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Length Determination of the Solar Greenhouse North Wall in Lanzhou
Available online at www.sciencedirect.com
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 205 (2017) 1230–1236
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Length Determination of the Solar Greenhouse North Wall in Lanzhou Xuting Gaoa, Huijun Yanga, Yong Guana, *, Jie Baia, Renli Zhanga, Wenju Hub a
Department of Building Environment and Energy Engineering, School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, PR China b Beijing Key Lab of Heating, Gas Supply Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China
Abstract In order to determine the optimal length of the greenhouse north wall in Lanzhou, this paper uses the Ecotect and Energyplus v8.6 to evaluate the effect of length of the greenhouse north wall on its indoor thermal environment. Then, the total heat storage/release capacity and the reduction rate of the shadow were proposed. The results show that under different weather conditions, the maximum shadow rate of the east and the west walls of the winter solstice appear at 10:00 in Lanzhou. With the length of the north wall increases, the reduction rate of the shadow gradually decreases, and when the length increases to about 90m, the shadow reduction rate changes smaller. The total heat storage capacity basically equals to its total heat release in a sunny day. While the length is more than 90m, the total heat release is greater than the total heat storage, and the indoor air temperature is lower than that of 90m in next day morning in typical cloudy day. Combined with the needs of the ventilation and insulation in solar greenhouse, the optimal length of north wall of solar greenhouse is about 90m in Lanzhou. The results can provide a reference for the scientific construction of the solar greenhouse in Lanzhou. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 10th International Symposium on Heating, Ventilation Peer-review under responsibility ofElsevier the scientific and Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Air Conditioning.
Keywords: Solar greenhouse; North wall; Heat storage/release; Optimal length
* Corresponding author. Tel.: + 86-931-4956743; fax: +86-931-4956017 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.361
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Xuting Gao et al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al. / Procedia Engineering 00 (2017) 000–000
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1. Introduction The solar greenhouse originated in the northern China in Ref. Pan and Li et al. [1-2]. There are some deficiencies for light and temperature environment and its building structures in traditional solar greenhouses. Therefore, a lot of researches have been carried out by many science and technology workers. For example, Gao and Tong et al. [3-4] numerically studied the illumination characteristics and the radiance of each surface in solar greenhouse. Li et al. [5]studied the heat transfer process of soil wall in solar greenhouse and its theoretical simplification methods. They pointed out that the soil wall can be divided into heat storage layer and heat insulation layer; the heat storage layer had large temperature fluctuation and can be used for storing heat during daytime and release heat into the solar greenhouse during nighttime. Jiang et al. [6] compared the insulation of different structure of solar greenhouse in the eastern part of the Hexi Corridor, the results show that the thermal insulation effect of the wall is better than grass brick; the building structure determines the land area of the solar greenhouse, the light incidence and other factors. Sun et al. [7] summarized the structural design parameters of the solar greenhouse as the five degrees (Angle, height, span, length and thickness), four ratios (high span ratio, front and rear slope ratio, insulation ratio and shade ratio), and the results show that the length is one of the parameters. The diurnal variation of the ground shadows in solar greenhouse was analyzed by the Chen et al. [8] in early days, and obtained the diurnal variation of light transmittance and shadow rate with the length of solar greenhouse by experiments. Sun et al. [7] considered that the length of the greenhouse affected the cultivation area and the indoor shade ratio. Each greenhouse area should be more than 300m2, that is to say, when the greenhouse span is 7m, the length should be more than 50m, but greenhouse length is generally not more than 100m, the range of the optimal length is from 50m to 100m. Li et al. [9-10] calculated the sunshine area and sun shadow percentage of the east and west walls in the indoor north wall, moreover, the percentage of sun shadow with the different length in solar greenhouse were analyzed. In order to scientifically construct solar greenhouse in Lanzhou, the solar greenhouse models with the different length of north wall are established to explore the construction method of north wall length in Lanzhou, when keeping the greenhouse's span and azimuth angle constant. 2. Materials and methods 2.1. Meteorological conditions and greenhouse parameters The solar greenhouse is located in Lanzhou, China, with an average elevation 1517.2m, latitude 36.05°N, longitude 103.88°E. Lanzhou has a typical temperate, semi-arid continental monsoon climate. It is dry with plenty of sunshine. Lanzhou weather in winter is chilly with an average temperature of -7.3C while spring is ephemeral with changeable weather. Though historical records show that in some extreme cases the temperature can be as low as -19.7C, the annual average sunshine hours are 2446 hours, frost free period is 160 days or more. In this paper, the meteorological data is used obtained by Lanzhou meteorological station.
