Home Effect of North Wall Materials on the Thermal Environment in Chinese Solar Greenhouse (Part A: Experimental Researches)
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Effect of North Wall Materials on the Thermal Environment in Chinese Solar Greenhouse (Part A: Experimental Researches)

  • Xingan Liu , Yiming Li , Anhua Liu , Xiang Yue and Tianlai Li EMAIL logo
Published/Copyright: December 31, 2019
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Open Physics
From the journal Open Physics Volume 17 Issue 1

Abstract

In order to clarify the dependence relationship between the heat storage & preservation wall and the thermal environment, and to provide data base and theory foundation for the north wall construction of the Chinese solar greenhouse (CSG), the experimental measures has been employed to investigate the distributions of temperature, humidity and heat transfer of three different wall materials (i.e. perforated brick, fine coal ash brick, common clay brick). The dynamic variations of the heat-storage and heat-release processes were identified, and the thermal response characteristics were discussed. The effect of north wall materials on the thermal environment of the solar greenhouse in northern China was revealed. The results indicated that the daily heat-storage and heat-release of fine coal ash brick wall can reach ϕimput = 34.5~130.6 W·m−2 and ϕoutput = −24.15~-45 W·m−2, respectively. The daily heat-storage time can reach t = 5~8 h, and the wall temperature at night can be 3~4∘C higher than the air temperature. Moreover, the maximum indoor temperature of the fine coal ash brick wall can be maintained at t ≤ 16.7 ≤ 31.1∘C, the minimum humidity can be maintained at 29.75~45%. Fortunately, the construction cost is moderate, while the physical properties are obviously better than those of perforated brick and common clay brick in the CSG. The overall thermal performance of fine coal ash brick is the best of the three north wall materials, and it can make the most advantage of the heat-storage and heat-preservation performances of the CSG. As a consequence, the fine coal ash brick wall of the solar greenhouse has good promotion value in northern China and other high latitude, high altitude and long winter regions.

Keywords: Chinese solar greenhouse; wall material; heat storage; heat release; temperature
PACS: 88.05.Qr; 88.05.Gh; 88.05.Np; 88.05.Ec

1 Introduction

Most areas of northern China are in the cold and freezing natural climate, and the lowest temperature in winter are mostly in the range of below zero. The application of solar greenhouse is often accompanied with large energy consumption. Moreover, the freezing damage of solar greenhouse often occurs in the northern winter. Chinese solar greenhouse (CSG) is an important agricultural facility for the year-around cultivation of vegetables and fruits in these cold regions. The CSG can achieve high-efficiency energy utilization without any auxiliary heating, so it plays a great role in the winter agricultural production in the northern China [1]. According to the statistical results until 2017, the covers of solar greenhouses in China is up to 370.1×10 4 hm2, accounting for 33% of the total horticultural areas in China. Why CSG has the ability of overwintering production under the severe condition without auxiliary heating? It is mainly because the enclosing body of the solar greenhouse, especially the north wall, has the capacities of heat storage and heat preservation. The north wall (i.e. back wall) of CSG can maintain the temperature and humidity at an appropriate level to create a suitable environment for crop growth. So the CSG construction can receive the solar radiation energy to realize self-sufficiency and reduce the cost of artificial heating [2]. The structural arrangement of the north wall has direct relationship with the thermal environment characteristics of the CSG, and the energy benefits are superior to other retaining enclosures such as soil, north roof and south roof. As the main thermal mass in the CSG, the north wall must have excellent capacities of heat storage and heat preservation while in consideration of the structural load. On the one hand, the north wall is supposed to absorb a large amount of solar radiation energy during the daytime. The heat transfer coefficient should be low, and the heat preservation and thermal insulation performance should be qualified to reduce the heat loss. On the other hand, the north wall is required a reasonable thermal inertia, which can continuously release the accumulated heat into the greenhouse. In other words, the north wall of the CSG is identified the critical heat source to increase the thermal performance at night. Therefore, the material design of the north wall is of great significance to the CSG.

At present, there are many basic researches on the heat storage and heat preservation of the north wall. Cabeza et al. [3], Kalnaes and Jelle [4] summarized the various kinds of composite walls used in the buildings over the past decade. The early related researches mainly utilize experimental methods. And the research ideas tend to use the reference of civil buildings. Gourdo et al. [5], Ren et al. [6], Jolanta et al. [7], Beshada et al. [8], Singh and Tiwari [9] have proposed new wall materials which can improve the heat storage & preservation properties of the north wall in the solar greenhouse. Chen et al. [10], Banu et al. [11], Kondo et al. [12], Heim and Clarke [13], Alawadhi [14], Critten [15], Najjar and Hasan [16] have successively studied diversified kinds of phase-change wall materials. Although these composite materials and phase-change materials can improve the thermal environment inside the solar greenhouse, the investment costs a lot in the long run. What’ more, the construction is especially difficult. Therefore, the economy and practicability of general promotion are not available. Another unfortunate factor is that the high-cost active-heating greenhouses is insufficient to be introduced or manufactured in China due to the immature agricultural technology.

