Study on the Influence of Solar Array Tube on Thermal Environment of Greenhouse

Abstract

The stratum and microenvironment temperatures in a greenhouse are important factors that affect crop yield. In order to solve the problem of temperature imbalance caused by solar radiation in greenhouses, this paper proposes the application of a solar radiation array tube in a greenhouse. By adding water or phase change materials to the array tube, the influence of the array tube on the formation and microenvironment temperature changes was studied, and a 10-day test was carried out. A test group and control group were set up to monitor test results, and the ground was divided into six areas. The depths of each area were 10 cm, 30 cm, and 50 cm, and the heights of the greenhouse centers were 0 cm, 30 cm, 60 cm, 90 cm, 120 cm, 150 cm, and 180 cm. Via an analysis of the test results obtained for the formation and microenvironment temperature, the arrangement of the array tube was found to exert a constant temperature regulation effect on the microenvironment of the greenhouse at a formation depth of 10 cm and was able to improve this formation depth to a certain extent. The temperature at 30 cm and 50 cm plays a positive role in building a good vegetation growth environment.

1. Introduction

Chinese solar greenhouse (CSG) is a kind of solar greenhouse that uses solar radiation as a heat source [1,2]. It can create a suitable environment for crop growth by artificial means and promote the economic development of national crops [3]. The cross-season planting of crops is also a key technology [4], which is highly significant for the development of cross-season agriculture and realizes the cross-season development goal of agriculture and cross-regional supply of crop varieties, overcoming the restrictions of traditional outdoor agriculture [5,6]. Currently, one of the problems faced by greenhouses is the imbalance of energy utilization between day and night. When the outdoor environment temperature is low at night, the energy dissipation in a greenhouse is faster and the temperature sharply decreases, which is not conducive to plant growth. Therefore, it is necessary to take measures to maintain an optimal temperature in the greenhouse, especially when sunlight is strong during the day. The energy accumulation of solar radiation cannot be effectively released, leading to increased temperatures and inefficient storage of energy.

In Inner Mongolia, China, due to the large temperature difference between day and night, the heat flux between a greenhouse and its environment at night is large, resulting in a significant decrease in the microenvironment temperature and formation temperature in the CSG, which is not suitable for crop growth. To overcome this problem, research institutions in various countries mainly regulate the temperature of the CSG microenvironment and stratum via active or passive heating technology [7,8,9]. Active heating technology [10] requires external energy consumption to power the greenhouse and is most commonly applied in the form of an electric heating system. However, it is expensive and highly technical and has limitations in energy storage regarding greenhouse thermal environment regulation. Especially in summer, due to the high temperature in a greenhouse, its thermal environment is extremely hostile and not suitable for crop growth. Passive heating technology converts daytime radiation energy into heat energy for storage [11] and spontaneously maintains the environment and formation temperature of CSG through heat exchange at night to achieve successful energy-saving heating. For example, in order to improve the nighttime temperature of a CSG, W Xu et al. [12] added a solar water wall composed of hollow polycarbonate plates to its north wall and improved the underground water storage tank in order to conduct a control test. The results show that the average nighttime temperature increased by 3.3 °C compared to the case without a water wall. Ihoume et al. [13] arranged a solar copper coil heating system in a southward-facing CSG, relying on the heat transfer medium cycle occurring on the top of the greenhouse, storing heat during the day and releasing heat in the greenhouse via circulation at night. The results show that the nighttime temperature of the greenhouse increased by 4 °C.