Fig. 1. The model of the solar greenhouse
Xuting Gao et al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al./ Procedia Engineering 00 (2017) 000–000
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3
In order to determine the optimal length of the greenhouse north wall in Lanzhou, this paper uses the Ecotect and Energyplus v8.6 to model the indoor thermal environment of solar greenhouse and to determine the length of the northern wall of the solar greenhouse in Lanzhou. The model of the greenhouse lies in the north towards south, the azimuth is 0°, the height of ridge is 2.8m, and the span is 6m. The length of the slope of the roof is 1m, the width is 100mm which is made of concrete; the angle of the slope of the roof is 37°. The east and west wall is made of bricks, which width is 480mm, the height of north wall is 2.2m, which is made of the width of PCM wall board layer, block brick layer and heat preservation layer is 50mm, 900mm, 100mm respectively. The covered material of the front roof is EVA plastics, which thickness is 0.12mm. The heat preservation quilt is used from 16:00 to 10:00, but the time of opening and closing depends on weather conditions. The established model is as shown in Fig. 1. 2.2. The reduction rate of the shadow To determine the optimal length of the greenhouse north wall, the reduction rate of the shadow was proposed. The shadow of east and west wall on the north wall shapes an area like trapezoid because the east wall causes shadow in the morning and the west wall causes shadow in the afternoon is calculated by Eq. (1).
St =
1 ( X 1 + X 2）× H 2
(1)
where S t is the area of trapezoid,m2, X 1 is the length of shadow in the upside of north wall, m, X 2 is the length of shadow in the downside of north wall, m, H is the height of the north wall, m. The shadow ratio of east and west wall on the north wall is calculated by Eq.(2) .
Sy =
St L× H
(2)
where S y is the shadow ratio, L is the length of the north wall, m. The shadow of east and west on the north wall will affect the lighting and the heat storage/release of the north wall. In this study, we uses the Ecotect software, which is a multipurpose software with analyzing building energy consumption, sunlight and shadow, to simulate and calculate the ratio of east and west wall on the north wall. The calculation conditions are to change the length of the north wall of the solar greenhouse (i.e. the length change is 30150m from 10:00 to 16:00), and the other parameters remain constant. 2.3. The total heat storage/release capacity The north wall of solar greenhouse plays an important role in controlling its thermal environment as a major mass in heat storage/release mass. The length of the north wall can affect interior temperature of the solar greenhouse as the result of effecting heat storage/release capacity. Then, the Energy Plus v8.6, which can analyzed and calculate building energy consumption, is used to determine the appropriate length of the north wall of the solar greenhouse according the heat storage/release capacity and its interior temperature. In order to comparative analysis, the sunshine day (Dec. 19) and the cloudy day (Dec. 21) are chose. The daily accumulation of solar radiation is 15.35 MJ/m2 in the sunshine day and is 7.6 MJ/m2 in the cloudy day. The specific outdoor temperature and solar radiation illumination were shown in Fig. 2. The length of the north wall of the solar greenhouse is changed to calculate heat storage/release capacity in the sunshine and the cloudy day and analyze temperature variation with other parameters remaining constant. The heat storage is held essential in the stage of opening the heat preservation quilt and heat release is held essential in the stage of closing. The total heat storage/release capacity can be obtained through Eq. (3).
Qx/f = Qc + Qn + Qs
(3)
where Qx / f is the heat storage and release of the solar greenhouse, W, Qc is internal surface heat gain, W, Qn is internal surface radiant heat gain, W, Qs is internal surface solar radiation heat gain, W.
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Xuting Gao al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al. /etProcedia Engineering 00 (2017) 000–000
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Fig.2. Hourly outdoor air temperature and solar radiation for representative days
3. Results and discussions 3.1. The effect of the change of The greenhouse north wall on shadow rate The greenhouse north wall plays an important role in regulating the thermal environment of solar greenhouse by its heat storage/release. The change of the wall length of greenhouse will affect the shadow rate of the east and west walls in solar greenhouse. The relations of greenhouse wall length and wall shadow rate in winter solstice are as shown in Fig. 3. It can be seen that with the increase of the length of the north wall of greenhouse from 30m to 150m, the shadow rates of east and west walls on the north wall are gradually reduced, the maximum shadow rate appears at 10:00 am. Because the height of east and west walls are the same, the shade area of east and west walls on the north wall is changeless, The shadow area in the greenhouse is low than that of light area, when the length of the north wall of greenhouse reaches about 90 m, the effect of increasing its length on the shadow rate will be reduced.
