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Model and Experiment Study on Solar Heating Biogas Production in Rural China
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Procedia Engineering 205 (2017) 3525–3530
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10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 19-22 October 10th and Air 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and Air Conditioning, Conditioning, ISHVAC2017, ISHVAC2017, 19-22 19-22 October October 2017, Jinan, China 2017, Jinan, China 2017, Jinan, China
Model and Experiment Study on Solar Heating Biogas Production in Rural Model and Experiment Study on Solar Heating Biogas Production in Rural China China a a a a
a,* a a b Xing Xing Su Sua,*,, Hang Hang Li Lia,, Xu Xu Zhang Zhanga and and Bo Bo Song Songb Xing Sua,*, Hang Lia, Xu Zhanga and Bo Songb
School Mechanical School of of Mechanical Engineering, Engineering, Tongji Tongji University, University, Shanghai Shanghai 201804, 201804, China China b Academy of Building Research, Beijing 100013, China bChina School of Mechanical Engineering, Tongji University, Shanghai 201804, China China Academy of Building Research, Beijing 100013, China b b China Academy of Building Research, Beijing 100013, China
Abstract Abstract Abstract Biogas Biogas digesters digesters have have recently recently been been used used as as an an alternative alternative cooking cooking fuel fuel for for some some rural rural families families in in China. China. Biogas Biogas production production by by digesters digesters is is Biogas digesters have by recently been used as an alternative cooking fuelused for to some rural families in production China. Biogas bycold digesters is considerably affected fermentation temperature. Soar can improve the rates in climate considerably affected by fermentation temperature. Soar energy energy can be be used to improve the biogas biogas production ratesproduction in winter winter in in cold climate considerably affected by fermentation temperature. Soar energy can be used to improve the biogas production rates in winter in cold climate zone of China. A thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for zone of China. A thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for zone of sunlight China. Agreenhouse thermal balance model is developed compute the fermentation ambient and was solarconducted radiation for simple assisted biogas project, of experiments on in Xuzhou, to simple sunlight greenhouse assisted biogas project, aatoseries series of field field experiments temperature on aa biogas biogasbyproject project in temperature Xuzhou, China China was conducted to simple sunlight greenhouse assistedofbiogas project, a series of field experiments onaverage a biogas project inrise Xuzhou, China was conducted to investigate the thermal performance biogas digesters and to validate the model, the temperature between the inside and ambient investigate the thermal performance of biogas digesters and to validate the model, the average temperature rise between the inside and ambient investigate the thermal performance of biogas digesters and to validate the model, the average temperature rise between the inside and ambient is over 10 ℃ in winter. is over 10 ℃ in winter. is over 10 ℃ in winter. © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 The Published by Ltd. Ltd. Peer-review under responsibility of the Elsevier scientific committee 2017 The Authors. Published by Peer-review Authors. under responsibility of Elsevier the scientific committee of of the the 10th 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and and Air Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating,onVentilation and Air Conditioning. Peer-review under responsibility of the scientific committee of the 10th International Symposium Heating, Ventilation and Conditioning. Conditioning. Air Conditioning. Keywords: Field experiment; Keywords: Field experiment; Biogas; Biogas; Solar Solar assisted; assisted; Fermentation Fermentation temperature temperature Keywords: Field experiment; Biogas; Solar assisted; Fermentation temperature
Nomenclature Nomenclature Nomenclature C Caa C C Clal C Frrl F F H Hrt(θ) Ht(θ) K K t(θ) Kl Kl Kld Q Q d Qhd Qh Qhl Ql Qlm Qm Qm Qrr Qr Q Qwi wi Q rrrr wi r T Traa T T Tdad Td Tjj Tjl Tl Tl
the the specific specific heat heat capacity capacity of of air, air, 1005 1005 J/(kg•K) J/(kg•K) the specific heat heat capacity capacity of air, 1005 J/(kg•K) the specific of water, the specific heat capacity of water, J/(kg•K) J/(kg•K) the specific heat capacity of water, J/(kg•K) light transmittance transmittance of of polyethylene polyethylene film film the light 2 the light transmittance of polyethylene film the the amount amount of of daily daily solar solar radiation radiation on on the the surface surface of of slope, slope, W/m W/m22 2 the amount of daily solar radiation on theW/m surface 2•K of slope, W/m the heat transfer coefficient of envelope, the heat transfer coefficient of envelope, W/m2•K 2 the heat transfer coefficient of envelope,of W/m •K the heat transfer coefficient of envelope envelope of fermentation fermentation tank, tank, W/m W/m2•K •K the heat transfer coefficient of envelope of fermentation tank, W/m2•K the amount of heat transfer between greenhouse and soil the amount of heat transfer between greenhouse and soil the amount of heat transfer between greenhouse and soil the the amount amount of of heat heat transfer transfer between between greenhouse greenhouse and and external external wall wall the amount of heat transfer between greenhouse and external wall the amount of heat transfer between greenhouse and fermentation fermentation tank tank the amount of heat transfer between greenhouse and daylighting fermentationfilm tank the amount of heat transfer between greenhouse and daylighting film the heat amount of heat transferonbetween greenhouse and