In this study, in order to solve the challenges of a cold nighttime CSG microenvironment [14] and low formation temperature [15], the application of a solar radiation array tube assembly in CSG is proposed. Its basic principle is the heat transfer capacity of the heat exchange tube, where the array tube is placed in the soil. Driven by the temperature difference between the underground soil and the air in the greenhouse, on the one hand, the natural convection heat transfer capacity between the soil and the air is enhanced. On the other hand, the heat exchange tube above the ground realizes the absorption and release of energy through heat transfer with the air, which strengthens the heat transfer capacity between the air in the greenhouse and the underground soil. Additionally, the peak and trough of the air in the greenhouse are the strongest stages of heat transfer capacity, reaching the “peak clipping and valley filling” of energy fluctuation. This method involves storing solar radiation energy in a greenhouse during the day in the underground soil and strengthening the heat dissipation capacity of the soil through the heat exchange tube at night to increase the temperature in the greenhouse. Notably, this method is compared with the method of completely opening the greenhouse during the day, which reflects the full utilization of solar radiation energy. During the test, the heat storage medium was either water or phase change material (PCM) [16], which was used to compare the temperature regulation effect of the microenvironment in the CSG and its formation. The final test results show that a PCM can greatly increase the minimum temperature at night compared with water. At present, many studies from around the world have used solar heat storage technology in CSG to maintain a temperature regulation effect indoors, such as the study by M Kumar [17]. This study shows that the use of solar heat storage technology in CSG can not only improve the yield of crops but also ensure the sustainable and environmentally friendly production of energy. W Lu [18] developed a tank model with water as the heat storage medium, which converts solar radiation energy into heat and stores it in the tank to supplement the heat in the CSG at night in winter, thereby increasing the nighttime temperature. L Gourdo et al. [19] designed a black plastic sleeve solar heating system to heat the greenhouse at night. Compared with the control group, this heating system can increase the average night temperature by 3.1 °C. H Ling [20] proposed a heating system combining a PCM and a solar concentrator to provide constant-temperature heating for CSG. The experimental results show that PCMs play an important role in the constant-temperature heating process of CSG. In this paper, the influence of a solar radiation array tube on the thermal environment of an arched greenhouse was studied. Due to the large temperature difference between the daytime temperature and nighttime temperature in greenhouses, this experiment applies passive heat storage heating technology to CSG. By arranging solar radiation array tubes in greenhouses, the temperature in the greenhouse and at the depth of 10 cm in the stratum plays a constant role regarding temperature, and the temperatures at depths of 30 cm and 50 cm increase to a certain extent, creating an environment that is suitable for crop growth, aiding the cross-seasonal development of CSG agricultural planting.

2. Materials and Methods

2.1. Description of the Experimental Process

This experimental process mainly includes four stages: 1. The establishment of the test model, 2. Pre-test preparation, 3. Basic theory of test, 4. Data processing and analysis.

The first stage includes the size design and azimuth arrangement of the test CSG. (1) In the process of position arrangement, in order to avoid the experimental error of the shadow caused by solar radiation on the CSG for the test group, the experimental error was caused by the different initial environmental conditions of the test group. Here, the greenhouse spacing of the CSG test group is simulated, and a reasonable greenhouse spacing is set to address the influence of this interference on the test process. (2) The material selection, tube section optimization, size design, and array tube layout of the greenhouse film and array tube were carried out. In this experiment, PE material was selected for a greenhouse film, wall thickness was 2 mm, heat transfer coefficient was 7.35–7.5 W/(m² °C), and light transmittance was 80–90%. In the selection of the array tube, the thermal conductivity, corrosivity, and economy were comprehensively considered, and the material used is aluminum. Considering the influence of solar radiation angle on the thermal performance of the array tube during the test period, a Fluent simulation of the square tube section and the circular tube section was carried out to strengthen the radiation of the daytime short wave in the array tube.

The second stage involves the layout of measuring points. Firstly, the location of measuring points in the test and control groups was divided into six regions. Each region is equipped with temperature measuring points with depths of 10 cm, 30 cm, and 50 cm. A total of 18 measuring points are arranged to characterize the temperature of different formation depths in each region. Secondly, seven measuring points located 0 cm, 30 cm, 60 cm, 90 cm, 120 cm, 150 cm, and 180 cm from the ground were arranged in the greenhouse microenvironment to characterize the temperature change in the greenhouse microenvironment. In the case of no array tube in the experimental group, the initial condition conformance test was carried out between the built test and the control group. After completing the array tube arrangement, 10-day data sampling was performed. When the heat storage medium was water or a PCM, a 5-day test data acquisition was carried out.

In the third stage, combined with the test data results, the stratum temperature in different areas of the test group and the control group were compared and analyzed to explore the effect of array tubes on the stratum temperature. The heat storage medium of the array tube is water/PCM, and the test cycle is five consecutive days for each medium. Through the microenvironment temperature test data of the test group and the control group, the data of the temperature measurement points at different heights were analyzed and compared to explore the influence of array tubes on the microenvironment temperature in CSG. The change rule of the formation and microenvironment temperature was summarized, and the application prospect of array tubes was evaluated.

2.2. Test Model Establishment

The structure of the test model is an arch greenhouse, in which the east and west sides are 3 m in length, the north and south sides are 2 m in width, and the radius of the arch section is 1 m. As shown in Figure 1, the size and layout of the test and control greenhouses are taken into account. Considering the factors of soil heat conduction between the greenhouses and the influence of shadow occlusion, the transverse heat conduction formula (1) was introduced into the soil between the greenhouses, and the temperature measuring points with bottom depths of 0.3 m and 0.5 m are set inside and outside the greenhouse. Experimental data show that the temperature difference between the measuring points of 0.3 m and 0.5 m in and outside the greenhouse is 2.1–3.1 °C and 3.4–4.2 °C. The maximum temperature difference is substituted into the formula. The energy loss of 0.3 m and 0.5 m at 0.5 m from the greenhouse is 0.04 W and 0.05 W, and the temperature loss is 0.05 °C/h and 0.04 °C/h; therefore, the error caused by the heat exchange between CSG and the external environment through the formation can be ignored:

$$Q=\mathrm{S}K\_df\Delta T$$

(1)

where S is the cross-sectional area, m²; K_df is soil transverse heat transfer coefficient, W/(m² · K). Here, K_df is taken as 0.47 W/(m² · K).

