65.66 ± 2.46 d
Bottom (9 locations)	13.60 ± 0.08 a	13.40 ± 0.08 a	80.04 ± 0.03 b	73.65 ± 2.33 c	66.50 ± 3.91 d
Front	14.10 ± 0.05 a	13.81 ± 0.03 a	80.08 ± 0.01 b	73.05 ± 2.56 c	64.49 ± 2.21 d
Center	13.54 ± 0.01 a	13.42 ± 0.02 a	80.01 ± 0.03 b	70.42 ± 2.01 c	62.97 ± 0.36 d
Rear	13.18 ± 0.03 a	12.97 ± 0.05 a	80.03 ± 0.05 b	75.93 ± 2.31 c	68.34 ± 5.33 d
Average	13.78 ± 0.08	13.64 ± 0.12	80.26 ± 0.24	70.38 ± 2.47	62.92 ± 3.30

^a,b,c,d different letters are significantly different based on Tukey’s one-way comparisons (p ≤ 0.05).

indicates the spatial variability of humidity inside the closed chamber when the humidity level was reduced from 80% to 70% and lists the humidity distribution after the dehumidifier operation. After 7.5 min of operation, the maximum humidity was found to be 73.65% ± 2.33% in the closed-chamber bottom layer, whereas the minimum humidity was found to be 67.09% ± 1.64% in the closed-chamber top layer. Similarly, after 15 min of operation, the maximum humidity was found as 66.5% ± 3.91% in the closed-chamber bottom layer, whereas the minimum humidity was found as 60.11% ± 3.52% in the closed-chamber top layer. After 22 min of operation, all layers had a humidity of <70%.

To reduce the humidity level from 90% to 70%, the dehumidifier required 22 min. During the middle period (after 11 min of operation), the maximum and minimum humidity levels on the front, middle, and rear sides were recorded as 73.40%, 75.50%, and 79% and 67.80%, 63.50%, and 71%, respectively. When the average humidity level inside the closed chamber reached 70%, the maximum and minimum humidity levels in the front, middle, and rear sides were 67%, 68%, and 72% and 60%, 58%, and 60%, respectively (Figure 7). The humidity in the middle layer was found to be lower than that in the top and bottom layers. This vertical variability occurred during the operation because dehumidified air from the dehumidifier module was provided to the middle layer. After 22 min of operation, the overall humidity level was 70%. However, the rear side of the closed chamber had a humidity level of >70%.

Table 5. Spatial variability of temperature and humidity inside the closed chamber when the humidity level was reduced from 90% to 70%.

Layer	Temperature (°C)		Humidity (%)
	Starting	After 22 min	Starting	After 11 min	After 22 min
Top (9 locations)	16.31 ± 0.12 a	16.23 ± 0.13 a	87.16 ± 1.33 b	71.73 ± 1.41 c	64.67 ± 0.58 d
Front	16.54 ± 0.08 a	16.18 ± 0.08 a	87.11 ± 1.65 b	71.21 ± 1.21 c	64.81 ± 0.39 d
Center	15.93 ± 0.06 a	16.24 ± 0.12 a	87.24 ± 0.98 b	70.78 ± 1.65 c	64.89 ± 0.85 d
Rear	16.46 ± 0.16 a	16.27 ± 0.10 a	87.14 ± 1.23 b	73.21 ± 0.78 c	64.31 ± 0.34 d
Middle (9 locations)	16.44 ± 0.14 a	16.27 ± 0.12 a	91.86 ± 0.80 b	72.44 ± 3.39 c	67.33 ± 1.91 d
Front	16.17 ± 0.16 a	16.54 ± 0.10 a	91.35 ± 0.90 b	71.31 ± 4.32 c	66.81 ± 2.06 d
Center	16.78 ± 0.10 a	16.14 ± 0.23 a	91.83 ± 0.61 b	72.78 ± 2.21 c	67.78 ± 0.32 d
Rear	16.37 ± 0.08 a	16.13 ± 0.06 a	92.41 ± 1.10 b	73.23 ± 0.24 c	67.40 ± 1.06 d
Bottom (9 locations)	16.85 ± 0.16 a	16.71 ± 0.14 a	88.99 ± 0.81 b	73.94 ± 3.00 c	70.12 ± 2.41 d
Front	16.48 ± 0.23 a	16.92 ± 0.16 a	16.92 ± 0.39 b	88.76 ± 2.94 c	71.93 ± 2.97 d
Center	17.22 ± 0.12 a	16.31 ± 0.02 a	16.31 ± 0.69 b	89.21 ± 3.02 c	73.89 ± 1.23 d
Rear	16.85 ± 0.14 a	16.91 ± 0.07 a	16.91 ± 0.45 b	89.01 ± 1.45 c	76.01 ± 2.15 d
Average	16.50 ± 0.14	16.40 ± 0.11	89.59 ± 0.98	72.70 ± 2.64	67.16 ± 1.63