Fig. 3. The relations of greenhouse wall length and wall shadow rate in winter solstice
The shadow rates of 10:00 under different lengths of solar greenhouse are as shown in Table 1. It can be seen that when the length of the greenhouse is 30m, 90m and 150m, the shadow rates are 8.2%, 2.7% and 1.6%, respectively,
Xuting Gao et al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al./ Procedia Engineering 00 (2017) 000–000
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when the solar greenhouse length is less than 90m, its shadow rate decreases significantly with the length increasing. When the wall length of the greenhouse is increased from 30m to 90m, the shadow rate can be reduced by 5.5%, while the greenhouse wall length decreases from 90m to 150m, the shadow rate is reduced by only 1.1%. The greenhouse wall lengths are from 30m to 50m, 90m to 110m and 130 to 150m, the reduction rate of shadow are 40%, 18.18%, 13.33%, respectively. With the increase in the wall length of the greenhouse, the reduction rate of the greenhouse is also decreasing. Table 1. Shadow rates of 10:00 under different lengths of solar greenhouse. Length（m） Shadow rates（%）
30 8.2
50 4.9
70 3.5
90 2.7
110 2.2
130 1.9
150 1.6
From the above simulation results, the shadow area is small in the early morning, the shadow rate is 0% at noon, but, the shadow in the afternoon and morning is opposite. When the wall length of the greenhouse is short, the shadow caused by the east and west walls in the north wall is larger, but it will influence lighting in a large degree. The longer greenhouse wall is good at lighting, but it effects the ventilation and insulation, taking account of the operation convenience and the stability of the greenhouse, especially in Lanzhou and other regions which has large wind, the wall length of the greenhouse is too long to stable indoor heat environment of solar greenhouse, and cause much heat loss. From the view of shadow, the suitable wall length of solar greenhouse is 90m in Lanzhou. 3.2. The effect of different wall length of solar greenhouse on total heat storage/release capacity In order to seek the corresponding relationship between the greenhouse wall length variation and the total heat storage/release capacity of solar greenhouse, Fig. 4 and Fig. 5 show the simulation results. It can be seen from Figure 3, in the sunny day, with the length of the north wall of the greenhouse increases, the total heat storage capacity is also increasing, when the length is less than 90m, the total heat storage capacity is greater than that of the heat release capacity, when the length is more than 90m, the result is opposite. With the wall length of greenhouse increases, the heat dissipation much more than that of the storage capacity, and the indoor air temperature drops at night, this is harmful to crop growth because the temperature is too low. In the cloudy day, the heat storage/release capacity increases, with length of the greenhouse increasing, but the total greenhouse heat release capacity is much larger than that of the heat storage capacity because of the affection of solar radiation.
Fig. 4. The relations of greenhouse wall length and the total heat storage/release capacity for representative days
It can be seen from Fig. 5, the longer the north wall is, the lower of the greenhouse temperature is. When the wall length is 90m, the indoor air temperatures of solar greenhouse are the highest. This is because when the length is longer than 90m, the total heat dissipation is greater than the heat storage capacity. The heat storage capacity is
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basically equal to the heat release capacity when the length is 90m. Regardless of day or night, the length of the north wall has little effect on greenhouse’s indoor air temperature in the cloudy day. Therefore, the wall length of solar greenhouse should not be too long. As a result, the suitable construction of the greenhouse wall length is about 90m, considering the storage heat and heat dissipation on the thermal environment of solar greenhouse in Lanzhou.