The shadows generated by CSG at different times during the test period are simulated. Values for the model, latitude, and longitude of the site (40.48° N, 111.41° E), and the time zone (+8) were input into Comsol software to simulate the shadow changes of CSG over one day. The simulated shadows are shown in Figure 2. The shadow change map was exported every 2 h, ranging from 6:00 to 18:00 (the time of sunset). The figure shows that the longest interference shadow of CSG in the north–south direction is 14:00, and its shadow is 0.68 m < 2 m; therefore, the interference of shadow on the test can be avoided.

In this test, the array tube is made of aluminum, with a length of 1.5 m, a rectangular section (80 mm × 40 mm), and a wall thickness of 2 mm. The solar radiation section is 1 m above the ground, and the heat exchange section is 0.5 m below the ground. In the course of the experiment, considering factors such as the environment of the site and the solar elevation angle during the test period, the simulation analysis of the circular and square cross-section aluminum tubes was carried out to explore the temperature increase effect of the two cross-section aluminum tubes during the test period, and to determine the cross-section shape of the array tube. In order to ensure that the quality of the heat storage medium is consistent, a circular cross-section array tube with the same length, wall thickness, and volume as the square array tube was designed, in which the cross-section diameter (DN) is 64 mm, and the remaining boundary conditions in the simulation are fixed. The temperature cloud diagram of the tube wall is shown in Figure 3. The time taken for the circular cross-section array tube to reach the maximum temperature is 14:08, and the square array tube takes 13:22 to reach the maximum temperature, which is 46 min shorter than the time for the circular cross-section array tube wall surface to reach the maximum temperature. The reason for this phenomenon is that the experiment was conducted from the end of September to the middle of October when the days are long and the nights are short. The southward radiation time is long, while the square array tube is arranged southward on one side of a large area; therefore, the square section array tube can absorb more radiation during the day and shorten the time required to reach the maximum temperature.

2.3. Test Preparation

In order to avoid the errors caused by different soil properties during the test, the loosening and drying of soil were carried out on the test and control greenhouses. At the same time, in order to avoid the test error caused by radiation on the probe of the measuring point, it was subjected to radiation protection treatment. Then, the steel skeleton was welded and assembled, and the skeleton was placed in a pit slot. Finally, a greenhouse was built on the outer surface of the skeleton to complete the main structure. At the same time, the pipe wall surface is coated, and the selected heat storage medium is packaged. The construction drawing is shown in Figure 4. PCM uses organic paraffin. The physical parameters are shown in

Table 1. PCM physical parameters.

Test Items	Index	Unit
Phase transition temperature	24	°C
Operating temperature range	<100	°C
Latent heat	165.00	kJ/kg
Heat storage capacity (19–30 °C)	189.00	kJ/kg
Density (solid/liquid)	0.92/0.85	kg/L
Specific heat (solid/liquid)	2.20/2.67	kJ/kg·K
Thermal conductivity (solid/liquid)	0.25/0.20	W/m·K
Coefficient of volume expansion	7.82	%

A PCM is provided and tested by Heatmate New Energy Technology (No. 333, Haiyang 1st Road, Pudong New Area, Shanghai, China) Co, Ltd.

, and the performance test is shown in Figure 5.

In this experiment, the array tube and PT100 temperature probe are arranged as shown in Figure 6. Figure 6a is the top view of the control test greenhouse. In order to characterize the formation temperature of each region in CSG in detail, six measuring points are evenly arranged in the experiment. Because the formation is over 50 cm in depth, it is the root growth interval of general crops. Therefore, during the test, a PT100 probe is arranged at formation depths of 10 cm, 30 cm, and 50 cm at each measuring point. A total of 18 PT100 are used to characterize the formation temperature in a single CSG, which can ensure the error caused by the contingency of the test data. The data acquisition system in the test is composed of a PT100 temperature sensor and a PT700 data acquisition instrument. The model of the PT100 temperature sensor is ECR3100, its range is 50–200 °C, and its accuracy is ±0.02 °C. The measurement range of the PT700 data acquisition instrument is −999.9–1999.9 °C, and the measurement error is ± 0.01 °C. In the measurement process, direct measurement technology was used to explore the influence of array tubes on the temperature in the greenhouse by arranging measuring points at different heights in the test and control groups. The ground was divided into regions, and temperature measuring points were arranged at different depths in different regions to explore the influence of array tubes on the formation temperature. In order to simplify descriptions, the symbol and position of the measuring point are defined as follows: The triangle represents the temperature of one stratum position in the CSG area; the circle represents the formation temperature in the second region; the diamond in the vertical direction represents the temperature of three strata in the region; the pentagonal star represents four formation temperatures in the region; the pentagon represents the formation temperature of the fifth region; the square represents six formation temperatures in the area; and the horizontal diamond represents the microenvironment temperature. Additionally, different strata depth-measuring points have different forms of expression; for example, in a greenhouse, the strata depth in the first area, with a 10 cm measuring point, can be expressed as A1-10; a 30 cm measuring point is expressed as A1-30; the measuring point at 50 cm is expressed as A1-50; and the measuring point at a 10 cm depth in B greenhouse area 1 is expressed as B1-10. A 30 cm measuring point can be expressed as B1-30; a measuring point at 50 cm can be expressed as B1-50; and the method of measuring points at other positions is consistent with this.

