A Study on the Impact of Different Cooling Methods on the Indoor Environment of Greenhouses Used for Lentinula Edodes during Summer

Abstract

The shitake mushroom (lentinula edodes) industry in the Gobi Desert region of southern Xinjiang has experienced rapid development and has reached a certain scale. To clarify the laws governing different cooling methods in greenhouses and identify suitable cooling methods for mushroom production in the Gobi Desert region, this study focused on monitoring the environmental changes in greenhouses using three different cooling methods: natural ventilation cooling, water-sprinkling roof cooling, and a fan and pad cooling system. The results showed that when combined with external shading (shade netting), natural ventilation cooling, fan and pad cooling, and water-sprinkling roof cooling, respectively, reduced the air temperature by 8.6 °C, 14.0 °C, and 15.2 °C. They also increased the relative humidity by 15.3%, 43.3%, and 51.2%, resulting in cooling efficiencies of 28.5%, 56.3%, and 68.1%, respectively. The water-sprinkling roof cooling system demonstrated the best cooling effect and temperature uniformity and had higher economic benefits. Therefore, the use of the external sprinkler cooling method in double-skeleton greenhouses is suitable for summer lentinula edodes production in the Gobi Desert region of southern Xinjiang.

1. Introduction

From 2008 to 2017, global production of edible mushrooms increased from 6.9 million metric tons to 10.24 million metric tons [1]. China has emerged as a major producer of edible mushrooms, particularly lentinula edodes, and has been the world’s leading producer since 1990 [2]. Lentinula edodes in China accounted for the highest production among all cultivated mushrooms, representing 28.37% of the total production [3]. As lentinula edodes cultivation expands, production in the Gobi Desert region has gradually developed to make efficient use of land resources. Due to the specific environmental requirements of lentinula edodes, such as low temperatures, high humidity, and significant temperature fluctuations, studying the ideal greenhouse environment for mushroom cultivation becomes crucial as the industry progresses.

The optimal temperature for lentinula edodes spore germination is 22–26 °C, while the temperature range for mycelium growth is 5–35 °C, with the optimal range being 23–25 °C [4]. Different growth stages have varying requirements for relative humidity: around 70% during the growth and development stage and 85–90% during the fruiting stage is most suitable [5]. Xinjiang is an important part of the arid region in central Asia, characterized by hot and dry summers [6]. Without implementing cooling measures, lentinula edodes cannot grow properly. Therefore, studying cooling methods for the greenhouse environment in the Gobi Desert region of Xinjiang is crucial. Various cooling measures can be employed in greenhouses, including natural ventilation cooling, shading, evaporative cooling, and air conditioning [7].

Natural ventilation cooling (NVC) is the most cost-effective method for cooling in greenhouses, as it requires no additional energy [8,9]. Research by McCartney et al. [10] has shown that NVC can reduce temperatures by 1.3 to 3.6 degrees Celsius and increase relative humidity by 5.7–17.7%. Furthermore, NVC helps regulate temperature, humidity, and gas concentrations such as carbon dioxide, resulting in effective cooling and improved greenhouse conditions. However, the cooling effect of NVC is greatly influenced by external environmental factors [11].

Shade cooling is a method of cooling in greenhouse facilities by means of preventing direct sunlight from entering the structure using shade nets or similar materials. Research by Hatem et al. [12] examined the impact of external shade nets on indoor air temperature and relative humidity. The results showed that the addition of external shade nets reduced the internal temperature of the greenhouse by approximately 3 °C to 5 °C while increasing the relative humidity by 2% to 5%. Chen et al. [13] compared the temperature difference between indoor and outdoor environments with and without shading. The results indicated that, with an outdoor temperature of 35 degrees Celsius and solar radiation of 850 W/m², the shaded greenhouse had an approximately 3 °C lower temperature difference between the indoor and outdoor environments compared to the unshaded greenhouse. Ahmed et al. [14] studied the radiation distribution in greenhouses under different shading configurations. The results showed that external shading was more effective at reducing thermal radiation than internal shading. Using external shading could reduce thermal radiation by 15% during the day and 21% during the night in the greenhouse.

Evaporative cooling is a method of achieving cooling by means of utilizing heat absorption during the evaporation of water. The evaporation of 1 g of liquid water into 1 m³ of water vapor can lower its temperature by 2.5 degrees Celsius [15]. Commonly used evaporative cooling systems include the fan and pad cooling (FPC) system, misting cooling system, and water-sprinkling roof cooling (WSC) system [8,16]. Saberian et al. [17], through CFD simulations in a subtropical desert region, found that the FPC can reduce the indoor temperature of a greenhouse by approximately 15 to 25 °C. The cooling efficiency ranged between 57% to 77% within a day, and during noon, the cooling efficiency remained between 58% to 75%. The system was able to maintain greenhouse temperatures within the range of 27 to 30 °C with a relative humidity of around 60%. In a roof exposed to solar radiation, the maximum temperature of an unmodified roof reached 57.8 °C, while a roof equipped with a roof spraying system had a maximum temperature of approximately 37.2 °C, resulting in a temperature reduction of around 20.6 °C with the use of a sprinkler cooling system.