daylighting film of solar radiation the surface of polyethylene film the heat of solar radiation on the surface of polyethylene film the heat of solar radiation onbetween the surface of polyethylene film the the amount amount of of heat heat transfer transfer between greenhouse greenhouse and and back back slope slope the proportion amount of heat transfer between greenhouse and back slope of solar solar radiation radiation absorbed absorbed by by air air in in greenhouse greenhouse the proportion of the proportion of solar radiation absorbed by air in greenhouse air temperature, ℃ the air temperature, ℃ 3 the air temperature,surface ℃ the the the temperature temperature of of surface of of ground, ground, ℃V ℃Vaa the volume volume of of greenhouse, greenhouse, m m3 the temperature of liquid, surface ℃ of ground, ℃Va the volume of greenhouse, m3 the temperature of liquid, ℃ the temperature of liquid, ℃ the the temperature of feedstock, feedstock, ℃ ℃ the temperature of feedstock, ℃
* * Corresponding Corresponding author. author. Tel.:86-13764600919 Tel.:86-13764600919 * Corresponding author. Tel.:86-13764600919 E-mail E-mail address:[email protected] address:[email protected] E-mail address:[email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 © The Authors. by Ltd. 1877-7058 © 2017 2017 Theresponsibility Authors. Published Published by Elsevier Elsevier Ltd.committee of the 10th International Symposium on Heating, Ventilation and Peer-review under of the scientific 1877-7058 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of Peer-review the scientific committee Peer-review under responsibility of the scientific committee of the the 10th 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and and Air Air Conditioning. Conditioning. Air Conditioning. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
10.1016/j.proeng.2017.09.921
Xing Su et al. / Procedia Engineering 205 (2017) 3525–3530
3526
Vl α ρa ρl τ
Xing Su, et al. / Procedia Engineering 00 (2017) 000–000
the volume of liquid in fermentation tank, m3 the temperature difference correction factor the air density, 1.205 kg/m3 the water density, 1000 kg/m3 represents the time, s
1. Introduction There are large biomass resources in China. In 2007 China produced over 900 million tons of crop straw, disposal of this vast amount of straw will be a serious problem, farmers burned straw without any environment protection devices in fields after harvesting lead to a serious air pollution problem like PM2.5 becoming seriously in recent years. Biogas digesters have recently been used as an alternative cooking fuel for some rural families in China. However, these benefits can only be realized with a small investment by farmers living in tropical climates, those who live in cold climates and higher altitudes cannot take advantage of the same type of digesters due to biogas production by digesters is considerably affected by fermentation temperature. In the cold regions of China, traditional biogas digesters are the most common model developed and used. However, their performances were severely restricted by the ambient temperatures for lack of insulation and warming measures in winter, the biogas production cannot satisfy cooking fuel demands of farmers. Thus, the design of enhancing the fermentation temperature should be adapted to the traditional biogas digesters. There have been a number of studies of using clean energy to improve the fermentation temperature. Shi et al. [1] used a ground source heat pump to heat the anaerobic fermentation tank in winter, the fermentation temperature was 32±2℃ during the experimental cycle. Li et al. [2] proposed to heat the anaerobic fermentation system by using solar energy combined with biomass boiler for ensuring biogas fermentation tank working normally in winter. When solar energy cannot satisfy the heat demand of fermentation tank, the biogas boiler will be switched on and complement the deficit of heat demand. When the demand for heating cannot be met through solar energy alone, the biogas boiler will be switched on and send heat to the fermentation tank. An air source heat pump was used to transfer the solar heat gains of a greenhouse to a 30 m3 digester in Canada was simulated by Curry and Pillay [3]. Feng et al.[4]conducted an experimental study on over-ground household solar water heating thermostatic biogas system. Experiments of two-stage solar heating technology were conducted in rural ecological campus biogas system by Qiu et al. [5]. The average fermentation temperature was 10 ± 0.5 °C, and 6 ± 1.0 °C higher than that without heating. However, these methods are complicate, the cost, operation and maintenance, corrosion problem and the heat transfer properties makes it uneconomical. Installation of PVC greenhouse type structure over a biogas plant allowed solar heating of the house. Due to most of the similar studies were conducted at laboratory scale and in theory, this paper aims to conduct a field experiment study on fermentation temperature and biogas production characteristic in the coldest time in cold rural regions in China. 2. Methods A middle scale biogas power project in Xuzhou is chosen as the case object in this study, the sizes of these digesters is 500 m3, which can supply the biogas for cooking demand of about 240 households. Fig. 1 shows the structure of the solar-assisted polyethylene digesters, which is composed of a polyethylene green house, a semi-underground steel digester, a storage tank of biogas, and other accessories. An extruded polystyrene board (XPS) with 6 cm depth was placed on the bottom side surface of the steel digester. The running strategy of biogas project in winter is as follows: in the daylight, the polyethylene film is bare to absorb the solar radiation as much as possible, in the light, the polyethylene film is covered by insulation layer for preventing the heat loss from greenhouse to surrounding environment.