Figure 6b shows the layout of array tubes and stratum measuring points in the experimental group. In order to explore the influence of array tubes on stratum temperature, the arrangement of stratum measuring points in the experimental greenhouse is consistent with that of the control group. Because this experiment uses a large-scale promotion experiment, considering the actual situation of crop demand for light, the number of array tubes on the south side is small, and the specific arrangement method is shown in the figure. Sixteen array tubes were designed, the volume of the heat storage medium contained in each array tube is 0.0048 m³, and the total volume is 0.0768 m³. The volume of greenhouse is 11.91 m³, and the ratio of heat storage medium volume to greenhouse volume is 0.0064.

Figure 6c shows the east–west side view of the central position of the greenhouse. Seven PT100 measuring points are arranged here to characterize the temperature stratification of the CSG microenvironment. The microenvironment temperature at different heights is defined as follows: The temperature of the measuring point near the ground of the test greenhouse is expressed as ETB-0; the measuring point at 30 cm height is ETB-30; the temperature of the measuring point near the ground of the control greenhouse is expressed as ETA-0; the measuring point at a height of 30 cm is ETA-30; and the remaining measuring points are represented in the same way.

Before the test, the test group and the control group CSG were compared. Figure 7 shows the temperature comparison of the test points in different regions of the CSG for the test group and the control group. In the control process, the temperature of each measuring point was collected every hour, and the measuring points at different depths in each region were compared and analyzed. It can be seen from the figure that the temperature change at the formation depth of 10 cm is large, and the temperature drop during the nighttime exothermic process is large, but the temperature of the formation at 30 cm and 50 cm does not change much during the heating and cooling process, so it can be used as an effective heat storage layer. Figure 8 shows the 3D wall shape diagram of each temperature measuring point in the microenvironment of the experimental group and the control group, where a is the heating process in the greenhouse, and b is the cooling process diagram. In the whole control experiment, the microenvironment temperature difference between the test and the control CSG at different times is less than 1 °C, which mostly ignores the error caused by the different initial microenvironment temperatures in the subsequent experiment.

2.4. Basic Test Theory

To understand the changes in the external environment temperature at different time points, the real working conditions of the external environment were accurately established by fitting the temperature data of the measured external environment during the test.

Table 2. Temperature fitting: Environmental temperature data are measured values provided by the weather station.

Test Time	External Environment Temperature Data (°C)	Fitting Function	R2
27 September 2022 8:00–28 September 2022 8:00	13 18 21 22 23 25 25 26 25 25 22 18 17 17 17 16 13 12 11 10 9 8 9 11 13	y = −0.0018x4 + 0.1187x3 − 2.63x2 + 20.459x − 6.8479	0.9683
28 September 2022 8:00–29 September 2022 8:00	13 17 20 21 23 23 25 26 25 25 22 18 17 15 17 15 15 12 12 11 10 10 10 10 15	y = −0.0018x4 + 0.1228x3 − 2.7653x2 + 21.837x − 10.559	0.944
29 September 2022 8:00–30 September 2022 8:00	15 17 21 23 23 25 27 27 27 24 23 20 19 18 16 16 15 12 11 11 11 10 10 11 16	y = −0.0012x4 + 0.0879x3 − 2.1341x2 + 17.788x − 1.8772	0.9696
30 September 2022 8:00–1 October 2022 8:00	16 19 21 22 24 25 26 27 27 27 25 22 18 16 17 17 15 16 16 15 14 14 14 13 15	y = −0.0022x4 + 0.1389x3 − 3.0095x2 + 23.165x − 11.161	0.911
1 October 2022 8:00–2 October 2022 8:00	15 18 24 25 26 27 27 27 27 25 23 21 18 19 16 13 13 14 15 14 14 14 12 13 16	y = −0.0019x4 + 0.1214x3 − 2.6702x2 + 20.799x − 6.9945	0.9589
3 October 2022 8:00–4 October 2022 8:00	5 5 6 8 9 10 10 9 9 6 5 4 3 3 1 1 0 −2 −2 −2 −2 −4 −4 −4 0	y = 0.0066x3 – 0.2734x2 + 2.6101x +1.7244	0.9602
4 October 2022 8:00–5 October 2022 8:00	0 3 4 6 8 8 9 10 9 11 8 7 5 0 2 0 1 −1 −2 −1 −2 −3 −3 −3 2	y = 0.0092x3 – 0.3945x2 + 4.3158x -4.6788	0.9243
5 October 2022 8:00–6 October 2022 8:00	2 5 8 9 10 11 12 12 12 11 10 9 8 7 8 5 5 5 4 2 0 −2 −2 −1 6	y = 0.0002x4 -0.0042x3 – 0.1387x2 + 2.7039x +0.2636	0.9033
6 October 2022 8:00–7 October 2022 8:00	6 8 10 11 12 14 14 14 13 14 13 10 8 6 4 2 1 0 −1 −1 −1 −1 −1 −2 3	y = 0.0104x3 – 0.4384x2 + 4.5099x +0.5649	0.9736
7 October 2022 8:00–8 October 2022 8:00	3 7 9 11 12 14 15 16 16 15 13 9 9 10 11 10 10 10 7 6 6 4 4 3 6	y = −0.0003x4 +0.0192x3 – 0.5159x2 + 5.0348x -1.5672	0.8758