The recent researches has turned into the effect of thermal property of north wall on the internal environment. Since the thermal environment plays the most important role in the solar greenhouse, people gradually increase their efforts to investigate the heat storage and heat preservation of the north wall. Chen [17], Yu et al. [18] and Santamouris et al. [19] have studied the north wall thermal performance in the solar greenhouse, and discussed the heat-storage characteristics during the daytime and the heat-release characteristics at night. Jolanta et al. [20], White et al. [21] researched the thermal insulation properties of the north wall structures. Mei [22], Mihalakakou [23], Newman and Wilson[24] studied the active-heating features of the north wall.The researches of Chen et al. [25, 26], Zhang et al. [27], Chauhan and Kumar [28], Nayak and Tiwari [29], Hassanain et al. [30] have pointed out that passive-heating north wall can effectively reduce energy consumption and cave the operating cost compared to the active-heating north wall. Some other researchers like Tong et al. [31], Xu et al. [32] and Saberian Sajadiye [33] have employed numerical simulations to predict the thermal performance of the north wall in the solar greenhouse. The north wall material has higher requirements for heat storage and heat preservation in northern China, because the natural environment is especially awful and the temperature is below zero for a long time in winter. But unfortunately, the generalized solar greenhouse limits the construction cost of the wall materials. As a result, there are very few usable wall materials for the CSG promotion in northern China. The north wall is still dominated by common clay brick, perforated brick and fine coal ash brick today. They account for more than 80% of the CSG wall materials in northern China [34]. The optimal method to research the effect of the north wall materials on the thermal environment inside the solar greenhouse is the combination of experiment measures and numerical simulations. One should firstly obtain reliable basic data by experimental testing. Secondly, the relationship between the north wall materials and the thermal environment inside the greenhouse is defined. And then the mathematical model can be established based on these results in numerical simulation. When the numerical model is verified accurate and reliable compared with the experimental data, the advantages of numerical simulation can be taken to research the flexibility of parameter variation and quantitative description. It can be seen that an important prerequisite of this methodology is to obtain the basic data through experiments and to understand the relationship between the north wall material and the greenhouse thermal environment. Nevertheless, there is no systematic research on the effect of wall materials (i.e. common clay brick, perforated brick and fine coal ash brick) which widely used in solar greenhouses in northern China. The previously reported experimental researches lack integrity and fail to reveal the essential relationship between north wall material and greenhouse thermal environment. It is insufficient to support the subsequent model establishment and simulation calculation. So it needs to be solved urgently.

Different north wall materials not only affect the heat-storage and heat-release characteristics of the solar greenhouse, but also directly determine the crop yield and energy consumption. In recent years, although some researchers have made some related efforts on the influence of the north wall material on the CSG thermal environment, it is far from complete and has not yet formed a complete theoretical system. Moreover, the suitable wall material property of the CSG for northern China has not yet been determined. Based on the above reasons, this study has investigated the north wall thermal performance through experimental measurements (for the thermal mathematical model of the CSG, it will be described in detail in another article of Part B using numerical simulation). The daily average heat-storage and heat-release characteristics, the heat-storage time, the maximum maintainable temperature, the humidity and the construction cost of different north wall materials in the CSG have been discussed in this experimental research. At the same time, the thermal performances of the different north wall materials can be systematically studied to provide the theory foundation for agricultural applications.

2 Experimental system and testing method

Chinese solar greenhouse in the experimental testing

The experimental CSG is located at 41∘49′N, 123∘34′E, facing to north and south, and the azimuth is south by west 7∘. In this experimental testing, three kinds of solar greenhouses are shown in Figure 1. All the outer layer materials of the north walls are respectively the polystyrene board, and the inner layer materials are perforated brick wall, fine coal ash brick wall and common clay brick wall. The polystyrene board is an excellent thermal insulation material with low price (only 20 RMB/720 cm2), low density (only 18 kg/m3) and low heat-transfer coefficient (only 0.041 W/m·K). At present, the specific thermal insulation on the outer layer of the north wall has been widely used in the CSG [4, 17, 25, 26, 27, 31, 32]. First of all, the external insulation layer can enhance the sealing property of the north wall, reducing the thermal loss and improving the thermal insulation performance. Secondly, the maintenance and replacement of the external insulation layer are very convenient, which greatly enhances the capacity of resisting the ambient environment damage. So the building life of the CSG is prolonged. Thirdly, the thermal insulation is built at the external layer with simple structure, short construction period and low invested cost. Therefore, the north wall in the CSG mainly uses polystyrene board as thermal insulation material at the outer layer. And the three north walls in the experiments are perforated brick& polystyrene board, fine coal ash brick & polystyrene board, common clay brick & polystyrene board. The ridge height is 4m, the north wall height is 2.7m, the span is 8m, and the length is 60m. The structural framework uses arch steel truss without pillar inside. The other structural parameters and material properties of the three experimental solar greenhouses are all the same.

Figure 1 Three kinds of solar greenhouses with different north wall materials in the experimental measurements: (a) perforated brick north wall, (b) fine coal ash brick north wall, (c) common clay brick north
Figure 1

Three kinds of solar greenhouses with different north wall materials in the experimental measurements: (a) perforated brick north wall, (b) fine coal ash brick north wall, (c) common clay brick north

2.1 Main structural and material parameters of the CSG

The internal structure of the CSG is shown in Figure 2. In the experiment, the south roofs (i.e. lighting surfaces) of three different north walls are made of the same materials. In the daytime, the south roof uses Japanese PO film for the covering material. It is a kind of modified polyolefine composite material mainly composed of ethylene, propylene, butene and other homopolymers, with good toughness, low temperature resistance, high softening temperature, excellent thermal and light stability, and good corrosion resistance. In the night, the south roof is covered with thermal insulation material (i.e. three layers of composite insulation quilt): two layers of non-woven fabric and one layer of sprayed cotton in the middle. The main material properties in the experimental greenhouses are shown in Table 1 and Table 2.