Through conducting experiments and measuring environmental parameters, this study evaluated the effects of different cooling methods on the greenhouse environment, including temperature and humidity. Through comparative analysis, it aimed to determine which cooling method would have better cooling performance in the production of lentinula edodes in the Gobi region during the summer, creating a more suitable growth environment and providing theoretical and practical support for summer lentinula edodes production in the Gobi region.

2. Material and Methods

2.1. Overview of the Lentinula Edodes Greenhouse

The test site was located at Bei Kang Microbial Technology Co., Ltd.’s double-layer plastic lentinula edodes greenhouse in Hotan Prefecture, Xinjiang (E 78.59°, N 37.4°, altitude 1413.9 m) (Figure 1). The lentinula edodes greenhouse was oriented in a north–south direction and constructed with a double-layer steel frame. The endothecium greenhouse was covered with thermal insulation and anti-aging plastic film, while the outer-field greenhouse used anti-aging plastic film in autumn and winter, and a double-layer shade net in spring and summer. The endothecium greenhouse had a length of 30 m, a width of 12 m, and a ridge height of 4 m. The outer-field greenhouse had a length of 32 m, a width of 14 m, and a ridge height of 5 m. Three greenhouses were used in the experiment, and all of them were equipped with internal spray systems for humidification and to cope with extremely high temperatures. One greenhouse utilized fan and pad cooling (FPC), another greenhouse used water-sprinkling roof cooling (WSC), and the third greenhouse relied on natural ventilation cooling (NVC).

The FPC greenhouse was equipped with two negative pressure fans (single-phase 550 W, air volume 28,000 m²/h), each mounted on the inner shed columns and horizontal columns, 0.45 m above the ground, with a gap of 7.8 m between them. The evaporative cooling pads were positioned on both sides of the entrance to the inner shed, horizontal columns of the inner shed. In the WSC greenhouse, four spray pipes (Φ32 PE black micro-nozzles) were laid between the inner shed and the outer shed. Each of these spray pipes had 24 atomized polyline nozzles with a spacing of 1.25 m. Water control for the spray pipes was managed using separate ball valves (Φ50). The NVC greenhouse relied mainly on ventilation windows located at the ends of the greenhouse (2 m long, 0.9 m wide) and bottom-side ventilation windows. Each greenhouse had lentinula edodes racks, along the east–west axis of each shed, there were five groups of assembled synthesis frames, with each frame measuring 6 m in length, 1 m in width, and 1.8 m in height (Figure 2).

During the testing process, the transmittance of the three types of greenhouses and the materials used for plastic film and shade net were consistent. The cooling systems of the three greenhouses were activated at 10:00 in the morning and deactivated at 20:00.

2.2. Test Equipment and Layout of Measuring Points

In July 2022, the indoor and outdoor environmental conditions of the mushroom greenhouse were monitored. The testing parameters include temperature, humidity, and the time interval for all measurement points was 10 min. The temperature was collected using the PDE-R4 temperature data recorder (Harbin Wage Electronic Technology Co., Ltd., Heilongjiang, China, temperature measuring range: −20–60 °C, accuracy: ±0.5 °C, resolution: 0.1 °C), The humidity was recorded using the PDE-SH thermal environment monitoring recorder (Harbin Wage Electronic Technology Co., Ltd., Heilongjiang, China, humidity measuring range: 0–99%, accuracy: ±3%, resolution: 1%).

The outdoor temperature and humidity sensors were installed at a distance of 1 m from the greenhouse and 0.9 m above the ground. The indoor sensors were installed at different positions, but the instrumentation was the same for all three greenhouses, as shown in Figure 3. Six temperature sensors are installed in each greenhouse, placed at the following locations: along the length at 8 m, 16 m, and 24 m; along the width at 7 m, at a height of 0.9 m above the ground; along the width at 3.5 m and 10.5 m, along the length at 16 m, at a height of 0.9 m above the ground; and along the length at 16 m, width at 7 m, at a height of 2.5 m above the ground. Three air relative humidity sensors are installed in each greenhouse, located at positions along the length of 8 m, 16 m, and 24 m, and along the width of 7 m, at a height of 0.9 m above the ground.