shows the external environment temperature data and fitting function during the test. In order to ensure fitting accuracy, data were collected every hour.

The energy equation and radiation model are involved in the test, in which the energy equation includes the air energy balance equation in CSG and the soil energy balance equation. The air energy exchange in the CSG mainly includes the heat exchange between the air and the soil layer, the heat exchange with the air outside the film, the heat exchange with the solar shortwave radiation, and the heat exchange between the array tubes, as shown in Equation (2).

$${\rho }_{ET}{c}_{ET}{V}_{ET}\frac{d{T}_{ET}}{dt}=-Q_{ET-S}-Q_{ET-PE}+Q_{ET-R}-Q_{ET-P}$$

(2)

where ${\rho }_{ET}$ is CSG microenvironment air density—kg/m³; $c_{ET}$ is the specific heat capacity of air at constant pressure—J/(kg·K), with a value of 1000.4 J/(kg·K).$Q_{ET-S}$ is the heat exchange between the air and the formation—W; $Q_{ET-PE}$ is the heat exchange capacity between the air and the environment outside the greenhouse film—W; $Q_{ET-R}$ is that the air receives solar short-wave radiation energy—W; $Q_{ET-P}$ is the heat exchange between the air and the array tube—W.

The lateral heat transfer response test of the soil layer in Equation (1), shows that the lateral heat transfer of the soil layer is very small and can be mostly ignored. Therefore, the energy balance model of the soil layer conforms to the following equations. The energy exchange includes the acceptance of solar short-wave radiation energy, the energy of the air to the soil layer, and the heat transfer of the array tube to different soil depths, as shown in Equation (3).

$${\rho }_S{c}_S{A}_S{d}_{S\left[0\right]}\frac{d{T}_{S\left[0\right]}}{dt}=Q_{S-R}+Q_{S-ET}-Q_{S\left[100\right]}$$

(3)

where ${\rho }_S$ is the density of soil layer—kg/m³; $c_S$ is the specific heat capacity of soil layer—J/(kg·K); $A_S$ is the surface area of soil layer in the CSG—m²; $d_{S\left[0\right]}$ represents the surface—m; $Q_{S-R}$ is the short wave radiation energy received by the soil layer—W; $Q_{S-ET}$ is the air heat exchange capacity between the soil layer and the CSG—W; $Q_{S\left[100\right]}$ is the heat transfer of the array pipe to the stratum at 100 mm—W.

3. Results

3.1. Test and Control Group Regional Strata Temperature Contrast Analysis

3.1.1. Water as Heat Storage Medium

The test data from 8:00 am on 27 September to 8:00 am on 2 October were analyzed. At this stage, the heat storage medium in the array tube was water. In order to study the temperature variation in the measuring points in each area of the test and control greenhouses in different time periods, data collection was performed every 3 h, as shown in Figure 9.

As shown in the figure, the formation depth of 10 cm at the regional temperature change with time is the largest, this is because the daytime surface of the solar radiation heat can be quickly transmitted to the formation of 10 cm, in the control group within five days in different regions daytime maximum temperature can reach 28.1~36.2 °C. In the test group, part of the heat storage medium water in the underground section of the array tube contacted the 10 cm formation and exchanged heat energy. This reduced the formation temperature at high-temperature periods during the day, and the temperature in different regions was reduced by 2.1~9.2 °C. At the same time, at low-temperature periods during the night, the heat loss rate of the control group at 10 cm depth was faster, and the lowest temperature in different regions reached 16.8~21.1 °C. In the test group, the heat distribution in the array tube at nighttime is effective, and the lowest temperature reached at night was 19.2~23 °C. Therefore, when the array tube is arranged in the CSG, it can play an effective role in cutting the temperature peak and filling the valley at 10 cm of the formation. This can weaken the temperature fluctuation degree, maintaining the formation temperature constant.