Figure 2 The structure diagram of the Chinese solar greenhouse
Figure 2

The structure diagram of the Chinese solar greenhouse

Table 1

The main materials of the CSG

greenhouse structure material
south roof by day Japanese PO plastic film
south roof by night Non-woven & sprayed cotton
back slope steel structure
back roof concrete
north wall perforated brick & polystyrene board fine coal ash brick & polystyrene board common clay brick & polystyrene board
brick wall composition clay, shale fine coal ash, lime clay
Table 2

The main material property parameters of the CSG

Parameter thermal conductivity K W/m·K specific heat Cp J/(kg·K) density ρ kg/m3 experimental thickness mm kinetic viscosity kg/(m·s) emissivity (-)
PO plastic film 0.19 - 920~960 0.08~0.8 - 0.7
heat preservation quilt 0.037~0.095 1600 70 16~55 - -
back roof 0.93 1050 1800 300 - 0.8
common clay brick north wall 0.81 1050 1800 370 - 0.93
perforated brick north wall 0.58 1061.1 1400 370 - 0.93
fine coal ash brick north wall 0.50 1050 1600 370 - 0.93
polystyrene board 0.041 2414.8 18 110 - -
soil 0.76~1.16 1010 1500~2000 2200 - 0.92
air inside the greenhouse 0.02 1013 1.205 - 1.81×10−5 -

2.2 Experimental equipment

The temperature and humidity in the experimental CSG are recorded by using an automatic HOBO recorder produced by Onset Company in the United States. The detective data is continuously recorded at one-minute intervals, and the collected data is preserved to the output text in the form of excel table. The instrument temperature probe is treated with radiation protection during the measurement and has a standard accuracy of ±0.2∘C. The other temperature measuring instruments is type-T thermocouple. The employed device has many advantages such as high measurement accuracy, fast thermal response time, high mechanical strength and good voltage resistance. The CR1000 and CR3000 data collector produced by Campbell Company are used for data acquisition. The HFP01 sensor is used to measure the heat flux data in soil, walls and other buildings, which uses a ceramic and plastic material shell produced by Hukseflux Company in the United States. The regular voltage signals are transmitted to the surrounding medium so as to collect heat flux data. The CR1000 and CR3000 data collector are used for data acquisition, too.

2.3 Characteristics and testing methods of experimental greenhouse management

The experimental period in this experimental investigation is the most representative severe winter in northern China from December 2016 to March 2017. The greenhouse management methods are the same for three different north walls in the experimental testing. The heat preservation quilt was rolled up at 8:30 a.m. and put down at 4:00 p.m. The heat fluxes of south roof, north wall, back slope and soil are monitored for 24 hours continuously. For the monitoring of various conventional meteorological factors, the time interval of data acquisition is 10 minutes. Figure 3 shows the layout of the detection points inside and outside the greenhouse, and the corresponding relevant location information is shown in Table 3.

Figure 3 Layout of temperature sensors and humidity sensors inside and outside the CSG
Figure 3

Layout of temperature sensors and humidity sensors inside and outside the CSG

Table 3

Locations and contents of measuring points inside and outside the CSG

Measuring point marks Test contents Measuring point positions Measuring point marks Test contents Measuring point positions
1 North wall surface temperature 0.50m from the surface and 0.00m from the north wall 11 South roof surface temperature 4.00m from the north wall
2 North wall surface temperature 1.35m from the surface and 0.00m from the north wall 12 South roof heat flux 4.00m from the north wall
3 Heat flux on the north wall surface 1.35m from the surface and 0.00m from the north wall 13 Back slope surface temperature 0.75m from the north wall
4 North wall surface temperature 2.25m from the surface and 0.00m from the north wall 14 Back slope heat flux 0.75m from the north wall
5 Indoor air temperature and humidity 1.00m from the surface and 1.50m from the north wall 15 Surface soil temperature 0.00m from the ground and 4.00m from the north wall
6 Indoor air temperature and humidity 1.00m from the surface and 4.00m from the north wall 16 Deep soil temperature 0.15m from the surface and 4.00m from the north wall
7 Indoor air temperature and humidity 1.00m from the surface and 6.50m from the north wall 17 Deep soil temperature 0.30m from the surface and 4.00m from the north wall
8 Indoor air temperature and humidity It is 2.00 m from the surface and 1.50 m from the north wall. 18 Surface soil heat flux 0.00m from the surface and 4.00m from the north wall
9 Indoor air temperature and humidity 2.00m from the surface and 4.00m from the north wall 19 Outdoor air temperature and humidity 1.00m from the ground and 9.98m from the north wall
10 Indoor air temperature and humidity 3.00m from the surface and 1.50m from the north wall

2.3.1 The locations and methods of the temperature and humidity measuring points

The temperature and humidity sensors in the experimental CSG are set in north to south direction. The distances from the north wall are 1.5 m, 4 m and 6.5 m, respectively. The hanging thermometer and hygrometer are 1.0 m from the indoor ground, 2 m and 3 m from the ground. Underground temperature sensors in the middle are placed in 0m, 0.15m and 0.3m depth. These sensors are evenly disposed inside and outside the CSG to capture the temperature and humidity distributions in every corner including the air, wall and soil. And this kind of the layout scheme of the sensor positions has been demonstrated reliable by many researchers [11, 12, 21, 26, 27].

2.3.2 The positions and methods of thermocouple arrangements

Figure 4 is the distribution diagram of the thermocouples. The heat flux plates are respectively arranged on the inner surface of the north wall, the inner surface of the back slope, front roof and soil surface. The arrangement positions of the thermocouples inside the walls are selected in the middle of the greenhouse (i.e. from east to west 30 meters). Therefore, the temperature of the wall cross section at different thicknesses can be measured, and the data are collected every 10 minutes. The probe distributions in the vertical direction are respectively 0.5m, 1.35m and 2.2m height above the ground. And the distances from the wall surface in the horizontal direction are 0.01m, 0.02m, 0.05m, 0.10m, 0.20m and 0.30m, respectively.

