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Study of solar heated biogas fermentation system with a phase change thermal storage device
Accepted Manuscript Study of Solar Heated Biogas Fermentation System with a Phase Change Thermal Storage Device Ye Tian, Yong Lu, Haowei Lu, Lei Wu, Xianlin Li PII:
S1359-4311(14)01194-6
DOI:
10.1016/j.applthermaleng.2014.12.065
Reference:
ATE 6259
To appear in:
Applied Thermal Engineering
Received Date: 11 June 2014 Revised Date:
3 December 2014
Accepted Date: 26 December 2014
Please cite this article as: Y. Tian, Y. Lu, H. Lu, L. Wu, X. Li, Study of Solar Heated Biogas Fermentation System with a Phase Change Thermal Storage Device, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2014.12.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Study of Solar Heated Biogas Fermentation System with a Phase Change Thermal Storage Device
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Ye Tian, Yong Lu*, Haowei Lu , Lei Wu and Xianlin Li School of Energy and Environment, Southeast University, Nanjing, 210096, China Email:[email protected]
Abstract
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A new technique of solar heating biogas fermentation system integrated with a phase change thermal storage device is introduced for improving the low efficiency of the traditional biogas fermentation devices during winter. For a designed solar heated biogas system, its performance of anaerobic fermentation process under the different natural cooling conditions has been assessed by varying the thicknesses of insulation materials. And the optimum relative proportion between the heat supplied by solar and that stored in phase change heat storage device has been also investigated. The preliminary results based on the numerical simulations performed with meteorological data from Anhui Province, China show that the proposed PCM device with 3m³paraffin wax as phase change material (PCM) and 20m2 solar collector areas can satisfy the heat required by producing 5m³ biogas per day corresponding to a solar fraction (f) of 0.8 in winter. It indicates that the proposed solar heating system could be a promising approach for promoting biogas technology applications in cold rural regions of China.
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Introduction
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Keywords: anaerobic fermentation; collector area; PCM volume; solar fraction
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Biomass like straw is rich and burnt without any environment protection devices each year in China, which results to air pollution problem like PM2.5 becoming seriously in recent years. In view of energy shortages and the abundant resources of straw in China, the biogas industry can be an effective way to meet the demand for clean energy in rural areas [1]. Biogas is a combustible gas which is produced by the anaerobic (i.e. oxygen free) microbial fermentation of organic substances at a certain temperature, humidity and pH value. There are many factors influencing the fermentation process, among which the fermentation temperature is particularly important. Sufficient warmth is the key to the fermentation process, so researchers need to find ways to meet the energy requirements at minimum cost. Another problem encountered in such a bioreactor is how to minimize temperature fluctuations, because they adversely affect the bacterial fermentation process[2]. The temperature fluctuations should not exceed 2~3 per hour. If the fermentation temperature fluctuations exceed 5 in a short period of time, biogas yield will decrease significantly, so a constant fermentation temperature is required [3~5]. Different kinds of fermentation technologies achieve anaerobic digestion at different temperatures: room temperature (15 ), medium temperature (30~38 ) and high temperature (50~55 ). In rural areas, the medium temperature fermentation process has been shown to be more economical with an optimum temperature of 35 [6]. Therefore, it is important, in order to grow and sustain the mesophilic bacteria and obtain optimum biogas production, to heat the bioreactor to a temperature of about 35 . The focus of current research in biogas technology is to use clean energy to improve the fermentation temperature and maintain an efficient biogas production rate in winter. This can reduce 1
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System description