In the formation depth of 30 cm, the temperature fluctuation is weak since the formation at 30 cm cannot easily receive radiation heat during the day, and the nighttime cooling rate is slow. Therefore, heat is easily retained on the surface, which acts as a good heat storage layer. In the control group, the minimum and peak temperatures in different regions within five days are approximately 16.1~22.2 °C. In the experimental group, by arranging array tubes, the radiation heat received by the above-ground section can be transmitted to the underground section through the medium water during the day, thereby increasing its temperature to a certain extent, and the minimum and peak temperatures in different regions can reach 20~26.2 °C.

At a formation depth of 50 cm, the greenhouse is less affected by solar radiation, and the fluctuation range is very weak. For the formation depths of 10 cm and 30 cm, as the formation depth increases, the temperature fluctuation range gradually decreases. In the formation of 50 cm, the temperature is low, and in the control group within five days of different regional temperature fluctuation range is only 15.6~20 °C. Meanwhile, the test group in different regions of the temperature range can reach 18.1~23 °C, a certain increase in the formation temperature.

3.1.2. PCM as Heat Storage Medium

The test data from 8:00 a.m. on 3 October to 8:00 a.m. on 8 October were analyzed. In this stage, the heat storage medium filled in the array tube was a PCM, and its physical parameters are shown in Table 1. Compared with the heat storage medium for the water test period, the external environment temperature significantly decreased, and its specific parameters are shown in Table 2.

As shown in Figure 10, at a formation depth of 10 cm, the maximum daytime temperature in different regions of the control group can reach 21.5~28 °C. In the experimental group, the peak daytime temperature can be reduced by 0.8~4 °C via the addition of a PCM heat storage medium to the array tube. Compared with the case where the heat storage medium is water, the peak clipping ability was significantly reduced. At low temperatures during the night, the minimum temperature in the control group was 3~15 °C, while in the experimental group, the minimum temperature could reach 11~17 °C. Compared with the case when the heat storage medium was water, the valley filling ability was enhanced, and the minimum temperature at night was increased to a certain extent. Therefore, whether the heat storage medium is water or PCM, it can have an effect of peak clipping and valley filling for the temperature at the depth of 10 cm.

At a 30 cm depth of the stratum, the temperature fluctuation range of different regions in the control group was 15~20 °C in five days, and the fluctuation range in the experimental group was 18.5~22 °C. At the layer of 50 cm, the temperature fluctuation range of different regions in the control group was 15.5~19.8 °C over five days, which was not conducive to the growth of vegetation roots in cross-seasonal planting, while the temperature fluctuation range of different regions in the experimental group was 18~21 °C over five days, which increased the temperature fluctuation range to a certain extent.

In summary, the arrangement of array tubes in the CSG can play the roles of peak shaving and valley filling in the 10 cm formation, and to a certain extent, maintain its temperature constant. At 30 cm and 50 cm of the stratum, the temperature can be increased in different areas of the CSG, which can solve the problem of a low temperature in the growth area of vegetation roots during the cross-season planting to a certain extent.

3.2. Test and Control Group Micro Environment Temperature Contrast Analysis

3.2.1. Water as Heat Storage Medium

Because the high temperature of the daytime microenvironment and low temperature at nighttime is not conducive to the growth of crops, the decrease in daytime peak temperature and increase in nighttime minimum temperature in the CSG microenvironment have great impacts on the yield of CSG crops. Figure 11 shows the temperature contrast curves of measuring points at different heights in the microenvironment of the test group and the control group when the heat storage medium is water. Figure 11a is the surface temperature of the ground change curve of the experimental group and the control group in five days. In the control group, the ground surface daytime temperature range is 54.8~56.3 °C, and the lowest temperature range at nighttime is 12.2~14.1°C. The highest temperature range of the experimental group was 45.8~46.3 °C, and the lowest temperature range was 17.2~21.2 °C. Compared with the control group, the daytime temperature of the experimental group can be reduced by 9.8 °C in a single day and increases by 7.2 °C at night. At the same time, it can be seen from the figure that, at other temperature measuring points at different heights, adding heat storage medium water to the array tube can play a role in peak shaving during the day and valley filling at night to a certain extent, and the temperature changes in the experimental group are slower than those in the control group. In order to more clearly quantify the peak clipping and valley filling effects of the array tube on the CSG microenvironment,

Table 3. Temperature amplitude interval and maximum variation when the heat storage medium is water.