Fig. 5. Variations of indoor air temperature of solar greenhouse under different wall length of greenhouse for representative days
4. Conclusions This paper uses the Ecotect and Energyplus v8.6 to evaluate the effect of length of the greenhouse north wall on its indoor thermal environment. The total heat storage/release capacity and the reduction rate of the shadow were proposed. Based these evaluation parameters, the optimum length has obtained by numerically simulation, the main conclusions of this study are as follows: (i) With the increase of the length of the north wall of greenhouse from 30m to 150m, the shadow rates of east and west walls on the north wall are gradually reduced, when the length of the north wall of greenhouse reaches about 90 m, the effect of increasing its length on the shadow rate will be reduced. (ii) In the sunny day, with the length of the north wall of the greenhouse increases, the total heat storage capacity is also increasing, when the length is less than 90m, the total heat storage capacity is greater than that of the heat release capacity, while the wall length is more than 90m, the result is opposite. (iii) Considering the storage heat and heat dissipation on the thermal environment of solar greenhouse, the suitable construction of the greenhouse wall length is about 90m in Lanzhou. Acknowledgements This study is supported by the National Natural Science Foundation of China (No: 51468028), the Science and Technology Plan of Gansu Province (No. 1506RJZA065) and Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering (No: NR2015K05). References [1] B.T. Pan, X.H. Liu, D.S. Yin. Research status and development trend of energy-saving sunlight greenhouse in northern China, Greenh. Hortic. 2 (2005) 15-17. [2] T.L. Li. Current situation and prospect of solar greenhouse industry in China, J. Shenyang Agric. Univ. 36(2) (2005) 131-138. [3] Q.L. Gao, Y.J. Liang, A.W. Duan. Study on the characteristics of light intensity in solar greenhouse and its variation, Trans. Chinese Soc. Agric. Eng. 19(3) (2003) 200-204. [4] G.H. Tong, B.M. Li. Simulation of solar radiance in surface of solar greenhouse, J. China Agric. Univ. 11(1) (2006) 61-65.
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[5] M. Li, J.C. Zhou, T. Zhou. Heat transfer process of soil wall in solar greenhouse and its theoretical simplification methods, Trans. Chinese Soc. Agric. Eng. 32(3) (2016) 175-181. [6] J.F. Jiang, Y.G. Wei. Comparison of insulation in sunlight greenhouse with different structures in the east of Hexi Corridor, Arid Weather. 33(2) (2015) 317-323. [7] Z.Q. Sun, J.Q. Wang. Design and performance of Huanghuai modified sunlight greenhouse, Trans. Chinese Soc. Agric. Eng.12 (1997) 41-47. [8] Q.Y. Chen. Numerical experiments on illumination environment of single roof greenhouse, Trans. Chinese Soc. Agric. Eng. 9(3) (1993) 96101. [9] X.F. Li, Q.Y. Chen. Effect of the east and west wall of sunlight greenhouse on direct radiation heat gain, Proceedings of the International Conference on Agricultural Bio - environment and Energy Engineering, 2003, 220-225. [10] X.F. Li, Q.Y. Chen. Effects of solar gates on the radiation of solar radiation, Trans. Chinese Soc. Agric. Eng. 20(5) (2004) 241-245.
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 205 (2017) 1230–1236
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Length Determination of the Solar Greenhouse North Wall in Lanzhou Xuting Gaoa, Huijun Yanga, Yong Guana, *, Jie Baia, Renli Zhanga, Wenju Hub a
Department of Building Environment and Energy Engineering, School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, PR China b Beijing Key Lab of Heating, Gas Supply Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China
Abstract In order to determine the optimal length of the greenhouse north wall in Lanzhou, this paper uses the Ecotect and Energyplus v8.6 to evaluate the effect of length of the greenhouse north wall on its indoor thermal environment. Then, the total heat storage/release capacity and the reduction rate of the shadow were proposed. The results show that under different weather conditions, the maximum shadow rate of the east and the west walls of the winter solstice appear at 10:00 in Lanzhou. With the length of the north wall increases, the reduction rate of the shadow gradually decreases, and when the length increases to about 90m, the shadow reduction rate changes smaller. The total heat storage capacity basically equals to its total heat release in a sunny day. While the length is more than 90m, the total heat release is greater than the total heat storage, and the indoor air temperature is lower than that of 90m in next day morning in typical cloudy day. Combined with the needs of the ventilation and insulation in solar greenhouse, the optimal length of north wall of solar greenhouse is about 90m in Lanzhou. The results can provide a reference for the scientific construction of the solar greenhouse in Lanzhou. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 10th International Symposium on Heating, Ventilation Peer-review under responsibility ofElsevier the scientific and Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Air Conditioning.