2.3. Statistical Analyses

Cooling Efficiency Calculation

Efficiency of cooling, n, is an analysis of the cooling performance among different types of greenhouse cooling systems. The ratio of the differences between the temperatures of dry bulbs within and outside of the greenhouse to those between the temperatures of moist bulbs inside and outside of the greenhouse is known as $n$. The formula for determining the greenhouse’s cooling efficiency is as follows [18,19,20,21]:

$$n=\frac{T_{db,o}-T_{db,c}}{T_{db,o}-T_{wb,c}}\times 100\%$$

(1)

where $n$ represents the cooling efficiency of three different cooling systems; $T_{db,c}$ and $T_{wb,c}$ are the dry and wet bulb temperatures inside l the greenhouse in °C; $T_{db,o}$ is the dry bulb temperature of the cooled air outside the greenhouse in °C. The formula below illustrates how to calculate wet bulb temperature using Stull’s [22] conversion method:

T_wb,c = T × atan[0.151977(RH% + 8.313659)^1/2] + atan(T + RH%) − atan(RH% − 1.676331) + 0.003918(RH%)^3/2 × atan(0.023101RH%) − 4.686035

(2)

where T is the greenhouse’s air temperature and RH% is the relative humidity of the atmosphere there.

3. Results and Analysis

3.1. Microclimate Analysis of Different Cooling Greenhouses

From 21 July to 25 July 2022, environmental monitoring data for the greenhouse was collected five days in a row. The daily temperature change in the room is represented by a three-point average of 8 m, 16 m, and 24 m in the length direction, 7 m in the span direction, and 0.9 m above the ground (racks for lentinula edodes along height 1/2).

3.1.1. Temperature Changes in Different Cooling Methods

Temperature is one of the key indicators for evaluating the cooling effect in the greenhouse. In mushroom production, controlling the temperature inside the greenhouse is crucial. Figure 4 shows the temperature variations inside the greenhouse and outside the greenhouse for 5 consecutive days from 21 July to 25 July 2022, using different cooling methods.

Outside the greenhouse, the highest temperatures occur around 17:00 each day, reaching a peak of 45.1 °C at noon on the 25th. The lowest outdoor temperatures are observed between 7:00 and 8:00 in the morning, with the lowest temperature recorded at 7:50 on the 24th at 21.3 °C. The temperature trends for the three different cooling methods align with the outdoor variations. There is little difference in nighttime temperatures, but significant differences are observed during the daytime. The NVC and FPC reach their highest temperatures around 17:00, reaching 37 °C and 31.1 °C, respectively. The WSC greenhouse reaches its highest temperature between 18:00 and 19:00, reaching 27.2 °C. The lowest temperatures for all three methods occur between 7:00 and 8:00 in the morning, with the NVC, FPC, and WSC recording minimum temperatures of 21.2 °C, 19.1 °C, and 20 °C, respectively.

During the testing period, the NVC, FPC, and WSC greenhouses experienced about 45.6%, 85.8%, and 78.6% of the temperature readings that were below 26 °C, respectively. The temperatures inside the FPC and WSC greenhouses were mostly maintained within the optimal range for mushroom growth (The optimal temperature for lentinula edodes spore germination is 22–26 °C [4]). However, the FPC greenhouse exceeded 30 °C for one hour at noon on the 25th, while the temperature remained below 30 °C for the rest of the time. This may be attributed to a sudden increase in outdoor temperature around 17:00. In the WSC greenhouse, the peak temperature was observed about one hour later, indicating that the external spraying cooling method has a stronger buffering effect. Therefore, the FPC greenhouse demonstrated good cooling performance but was not as effective as the WSC greenhouse, while the NVC greenhouse had the least effective cooling performance.

3.1.2. Cooling Efficiency of Different Cooling Methods

When evaluating the cooling performance of different cooling methods, the cooling efficiency is an important indicator. Under the same conditions, a higher cooling efficiency indicates better cooling performance. We have selected the data from 23 July 2022 (Major Heat) to analyze the variation in cooling efficiency for different cooling methods (Figure 5). The calculation of cooling efficiency ($n$) is based on Formulas (1) and (2).