Fig. 1. The test biogas system at Xuzhou
Xing Su et al. / Procedia Engineering 205 (2017) 3525–3530
Xing Su, et al. / Procedia Engineering 00 (2017) 000–000
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3. Thermal model For the greenhouse is not ventilated to enhance the temperature in winter, the model makes following assumptions in order to simplify the system’s physics: (1) (2) (3) (4) (5) (6) (7) (8)
The density, pressure, specific heat capacity and specific humidity are assumed to constant values. The ground, walls, and polyethylene film are assumed as isotropic gray body. Heat loss through the long wave radiation among the surface of polyethylene film, sky and surrounding objects is neglected in the daytime. Solar radiation reflected inside the system is neglected. The effect of infiltration wind is neglected. The heat storage of wall and soil are neglected for they have small storage capacity. It is assumed that the digester does not affect the soil temperature. The physical parameters of soil are assumed constant.
The thermal balance equation of air in the greenhouse can be described as below:
�� �� ��
��� ��
� �� �� + �� + �� + ∑���� ��� + �� + ��
(1)
where ρα is the air density, 1.205 kg/m3; Vα is the volume of greenhouse, m3; Cαis the specific heat capacity of air, 1005 J/(kg•K); Tα is the air temperature, ℃; τ represents the time, s; and rr is the proportion of solar radiation absorbed by air in greenhouse.Qris the heat of solar radiation on the surface of polyethylene film, which can be calculated by using Eq. (2) according to Klein’s research [6].
�� � ��(�) �� ��
(2)
where Ht(θ) is the amount of daily solar radiation on the surface of slope, W/m2; Fris the light transmittance of polyethylene film, in this research, the thickness of polyethylene film is 0.12mm, the average light transmittance is 0.7. While the geometric parameter of project and physical parameter of each materials are acquired, the air temperature in greenhouse and fermentation temperature can be calculated based on Eq. (8) and Eq. (9). Qd,Qh,Qwi,Qm,Ql represent the amount of heat transfer between greenhouse and soil, external wall, back slope, daylighting film, and fermentation tank, respectively. They can be calculated by using Eq. (3) - Eq. (7)
Q � � �� F� (T� − T� )
Q � � �� F� (T� − T�� )
Q �� � ���� F�� (T� − T�� )
Q � � �� F� (T� − T�� ) Q � � �� F� (T� − T� )
(3) (4) (5) (6) (7)
Where K is the heat transfer coefficient of envelope, W/m2•K;Td is the temperature of surface of ground, ℃; α is the temperature difference correction factor. Based on Eq. (1) - Eq. (7), the thermal balance equation of air in the greenhouse can be described by the Eq. (8): ��
�� �� �� � � �� �� ��� �� − �� �� (�� − �� ) − �� �� (�� − ��� ) − ∑ ��� ��� (�� − ��� ) − �� �� (�� − ��� ) − �� ��� � (�� − �� ) For the fermentation tank, the thermal balance equation can be described as Eq. (9)
Xing Su, et Engineering 00 (2017) 000–000 Xing Sual.et/ Procedia al. / Procedia Engineering 205 (2017) 3525–3530
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࣋ ࢂ
ࢊࢀ ࢊ࣎
ࡼ ࢂ ሺࢀ െ ࢀ ሻ ൌ ሺ െ ࢘࢘ ሻࡽ࢘ ࡽ
(9) where ρl is the water density, 1000 kg/m ; Vl is the volume of liquid in fermentation tank, m ; Cl is the specific heat capacity of water, J/(kg•K); Tl is the temperature of feedstock, ℃; Tj is the temperature of liquid, ℃; Kl is the heat transfer coefficient of envelope of fermentation tank, W/m2•K. While the geometric parameter of project and physical parameter of each materials are acquired, the air temperature in greenhouse and fermentation temperature can be calculated based on Eq. (8) and Eq. (9). 3
3
4. Experiments The field continuous experiment was conducted in December 25, 2015 to January 7, 2016, about 2 weeks. The goal of this field experiment was to capture both the local ambient climatological conditions and, simultaneously, a representative sample of temperatures within the digester to ascertain the thermal performance of the digester over time. The testing parameters include outdoor air temperature, solar radiation, air temperature in the greenhouse and fermentation temperature. During the testing period, the ambient climate include sunshine, cloudy and rainy days. 5. Results The testing results of outdoor air temperature and solar radiation during the experiment period are shown in Figure 2 and Figure 3, respectively. By applying the ambient climatological conditions and geometric parameters of projects to the Eq. (8) and Eq. (9). The
air temperature of greenhouse and fermentation temperature can be calculated, Figures 4 shows the difference of Fig. 2. Outdoor air temperature during experiment priod
fermentation temperature between calculation and experiment, the measured fermentation temperature fluctuated within a day, while the calculation value is relatively stable, this error is caused mainly by the simplification of calculation model, however, the error between testing and calculation value is less than 10% for all the results.