Keywords: Solar greenhouse; North wall; Heat storage/release; Optimal length
* Corresponding author. Tel.: + 86-931-4956743; fax: +86-931-4956017 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.361
2
Xuting Gao et al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al. / Procedia Engineering 00 (2017) 000–000
1231
1. Introduction The solar greenhouse originated in the northern China in Ref. Pan and Li et al. [1-2]. There are some deficiencies for light and temperature environment and its building structures in traditional solar greenhouses. Therefore, a lot of researches have been carried out by many science and technology workers. For example, Gao and Tong et al. [3-4] numerically studied the illumination characteristics and the radiance of each surface in solar greenhouse. Li et al. [5]studied the heat transfer process of soil wall in solar greenhouse and its theoretical simplification methods. They pointed out that the soil wall can be divided into heat storage layer and heat insulation layer; the heat storage layer had large temperature fluctuation and can be used for storing heat during daytime and release heat into the solar greenhouse during nighttime. Jiang et al. [6] compared the insulation of different structure of solar greenhouse in the eastern part of the Hexi Corridor, the results show that the thermal insulation effect of the wall is better than grass brick; the building structure determines the land area of the solar greenhouse, the light incidence and other factors. Sun et al. [7] summarized the structural design parameters of the solar greenhouse as the five degrees (Angle, height, span, length and thickness), four ratios (high span ratio, front and rear slope ratio, insulation ratio and shade ratio), and the results show that the length is one of the parameters. The diurnal variation of the ground shadows in solar greenhouse was analyzed by the Chen et al. [8] in early days, and obtained the diurnal variation of light transmittance and shadow rate with the length of solar greenhouse by experiments. Sun et al. [7] considered that the length of the greenhouse affected the cultivation area and the indoor shade ratio. Each greenhouse area should be more than 300m2, that is to say, when the greenhouse span is 7m, the length should be more than 50m, but greenhouse length is generally not more than 100m, the range of the optimal length is from 50m to 100m. Li et al. [9-10] calculated the sunshine area and sun shadow percentage of the east and west walls in the indoor north wall, moreover, the percentage of sun shadow with the different length in solar greenhouse were analyzed. In order to scientifically construct solar greenhouse in Lanzhou, the solar greenhouse models with the different length of north wall are established to explore the construction method of north wall length in Lanzhou, when keeping the greenhouse's span and azimuth angle constant. 2. Materials and methods 2.1. Meteorological conditions and greenhouse parameters The solar greenhouse is located in Lanzhou, China, with an average elevation 1517.2m, latitude 36.05°N, longitude 103.88°E. Lanzhou has a typical temperate, semi-arid continental monsoon climate. It is dry with plenty of sunshine. Lanzhou weather in winter is chilly with an average temperature of -7.3C while spring is ephemeral with changeable weather. Though historical records show that in some extreme cases the temperature can be as low as -19.7C, the annual average sunshine hours are 2446 hours, frost free period is 160 days or more. In this paper, the meteorological data is used obtained by Lanzhou meteorological station.
Fig. 1. The model of the solar greenhouse
Xuting Gao et al. / Procedia Engineering 205 (2017) 1230–1236 Xuting Gao et al./ Procedia Engineering 00 (2017) 000–000
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3
In order to determine the optimal length of the greenhouse north wall in Lanzhou, this paper uses the Ecotect and Energyplus v8.6 to model the indoor thermal environment of solar greenhouse and to determine the length of the northern wall of the solar greenhouse in Lanzhou. The model of the greenhouse lies in the north towards south, the azimuth is 0°, the height of ridge is 2.8m, and the span is 6m. The length of the slope of the roof is 1m, the width is 100mm which is made of concrete; the angle of the slope of the roof is 37°. The east and west wall is made of bricks, which width is 480mm, the height of north wall is 2.2m, which is made of the width of PCM wall board layer, block brick layer and heat preservation layer is 50mm, 900mm, 100mm respectively. The covered material of the front roof is EVA plastics, which thickness is 0.12mm. The heat preservation quilt is used from 16:00 to 10:00, but the time of opening and closing depends on weather conditions. The established model is as shown in Fig. 1. 2.2. The reduction rate of the shadow To determine the optimal length of the greenhouse north wall, the reduction rate of the shadow was proposed. The shadow of east and west wall on the north wall shapes an area like trapezoid because the east wall causes shadow in the morning and the west wall causes shadow in the afternoon is calculated by Eq. (1).