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conflicts between energy combustion and environmental conservation. And in some regions, where there is plenty of sunshine throughout the year, solar energy is a relatively economical source for heating purposes. In large and medium-sized methane gas projects, the ground source heat pump, boiler and solar greenhouse etc. are used to improve the fermentation temperature.Liu Jianyu etal.(2013) used a groundwater source heat pump to heat the anaerobic fermentation tank in winter, and the total savings in standard coal consumption was only 44% of the demand of standard coal using coal-boilers directly during the experimental cycle.[7].Li Jicheng etal.(2014) proposed a idea that to heat the anaerobic fermentation system by uniting the solar energy and biomass boiler, which is utilized to ensure the gas production rate of large-scale biogas fermentation tank under normal conditions in winter. 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[8].Ling Qiu etal.(2008) designed a double-effect solar heated biogas fermentation system consisting of a solar greenhouse and a hot chamber. The solar greenhouse is used to retain heat from the sun and improve the ambient air temperature around the bioreactor, thus reducing heat loss from the fermentation tank and improving the heat utilization efficiency respectively [9]. However, the ground source heat pump, boiler and solar greenhouse etc are not appropriate in small-sized and household methane gas projects. P.Axaopoulos etal. (2001) designed a solar powered household biogas fermentation system, using flat-plate collectors as an integral part of the roof structure of a swine manure digester [10].In order to solve the problems of fermentation process in winter in the northeast China, Li B(2014)studied the gas production rate of fermentation tank with solar heating to improve the temperature, and the experimental investigations were mainly based on the climatic conditions in northeast China[11]. This paper presents a solar heated small-sized biogas fermentation system with a phase change thermal storage device in Anhui province, located in central China. Different insulation materials were studied under natural coolingto ascertain the optimum, most cost effective material and material thickness, as well as the best relative sizes for the three components of the system, namely: the fermentation tank, the solar heat collectors and the phase change heat storage system.In this study, the appropriate insulation material, the optimal insulation layer thickness and the most suitable solar fraction value are selected. The solar heated phase change thermal storage device can ensure a continuous and efficient fermentation process, which can effectively cope with changes in solar energy resources resulting from diurnal changes, cloud cover, days of inconstant sunshine and seasonal fluctuations.
Fig.1.Schematic of Solar Heated Biogas System with PCM storage tank
The system mainly consists of three subsystems—heating system, heat storage system and fermentation system, as shown in fig.1. The parabolic trough condenser is the main component of 2
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heating system and it translates the solar energy into heat. The heat storage device accumulates the surplus heat and supplies heat when there is not sufficient heat-collecting capacity. Furthermore, the storage device reduces the influence of the relatively unstable solar energy heat source and increases the stability of the system. The collected heat is transferred to the heat storage device when the fermentation temperature is higher than 35 . When the fermentation temperature is lower than 35 , it is used for heating the straw fermentation tank. In summer, the fermentation temperature can reach temperatures above 35 , so the biogas production rate is guaranteed and no additional heating is required. The oversupply of solar heat is then transferred to the phase change heat storage device.This process is the heat storage process, as shown in loop. On the one hand, the heat stored on sunny, warm days, will be taken out to replenish the heat loss of the fermentation tank when the loop. On the other hand, heat-collecting capacity cannot meet the heat load in winter, as shown in when the heat-collecting capacity can meet the heat load, the heat-collecting capacity is supplied directly to the fermentation tank, as shown in loop. Thermal Analysis
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Meteorological data and basic value of the system parameters
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In this study, the system consisted of three main parts shown in Fig.1. And a fermentation tank was designed to produce 5m3 biogas a day. It was a metallic cylindrical tank with 1.1m inner diameter and 1.98m height. In order to keep the efficient fermentation temperature in winter, the fermentation tank needs to be not only insulted by using the suitable thickness insulation material but also heated by the thermal storage device. This device is designed to be filled paraffin wax which is used to keep the thermal energy collected by solar collectors in summer and autumn, since the inside temperature of fermentation tank can work normally due to the high ambient air temperature. When the ambient air temperature drops below the normal fermentation temperature in winter and the heat collected by solar collectors temporally is not enough, some additional heat from the paraffin wax thermal storage device will be provide to satisfy that required by the fermentation processing in the tank. Table 1 The meteorological data in Anhui Province, China 1 3.0