Figure 4 The locations of the measuring points in the north wall of the CSG
Figure 4

The locations of the measuring points in the north wall of the CSG

2.4 Calculation methods and formulas

The accumulated time of the instrument is slightly different from the daily timing method (24-hour timing method, unit: hour). The conversion formula between the two methods is as follows:

(1) t={ ttime(first day)ttime+24h(second day)

where t is the accumulated time value of the instrument, ttime is the daily timing method.

To reflect the fluctuation range of temperature change in the CSG, the limit temperature difference between days is calculated in the experiment, and the formula is as follows:

(2) ΔTmax−min=Tmax−Tmin

where ΔTmax − min is the limit temperature difference value, Tmax is daily maximum temperature of greenhouse, and Tmin is the minimum temperature of the greenhouse during the daytime.

In order to evaluate the influence of the external environment, the heat-release performance and the speed rate of reaction temperature, the daily heating rate and cooling rate of the greenhouse must be calculated. The specific calculation methods are as follows:

(3) kTup=| Tstart(up)−Tend(up) |ΔtupΔtup
(4) kTdown=Tstart(down)−Tend(down)ΔtdownΔtdown

where kTup and kTdown are the daily heating rate and daily cooling rate of the greenhouse. Tstart(up) and Tend(up) are the temperature starting and ending values in the heating stage. Tstart(down) and Tend(down) are the temperature starting and ending values in the cooling stage. Δtup is the required time to heat up. Δtdown is the required time to cool down.

In order to evaluate the thermal performance of each experimental greenhouse, the maximum average temperature and the lowest average temperature are calculated. The average heating rate and average cooling rate are analyzed. The time range of the average heating rate is from 8: 30 a.m. to 1: 00 p.m., and the corresponding time range of the average cooling rate is from 1:00 p.m. to 6: 00 a.m. of the next day. The specific calculation methods of the average heating rate and the average cooling rate are calculated as follows:

(5) φ¯top=(T¯13:00−T¯8:30)7.5
(6) φ¯down=(T¯13:00−T¯6:00)17

where φ¯top is average heating rate of the CSG, φ¯down is the average cooling rate. T¯13:00 is the air average temperature of greenhouse at 1:10 p.m. for 10 consecutive days. T¯8:30 is the air average temperature of greenhouse at 8:30 a.m. for 10 consecutive days (the temperature when picking up curtain). T¯6:00 is the air temperature of greenhouse at 6:00 a.m. for 10 consecutive days.

In order to study the heat-storage and heat-release performance of the north wall,the heat flux (per unit time and per unit area) is measured by heat flux sensor, and the calculation method of heat flux along the vertical direction of the wall is expressed as follows:

(7) q=t1−tn+1∑i=1nbiλiA

where q is the heat flux at the measuring points. t1 and tn+1 are respectively the temperatures in front and back of the measuring points. bi is the wall thickness of the i layer. λi is the thermal conductivity of the wall. A is the contact area of the measuring point.

The relative humidity of the air in the greenhouse is an important meteorological factor to evaluate the microclimate of the greenhouse in the agricultural facilities. The high or low air relative humidity will become a common obstacle factor in the production. The relative humidity of the air used in the experiment is the ratio of absolute humidity to highest humidity, and the air humidity value represents the saturation level of the water vapor in the greenhouse. The specific calculation formula is as follows:

(8) Rh=ρwρw,max⋅100%

where Rh is air relative humidity. ρw is the absolute humidity of the greenhouse. ρw,max is the maximum humidity.

3 Experimental results and analysis

3.1 The heat-storage and heat-release characteristics of three north walls

3.1.1 The heat-storage and heat-release characteristics of the north wall under the condition of sunny day

The energy conversion modes in the CSG consist of the heat-storage and the heat-release processes of the north wall and the soil. The energy transfer forms in solar greenhouse are composed of solar radiation, heat conduction of the enclosures, natural and forced convection inside and outside the greenhouse, and so on. The heat generated by the greenhouse at night comes from the heat release from the walls and the soil. The instantaneous heat-release energy of the wall is collected by setting the heat flux plate at the height of 0.5m, 1.35m and 2.25m above the ground, so the arithmetic mean value of the heat flux at each moment is obtained.