The average cooling efficiencies of the NVC, WSC, and FPC greenhouses on 23 July are 24.6%, 65.2%, and 48%, respectively. The average cooling efficiencies during the cooling operation period (10:00 to 20:00) are 28.5%, 68.1%, and 56.3%, respectively. The maximum values are 43.2%, 79.2%, and 72.3%, respectively. The minimum values are 14.5%, 56.6%, and 41.1%, respectively. The results indicate that after the start of the cooling operation, the cooling efficiency of the WSC greenhouse is higher than that of the FPC greenhouse, which is higher than that of the NVC greenhouse. The cooling efficiency of the FPC and WS greenhouses is relatively close, but the cooling efficiency of the external spraying system (WSC) is more stable. After the cooling operation starts, the cooling efficiency of the NVC and FPC greenhouses increases significantly, while the cooling efficiency of the WSC greenhouse increases less significantly. This is mainly because the nighttime cooling efficiency of the WSC greenhouse is higher than that of the NVC and FPC greenhouses. Therefore, the cooling performance is best when using the external spraying cooling method, followed by the wet curtain fan cooling system, and the natural ventilation cooling system has the lowest cooling performance.

3.1.3. Relative Humidity Changes in Different Cooling Methods

The growth of lentinula edodes is highly sensitive to humidity, making the study of relative humidity particularly important. Figure 6 shows the variations in indoor and outdoor relative humidity over five consecutive days from 21 July to 25 July 2022. The humidity trends in the NVC, WSC, and FPC greenhouses, which employ different cooling methods, align with the outdoor conditions. The relative humidity is higher during the morning and evening, while it decreases around noon.

The period around 16:00 each day experiences the lowest outdoor relative humidity, with the lowest recorded at 16:30 on 22 July, reaching 7%. The highest outdoor relative humidity occurs in the morning, around 7:30, with the peak value of 42% recorded at 7:40 on 22 July. The trends in indoor relative humidity in the NVC, WSC, and FPC greenhouses resemble those observed outdoors. The lowest indoor relative humidity in the NVC, WSC, and FPC greenhouses occurs around 16:00, with values of 18.3%, 38.3%, and 41%, respectively. The highest indoor relative humidity in the NVC, WSC, and FPC greenhouses occurs in the early morning, with peak values of 56.3%, 93.3%, and 80.3%, respectively. Throughout the five-day monitoring period, the indoor relative humidity in the NVC greenhouse remains below 60%. The majority of the time, the humidity in the WSC and FPC greenhouses is above 60%. However, the FPC greenhouse reaches relative humidity above 75% only 9.7% of the time, whereas the WSC greenhouse reaches this level 48.9% of the time. The average outdoor relative humidity over the five days is 20.8%. The average indoor relative humidity in the NVC, WSC, and FPC greenhouses is 36.1%, 72.0%, and 64.1%, respectively. Therefore, the WSC greenhouse demonstrates the most effective relative humidity control, followed by the FPC greenhouse, while the NVC greenhouse exhibits the poorest relative humidity control.

3.2. Analysis of Temperature Uniformity of Different Cooling Greenhouses

In greenhouse production, there is often regional variability within the crops, which leads to difficulties in quality and management. One of the reasons for this variability is the uneven distribution of temperature inside the greenhouse. Therefore, it is crucial to analyze the temperature uniformity under different cooling methods for cultivating lentinula edodes. On 23 July 2022, a comparison was made between different positions inside and outside the greenhouse in terms of length direction, span direction, and vertical direction. In the length direction, three points were selected: front (F), middle (M), and back (B) (at 8 m, 16 m, and 24 m along the length, respectively, at a height of 0.9 m from the ground). In the span direction, three points were chosen: left (L), middle (M), and right (R) (at 16 m along the length and 3.5 m, 7 m, and 10.5 m along the span, respectively, at a height of 0.9 m from the ground). In the vertical direction, two points were taken: top (T) and bottom (Bo) (at 16 m along the length, 7 m along the span, at heights of 0.9 m and 2.5 m from the ground, respectively). Outside the greenhouse, the highest temperature was recorded at 17:00, reaching 40.7 °C, while the lowest temperature occurred at 07:50 and was 21.8 °C.

3.2.1. NVC, WSC, and FPC Temperature Changes in the Length Direction

Figure 7 shows the temperature variation trend along the length direction in different cooling methods in the greenhouse during the summer season on 23 July 2022. The results demonstrate that the temperature variation trend along the length direction in different cooling methods is consistent with the outdoor conditions, indicating uniform temperature changes. At noon, the temperature in the NVC greenhouse is higher than that in the WSC and FPC greenhouses. Additionally, during the night, the temperature in the FPC greenhouse is higher than that in the WSC greenhouse.

Figure 7a represents the temperature variation along the length direction in the NVC greenhouse. The highest temperature recorded at the F of the greenhouse is 32.2 °C, while the highest temperatures at the B and M of the greenhouse are 31.9 °C. The lowest temperatures recorded at F, M, and B are 23 °C, 22.5 °C, and 22.5 °C, respectively. The temperature difference along the length direction in the NVC greenhouse at the highest temperature is 0.3 °C, which is significantly lower compared to the outdoor temperature difference of 8.8 °C at the highest temperature. The temperature difference along the length direction in the NVC greenhouse at the lowest temperature is 0.5 °C, while the outdoor temperature difference at the lowest temperature is 0.7 °C. Furthermore, during the night, the indoor temperature in the NVC greenhouse closely matches the ambient temperature.