2 5.4
3 9.5
4 16.3
5 22.1
6 25.4
7 28.0
8 27.3
9 23.9
10 17.8
11 10.8
12 4.5
198.3
222.7
295.8
401.4
501.8
427.4
460.7
499.3
368.5
328.6
246.7
206.1
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Month Average temperature( ) Total solar radiation(MJ/m2)
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The system thermal analysis was generally conducted under the local meteorological conditions in Anhui Province, China, where the demonstration system will be located. The typical data of solar radiation as well as ambient air temperature are listed in Table 1. And the average temperature and the total solar radiation in the local winter are around 5 °C and 627 MJ/m2 respectively. Heat demand and dissipation of the ferementation tank in winter
The heat requirement of the fermentation system is shown in Fig. 2. Here we do not consider that the heat loss due to biogas removal from the bioreactor and the heat generated by the fermentation process. They are negligible and have little influence on the fermentation process～[13].Therefore, the system heat load includes the heat loss (ܳଵ) dissipating from the insulation enclosure and the heat load (ܳଶ )～of biomass digestive fluid which need to be heated to reach the required operating temperature 35 from the ambient air temperature before entering the bioreactor fermentation tank. Then the heat loss (ܳଵ) and the system total heat demand (ܳଶ ) can be described as:
Q1 = Q1a + Q1b 3
(1)
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(2)
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Where, ܳଵ is the heat loss from the fermentation tank,～ܳଵ is the heat loss in the pipeline loop and ܳଶ is the heat required by heating biomass digestive fluid.
Fig.2.The analysis of system heat load
3.2.1 Heat loss and the fermentation tank insulation design
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The bottom and side areas of the designed fermentation tank are 5.98m2 and 5.72m2respective, since it is a cylindrical tank with 1.1m inner diameter and 1.32m height. The initial temperature inside the fermentation tank and the ambient air temperature in winter are supposed as 35 and 5 respectively. The standard heat resistant equations for multi-layer flat wall and multi-layer cylindrical wall are written as [12]:
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K Flat =
δ δ 1 ( + 1+ 2+ ) α1 λ1 λ2 α 2
K Cylinder =
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1 d1 d 2 d 1 d1 d1 1 + ln + ln 2 + α1 d 0 2λ1 d 0 2λ2 d1 α 2
(3)
(4)
where, ܭி௧ represents the combined heat transfer coefficient of a multi-layer flat wall; K ௬ௗ represents the combined heat transfer coefficient of a multi-layer cylindrical wall;ߙଵ and ߙଶ are the heat transfer coefficients of the tank internal surface and the outside convection heat transfer coefficient, W·m-2·K-1; ߜଵ and ߜଶ are the thicknesses of the tank and the external thermal insulation layer, units of meters (m); ݀ , ݀ଵ and ݀ଶ are the diameters of each layers, m; λଵ and λଶ are the heat conductivity coefficients of the fermentation tank and external thermal insulation layer, W·m-1·K-1. Among traditional industrial insulation materials like such as polyurethane foam, polystyrene foam, polyethylene foam and rock wool, polyurethane foam is usual the best choice for thermal insulation due to its smallest thermal conductivity coefficient (0.0035 w·m-1·K-1). Here,～ rigid polyurethane foam was used to insulate the fermentation tank. In the case of its thickness ranging from 20 to 80mm, the thermal insulation performance was investigated through analysing the natural temperature dropping processing of the fermentation tank as shown in Fig. 3. The longer the natural cooling process lasts, the better the thermal insulation performance is. It is obviously that the thermal 4
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insulation performance is improved with the augment the thickness of～polyurethane foam. But their augment relationship is not linear proportional, since the thermal insulation performance of 100mm polyurethane foam is similar to that of 80mm polyurethane foam with 20mm polystyrene. For the unit mass price of polyurethane foam is higher than that of polystyrene, we chose 80mm polyurethane foam as the insulation material for the fermentation tank for balance between efficiency and economy.