Xing Su et al. / Procedia Engineering 205 (2017) 3525–3530
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Actually, due to the difference of solar radiation in the sunny day and raining day, the errors between testing and calculation is different. The daily variation of air temperature of greenhouse and fermentation temperature in a sunny day and raining day are compared. Figure 5 shows the relationship between indoor air temperature and outdoor air temperature, the real testing daily average outdoor temperature was 2.7 ℃, the lowest temperature was -5.6 ℃, the highest temperature was 17.1 ℃, while the average air temperature in greenhouse was 15.9 ℃, the lowest temperature was 6.9 ℃, the highest temperature was 36.9 ℃. The average temperature of solar greenhouse increased by 12.56 ℃ in 24 hours. The trends of indoor air temperature is similar as outdoor air temperature, the lowest air temperature is in 7:00, after that insulation cover of polyethylene film was removed, the indoor air temperature increased rapidly when the outdoor air temperature and solar radiation rising. In rain day, the daily range of outdoor air temperature is lower than in sunny day. As shown in Figure 6,
Fig. 3. Solar radiation measured on the horizontal surface during experiment period
the lowest outdoor air temperature was 0.7 ℃ at about 7:00 am and the lowest temperature, while the indoor air temperature is 9.6 ℃. The average temperature of solar greenhouse increased by 5.6 ℃ in 24 hours.
Fig. 4. Comparisons of fermentation temperature between calculation and experiment
Fig. 5. Comparisons of air temperature between indoor and outdoor in sunny day
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5. Discussion The data calculated by heat transfer model of greenhouse and fermentation tank agreed with measure data, the errors of fermentation temperature between calculation and experiment is less than 10% during experiment period. The temperature increasing effect of greenhouse in sunny day is better than that in rain day (12℃ vs. 5.6℃). The fluctuation of indoor air temperature is higher than that of outdoor air temperature, while in the rain day is just the opposite. The main reason is the solar radiations has the more effect on the indoor air temperature than outdoor air temperature, the indoor temperature increased rapidly when the when solar radiation is strong, the heat loss of convection in the greenhouse is far lower than outdoor. For the daily range of outdoor air temperature and solar radiation in sunny are higher than in rain day, the fluctuation of indoor air temperature in rain day is lower than in sunny day. The temperature enhancing effect of polyethylene film is obvious, during the test period, the daily outdoor average temperature was 4.2℃, the total solar radiation was 10079MJ/m2, the daily average temperature of indoor air was 12.7 ℃, the daily average fermentation temperature was 16.2 ℃. Although the solar radiation is very weak in rain day, the fermentation temperature decreased slowly due to the great heat inertia of the fermentation tank system. 6. Conslusions
In this study, a thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for simple sunlight greenhouse assisted biogas project, and verified by field experiment, the results calculated by the thermal balance model is in good agreement with measurements. The polyethylene film has a very obvious effect on enhancing the fermentation temperature, it can increase the slurry temperature and decrease temperature fluctuations. A series of field experiments on a biogas project in Xuzhou, China was conducted to investigate the thermal performance of biogas digesters, the temperature rise between the fermentation temperature and ambient air temperature is about 12 ℃ in sunny day and 5.6 ℃ in rain day in winter. On this conditions, the biogas production reliability can be improved during the worst time of the year. Acknowledgment This research has been supported by the National Major Project of Scientific and Technical Supporting Programs of China during the 12th Five-year Plan Period (Grant No. 2015BAL02B03). References [1] H. Shi, T. Wang, H. Zhu. Heating system of biogas digester by ground source heat pump, Transactions of the CSAE. 26 (2010) 268-273. [2] B. Li, X. Z. Zhou, W. Yan. Experimental study on using solar to improve producing methane in northeast China, Advanced Materials Research. 953 (2014) 132-135. [3] N. Curry, P. Pillay. Integrating solar energy into an urban small-scale anaerobic digester for improved performance, Renewable Energy. 83 (2015) 280-293. [4] R. Feng, J. Li, T. Dong, X. Z. Li. Performance of a novel household solar heating thermostatic biogas system, Applied Thermal Engineering. 96 (2016) 519-526. [5] L. Qiu, Y. Liang, Y. Deng, T. Luo. Effects of solar two-stage heating on temperature rising for biogas fermentation, Transactions of the Chinese Society of Agricultural Engineering. 27 (2011) 166-171. [6] S.A. Klein. Calculation of monthly average insolation on tilted surfaces, Solar Energy. 19 (1977) 325-329.