Figure 7b illustrates the temperature variation along the length direction in the WSC greenhouse. The highest temperatures recorded at F, M, and B of the greenhouse are 25.7 °C, 26.4 °C, and 26.4 °C, respectively. The lowest temperatures recorded at F, M, and B are 19.4 °C, 19.6 °C, and 19.6 °C, respectively. The temperature difference along the length direction in the WSC greenhouse at the highest temperature is 0.7 °C, while the temperature difference between the WSC greenhouse and the outdoor temperature at the highest temperature is 15 °C. At the lowest temperature, the temperature difference along the length direction in the WSC greenhouse is 0.2 °C, whereas the temperature difference between the WSC greenhouse and the outdoor temperature at the lowest temperature is 1.4 °C.

Figure 7c depicts the temperature variation along the length direction in the FPC greenhouse. The highest temperature recorded at F of the greenhouse is 29.5 °C, while both the B and M of the greenhouse have the highest temperature of 29.3 °C. The lowest temperatures recorded at F, M, and B are 20.6 °C, 20.6 °C, and 20.7 °C, respectively. The temperature difference along the length direction in the FPC greenhouse at the highest temperature is 0.2 °C, while the temperature difference between the FPC greenhouse and the outdoor temperature at the highest temperature is 11.2 °C. At the lowest temperature, the temperature difference along the length direction in the FPC greenhouse is 0.1 °C, whereas the temperature difference between the FPC greenhouse and the outdoor temperature at the lowest temperature is 1.1 °C.

3.2.2. NVC, WSC, and FPC Temperature Changes in the Span Direction

Figure 8 represents the temperature variation trend along the span direction in different cooling methods in the greenhouse on 23 July 2022. The results demonstrate that the temperature variation trend along the span direction in different cooling methods is consistent with the outdoor conditions. During the daytime, the WSC greenhouse exhibits the best uniformity along the span direction, followed by the NVC greenhouse, while the FPC greenhouse shows the least uniformity. During the nighttime, all three cooling methods in the greenhouse display uniform temperature variations along the span direction.

Figure 8a illustrates the temperature variation along the span direction in the NVC greenhouse. The results show that at the L, M, and R positions of the greenhouse, the highest temperatures are 35 °C, 33 °C, and 33.9 °C, respectively. The lowest temperatures recorded at L, M, and R are 20.9 °C, 20.9 °C, and 21 °C, respectively. At the highest temperature along the span direction, the temperature difference is 2 °C, with a difference of 5.7 °C compared to the outdoor highest temperature. At the lowest temperature along the span direction, the temperature difference is 0.1 °C, with a difference of 0.9 °C compared to the outdoor lowest temperature.

Figure 8b displays the temperature variation along the span direction in the WSC greenhouse. From the results, we observe that the highest temperatures at L, M, and R positions are 26.6 °C, 26.4 °C, and 27.5 °C, respectively. The lowest temperatures at these positions are 19.4 °C, 19.6 °C, and 19.4 °C, respectively. In the span direction, the temperature difference at the highest temperature is 1.1 °C, with a difference of 13.2 °C compared to the outside maximum temperature. At the lowest temperature, the temperature difference along the span direction is 0.2 °C, with a difference of 2.4 °C compared to the outside minimum temperature.

Figure 8c represents the temperature variation along the span direction in the FPC greenhouse. From the results, we observe that the highest temperatures at the L, M, and R positions are 34.3 °C, 29.3 °C, and 30.9 °C, respectively. The lowest temperatures at these positions are 21.1 °C, 20.6 °C, and 20.9 °C, respectively. In the span direction, the temperature difference at the highest temperature is 5 °C, with a difference of 11.4 °C compared to the outside maximum temperature. At the lowest temperature, the temperature difference along the span direction is 0.5 °C, with a difference of 1.2 °C compared to the outside minimum temperature.

3.2.3. Vertical Temperature Fluctuations of the NVC, WSC, and FPC

Figure 9 depicts the temperature variation along the vertical direction in different cooling greenhouses on 23 July 2022, during the summer season. The results demonstrate that the temperature trends along the height direction in the different cooling greenhouses are consistent with the outdoor conditions. During the daytime, there are significant temperature differences between the top and bottom of the three cooling greenhouses. The top temperature of the NVC greenhouse occasionally exceeds or equals the outdoor temperature within a short period. The upper part of the FPC greenhouse exhibits higher temperatures compared to the WSC greenhouse. During the nighttime, the temperature variation along the vertical direction is uniform for all three cooling greenhouses.