Measuring Point Position	MAXTC (°C)	MAXTT (°C)	Maximum Reduction (°C)	MINTC (°C)	MINTT (°C)	Maximum Lift (°C)
ET0	54.8~56.3	45.8~46.3	9.8	12.2~14.1	17.2~21.2	7.2
ET30	44.1~44.2	36.4~38.1	7.1	9.8~15.8	16.1~17.7	7.9
ET60	43.7~44.4	37.2~38.3	7.2	9.9~14.7	14.8~16.2	6.2
ET90	40.2~46	38.1~40.8	6.9	9.6~15.1	14.8~17.2	6.5
ET120	42~44.2	36.6~38.3	6.2	9.3~15.7	16.1~18.2	8.3
ET150	42.1~43.8	35.7~38.8	5.3	9.4~13.2	14.4~17.1	6.1
ET180	44~46.1	36.8~41.2	5.2	10.2~16.4	15~20.1	4.8

The above data are calculated from measured values.

shows the highest daytime temperature range and the lowest nighttime temperature range within five days, the maximum reduction in the highest temperature range, and the maximum increase in the lowest temperature range within a single day in the test and control groups. The maximum temperature range of the test group is simplified as MAXTT, and the minimum temperature range is simplified as MINTT. The maximum temperature range of the control group is simplified to MAXTC, and the minimum temperature range is simplified to MINTC.

3.2.2. PCM as Heat Storage Medium

Figure 12 shows the temperature curves of the measuring points at different heights in the microenvironment of the test and control groups when the heat storage medium is PCM. Table 3 and

Table 4. Temperature amplitude interval and maximum variation when the heat storage medium is PCM.

Measuring Point Position	MAXTC (°C)	MAXTT (°C)	Maximum Reduction (°C)	MINTC (°C)	MINTT (°C)	Maximum Lift (°C)
ET0	28.3~38.6	24.3~29.1	9.7	0.1~8.2	8.4~12.2	8.3
ET30	22.8~33.1	20.7~29.2	4.1	−3.8~6.1	6~13.1	9.8
ET60	22.6~33.2	20.5~27.6	4.6	−4.2~5	6.1~9.2	10.3
ET90	24.2~35.4	21.7~31.4	4.2	−4~6.1	5.2~10.5	9.2
ET120	22.2~32.7	21~27.8	5.6	−3.8~4.1	5.3~9	9.1
ET150	23.2~32.8	20.8~28	4.8	−3.7~1.8	5.7~9.8	9.4
ET180	22.9~32.7	21.7~28.4	5.5	−3.8~5	6.1~10.6	9.9

The above data are calculated from measured values.

show that the peak clipping degree of the maximum temperature of the PCM heat storage medium is weaker than that of the water heat storage medium during the daytime, and the peak clipping effect on the surface is at a maximum regardless of the heat storage medium. At the same time, when the heat storage medium is water, the peak clipping degree gradually decreases with the increase in the measuring point position, and when the heat storage medium is a PCM, the peak clipping degree is the weakest at a height of 30~90 cm. In the case of the lowest temperature at night, the valley filling effect is clearly enhanced when the heat storage medium is a PCM compared with when the heat storage medium is water. According to Table 3 and Table 4, when the heat storage medium is water, the lowest temperature at night increased by 8.3 °C at a height of 120 cm, and the lowest increase was 4.8 °C at a height of 180 cm. For the case where the heat storage medium is a PCM, the maximum temperature can reach 10.3 °C at a height of 60 cm, and the minimum temperature on the ground surface is 8.3 °C. In summary, a PCM has a weaker peak clipping effect in the daytime than water, but the valley filling effect is enhanced at night.

4. Discussion

The test results show that the arrangement of solar radiation array tubes in the CSG can cause the temperature of the stratum and the microenvironment in the greenhouse to influence peak shaving and valley filling. The daytime array tube transfers the radiation energy to the underground section through the heat storage medium, and the contact heat transfer between the underground section and the stratum stores heat in the soil to enhance the heat absorption capacity of the stratum. For shorter depths of the formation, the radiation energy received by the surface is transferred to this position faster, resulting in a large temperature fluctuation over a single day, causing a high temperature during the day and a low temperature at night. Meanwhile, in the test group, energy from the high-temperature soil layer during the day was transferred due to the temperature difference in the surface of the array tube, thereby reducing the peak temperature of the tube. At the same time, the temperature of the shallow position rapidly dissipates at night. During this time, the high-temperature heat storage medium in the array tube begins to release heat, thereby maintaining the lowest temperature at night and increasing the temperature of the shallow position at night. The temperature fluctuation range at the deep position gradually decreases with the increase in depth, and no matter what kind of heat storage medium is used, the average temperature increases. However, in practical applications, for different kinds of crops, the optimal temperature range for root growth is different. When the heat storage medium is water, it cannot be regulated according to the required temperature range. PCM has the characteristic of a constant-temperature heat storage/release, and currently, PCM can be developed with different phase transition temperatures according to demand. Therefore, if the PCM with a suitable phase transition temperature can be selected for different kinds of crops, the formation temperature can be maintained in the suitable growth temperature range of crops, which is beneficial to the growth of crops and the yield of crops. However, due to the long test period required for such experiments, the relationship between the application of array tubes and yield needs further study.