ScienceDirect ScienceDirect ScienceDirect
Available online at www.sciencedirect.com Procedia Procedia Engineering Engineering 00 00 (2017) (2017) 000–000 000–000 Procedia Engineering 00 (2017) 000–000
ScienceDirect
Procedia Engineering 205 (2017) 3525–3530
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 19-22 October 10th and Air 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and Air Conditioning, Conditioning, ISHVAC2017, ISHVAC2017, 19-22 19-22 October October 2017, Jinan, China 2017, Jinan, China 2017, Jinan, China
Model and Experiment Study on Solar Heating Biogas Production in Rural Model and Experiment Study on Solar Heating Biogas Production in Rural China China a a a a
a,* a a b Xing Xing Su Sua,*,, Hang Hang Li Lia,, Xu Xu Zhang Zhanga and and Bo Bo Song Songb Xing Sua,*, Hang Lia, Xu Zhanga and Bo Songb
School Mechanical School of of Mechanical Engineering, Engineering, Tongji Tongji University, University, Shanghai Shanghai 201804, 201804, China China b Academy of Building Research, Beijing 100013, China bChina School of Mechanical Engineering, Tongji University, Shanghai 201804, China China Academy of Building Research, Beijing 100013, China b b China Academy of Building Research, Beijing 100013, China
Abstract Abstract Abstract Biogas Biogas digesters digesters have have recently recently been been used used as as an an alternative alternative cooking cooking fuel fuel for for some some rural rural families families in in China. China. Biogas Biogas production production by by digesters digesters is is Biogas digesters have by recently been used as an alternative cooking fuelused for to some rural families in production China. Biogas bycold digesters is considerably affected fermentation temperature. Soar can improve the rates in climate considerably affected by fermentation temperature. Soar energy energy can be be used to improve the biogas biogas production ratesproduction in winter winter in in cold climate considerably affected by fermentation temperature. Soar energy can be used to improve the biogas production rates in winter in cold climate zone of China. A thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for zone of China. A thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for zone of sunlight China. Agreenhouse thermal balance model is developed compute the fermentation ambient and was solarconducted radiation for simple assisted biogas project, of experiments on in Xuzhou, to simple sunlight greenhouse assisted biogas project, aatoseries series of field field experiments temperature on aa biogas biogasbyproject project in temperature Xuzhou, China China was conducted to simple sunlight greenhouse assistedofbiogas project, a series of field experiments onaverage a biogas project inrise Xuzhou, China was conducted to investigate the thermal performance biogas digesters and to validate the model, the temperature between the inside and ambient investigate the thermal performance of biogas digesters and to validate the model, the average temperature rise between the inside and ambient investigate the thermal performance of biogas digesters and to validate the model, the average temperature rise between the inside and ambient is over 10 ℃ in winter. is over 10 ℃ in winter. is over 10 ℃ in winter. © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 The Published by Ltd. Ltd. Peer-review under responsibility of the Elsevier scientific committee 2017 The Authors. Published by Peer-review Authors. under responsibility of Elsevier the scientific committee of of the the 10th 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and and Air Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating,onVentilation and Air Conditioning. Peer-review under responsibility of the scientific committee of the 10th International Symposium Heating, Ventilation and Conditioning. Conditioning. Air Conditioning. Keywords: Field experiment; Keywords: Field experiment; Biogas; Biogas; Solar Solar assisted; assisted; Fermentation Fermentation temperature temperature Keywords: Field experiment; Biogas; Solar assisted; Fermentation temperature
Nomenclature Nomenclature Nomenclature C Caa C C Clal C Frrl F F H Hrt(θ) Ht(θ) K K t(θ) Kl Kl Kld Q Q d Qhd Qh Qhl Ql Qlm Qm Qm Qrr Qr Q Qwi wi Q rrrr wi r T Traa T T Tdad Td Tjj Tjl Tl Tl
the the specific specific heat heat capacity capacity of of air, air, 1005 1005 J/(kg•K) J/(kg•K) the specific heat heat capacity capacity of air, 1005 J/(kg•K) the specific of water, the specific heat capacity of water, J/(kg•K) J/(kg•K) the specific heat capacity of water, J/(kg•K) light transmittance transmittance of of polyethylene polyethylene film film the light 2 the light transmittance of polyethylene film the the amount amount of of daily daily solar solar radiation radiation on on the the surface surface of of slope, slope, W/m W/m22 2 the amount of daily solar radiation on theW/m surface 2•K of slope, W/m the heat transfer coefficient of envelope, the heat transfer coefficient of envelope, W/m2•K 2 the heat transfer coefficient of envelope,of W/m •K the heat transfer coefficient of envelope envelope of fermentation fermentation tank, tank, W/m W/m2•K •K the heat transfer coefficient of envelope of fermentation tank, W/m2•K the amount of heat transfer between greenhouse and soil the amount of heat transfer between greenhouse and soil the amount of heat transfer between greenhouse and soil the the amount amount of of heat heat transfer transfer between between greenhouse greenhouse and and external external wall wall the amount of heat transfer between greenhouse and external wall the amount of heat transfer between greenhouse and fermentation fermentation tank tank the amount of heat transfer between greenhouse and daylighting fermentationfilm tank the amount of heat transfer between greenhouse and daylighting film the heat amount of heat transferonbetween greenhouse and daylighting film of solar radiation the