^a,b,c,d different letters are significantly different based on Tukey’s one-way comparisons (P ≤ 0.05).

indicates the spatial variability of humidity inside the closed chamber when the humidity level was reduced from 90% to 70% and shows how the humidity changed after the dehumidifier operation. According to Table 5, the humidity levels at the three different layers were different, but the humidity levels at the individual layers were relatively the same. After 11 min of operation, the maximum humidity was found to be 73.94% ± 3.00% in the closed-chamber bottom layer, whereas the minimum humidity was found to be 71.73% ± 1.41% in the closed-chamber top layer. Similarly, after 22 min of operation, the maximum humidity was found to be 70.12% ± 2.41% in the closed-chamber bottom layer, whereas the minimum humidity was found to be 64.67% ± 0.58% in the closed-chamber top layer. After 22 min of operation, except for the bottom layer, all layers had a humidity level of <70%.

3.2.2. Small Greenhouse

During the experiment to reduce the humidity level from 80% to 70%, the outside humidity increased by 2.4%, and the inside humidity decreased by ~10%. Similarly, when the humidity was reduced from 90% to 70%, the outside humidity increased by 4.90%, and the inside humidity decreased by 20.00%.

After the humidity level was stabilized at 80% under equilibrium conditions, the dehumidifying operation was started inside the closed chamber. During the initial period, the average humidity was ~80.00–80.30% for the bottom layer and 79.70% for the top layer. During the middle period (after 9 min of operation), on the front side, the maximum humidity was found in the middle layer (75.30%), and the minimum humidity was recorded near the front layer (65.10%) (Figure 8). After 18 min of operation, the overall humidity level was determined to be 70.00%. However, the rear side of the greenhouse had a humidity level of >70.00%.

Table 6. Spatial variability of temperature and humidity inside the small greenhouse when the humidity level was reduced from 80% to 70%.

Layer	Temperature (°C)		Humidity (%)
	Starting	After 18 min	Starting	After 9 min	After 18 min
Top (9 locations)	19.88 ± 0.29 a	19.56 ± 0.12 a	80.76 ± 0.21 b	74.99 ± 1.72 c	69.73 ± 1.50 d
Front	19.59 ± 0.02 a	19.18 ± 0.08 a	80.85 ± 0.22 b	74.14 ± 1.23 c	68.48 ± 1.25 d
Center	19.93 ± 0.09 a	19.42 ± 0.10 a	80.46 ± 0.20 b	74.82 ± 0.98 c	69.82 ± 0.92 d
Rear	20.12 ± 0.03 a	20.08 ± 0.09 a	80.97 ± 0.23 b	76.01 ± 1.65 c	70.89 ± 1.22 d
Middle (9 locations)	19.91 ± 0.06 a	19.62 ± 0.19 a	80.50 ± 0.11 b	73.86 ± 0.72 c	70.20 ± 1.54 d
Front	19.58 ± 0.06 a	19.20 ± 0.10 a	80.45 ± 0.09 b	73.68 ± 0.92 c	69.12 ± 1.87 d
Center	19.80 ± 0.04 a	19.40 ± 0.20 a	80.21 ± 0.07 b	73.68 ± 0.94 c	70.18 ± 1.69 d
Rear	20.35 ± 0.01 a	20.26 ± 0.09 a	80.84 ± 0.09 b	74.22 ± 0.64 c	71.30 ± 1.41 d
Bottom (9 locations)	20.15 ± 0.21 a	19.88 ± 0.11 a	80.73 ± 0.19 b	74.86 ± 1.67 c	70.20 ± 1.60 d
Front	19.85 ± 0.02 a	19.31 ± 0.12 a	80.45 ± 0.20 b	73.41 ± 1.91 c	69.48 ± 1.51 d
Center	19.88 ± 0.04 a	19.64 ± 0.09 a	80.21 ± 0.20 b	74.85 ± 1.25 c	69.89 ± 1.98 d
Rear	20.72 ± 0.09 a	20.69 ± 0.11 a	80.84 ± 0.18 b	76.32 ± 0.97 c	71.23 ± 2.11 d
Average	19.98 ± 0.25	19.68 ± 0.14	80.51 ± 0.17	74.68 ± 1.37	70.06 ± 1.55

^a,b,c,d different letters are significantly different based on Tukey’s one-way comparisons (p ≤ 0.05).