Figure 9a presents the temperature variation along the vertical direction in the NVC greenhouse. Along the vertical direction, it can be observed that the temperature at the T of the greenhouse occasionally reaches or equals the outdoor temperature during midday. The maximum temperature at the upper part is 37 °C, while the maximum temperature at the Bo is 33 °C. The lowest temperature recorded at the upper part is 21.2 °C, and at the lower part is 20.9 °C. In the vertical direction, the maximum temperature difference at the highest temperature is 4 °C, with a difference of 3.7 °C compared to the outdoor maximum temperature. At the lowest temperature, the temperature difference along the vertical direction is 0.3 °C, with a difference of 0.9 °C compared to the outdoor minimum temperature.

Figure 9b presents the temperature variation along the vertical direction in the WSC greenhouse. In the vertical direction, the maximum temperature is 32.6 °C for T and 26.4 °C for Bo. Both T and Bo have a minimum temperature of 19.6 °C. Regarding the vertical direction, the temperature difference between Bo and T at the highest temperature is 6.2 °C, with a temperature difference of 14.3 °C compared to the ambient temperature. At the lowest temperature, there is no temperature difference between Bo and T (0 °C), while the temperature difference from the ambient temperature is 2.2 °C.

Figure 9c presents the temperature variation along the vertical direction in the FPC greenhouse. When examining the vertical axis, the maximum temperature recorded at T is 34.3 °C, while Bo reaches a maximum temperature of 29.3 °C. The respective minimum temperatures for T and Bo are 20.8 °C and 20.6 °C. In the vertical direction, the temperature difference between Bo and T at their highest temperatures is 5 °C, with a temperature difference of 11.4 °C compared to the ambient temperature at its peak. At the lowest temperature, the temperature difference between Bo and T in the vertical direction is 0.2 °C, while the temperature difference from the ambient temperature is 1.2 °C at its lowest point.

The NVC greenhouse exhibits uniform temperature variation along the length direction, with temperature differentials of less than 1 °C. However, there are slight temperature variations along the span direction, with L having a higher temperature than R and M. At its highest temperature, L differs from R by 1.6 °C and from M by 2 °C. Along the vertical direction, the temperature variation is uneven, with T and Bo having a maximum temperature difference of 4 °C. In the WSC greenhouse, temperature changes are uniform along the length and span directions, but there are some differences along the vertical direction. Around noon, before and after 17:00, there is a temperature difference of 6.2 °C between the upper and lower parts of the greenhouse. The upper part of the greenhouse has higher temperatures but is lower than the FPC greenhouse. The temperature in the lentinula edodes cultivation area is maintained below 27 °C, reaching a maximum of 26.6 °C, but only momentarily, achieving effective cooling and minimizing the impact on lentinula edodes. In the FPC greenhouse, there are significant temperature variations along the span and vertical directions. Before 17:00, the temperature difference between L and R, as well as between L and M in the span direction, reaches 3.4 °C and 5 °C, respectively, indicating large temperature differentials. The temperature difference along the vertical direction reaches 5 °C, with T having higher temperatures. Therefore, the WSC greenhouse exhibits the best overall uniformity, followed by the FPC greenhouse, while the NVC greenhouse shows the lowest level of uniformity.

3.3. Cost Analysis of Different Cooling Greenhouses

Increasing profit is one of the objectives of agricultural production, and it can be achieved through reducing construction and operational costs. Therefore, it is crucial to compare the construction and operational costs of different cooling greenhouses. The construction materials and methods for the three types of cooling greenhouses are consistent, with the only difference being the cooling equipment. Therefore, we will only consider the costs of the cooling equipment and operation when comparing the costs. In the case of the NVC greenhouse, it relies on external conditions for cooling and does not require additional costs for cooling, so the cooling cost is negligible. The main comparison for cooling costs will be between the WSC and FPC greenhouses, since both of them use evaporation as a cooling method, and any excess water is recycled and reused. Therefore, we do not consider any differences in water evaporation between the two.

Table 1. Cost of cooling equipment for different cooling greenhouses.

Cooling Type	Name	Unit Price	The Amount	Total Price
NVC				USD 0
WSC	water pipelines	USD 1.41	120	USD 406.83
	sprinkler heads	USD 0.28	96
	Water pump	USD 210.75	1
FPC	negative pressure fans	USD 112.4	2	USD 686.36
	evaporative cooling pad	USD 35.83	7
	Water pump	USD 210.75	1

presents the cooling equipment costs for the different cooling greenhouses.