surface of polyethylene film the heat of solar radiation on the surface of polyethylene film the heat of solar radiation onbetween the surface of polyethylene film the the amount amount of of heat heat transfer transfer between greenhouse greenhouse and and back back slope slope the proportion amount of heat transfer between greenhouse and back slope of solar solar radiation radiation absorbed absorbed by by air air in in greenhouse greenhouse the proportion of the proportion of solar radiation absorbed by air in greenhouse air temperature, ℃ the air temperature, ℃ 3 the air temperature,surface ℃ the the the temperature temperature of of surface of of ground, ground, ℃V ℃Vaa the volume volume of of greenhouse, greenhouse, m m3 the temperature of liquid, surface ℃ of ground, ℃Va the volume of greenhouse, m3 the temperature of liquid, ℃ the temperature of liquid, ℃ the the temperature of feedstock, feedstock, ℃ ℃ the temperature of feedstock, ℃
* * Corresponding Corresponding author. author. Tel.:86-13764600919 Tel.:86-13764600919 * Corresponding author. Tel.:86-13764600919 E-mail E-mail address:[email protected] address:[email protected] E-mail address:[email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 © The Authors. by Ltd. 1877-7058 © 2017 2017 Theresponsibility Authors. Published Published by Elsevier Elsevier Ltd.committee of the 10th International Symposium on Heating, Ventilation and Peer-review under of the scientific 1877-7058 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of Peer-review the scientific committee Peer-review under responsibility of the scientific committee of the the 10th 10th International International Symposium Symposium on on Heating, Heating, Ventilation Ventilation and and Air Air Conditioning. Conditioning. Air Conditioning. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
10.1016/j.proeng.2017.09.921
Xing Su et al. / Procedia Engineering 205 (2017) 3525–3530
3526
Vl α ρa ρl τ
Xing Su, et al. / Procedia Engineering 00 (2017) 000–000
the volume of liquid in fermentation tank, m3 the temperature difference correction factor the air density, 1.205 kg/m3 the water density, 1000 kg/m3 represents the time, s
1. Introduction There are large biomass resources in China. In 2007 China produced over 900 million tons of crop straw, disposal of this vast amount of straw will be a serious problem, farmers burned straw without any environment protection devices in fields after harvesting lead to a serious air pollution problem like PM2.5 becoming seriously in recent years. Biogas digesters have recently been used as an alternative cooking fuel for some rural families in China. However, these benefits can only be realized with a small investment by farmers living in tropical climates, those who live in cold climates and higher altitudes cannot take advantage of the same type of digesters due to biogas production by digesters is considerably affected by fermentation temperature. In the cold regions of China, traditional biogas digesters are the most common model developed and used. However, their performances were severely restricted by the ambient temperatures for lack of insulation and warming measures in winter, the biogas production cannot satisfy cooking fuel demands of farmers. Thus, the design of enhancing the fermentation temperature should be adapted to the traditional biogas digesters. There have been a number of studies of using clean energy to improve the fermentation temperature. Shi et al. [1] used a ground source heat pump to heat the anaerobic fermentation tank in winter, the fermentation temperature was 32±2℃ during the experimental cycle. Li et al. [2] proposed to heat the anaerobic fermentation system by using solar energy combined with biomass boiler for ensuring biogas fermentation tank working normally in winter. When solar energy cannot satisfy the heat demand of fermentation tank, the biogas boiler will be switched on and complement the deficit of heat demand. When the demand for heating cannot be met through solar energy alone, the biogas boiler will be switched on and send heat to the fermentation tank. An air source heat pump was used to transfer the solar heat gains of a greenhouse to a 30 m3 digester in Canada was simulated by Curry and Pillay [3]. Feng et al.[4]conducted an experimental study on over-ground household solar water heating thermostatic biogas system. Experiments of two-stage solar heating technology were conducted in rural ecological campus biogas system by Qiu et al. [5]. The average fermentation temperature was 10 ± 0.5 °C, and 6 ± 1.0 °C higher than that without heating. However, these methods are complicate, the cost, operation and maintenance, corrosion problem and the heat transfer properties makes it uneconomical. Installation of PVC greenhouse type structure over a biogas plant allowed solar heating of the house. Due to most of the similar studies were conducted at laboratory scale and in theory, this paper aims to conduct a field experiment study on fermentation temperature and biogas production characteristic in the coldest time in cold rural regions in China. 2. Methods A middle scale biogas power project in Xuzhou is chosen as the case object in this study, the sizes of these digesters is 500 m3, which can supply the biogas for cooking demand of about 240 households. Fig. 1 shows the structure of the solar-assisted polyethylene digesters, which is composed of a polyethylene green house, a semi-underground steel digester, a storage tank of biogas, and other accessories. An extruded polystyrene board (XPS) with 6 cm depth was placed on the bottom side surface of the steel digester. The running strategy of biogas project in winter is as follows: in the daylight, the polyethylene film is bare to absorb the solar radiation as much as possible, in the light, the polyethylene film is covered by insulation layer for preventing the heat loss from greenhouse to surrounding environment.