The aforementioned findings also show that the arrangement of array tubes can be used to maintain the temperature of the microenvironment in a CSG. This is because solar radiation accumulates heat in the greenhouse due to the greenhouse effect. The array tube directly absorbs solar radiation and carries out a natural convection heat transfer with the air in the greenhouse, leading to heat conduction and heat transfer with the heat storage medium in the array tube. The array tube absorbs and transmits the heat in the greenhouse to the underground soil; therefore, the air in the experimental group, the surface temperature of the ground, and the temperature in the 10 cm formation are lower than the temperatures at the corresponding positions in the control group, and the formation depths are 30 cm and 50 cm. The temperatures at these positions are higher than those in the control group. Because the underground section cannot accept solar radiation, the temperature is lower than that of the ground section, and the heat transfer and viscosity change with temperature [21]. Therefore, if the heat storage capacity of the heat storage medium in the underground section is improved, it will become the objective of future research. For example, the heat exchange capacity of the heat storage medium between the ground section and the underground section can be strengthened by setting up a hollow copper tube in the vertical direction of the array tube, or by improving the physical parameters of the PCM. The solid density is less than the liquid density, and it does not easily agglomerate at a low temperature after multiple thermal cycles. In this way, the PCM of the underground section can float on the ground section of the array tube via suspension force, thereby strengthening the natural convection [22] of the heat storage medium between the underground section and the ground section. By adding copper oxide to the PCM matrix, its atomic mobility can be increased, and the heat transfer in the heat storage medium can be increased [23,24] to further improve the heat storage capacity of the underground PCM. At the same time, the heat storage medium begins to release heat at night, which can improve the microenvironment temperature at different heights, and the maximum increase is 8.3 °C when the heat storage medium is water. The maximum increase in the PCM is 10.3 °C, and the PCM [25] shows stronger temperature regulation ability than water during the night.

In future applications of a solar radiation array tube in a CSG, we will continue to study ways to strengthen the thermal storage performance [26], further improve the temperature regulation effect of the array tube in CSG, and explore the relationship between temperature regulation effect and crop yield [27]. By coupling active heating technology to achieve the effect of CSG cross-seasonal constant temperature under the condition of maximum energy efficiency ratio, CSG energy consumption transformation is realized.

5. Conclusions

In this paper, the influence of the array tube arrangement on the formation temperature of CSG is explored by dividing different depth measuring points in each area of the control and test groups, and the influence of an array tube on the microenvironment of CSG is explored by setting temperature measuring points at different heights. The effects of other factors on the initial conditions of the test and control groups were first excluded in the pre-test preparation. Then, the test data of the two heat storage media were monitored for 5 days, respectively; finally, via a data post-processing analysis. The results show that an array tube arranged in the CSG can have a temperature regulation effect on the formation depth and microenvironment.

The main results are summarized as follows:

• In the control group, during the test period in which water was used as the heat storage medium, the temperature fluctuation range of the formation with a depth of 10 cm is 16.8~36 °C, of 30 cm is 20~26 °C, and of 50 cm is 15.6~20 °C. Therefore, the smaller the formation depth, the more easily it is affected by daytime solar radiation, and the fluctuation range of formation temperature gradually decreases with the increase in depth.
• By adding the two kinds of heat storage medium mentioned into the medium mentioned in the above study to the array tube, the temperature at a depth of 10 cm can be peak-shaving and valley-filling, and the temperature range can be increased to a certain extent at depths of 30 cm or 50 cm.
• The arrangement of array tubes in the CSG can play a temperature regulation role at different heights of the microenvironment to a certain extent and can reduce the fluctuation range of the microenvironment. When the heat storage medium is water, the maximum weakening degree of the maximum temperature is 9.8 °C, and the maximum increase in the minimum temperature is 8.3 °C. When the heat storage medium is PCM, the maximum weakening degree is 9.7 °C, and the maximum increase is 10.3 °C.
• When the heat storage medium is water, the weakening range of peak temperature is 2~9 °C. When the heat storage medium is PCM, the weakening range is 0.8~4 °C. Compared with water, PCM heat storage medium has a poor weakening effect on the depth of the 10 cm stratum and the maximum temperature of the microenvironment during the day.