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Fig.3.Temperature dropping process of fermentation tank under the different insulation thicknesses 3.2.2 Heat demand of heating biomass digestive slurry
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Under the condition of 80mm thickness polyurethane foam, the heat transfer coefficients are 0.49 and 0.47 w·m-2·K-1 respectively according to Eq. (1) and Eq. (2). The calculated heat loss from the fermentation tank (ܳଵ ) would be 145.47 W by using Eq. (5). Furthermore, there is a circulation loop of biomass digestive liquid, its flow volume is designed as 5 m3·d-1. The flow velocity would be 8 m·s-1 in a pipe system with 20mm diameter. The heat loss in the pipeline loop (Qଵ ) is figured out as 175 W and the total heat loss (Qଵ ) is 320 W. Q1a = K1A1 t gi -t f +K 2 A 2 t gi -t f
(5)
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where, ܭଵ and ܭଶ are heat transfer coefficients at the bottom and side of fermentation tank, W·m-2·K-1; ܣଵ and ܣଶ are the bottom and side areas of fermentation tank, m2; ݐ is the fermentation tank temperature, K; ݐ is the ambient air temperature, K. Since biomass digestive slurry～need to be heated up to 35 before entering into the fermentation tank, its heat requirement is given by Eq. (6). Q2 = ρ s qs Cs (t gi - ts ) (6) where,～ߩ௦ is the density of biomass digestive slurry in the reactor, kg·m-3,ݍ௦ is the flow rate of the biomass digestive liquid, m3·s-1; ܥ௦ is the specific heat capacity of the biomass digestive liquid, J·kg·K-1; ݐ is thefermentation tank temperature, K; and ݐ௦ is the initial temperature of biomass digestive liquid, K. The basic values of parameters used in the calculation are listed in Table 2. Table 2 Basic values of parameters used in the heating load calculations Parameter
Unit
Biogas production Biogas production rate Biomass fermentation concentration The temperature of biomass digestive liquid Specific heat capacity of biomass digestive liquid
5
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m3·d Nm3-biogas/kg-straw TS °C J· kg· K-1
Value 5 0.2 15% 35 4200
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In the case of 35°C fermentation temperature, 5 m3 biogas production per day need the minimum consumption of 25 kg straw under 0.2 straw-biogas production rate. The total 167 kg biomass digestive slurry～with 15% biomass concentration need to be replenished into the fermentation tank. Based on Eq. (5) and Eq. (6), the heat load for heating the biomass digestive slurry～(ܳଶ ) and the total heating load (ܳௗ ) are figured out as 122 W and 442 W separately.
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The system optimal solar fraction
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Solar fraction is defined as the ratio of thermal energy supplied by solar collectors temporally. Since the paraffin wax thermal storage device and solar collectors are integrated to provide the heat demand by the fermentation system. The system thermal balance is illustrated in Figure 4. As for the thermal storage device, it is designed to work in two operation modes. When solar radiation is strong and solar collectors can transfer enough thermal to satisfy the system thermal demand of producing biogas in summer and autumn, the thermal storage device works in the storage mode in which it stores the additional thermal energy collected by solar collectors. Correspondingly, when solar radiation is weak and temporally variable in winter, only solar collectors cannot keep the fermentation processing steady. The thermal storage device must work in the thermal supply mode and complement the system thermal demand beyond the thermal supply from solar collectors. Thus, it is necessary to study the optimal solar fraction to rate the proportion of thermal energy supplied by thermal storage device and solar collectors.
Fig.4.Energy balance of the designed fermentation system
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3.3.1 Solar collector areas
Since the system thermal demand is determined, the minimum solar collector area can be figured out under a specific value of solar fraction. The estimated formulae for the required collector area of a direct system and an indirect system can be described as Eq. (7) and Eq. (8) [14].