Fig. 1. The test biogas system at Xuzhou
Xing Su et al. / Procedia Engineering 205 (2017) 3525–3530
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3. Thermal model For the greenhouse is not ventilated to enhance the temperature in winter, the model makes following assumptions in order to simplify the system’s physics: (1) (2) (3) (4) (5) (6) (7) (8)
The density, pressure, specific heat capacity and specific humidity are assumed to constant values. The ground, walls, and polyethylene film are assumed as isotropic gray body. Heat loss through the long wave radiation among the surface of polyethylene film, sky and surrounding objects is neglected in the daytime. Solar radiation reflected inside the system is neglected. The effect of infiltration wind is neglected. The heat storage of wall and soil are neglected for they have small storage capacity. It is assumed that the digester does not affect the soil temperature. The physical parameters of soil are assumed constant.
The thermal balance equation of air in the greenhouse can be described as below:
�� �� ��
��� ��
� �� �� + �� + �� + ∑���� ��� + �� + ��
(1)
where ρα is the air density, 1.205 kg/m3; Vα is the volume of greenhouse, m3; Cαis the specific heat capacity of air, 1005 J/(kg•K); Tα is the air temperature, ℃; τ represents the time, s; and rr is the proportion of solar radiation absorbed by air in greenhouse.Qris the heat of solar radiation on the surface of polyethylene film, which can be calculated by using Eq. (2) according to Klein’s research [6].
�� � ��(�) �� ��
(2)
where Ht(θ) is the amount of daily solar radiation on the surface of slope, W/m2; Fris the light transmittance of polyethylene film, in this research, the thickness of polyethylene film is 0.12mm, the average light transmittance is 0.7. While the geometric parameter of project and physical parameter of each materials are acquired, the air temperature in greenhouse and fermentation temperature can be calculated based on Eq. (8) and Eq. (9). Qd,Qh,Qwi,Qm,Ql represent the amount of heat transfer between greenhouse and soil, external wall, back slope, daylighting film, and fermentation tank, respectively. They can be calculated by using Eq. (3) - Eq. (7)
Q � � �� F� (T� − T� )
Q � � �� F� (T� − T�� )
Q �� � ���� F�� (T� − T�� )
Q � � �� F� (T� − T�� ) Q � � �� F� (T� − T� )
(3) (4) (5) (6) (7)
Where K is the heat transfer coefficient of envelope, W/m2•K;Td is the temperature of surface of ground, ℃; α is the temperature difference correction factor. Based on Eq. (1) - Eq. (7), the thermal balance equation of air in the greenhouse can be described by the Eq. (8): ��
�� �� �� � � �� �� ��� �� − �� �� (�� − �� ) − �� �� (�� − ��� ) − ∑ ��� ��� (�� − ��� ) − �� �� (�� − ��� ) − �� ��� � (�� − �� ) For the fermentation tank, the thermal balance equation can be described as Eq. (9)
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࣋ ࢂ
ࢊࢀ ࢊ࣎
ࡼ ࢂ ሺࢀ െ ࢀ ሻ ൌ ሺ െ ࢘࢘ ሻࡽ࢘ ࡽ
(9) where ρl is the water density, 1000 kg/m ; Vl is the volume of liquid in fermentation tank, m ; Cl is the specific heat capacity of water, J/(kg•K); Tl is the temperature of feedstock, ℃; Tj is the temperature of liquid, ℃; Kl is the heat transfer coefficient of envelope of fermentation tank, W/m2•K. While the geometric parameter of project and physical parameter of each materials are acquired, the air temperature in greenhouse and fermentation temperature can be calculated based on Eq. (8) and Eq. (9). 3
3
4. Experiments The field continuous experiment was conducted in December 25, 2015 to January 7, 2016, about 2 weeks. The goal of this field experiment was to capture both the local ambient climatological conditions and, simultaneously, a representative sample of temperatures within the digester to ascertain the thermal performance of the digester over time. The testing parameters include outdoor air temperature, solar radiation, air temperature in the greenhouse and fermentation temperature. During the testing period, the ambient climate include sunshine, cloudy and rainy days. 5. Results The testing results of outdoor air temperature and solar radiation during the experiment period are shown in Figure 2 and Figure 3, respectively. By applying the ambient climatological conditions and geometric parameters of projects to the Eq. (8) and Eq. (9). The
air temperature of greenhouse and fermentation temperature can be calculated, Figures 4 shows the difference of Fig. 2. Outdoor air temperature during experiment priod
fermentation temperature between calculation and experiment, the measured fermentation temperature fluctuated within a day, while the calculation value is relatively stable, this error is caused mainly by the simplification of calculation model, however, the error between testing and calculation value is less than 10% for all the results.