Figure 5 displays the daily variation of instantaneous heat flux of different wall materials under sunny days. As shown in Figure 5(A), the positive heat flux indicates that the north wall of the greenhouse is in the heat-storage state (ϕ+). The north wall of greenhouse B is the first to store heat (t = 2.5h, 8:30 a.m.). The second is the greenhouse C (t = 3.3h, 9:18 a.m.) and the third is the greenhouse A (t = 3.5, 9:30 a.m.). The heat-storage characteristics are mainly related to the structural feature, thermal conductivity, specific heat capacity and other physical parameters of the north wall. The thermal resistance and specific heat capacity of the three kinds of wall materials are closed to each other, but the wall structure of fine coal ash brick is more conducive to the accumulation of solar radiation energy. So the heat-storage performance is the best. After 8:30 a.m., the thermal preservation quilt of the south roof is opened, and the heat flux of the north wall increases rapidly under the solar radiation, but the time period of reaching the peak value of the three greenhouses are slightly different. The north wall of greenhouse A reaches the maximum ϕ+ = 119.5 W·m−2 at t = 7.3h (i.e. 1:18 p.m.). The north wall of greenhouse B reaches the maximum ϕ+ = 130.6 W·m−2 at t = 7h (i.e. 1:00 p.m.). The north wall of greenhouse C reaches the maximum ϕ+ = 101.5 W·m−2 at t = 6.8h (i.e. 0:48 p.m.). In addition, the negative heat flux indicates that the north wall is in a heat-release state (ϕ−). After 4:00 p.m., the instantaneous heat flux of the north wall of three greenhouses gradually changed from positive to negative value. With the decrease of the ambient temperature, the solar radiation is weakened and the internal temperature of the greenhouse is accordingly reduced. In order to maintain the heat balance of the greenhouse, the wall is changed from heat-storage to heat-release. The north wall of greenhouse A and greenhouse C are the first to realize heat release (t = 10 h, 4:00 p.m.), and then is the greenhouse B (t = 10.2 h, 4:12 p.m.). After 8:00 p.m., the north wall heat-release of greenhouse B tends to be stabilized, and the heat-release of greenhouse B is the largest among the three kinds of wall materials (ϕ− = 50W·m−2). The heat-release characteristics are directly related to the crop growth in the CSG at night. The excellent heat-release characteristics of fine coal ash brick wall are helpful to create the best growth environment for crops. As shown in Figure 5(B) and Figure 5(C), four typical moments of t = 3.0h, t = 6.8h, t = 9.5h and t = 11.45h have been selected in the experimental measurements. These four typical time correspond to the heat-release valley, the heat-storage peak and the transition stage of the north wall. As shown in Figure 5(C), the difference of heat storage among the three greenhouse north wall materials is the largest at t = 6.8h (i.e. 0:48 p.m.). The difference of heat release among the three greenhouse north wall materials is the greatest at t = 11.45h (i.e. 5:27 p.m.).

Figure 5 Daily variation of instantaneous heat flux of different wall materials under sunny days (Observation time: from 6:00 a.m. to 6: 00 a.m. the next day, 24 h)
Figure 5

Daily variation of instantaneous heat flux of different wall materials under sunny days (Observation time: from 6:00 a.m. to 6: 00 a.m. the next day, 24 h)

3.1.2 The heat-storage and heat-release characteristics of the north wall under the condition of cloudy day

Figure 6 displays the daily variation of instantaneous heat flux of different wall materials under cloudy days. As shown in Figure 6(A), due to the lack of heat source in the greenhouse, the wall heat-storage time (only 3~5h) is shorter than that in sunny days. Greenhouse B has the longest heat-storage time and greenhouse C has the shortest heat-storage time. The heat-storage fluxes of the north wall in greenhouse A and greenhouse C reach the maximum ϕ+ = 20.7 W·m−2 at t = 6.9h (i.e. 0:54 p.m.). Nevertheless, the heat flux of the north wall in greenhouse B reaches the maximum ϕ+ = 34.5 W·m−2 at t = 6.8h (i.e. 0:48 p.m.). In cloudy days, the solar radiation energy is much less than the sunny days, so the heat mass entering the greenhouse is less. As a consequence, the temperature in the greenhouse is much lower than that of sunny days. In the conditions of solar radiation energy absence under cloudy days, the heat-storage performance of the perforated brick wall and the common clay brick wall is more obvious, and external wall will lose more energy than that of the fine coal ash brick wall. Most of the time period of the three north walls is in the heat-release state in the condition of cloudy day. The north wall heat-release of the greenhouse B is the maximum (i.e. ϕ− = 24.15 W·m−2), and the greenhouse C is the minimum (i.e. ϕ− = 17.25 W·m−2). The amount of heat accumulated by the fine coal ash brick north wall is the most in the daytime, so the amount of heat released at night is also the most. The north walls of greenhouse A and greenhouse C are the first to release heat (i.e. t = 8.4h, 1:24 p.m.), and then is greenhouse B (i.e. t = 9.4h, 3:24 p.m.). After 9:00 p.m., the heat flux of the north wall tends to be stable. As shown in Figure 6 (B) and Figure 6 (C), four typical moments of t = 4.0h, t = 6.9h, t = 10h and t = 11.6h are selected respectively. These four typical moments also correspond to the heat release valley, the heat storage peak and the transition stage of the north wall, respectively. As shown in Figure 6(C), the heat-storage difference of the three north wall materials is the largest at t = 6.9 h (i.e. 0:54 p.m.). The north wall materials of three greenhouses are in the conversion stage of heat-storage and heat-release at t = 11.6h (i.e. 5:36 p.m.). The amount of solar radiation on cloudy days is the weakest, which is almost only scattering radiation. The north wall of fine coal ash brick also has better heat storage and heat preservation capacities under cloudy conditions.

Figure 6 Daily variation of instantaneous heat flux of different wall materials under cloudy days (Observation time: from 6:00 a.m. to 6: 00 a.m. the next day, 24 h)
Figure 6

Daily variation of instantaneous heat flux of different wall materials under cloudy days (Observation time: from 6:00 a.m. to 6: 00 a.m. the next day, 24 h)

To further compare the heat releasing difference of the north wall materials, the heat-release characteristics of the three greenhouses in the whole winter are comprehensively evaluated. The contribution of the monthly average released heat of the three kinds of north walls to the greenhouse is displayed in Figure 7. According to the results of heat-release test, the heat-release capacities of the three north walls increase month by month. The most obvious increasing is greenhouse B and the growth rate is 22%. The average monthly released heat of greenhouse A and greenhouse C is closed to each other. In January, February and March, the average monthly heat-release capacity in the three greenhouses is: greenhouse B>greenhouse A>greenhouse C.