From Table 1, it can be observed that the cooling equipment required for WSC includes 120 m of water pipes for laying the sprinkler system, 96 spray nozzles, and one water pump, with a total value of USD 406.83. The cooling equipment required by the FPC includes two negative pressure fans, two evaporative cooling pads (each measuring 3.5 m²), and one water pump, with a total value of up to USD 686.36. Therefore, in terms of cooling equipment costs, FPC has the highest cost, followed by WSC, while NVC has a construction cost of zero. During operation, power consumption is a primary consideration for WSC and FPC (depreciation costs are ignored). WSC consumes 21 kW·h of electricity per day (11 kW·h for the fans and 10 kW·h for the water pump). Additionally, the WSC greenhouse requires an extra 10 kW·h of electricity per day for water pump pressurization. Based on the above analysis, it can be concluded that NVC is the most economical option, although its cooling effectiveness is not good. FPC has higher cooling equipment and operational costs compared to WSC, without achieving higher benefits.

4. Discussion

Cooling is considered a fundamental requirement for overcoming high-temperature issues in greenhouse crop production [23], and a well-designed greenhouse (including its shape, size, and roof structure) combined with appropriate cooling techniques is key to addressing summer greenhouse production challenges in high-temperature regions [24]. In this experiment, three methods were used to cool the greenhouse: external shading combined with natural ventilation, a water-sprinkling roof, and a fan and pad cooling system.

A shading net can reduce the heat entering the greenhouse through blocking a portion of direct sunlight. When solar radiation passes through the shading net, some of the light is reflected, scattered, or absorbed, causing the surface temperature of the shading net to rise. As a result, some of the heat on the shading net is taken away, and not all of it is transmitted into the interior of the greenhouse. This helps reduce the solar heat load inside the greenhouse and lowers the temperature. Additionally, the shading net contributes to slowing down the greenhouse effect. The greenhouse effect refers to the trapped heat inside the greenhouse, which leads to a continuous rise in the temperature inside. The use of a shading net can, to a certain extent, prevent excessive heating inside the greenhouse and alleviate the impact of the greenhouse effect.

Natural ventilation cooling (NVC) causes the air inside and outside the greenhouse to form convection, with cold air descending and hot air rising. Through ventilation facilities such as vents and openings, cold air enters the greenhouse, pushing the hot air upwards and allowing it to be expelled through the ventilation openings. This convection heat exchange process effectively transfers heat from the interior of the greenhouse to the outside air, thereby reducing the temperature inside the greenhouse. Natural ventilation also helps control the thermal conduction of the interior surfaces and walls of the greenhouse. In hot weather, the greenhouse surface may absorb a significant amount of heat. Through natural ventilation, this heat can be rapidly dissipated to the external environment via thermal conduction, thereby reducing the heat load inside the greenhouse.

The fan and pad cooling (FPC) works as follows: When external air passes through the evaporative cooling pad, water on the surface of the evaporative cooling pad evaporates into water vapor, absorbing heat from the air. This process is an endothermic process, thereby reducing the temperature of the air. Water in the evaporative cooling pad is continually replenished from a water tank or pipeline, keeping the evaporative cooling pad in a moist state for continuous evaporative cooling. The FPC circulates the cooled air into the greenhouse using fans. Fresh and cool air enters the greenhouse through the evaporative cooling pad, pushing the hot air inside the greenhouse upwards, creating an air convection. As a result, the hot air inside the greenhouse is expelled, while fresh and cool air is continuously supplied, forming a circulation. This air circulation process helps uniformly lower the temperature inside the greenhouse, maintaining fresh and cool air within the greenhouse.

Water-sprinkling roof cooling (WSC) sprays water mist onto the greenhouse roof. When the water mist encounters hot air, it rapidly evaporates into water vapor; this evaporation process requires heat absorption, thus taking away some heat from the surrounding air and reducing the air temperature. This evaporation cooling effect leads to a decrease in the air temperature inside the greenhouse. During the spraying process, some of the water mist absorbs the heat from the interior of the greenhouse before evaporating. This creates a protective layer of mist inside the greenhouse, blocking direct sunlight radiation from entering the interior. As a result, the greenhouse’s solar heat gain is reduced. Additionally, the water mist absorbs the heat from the interior, slowing down the temperature rise inside the greenhouse and maintaining a lower temperature.

In the study, NVC can reduce the average daily temperature by 3.4 °C and achieve a maximum temperature difference of 8.6 °C between the inside and outside of the greenhouse, the research results are similar to those of Lucas McCartney: the cooling effect of the greenhouse varies between 1.9 °C and 12.6 °C [25]. It provides some cooling effect, but the cooling efficiency is low. In hot summer seasons, when external temperatures are excessively high, relying solely on natural ventilation may not fulfill the temperature requirements for mushroom growth. Nevertheless, natural ventilation offers advantages such as low energy consumption and practicality [26]. It can be effectively utilized during the spring and autumn seasons to reduce production costs.