St =
1 ( X 1 + X 2）× H 2
(1)
where S t is the area of trapezoid,m2, X 1 is the length of shadow in the upside of north wall, m, X 2 is the length of shadow in the downside of north wall, m, H is the height of the north wall, m. The shadow ratio of east and west wall on the north wall is calculated by Eq.(2) .
Sy =
St L× H
(2)
where S y is the shadow ratio, L is the length of the north wall, m. The shadow of east and west on the north wall will affect the lighting and the heat storage/release of the north wall. In this study, we uses the Ecotect software, which is a multipurpose software with analyzing building energy consumption, sunlight and shadow, to simulate and calculate the ratio of east and west wall on the north wall. The calculation conditions are to change the length of the north wall of the solar greenhouse (i.e. the length change is 30150m from 10:00 to 16:00), and the other parameters remain constant. 2.3. The total heat storage/release capacity The north wall of solar greenhouse plays an important role in controlling its thermal environment as a major mass in heat storage/release mass. The length of the north wall can affect interior temperature of the solar greenhouse as the result of effecting heat storage/release capacity. Then, the Energy Plus v8.6, which can analyzed and calculate building energy consumption, is used to determine the appropriate length of the north wall of the solar greenhouse according the heat storage/release capacity and its interior temperature. In order to comparative analysis, the sunshine day (Dec. 19) and the cloudy day (Dec. 21) are chose. The daily accumulation of solar radiation is 15.35 MJ/m2 in the sunshine day and is 7.6 MJ/m2 in the cloudy day. The specific outdoor temperature and solar radiation illumination were shown in Fig. 2. The length of the north wall of the solar greenhouse is changed to calculate heat storage/release capacity in the sunshine and the cloudy day and analyze temperature variation with other parameters remaining constant. The heat storage is held essential in the stage of opening the heat preservation quilt and heat release is held essential in the stage of closing. The total heat storage/release capacity can be obtained through Eq. (3).
Qx/f = Qc + Qn + Qs
(3)
where Qx / f is the heat storage and release of the solar greenhouse, W, Qc is internal surface heat gain, W, Qn is internal surface radiant heat gain, W, Qs is internal surface solar radiation heat gain, W.
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Fig.2. Hourly outdoor air temperature and solar radiation for representative days
3. Results and discussions 3.1. The effect of the change of The greenhouse north wall on shadow rate The greenhouse north wall plays an important role in regulating the thermal environment of solar greenhouse by its heat storage/release. The change of the wall length of greenhouse will affect the shadow rate of the east and west walls in solar greenhouse. The relations of greenhouse wall length and wall shadow rate in winter solstice are as shown in Fig. 3. It can be seen that with the increase of the length of the north wall of greenhouse from 30m to 150m, the shadow rates of east and west walls on the north wall are gradually reduced, the maximum shadow rate appears at 10:00 am. Because the height of east and west walls are the same, the shade area of east and west walls on the north wall is changeless, The shadow area in the greenhouse is low than that of light area, when the length of the north wall of greenhouse reaches about 90 m, the effect of increasing its length on the shadow rate will be reduced.