indicates the spatial variability of humidity inside the small greenhouse when the humidity level was reduced from 80% to 70% and shows how the humidity changed after the dehumidifier operation. According to Table 6, the humidity levels at the three different layers were different, but the humidity levels at the individual layers were relatively the same. After 9 min of operation, the maximum humidity was found to be 74.99% ± 1.72% in the greenhouse top layer, whereas the minimum humidity was found to be 73.86% ± 0.72% in the greenhouse middle layer. Similarly, after 22 min of operation, the maximum humidity was found as 70.20% ± 1.60% in the greenhouse bottom layer, whereas the minimum humidity was found as 69.73% ± 1.50% in the greenhouse top layer. After 28 min of operation, except for the bottom layer, all layers had a humidity of <70%.

To reduce the humidity level from 90% to 70%, the dehumidifier required 35 min. During the middle period (after 17.5 min of operation), the maximum and minimum humidity levels in the front, middle, and rear sides were recorded as 81%, 81%, and 81.5%, and 71.5%, 72%, and 72.50%, respectively. When the average humidity level inside the closed chamber reached 70%, the maximum and minimum humidity levels on the front, middle, and rear sides were 73%, 75%, and 75%, and 66%, 63%, and 68%, respectively (Figure 9). After 35 min of operation, the overall humidity level was 70%. However, the rear top side of the greenhouse had a humidity level of >70%.

Table 7. Spatial variability of humidity inside the small greenhouse when the humidity level was reduced from 90% to 70%.

Layer	Temperature (°C)		Humidity (%)
	Starting	After 35 min	Starting	After 17.5 min	After 35 min
Top (9 locations)	20.15 ± 1.55 a	19.85 ± 1.06 a	90.63 ± 0.14 b	73.21 ± 2.02 c	67.62 ± 2.61 d
Front	20.31 ± 0.89 a	20.12 ± 1.01 a	90.47 ± 0.11 b	71.23 ± 2.03 c	65.63 ± 3.21 d
Center	20.02 ± 1.21 a	19.88 ± 1.06 a	90.61 ± 0.36 b	73.29 ± 1.65 c	67.41 ± 2.14 d
Rear	20.12 ± 2.21 a	19.55 ± 1.00 a	90.81 ± 0.10 b	75.11 ± 2.02 c	69.82 ± 0.26 d
Middle (9 locations)	21.42 ± 1.64 a	21.12 ± 1.04 a	90.45 ± 0.29 b	76.68 ± 2.75 c	69.48 ± 1.47 d
Front	21.50 ± 1.69 a	21.19 ± 1.01 a	90.07 ± 0.31 b	73.61 ± 2.98 c	67.61 ± 1.36 d
Center	21.58 ± 1.21 a	21.05 ± 0.65 a	90.50 ± 0.36 b	77.25 ± 1.39 c	69.28 ± 1.24 d
Rear	21.10 ± 1.02 a	21.12 ± 1.25 a	90.78 ± 0.34 b	79.18 ± 2.01 c	71.55 ± 1.68 d
Bottom (9 locations)	22.48 ± 1.40 a	22.38 ± 0.96 a	90.48 ± 0.15 b	77.36 ± 4.90 c	71.21 ± 2.65 d
Front	22.47 ± 1.65 a	22.26 ± 0.97 a	90.17 ± 0.21 b	72.25 ± 5.36 c	68.10 ± 2.36 d
Center	22.61 ± 1.25 a	22.59 ± 1.02 a	90.51 ± 0.10 b	78.25 ± 6.36 c	71.18 ± 2.97 d
Rear	22.35 ± 1.22 a	22.29 ± 1.05 a	90.76 ± 0.21 b	81.58 ± 5.55 c	74.35 ± 2.14 d
Average	21.35 ± 1.45	21.11 ± 1.03	90.52 ± 0.20	73.75 ± 3.22	69.43 ± 2.21

^a,b,c,d different letters are significantly different based on Tukey’s one-way comparisons (p ≤ 0.05).

indicates the spatial variability of humidity inside the small greenhouse when the humidity level was reduced from 90% to 70% and shows how the humidity changed after the dehumidifier operation. After 17.5 min of operation, the maximum humidity was found to be 77.36% ± 14.90% in the greenhouse bottom layer, whereas the minimum humidity was found to be 73.21% ± 2.02% in the greenhouse top layer. Similarly, after 35 min of operation, the maximum humidity was found to be 71.21% ± 2.65% in the greenhouse bottom layer, whereas the minimum humidity was found to be 67.62% ± 2.61% in the greenhouse top layer. After 35 min of operation, except for the bottom layer, all layers had a humidity of <70%.