Ac =
86400QDemand ⋅ f JTηcd (1-η L )
(7)
FRU L Ac ) U hX AhX
(8)
A IN =Ac (1+
where, ܣ and ܣூே represent the areas of the direct system and indirect system respectively, m2; f is the solar fraction, as shown in Table 3; ்ܬis the average daily amount of solar radiation, J·m-2; ߟௗ is the average collection efficiency of solar collector, which ranges from 0.25 to 0.50 according to the standard of flat plate solar collectors (GB/T 6424-2007) and here it is chosen as 0.4; ߟ is the 6
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average heat loss rate of solar collector, 0.20~0.30; ܨோ ܷ is the total heat loss coefficient of collectors, 2~6; ܷ௫ is the heat transfer coefficient of heat exchanger, W·m-2·K-1; and ܣ௫ is the area of the heat exchanger, m2. Table 3 There commended values of solar fraction in different regions Rank
Solar energy resources～affluent areas Solar energy resources～rich areas Solar energy resources～general areas Solar energy resources～poor areas
Solar fraction Short term heat storage system ≥50% 30% 50% 10% 30% 5% 10%
I II III IV
Seasonal heat storage system ≥60% 40% 60% 20% 40% 10% 20%
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3.3.2 The volume of thermal storage device
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The effective heat storage capacity is calculated by the Eq. (9). The effective heat storage capacity refers to the heat that can be utilized to cover the shortage of the heat-collecting capacity in winter, namely, the difference value between the heat load and the heat-collecting capacity. In Table 4, the design parameters of the PCM storage device are given [15].
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Table 4Thedesign parameters of PCM storage device Parameter The～heating cycle Heat storage temperature The average inlet/outlet temperature of PCM storage device The average inlet/outlet temperature of fermentation tank
Unit d °C K K
(9)
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QStorage = QDemand × (1 − f )
Value 60 55 303.15/323.15 323.15/303.15
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where, Qௌ௧ represents the effective storage heat capacity, W; Qௗ is the total heat load, W; f is the solar fraction. As a latent heat storage material, paraffin wax has many advantages, such as: high latent heat, good thermal stability, no supercooling phenomenon, low expense and small volume change in the solid-slurry phase change process[16].However, it also has some drawbacks: low thermal conductivity, low density and poor mobility have a certain impact on the heat extraction time [17, 18]. Reference [19]～demonstrates thatmixing the appropriate amount of graphite powder in paraffin can enhance heat transfer.By means of comparative analyses, we have selected paraffin wax RT54. Its average relative molecular mass is 377, melting point is 55 , the latent heat is 179 kJ·kg-1, the specific heat of solid and slurry are 1.8 kJ·kg-1·K-1 and 2.4 kJ·kg-1·K-1, the density of solid and slurry are 900 and 760 kg·m-3 and the thermal conductivity coefficient is 0.2 W·m-1·K-1. As the heat loss rate of the heat storage device is 30%, the effective heat storage capacity needs to be 644297 kJ [20].The volume of paraffin wax can be calculated by Eq. (10) and Eq. (11) below.