In the study, FPC can reduce the average daily air temperature inside the greenhouse by 6.8 °C. It can achieve a maximum temperature difference of 14 °C between the inside and outside of the greenhouse while increasing the relative humidity of the air inside the greenhouse. Similar to the research conducted by other researchers, in a high-temperature environment ranging from 32 °C to 42 °C during the summer, using wet curtains in a mushroom greenhouse can reduce the internal temperature to 23 °C to 30 °C, achieving a cooling effect of 9 °C to 12 °C [27]. In low-humidity desert regions, using an external shading system combined with a fan and pad cooling system can reduce the greenhouse’s internal temperature by 4–12 °C [23]. It has good cooling efficiency. However, the construction and operational costs of implementing wet curtains and fan cooling are high. Moreover, this cooling method exhibits significant temperature gradients. In the experiment, large temperature gradients were observed in both the span-wise and vertical directions, which aligns with previous research findings [28,29]. Therefore, it is not recommended to use this cooling method in summer lentinula edodes production.

In the study, WSC demonstrates high cooling efficiency. It can reduce the average daily temperature inside the greenhouse by 7.9 °C. Similar to the study of Yu et al., the greenhouse temperature can be reduced by 6.6 °C [30], with a maximum temperature difference of 15.2 °C between the inside and outside of the greenhouse. Additionally, the air temperature distribution inside the greenhouse is relatively uniform. Spraying water at the top of the greenhouse effectively removes a significant amount of heat from the greenhouse; the cooling effect is stronger than that in the research of Guo et al. [31]. However, it is important to note that many regions in Northwest China face severe water scarcity issues [32]. Since the present experiment did not investigate water flow rates, it is necessary to conduct further research to optimize the cooling effect through setting appropriate sprinkler flow rates with the aim of conserving water resources.

Internal sprinkler cooling is not employed in the greenhouse primarily because high relative humidity can negatively affect the quality of mushroom growth. Lentinula edodes require an optimal relative humidity of around 70% during their growth and development stages [4]. Through using internal sprinklers for cooling, the relative humidity inside the greenhouse would exceed 70%, potentially causing direct water contact with the lentinula edodes and impacting their growth. Internal sprinkler cooling is mainly used before mushroom substrate watering and during extreme weather conditions.

The development of automated agriculture is a necessity in the current era [33], and the mushroom industry is moving toward artificial intelligence. Therefore, there is a need for more precise environmental measurements. Currently, the monitored environment is that of the greenhouse, but there may be differences between the greenhouse environment and the environment inside the mushroom substrate. Therefore, future research should focus on studying the relationship between the greenhouse and the mushroom substrate, aiming to accurately monitor the internal environment of the mushroom substrate through monitoring the greenhouse environment. This will provide a theoretical basis for the implementation of artificial intelligence in mushroom production.

5. Conclusions

This study focuses on analyzing the internal environmental changes in greenhouses using different cooling methods, specifically, natural ventilation cooling, fan and pad cooling, and water-sprinkling roof cooling, in the arid Gobi region of Southern Xinjiang. The study was conducted in double-skeleton greenhouses used for mushroom cultivation. The following conclusions have been drawn:

(1)

The water-sprinkling roof cooling greenhouse was the most effective at greenhouse cooling and humidity control. The internal greenhouse temperature reduction achieved using the different cooling methods was 8.6 °C, 14.0 °C, and 15.2 °C for natural ventilation cooling, fan and pad cooling, and water-sprinkling roof cooling, respectively. Furthermore, the relative humidity was increased by 15.3%, 43.3%, and 51.2% for natural ventilation cooling, fan and pad cooling, and water-sprinkling roof cooling, respectively.

(2)

Water-sprinkling roof cooling exhibits the highest cooling efficiency, with an average cooling efficiency of 68.1% and a maximum of 79.2%. Fan and pad cooling follows with lower cooling efficiency, while natural ventilation cooling shows the lowest cooling efficiency.

(3)

Water-sprinkling roof cooling demonstrates the best uniformity in temperature changes within the greenhouse, while natural ventilation cooling and fan and pad cooling exhibit significant temperature gradients along both the height and span directions.

Based on these findings, it could be concluded that the water-sprinkling roof cooling system provides the best cooling effect, uniformity, and economic benefits, making it more suitable for summer shitake mushroom cultivation in the arid Gobi region of southern Xinjiang.