Figure 7 Comparison of the average released heat of three kinds of greenhouse walls in winter
Figure 7

Comparison of the average released heat of three kinds of greenhouse walls in winter

3.2 The response regularities of the internal temperature to the heat-storage and heat-release characteristics of different north wall materials

3.2.1 Effect of heat-storage and heat-release of perforated brick wall on greenhouse temperature

Figure 8(a) is the heat-release distribution in the north wall of greenhouse A at night (i.e. 6:00 p.m. to the next day 6:00 a.m.). For greenhouse A, the heat-release thickness of the north wall at 6:00 p.m. is 15cm, but the heat-release thickness at 9:00 p.m. is 20cm. After that, the heat-release thickness of the north wall is equal to or greater than 30 cm at 0:00, 3:00 and 6:00 in the morning. The heat-release temperature is gradually attenuated over time, suggesting that the solar energy accumulated in the north wall is transferred to the indoor environment. The high-temperature region of the north wall is mainly concentrated on the surface of 5cm near the inner wall, indicating that the heat accumulated in the brick wall tends to conduct to the outside. Therefore, it is critical important to increase the insulation property of the north wall in the solar greenhouse. It can be seen from Figure 8(b) that the air temperature in the greenhouse is higher than the surface temperature of the north wall during the time period of t = 0h~10h (i.e. 6:00 a.m. to 4:00 p.m.), indicating that the greenhouse wall is in the heat-storage stage due to the warming effect of the solar radiation. When t = 10h (i.e. 4:00 p.m.), the air temperature inside the greenhouse is 2~3∘C lower than the wall surface temperature, so the wall is in the heat-releasing stage at night.

Figure 8 The heat-storage and heat-release rule of perforated brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)
Figure 8

The heat-storage and heat-release rule of perforated brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)

3.2.2 Effect of heat-storage and heat-release of fine coal ash brick wall on greenhouse temperature

Figure 9(a) is the rule of heat-release in the north wall of greenhouse B at night (i.e. 6:00 p.m. to the next day 6:00 a.m.). The heat-release thickness of the north wall at 6:00 p.m. is 15cm and at 9:00 p.m. is 20cm. Like greenhouse A, the heat-release thickness of the north wall is greater than 30 cm at 0:00, 3:00 and 6:00. The heat-releasing high-temperature region is concentrated on 5~10cm of the inner wall near surface, so the distribution range of the heat-storage source inside the north wall body is wider than that of greenhouse A. Meanwhile, the heat preservation performance of the north wall in greenhouse B is satisfied to the CSG. As illustrated in Figure 9(b), the air temperature is 2~3∘C higher than the wall surface temperature during the time period t = 5~6h (i.e. 11:00 a.m. to 12:00 a.m.), suggesting that the north wall is in the heat-storage stage. For the time period of t = 0~5h (i.e. 6:00 a.m. to 11:00 a.m.) and t = 6~24h (i.e. 12:00 a.m. to the next day 6:00 a.m.), the north wall is in the heat-release stage, and the air temperature inside the greenhouse is lower than the wall surface temperature. The temperature difference between them is especially noticeable at night. The heat release ability of the wall is significant and lasting, and the surface temperature of the north wall is 3~4∘C higher than the indoor air temperature.

Figure 9 The heat-storage and heat-release rule of fine coal ash brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)
Figure 9

The heat-storage and heat-release rule of fine coal ash brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)

3.2.3 Effect of heat-storage and heat-release of common clay brick wall on greenhouse temperature

Figure 10(a) is the rule of heat-release in the north wall of greenhouse C at night (i.e. 6:00 p.m. to the next day 6:00 a.m.). The heat-release high-temperature region of the north wall is mainly concentrated on the 3cm of the inner wall, which indicates that the accumulated heat in the north wall is more conductive to the outside. Therefore, greenhouse C needs to enhance the wall insulation. According to Figure 10(b), the difference between air temperature and wall surface temperature is insignificant, and the heat-storage effect of wall is not obvious before t = 5.2h (i.e. 11:12 a.m.). Under the effect of solar radiation, the indoor air temperature is obviously higher than that of the north wall in the period of t = 5.2~11.3h (i.e. 11:12 a.m. to 5:18 p.m.). After t = 11.3h (i.e. 5:18 p.m.), the air temperature at night is 2~3∘C higher than that of the wall surface, and the wall is in heat-release stage.

Figure 10 The heat-storage and heat-release rule of common clay brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)
Figure 10

The heat-storage and heat-release rule of common clay brick wall: (a) Temperature variation of different cross sections of the north wall (b) Variation of surface temperature and air temperature (observation time: 6:00 a.m. to the next day 6:00 a.m.)

3.3 The response regularities of internal thermal environments to external environmental conditions of three north wall materials

3.3.1 Response regularities of air temperature to heat-storage and heat-release characteristics of different north wall materials