Q = C 1 M ∆ t1 + C 2 M ∆ t 2 + M r ～～～～
V = M / ρl
(10) (11)
where, Q represents the effective storage heat capacity, J;ܥଵ and ܥଶ are the specific heat of solid and liquid, J·kg-1·K-1; ∆ݐଵ is the difference value between the melting point temperature and the fermentation temperature; ∆ݐଶ is the difference value between the heat storage temperature and the melting point temperature; M is the quantity of paraffin wax, kg; r is the latent heat, J·kg-1; V is the volume of paraffin wax, m3; ρl is the paraffin wax liquid density, kg·m-3. 7
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Results and discussions
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The proposed solar heated biogas system was designed to produce 5m3 biogas a day. And the fermentation temperature should be kept around 35°C in 60 days in winter period. To satisfy the system thermal demand, a minimum thermal power of 442W should be supplied by both solar collectors and the thermal storage device. Since the thermal storage device must keep its temperature above 35°C in 60 days in Table 4, a 200mm thickness of polyurethane foam was used to insulate thermal loss from the thermal storage device enclosure. Then thermal conductivity coefficient of the thermal storage device enclosure with the cylindrical shape is also given by Eq. (4). Table 5 Collector areas and storage volumes under the different solar fraction Storage Volume (m3)
0.3 0.4 0.5 0.6 0.7 0.8 0.9
8 10 13 15 18 20 23
10.5 9 7.5 6 4 3 1.5
Thermal Conductivity Coefficient of thermal storage device enclosure (W·K-1) 5.17 4.58 4.41 3.40 2.71 1.89 1.36
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Collector Area (m2)
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Solar Fraction (f)
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Under the known condition of 442W system thermal requirement, the areas of the proposed solar collectors in section 3.3.1 and the volume of the thermal storage device described in section 3.3.2 are calculated according to Eq. (3)~(6) by different solar fraction. The results are listed in Table 5. It is obviously that the area of solar collectors increases while the volume of thermal storage decreases gradually with increasing solar fraction.
Fig.5.The ratio of solar collector area over thermal storage volume with respect to solar fraction
In order to choose an optimal solar fraction, the ratio of collector area over storage volume with respect to solar fraction is plotted in Figure 5. In general, the ratio is proportional to solar fraction. Especially, it will increase sharply when solar fraction is more than 0.8. It indicates that the less solar collector area is, the more the thermal storage volume will be used. When a larger solar fraction is chosen, the system economic performance will be weak since the possible thermal collected by solar collectors more than that required by thermal storage device in summer and autumn. In the case of a smaller solar fraction, the specific thermal storage device cannot store enough heat in summer and autumn to supply the fermentation system thermal demand in winter. Figure 6 shows the thermal balance between collection and storage in summer and autumn. It 8
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shows that the thermal energy collected by solar collectors is more than that required the thermal storage device when solar fraction is larger than 0.7. When solar fraction is equal to 0.7, there will be only 8% more thermal quantity than that required by the thermal storage device. And there will be 50% and 174% more additional thermal energy over the storage capability of the thermal storage device with respect to solar fraction 0.8 and 0.9 separately. Therefore, if the proposed fermentation system with 5m3 production a day works normally in winter, it is better to be integrated with two thermal supply units of 20 m2 solar collectors and 3 m3 paraffin wax thermal storage device under the condition of 0.8 solar fraction.
Fig.6.The thermal balance between collection and storage in summer and autumn
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To gain a better understanding of the proposed system performance, the thermal supply period of thermal storage device with 200mm insulation thickness of polyurethane foam has also been investigated in the case of 0.8 solar fraction, 65℃ temperature inside the thermal storage device and 5℃ ambient air temperature. The calculated results are shown in Figure 7. It shows that the temperature inside the thermal storage device more than 35℃ lasts about 61 days, which is roughly equal to the 60 days of thermal supply period. It denotes that the heat dissipation of the thermal storage device in winter is nearly equal to its 20% additional thermal quantity kept in summer and autumn. In this case, the system can operate normally without the auxiliary heating unit when f is 0.8.
Fig.7.The temperature drop curve of heat storage device
As for the uncertainty of the proposed system, we only consider the desired conditions such as the assumption of 40% solar-thermal efficiency and 5°C ambient air temperature etc. Further work is 9
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necessary to investigate the proposed system under the other conditions.