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Actually, due to the difference of solar radiation in the sunny day and raining day, the errors between testing and calculation is different. The daily variation of air temperature of greenhouse and fermentation temperature in a sunny day and raining day are compared. Figure 5 shows the relationship between indoor air temperature and outdoor air temperature, the real testing daily average outdoor temperature was 2.7 ℃, the lowest temperature was -5.6 ℃, the highest temperature was 17.1 ℃, while the average air temperature in greenhouse was 15.9 ℃, the lowest temperature was 6.9 ℃, the highest temperature was 36.9 ℃. The average temperature of solar greenhouse increased by 12.56 ℃ in 24 hours. The trends of indoor air temperature is similar as outdoor air temperature, the lowest air temperature is in 7:00, after that insulation cover of polyethylene film was removed, the indoor air temperature increased rapidly when the outdoor air temperature and solar radiation rising. In rain day, the daily range of outdoor air temperature is lower than in sunny day. As shown in Figure 6,
Fig. 3. Solar radiation measured on the horizontal surface during experiment period
the lowest outdoor air temperature was 0.7 ℃ at about 7:00 am and the lowest temperature, while the indoor air temperature is 9.6 ℃. The average temperature of solar greenhouse increased by 5.6 ℃ in 24 hours.
Fig. 4. Comparisons of fermentation temperature between calculation and experiment
Fig. 5. Comparisons of air temperature between indoor and outdoor in sunny day
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5. Discussion The data calculated by heat transfer model of greenhouse and fermentation tank agreed with measure data, the errors of fermentation temperature between calculation and experiment is less than 10% during experiment period. The temperature increasing effect of greenhouse in sunny day is better than that in rain day (12℃ vs. 5.6℃). The fluctuation of indoor air temperature is higher than that of outdoor air temperature, while in the rain day is just the opposite. The main reason is the solar radiations has the more effect on the indoor air temperature than outdoor air temperature, the indoor temperature increased rapidly when the when solar radiation is strong, the heat loss of convection in the greenhouse is far lower than outdoor. For the daily range of outdoor air temperature and solar radiation in sunny are higher than in rain day, the fluctuation of indoor air temperature in rain day is lower than in sunny day. The temperature enhancing effect of polyethylene film is obvious, during the test period, the daily outdoor average temperature was 4.2℃, the total solar radiation was 10079MJ/m2, the daily average temperature of indoor air was 12.7 ℃, the daily average fermentation temperature was 16.2 ℃. Although the solar radiation is very weak in rain day, the fermentation temperature decreased slowly due to the great heat inertia of the fermentation tank system. 6. Conslusions
In this study, a thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for simple sunlight greenhouse assisted biogas project, and verified by field experiment, the results calculated by the thermal balance model is in good agreement with measurements. The polyethylene film has a very obvious effect on enhancing the fermentation temperature, it can increase the slurry temperature and decrease temperature fluctuations. A series of field experiments on a biogas project in Xuzhou, China was conducted to investigate the thermal performance of biogas digesters, the temperature rise between the fermentation temperature and ambient air temperature is about 12 ℃ in sunny day and 5.6 ℃ in rain day in winter. On this conditions, the biogas production reliability can be improved during the worst time of the year. Acknowledgment This research has been supported by the National Major Project of Scientific and Technical Supporting Programs of China during the 12th Five-year Plan Period (Grant No. 2015BAL02B03). References [1] H. Shi, T. Wang, H. Zhu. Heating system of biogas digester by ground source heat pump, Transactions of the CSAE. 26 (2010) 268-273. [2] B. Li, X. Z. Zhou, W. Yan. Experimental study on using solar to improve producing methane in northeast China, Advanced Materials Research. 953 (2014) 132-135. [3] N. Curry, P. Pillay. Integrating solar energy into an urban small-scale anaerobic digester for improved performance, Renewable Energy. 83 (2015) 280-293. [4] R. Feng, J. Li, T. Dong, X. Z. Li. Performance of a novel household solar heating thermostatic biogas system, Applied Thermal Engineering. 96 (2016) 519-526. [5] L. Qiu, Y. Liang, Y. Deng, T. Luo. Effects of solar two-stage heating on temperature rising for biogas fermentation, Transactions of the Chinese Society of Agricultural Engineering. 27 (2011) 166-171. [6] S.A. Klein. Calculation of monthly average insolation on tilted surfaces, Solar Energy. 19 (1977) 325-329.