Figure 11 exhibits the indoor and outdoor temperature distributions of three different north wall materials in the sunny day. When the external environment temperature reaches the peak (i.e. t = 12.54h, Tfine(out) =0.3∘C), the internal temperature reaches the maximum value. Greenhouse B reaches the peak firstly (t = 12.16h). And then is greenhouse C (t = 12.33h). The last is greenhouse A (t = 12.92h). The comparison can be expressed as Tmax(B) = 35.1∘C > Tmax(A) = 31.1∘C > Tmax(C) = 26.9∘C. The peak value of internal temperature in greenhouse B is the largest, indicating that the instantaneous endothermic capacity of pulverized coal ash brick wall is the strongest. In the period of t = 7.6~17.1h, the internal temperature variations of the three greenhouses are the same, which is consistent with the external temperature changing trend. It’s worth noting that the temperature curve is unimodal. All the south roof quilts of three greenhouses are opened at 8:30 a.m. in the morning. As the increase of the solar height angle after t = 7.6 h, the indoor temperature is obviously affected by the external temperature rising. The greenhouse A and the greenhouse B have the same heating rate (i.e. kT(A)up = kT(B)up = 5.2∘C/h). The heating rate of greenhouse C is smaller (i.e. kT(C)up = 4.4∘C/h). It indicates that the fine coal ash brick wall and the common clay brick wall have the same thermal buffering capacity, but the response of the perforated brick wall to the ambient temperature changing is the weakest. The solar altitude angle decreases gradually after t = 13.3h (since 1:18 p.m.), the cooling rates of the three greenhouses are obviously higher than that of external ambient temperature (i.e. kT(B)down = 3.3∘C/h > kT(C)down = 3.2∘C/h > kT(A)down = 2.9∘C/h > kT(out)down = 1.3∘C/h). The temperature drop slows down at night after t = 17.1h (since 5:06 p.m.). At the same time, the temperature variations of greenhouse B and greenhouse C exhibit multi-peak distributions. The lowest internal temperature of the three greenhouses appears at t=7.6h (i.e. 7:36 a.m.), and the comparison can be expressed as Tmin(A) = 5.18∘C < Tmin(C) = 5.9∘C < Tmin(B) = 11.1∘C. In the daytime, the solar energy is stored inside the north wall to raise the temperature. At night, the heat in the north wall is released, and the wall temperature decreases. It can be seen that no matter daytime or night, greenhouse B heat is basically balanced, the fine coal ash brick wall stores the most amount of heat during the daytime and releases the most amount of heat at night.

Figure 11 Effect of the north wall material on the indoor temperature in the sunny day
Figure 11

Effect of the north wall material on the indoor temperature in the sunny day

Figure 12 shows the indoor and outdoor temperature distributions of three different north wall materials in the cloudy day. Due to the high cloud covering, the solar radiation intensity is weakened. As a consequence, the temperature difference inside and outside the greenhouse is reduced, and the limit temperature in the three greenhouses correspondingly decreases. The comparisons can be expressed as ΔTmax − min(A) = 11.2∘C (cloudy) < ΔTmax − min(A) = 25.9∘C (sunny), ΔTmax − min(B) = 9.1∘C (cloudy) < ΔTmax − min(B) = 24∘C (sunny) and ΔTmax − min(C) = 8.8∘C (cloudy) < ΔTmax − min(C) = 21∘C (sunny). Meanwhile, the heating rate of internal and external environment is smaller than that of sunny day. The internal heating rates of the three kinds of north wall materials are basically the same (i.e. kT(A)up = 2.4∘C/h, kT(B)up = 2.3∘C/h and kT(C)up = 2.3∘C/h). There is no significant difference in the peak indoor temperature between greenhouse A and greenhouse C (i.e. Tmax(A) = 16.6∘C and Tmax(C) = 16.3∘C). The time of the outdoor temperature peak (i.e. tmax(out) = 13.68h) lags behind that of the indoor temperature peak (i.e. tT max(A) = 12.92h, tT max(B) = 12.16h and tT max(C) = 12.16h).

Figure 12 Effect of the north wall material on the indoor temperature in the cloudy day
Figure 12

Effect of the north wall material on the indoor temperature in the cloudy day

Figure 13 illustrates the effects of different north wall materials on heating rate and cooling rate of the CSG. As discussed above, the heating process is in the morning, and the cooling process is in the afternoon and evening. The higher the heating rate is, the faster the thermal response of air temperature to external climate is, and the solar energy absorption by the north wall is insignificant. The higher the cooling rate is, the worse the thermal inertia is, and the heat-storage performance of the north wall is worse. As shown in statistical results, the average heating rate (i.e. φ¯topA= 5.008∘C/h) and the average cooling rate (i.e. φ¯downA= 1.356∘C/h) are both higher and the temperature fluctuation is more greatly in greenhouse A. The average heating rate and the average cooling rate of greenhouse B are about the same as those of greenhouse C (i.e. φ¯topB=φ¯topC= 4.382∘C/h and φ¯downB=φ¯downC= 1.304∘C/h). But they are smaller than the greenhouse A. It indicates that the north wall of greenhouse B and greenhouse C have better heat storage. The thermal properties of different north wall materials are different. For example, the thermal conductivity, the specific heat and so on. Due to the various thermal properties of the wall materials, the thermal preservation can be different under the same thermal resistance. In the cloudy days, the average heating rate and average cooling rate of the three greenhouses decrease significantly, which tend to be slower compared with the

Figure 13 Effects of different north wall materials on heating rate and cooling rate of the CSG (greenhouse A: perforated brick wall, greenhouse B: fine coal ash brick wall, greenhouse C: common clay brick wall, out: external environment, top: heating rate, down: cooling rate)
Figure 13

Effects of different north wall materials on heating rate and cooling rate of the CSG (greenhouse A: perforated brick wall, greenhouse B: fine coal ash brick wall, greenhouse C: common clay brick wall, out: external environment, top: heating rate, down: cooling rate)

sunny days (i.e. fine days). And the heating/cooling rate of greenhouse A is the most obviously among the three greenhouses. The temperature fluctuation of greenhouse B is the smallest. The heating rate and cooling rate are respectively φ¯topB= 1.936∘C/h and φ¯downB= 0.639∘C/h, which demonstrates that the heat-storage inertia of greenhouse B is significant.

3.3.2 Response regularities of air relative humidity to heat-storage and heat-release characteristics of different north wall materials

Figure 14 shows the relationship between temperature and humidity in the greenhouse B between December 2016 and March 2017. It can be found from the diagram that the variation of air relative average humidity is opposite to that of air temperature change. The changes in the air relative humidity will directly affect the growth and development of the plants. And it may even cause diseases.