Fig. 3. The relations of greenhouse wall length and wall shadow rate in winter solstice
The shadow rates of 10:00 under different lengths of solar greenhouse are as shown in Table 1. It can be seen that when the length of the greenhouse is 30m, 90m and 150m, the shadow rates are 8.2%, 2.7% and 1.6%, respectively,
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when the solar greenhouse length is less than 90m, its shadow rate decreases significantly with the length increasing. When the wall length of the greenhouse is increased from 30m to 90m, the shadow rate can be reduced by 5.5%, while the greenhouse wall length decreases from 90m to 150m, the shadow rate is reduced by only 1.1%. The greenhouse wall lengths are from 30m to 50m, 90m to 110m and 130 to 150m, the reduction rate of shadow are 40%, 18.18%, 13.33%, respectively. With the increase in the wall length of the greenhouse, the reduction rate of the greenhouse is also decreasing. Table 1. Shadow rates of 10:00 under different lengths of solar greenhouse. Length（m） Shadow rates（%）
30 8.2
50 4.9
70 3.5
90 2.7
110 2.2
130 1.9
150 1.6
From the above simulation results, the shadow area is small in the early morning, the shadow rate is 0% at noon, but, the shadow in the afternoon and morning is opposite. When the wall length of the greenhouse is short, the shadow caused by the east and west walls in the north wall is larger, but it will influence lighting in a large degree. The longer greenhouse wall is good at lighting, but it effects the ventilation and insulation, taking account of the operation convenience and the stability of the greenhouse, especially in Lanzhou and other regions which has large wind, the wall length of the greenhouse is too long to stable indoor heat environment of solar greenhouse, and cause much heat loss. From the view of shadow, the suitable wall length of solar greenhouse is 90m in Lanzhou. 3.2. The effect of different wall length of solar greenhouse on total heat storage/release capacity In order to seek the corresponding relationship between the greenhouse wall length variation and the total heat storage/release capacity of solar greenhouse, Fig. 4 and Fig. 5 show the simulation results. It can be seen from Figure 3, in the sunny day, with the length of the north wall of the greenhouse increases, the total heat storage capacity is also increasing, when the length is less than 90m, the total heat storage capacity is greater than that of the heat release capacity, when the length is more than 90m, the result is opposite. With the wall length of greenhouse increases, the heat dissipation much more than that of the storage capacity, and the indoor air temperature drops at night, this is harmful to crop growth because the temperature is too low. In the cloudy day, the heat storage/release capacity increases, with length of the greenhouse increasing, but the total greenhouse heat release capacity is much larger than that of the heat storage capacity because of the affection of solar radiation.
Fig. 4. The relations of greenhouse wall length and the total heat storage/release capacity for representative days
It can be seen from Fig. 5, the longer the north wall is, the lower of the greenhouse temperature is. When the wall length is 90m, the indoor air temperatures of solar greenhouse are the highest. This is because when the length is longer than 90m, the total heat dissipation is greater than the heat storage capacity. The heat storage capacity is
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basically equal to the heat release capacity when the length is 90m. Regardless of day or night, the length of the north wall has little effect on greenhouse’s indoor air temperature in the cloudy day. Therefore, the wall length of solar greenhouse should not be too long. As a result, the suitable construction of the greenhouse wall length is about 90m, considering the storage heat and heat dissipation on the thermal environment of solar greenhouse in Lanzhou.
Fig. 5. Variations of indoor air temperature of solar greenhouse under different wall length of greenhouse for representative days
4. Conclusions This paper uses the Ecotect and Energyplus v8.6 to evaluate the effect of length of the greenhouse north wall on its indoor thermal environment. The total heat storage/release capacity and the reduction rate of the shadow were proposed. Based these evaluation parameters, the optimum length has obtained by numerically simulation, the main conclusions of this study are as follows: (i) With the increase of the length of the north wall of greenhouse from 30m to 150m, the shadow rates of east and west walls on the north wall are gradually reduced, when the length of the north wall of greenhouse reaches about 90 m, the effect of increasing its length on the shadow rate will be reduced. (ii) In the sunny day, with the length of the north wall of the greenhouse increases, the total heat storage capacity is also increasing, when the length is less than 90m, the total heat storage capacity is greater than that of the heat release capacity, while the wall length is more than 90m, the result is opposite. (iii) Considering the storage heat and heat dissipation on the thermal environment of solar greenhouse, the suitable construction of the greenhouse wall length is about 90m in Lanzhou. Acknowledgements This study is supported by the National Natural Science Foundation of China (No: 51468028), the Science and Technology Plan of Gansu Province (No. 1506RJZA065) and Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering (No: NR2015K05). References [1] B.T. Pan, X.H. Liu, D.S. Yin. Research status and development trend of energy-saving sunlight greenhouse in northern China, Greenh. Hortic. 2 (2005) 15-17. [2] T.L. Li. Current situation and prospect of solar greenhouse industry in China, J. Shenyang Agric. Univ. 36(2) (2005) 131-138. [3] Q.L. Gao, Y.J. Liang, A.W. Duan. Study on the characteristics of light intensity in solar greenhouse and its variation, Trans. Chinese Soc. Agric. Eng. 19(3) (2003) 200-204. [4] G.H. Tong, B.M. Li. Simulation of solar radiance in surface of solar greenhouse, J. China Agric. Univ. 11(1) (2006) 61-65.
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