4. Discussion

Figure 10 shows the temperature and humidity comparison when the humidity level was reduced from 80% to 70% and 90% to 70% for the small greenhouse. The 80% to 70% operation was taken only 18 min. Due to the short period of operation; the moisture removal rate was low, and no statistically significant difference was found between the inside and outside humidity. During the experiment, the indoor temperature remained constant for both test sites, and the performance of the low-powered dehumidifier module was affected by the humidity ratio of indoor air. The evaporator performance was also a key factor in maintaining the dehumidifier performance [37]. A higher humidity ratio indicates a higher moisture removal rate. In the same manner, the dew point temperature and the evaporating temperature become higher accordingly. As a result, the coefficient of performance of the evaporator increases, which decreases the power and enhances the efficiency of the dehumidification system. The outside temperature shows less impact on the performance of the dehumidifier module. On the other hand, in the 90% to 70% condition, the operation time and moisture removal rate were higher than 80% to 70%. As a result, there has a significant difference between the inside and outside humidity.

The number of dehumidifiers, fan speed, and direction are the primary concerns for providing sufficient dehumidification. According to previous research, the humidity uniformity was also uneven even though it uses different high-powered and large-scale dehumidifiers [38,39]. Based on the spatial and vertical variability, the top and middle layers accrued relatively higher uniformity using the low-powered suspension-type dehumidifier. In contrast, bottom layer uniformity was lower for all conditions. Based on the result of using the same dehumidifier module in a crop condition [6], the bottom layer humidity level was an issue with uniform distribution. Therefore, to improve the uniformity level of the whole greenhouse, an additional dehumidifier module in the bottom layer would be a promising solution.

Table 8. Comparison of power consumption of various greenhouse dehumidifier research and the present study.

Study	Dehumidifier Type	Number of Dehumidifiers	Greenhouse Size (m3)	Mean Electrical Power (kW)	Energy Saves Using the Current Study (%)
[30]	Heat exchanger	4	400.00	3.68	42.60
[38]	Heat exchanger	2	468.00	24.00	694.87
[40]	Heat pump	1	877.00	12.00	112.08
[41]	Heat exchanger	1	1003.00	7.87	21.62
Current	Condensation cooling	1	108.50	0.70	-

shows the comparison of power consumption of various greenhouse dehumidifier studies and the present study. Based on the literature, the condensation cooling dehumidifier (current study) used the least power consumption than the other commercial greenhouse dehumidifier. On the other hand, the per-area power consumption was higher when using the heat pump dehumidification system. According to the analysis, the proposed low-powered condensation cooling system dehumidifier may depending on climate conditions, reduce power consumption from 20 to 700%.

5. Directions for Further Research

Based on the spatial and vertical variability of humidity, an extra dehumidifier is required at the bottom layer to make the uniform humidity distribution. However, in the future, an artificial simulation is necessary before installing this dehumidifier in large greenhouses with crops. For that purpose, we have to adapt the climatical status in order to secure an optimum environment for the crop. The artificial simulation needs to consider inside greenhouse water vapor transfer including air exchange processes through vents or leakage, plant transpiration, condensation, and evaporation. To analyze and predict the humidity condition, computational fluid dynamics (CFD) will be an effective method to simulate the dehumidification status inside greenhouses. CFD and the real test including our proposed dehumidifier would aid in the measure of the location of the multiple dehumidifiers for different crop conditions.

In addition, solar modules will be installed to act as the main power source for various sensors and dehumidifiers. The main purpose of using this low-powered dehumidifier is to use solar-generated electricity as well as deploy agrivoltaic systems.

6. Conclusions

In this study, we evaluated the performance of a low-powered suspension-type dehumidifier module in terms of the humidity changes over time and investigated the humidity distributions by spatial and vertical variability analysis for a closed chamber and a small greenhouse. A mass balance method was used to calculate the humidity response theoretically. The moisture removal rate, experimental area, condensation on the area cover, and the inner airspeed were considered in the theoretical calculations. The humidity response of the dehumidifier module indicated that the difference between the theoretical and experimental humidity response times for both 90% to 70% and 80% to 70% humidity reductions was 1 min. According to the spatial and vertical variability, the humidity change range at the top layer was greater than that at the middle and bottom layers. For the 90% to 70% and 80% to 70% operating conditions, the average humidity level decreased while maintaining the humidity response time. However, uniformity was not maintained because the dehumidifier air did not reach all the enclosed space, especially in the bottom layer. The results of this study would provide useful information for the development of low-power dehumidifier modules and demonstrate the feasibility of controlling humidity in various greenhouses using the presented guidelines.