Figure 14 The temperature and humidity distributions of greenhouse B (observation time period: December, 2016 to March, 2017)
Figure 14

The temperature and humidity distributions of greenhouse B (observation time period: December, 2016 to March, 2017)

Figure 15 describes the humidity relationship between the internal air and the outdoor environment in the sunny day. The relative average humidity curves in the three greenhouses are like a unimodal funnel. The relative air humidity reaches the maximum at t = 7.2h (i.e. 7:12 a.m.). Then the heat preservation quilt on south roof is rolled up, and the indoor air temperature rises. So that the air humidity decreases rapidly with the temperature increase. The humidity reaches the lowest value at t = 13.6h (i.e. 1:36 p.m.). As the heat preservation quilt is put down again, the humidity gradually increases with the decrease of temperature after t = 19.2 (i.e. 7:12 p.m.). The humidity remains at a high level until the heat preservation quilt is opened in the next day. Generally speaking, higher relative average humidity is easy to induce crop diseases in the CSG. The relative humidity of greenhouse A and greenhouse C maintains above 85% for most of the time period (i.e. 0:00 a.m. to 8:30 a.m.). So they are not suitable for crop growth compared with greenhouse B.

Figure 15 Humidity relationship between the indoor air and the outdoor air with different north wall materials in the sunny day
Figure 15

Humidity relationship between the indoor air and the outdoor air with different north wall materials in the sunny day

Figure 16 shows the humidity relationship between the internal air and the outdoor environment in the cloudy day. Due to the weakening of solar radiation intensity, the humidity of the external environment increases in cloudy days. The greenhouse A and the greenhouse C maintain the relative humidity above 84.5% for most of the time period (i.e. 0:00 a.m. to 8:30 a.m. and 5:00 p.m. to12:00 p.m.). The relative average humidity curves in greenhouse A and greenhouse C present unimodal funnel distribution. But the humidity distribution in greenhouse B is lower (i.e. greenhouse B < greenhouse A < greenhouse C). Moreover, the relative average humidity in greenhouse B at night rises again obviously in comparison with greenhouse A and greenhouse C.

Figure 16 Humidity relationship between the indoor air and the outdoor air with different north wall materials in the cloudy day
Figure 16

Humidity relationship between the indoor air and the outdoor air with different north wall materials in the cloudy day

As an important energy storage structure in the CSG, the north wall is the main heat source in the greenhouse at night, and it is also an important facility to maintain the temperature at night. Although the external environment in winter goes against to the crop growth, we can create a suitable growth environment for spontaneous regulation by the north wall. The key of this kind of spontaneous regulation is the heat-storage and heat-release characteristics of the north walls (i.e. heat storage flux ϕ+ and heat releasing flux ϕ−). The heat transfer characteristics of the north wall depend on the material, which are also related to the compensation of temperature and the adjustment of humidity. Due to the generalized use of CSG in northern China, the selection of the north wall materials must take into account of the economic benefits. Therefore, low construction cost is an indispensable factor in the north wall construction. The three different wall materials investigated in this investigation (i.e. perforated brick, fine coal ash brick, common clay brick) are all energy-saving building blocks which is advocated by the state. All of them are suitable for the north wall of the CSG. However, in terms of construction cost, the perforated brick is the lowest (i.e. 22, 100 RMB). The price of fine coal ash brick is moderate (i.e. 34, 100 RMB). The application of fine coal ash brick not only has good heat storage & preservation characteristics in sunny and cloudy days, but also meets the requirements of national energy saving and emission reduction. In a word, the fine coal ash brick can be widely popularized and applied as the north wall in the CSG and other similar passive solar greenhouses. To sum up, the fine coal ash brick wall has good promotion value for the high latitude, high altitude and long winter regions.

4 Conclusions

The present experimental investigation has studied the effect of north wall material on the thermal environment in the CSG. The main conclusions are as follows:

  1. The peak indoor temperature of greenhouse B is higher than that of greenhouse A and greenhouse C on both cloudy and sunny days. Despite the atmospheric counter-radiation in cloudy days, the minimum temperatures in greenhouse B and greenhouse C are basically constant at night, and the greenhouse B can maintain the thermal environment at a high level.

  2. The indoor temperature fluctuation of greenhouse A is remarkable in sunny days. But the heat storages of greenhouse B and greenhouse C are better. The temperature fluctuation of greenhouse B is the smallest in cloudy days, indicating the fine coal ash brick has better thermal inertia property than the perforated brick and common clay brick.

  3. In sunny days, the north wall material of greenhouse B is the fastest to store heat and the greenhouse A is the slowest. In cloudy days, the heat storage time of greenhouse B is the longest, and greenhouse C is the shortest. Moreover, the heat release of greenhouse B is the greatest, and the greenhouse C is the smallest.

  4. The relationship between the average relative humidity is: greenhouse B < greenhouse A < greenhouse C. Moreover, the recovery rate of the relative humidity in greenhouse B at night is obviously slower. The humidity level of greenhouse B is more beneficial to crop growth.

  5. Using fine coal ash brick wall as the north wall can give full play to the heat storage & preservation performance because it realizes the efficient utilization of light energy and consumes moderate construction cost. This kind of north wall has good promotion value for the high latitude, high altitude and long winter regions.

Acknowledgement

The present work is supported financially by the Postdoctoral Foundation of Xingan Liu under the Grant 1105/770218004.

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Received: 2019-09-30
Accepted: 2019-11-26
Published Online: 2019-12-31

© 2019 X. Liu et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/phys-2019-0079
Keywords for this article
Chinese solar greenhouse; wall material; heat storage; heat release; temperature
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