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Conclusion
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This work focuses on the design of a solar heated biogas fermentation system combined with a phase change thermal storage device which is filled with paraffin wax. A mathematical model was developed to estimate the heat demand and thermal dissipation of a pilot fermentation system at 35 temperature with 5m3 biogas production per day under the local metrological data. Both solar collectors and the thermal storage device are used to satisfy the system thermal requirement for operating the fermentation processing at the desired fermentation temperature 35°C during the local winter in Anhui Province, China. The thermal storage tank works in two ways. When solar radiation is strong and solar collectors can transfer enough thermal energy to keep the efficient fermentation temperature in summer and autumn, the thermal storage device works in the thermal storage model in which it stores the additional thermal energy over that required by the system fermentation processing. Correspondingly, when solar radiation is weak and temporally variable in winter, the thermal storage device must work in the thermal supply model and complement the system thermal demand beyond the thermal supply from solar collectors. Model simulations were conducted for investigating the thermal performance of solar collector area and the volume of thermal storage device under the different solar fraction. The results suggested that the proposed fermentation system with 5m3 production a day works normally in winter, it must be integrated with two thermal supply units of 20 m2 solar collectors and 3 m3 paraffin wax thermal storage device under the condition of 0.8 solar fraction.
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Ackonwledgements
References
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Financial support from the National Key Technology R&D Program (No. 2013BAJ10B12) is sincerely acknowledged.
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[1] Chen Y, Yang G, Sweeney S, et al. Household biogas use in rural China: a study of opportunities and constraints[J]. Renewable and Sustainable Energy Reviews, 2010, 14(1): 545-549. [2] Alkhamis, T.M., et al., Heating of a biogas reactor using a solar energy system with temperature control unit. SOLAR ENERGY, 2000. 69(3): p. 239-247. [3] Zhang Yangjun Qin Chaokui and Chen Zhiguang, Household Biogas Digester Heated By Solar-Collector, Gas Technology, 2011, 08 (18-23). [4] Zhou Mengjin and Zhang Ronglin, Biogas Practical Technology, Chemical industry press, 2004. [5] Sreekrishnan T R, Kohli S, Rana V. Enhancement of biogas production from solid substrates using different techniques––a review[J]. Bioresource technology, 2004, 95(1): 1-10. [6] Liu Jianmin, Principle of Solar Energy Utilization, Technology and Engineering, Beijing: Electronic industry press, 2010. [7] Jicheng L, Wenzhe L. Experimental Study on United Heating System of Solar Energy and Biomass Boiler[J]. Journal of Agricultural Mechanization Research, 2014, 10: 056. [8] Li B, Zhou X Z, Yan W. Experimental Study on Using Solar to Improve Producing Methane in Northeast China[C], Advanced Materials Research. 2014, 953: 132-135.
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[9] Liu J, Li W, Chen Z, et al. Heating mode of biogas plant in alpine region based on underground water source heat pump[J]. Transactions of the Chinese Society of Agricultural Engineering, 2013, 29(5): 163-169. [10] Axaopoulos, P., et al., Simulation and experimental performance of a solar-heated anaerobic digester. Solar Energy, 2001. 70(2): p. 155-164. [11] Qiu Ling and Wang Lanying, Heat Equilibrium Calculation of Solar Two Calefacient Effects on Biogas Digester, Proceeding of International Seminar on Rural Biomass Energy and ASEAN Plus Three (China, Japan and Korea) Forum on Biomass Energy, Beijin, 2008, 228-232. [12] Yang Shiming and Tao Wenquan, Heat Transfer (third edition), Higher education press, 1998, 207-212. [13] Su, Y., R. Tian and H.Y. Xiao, Research and Analysis of Solar Heating Biogas Fermentation System. 2011. [14] He Zinian and Zhu Dunzhi, Solar heating Application Manual, Chemical industry press, 2009. [15] Wang Zengyi, et al, Experimental study on heat transfer characteristics of the process of charging/discharging of a heat pipe heat exchanger with latent heat storage, Journal of engineering thermophysics, 2005(06), 989-991. [16] Oro, E., et al., Thermal energy storage implementation using phase change materials for solar cooling and refrigeration applications. 1st International conference on solar heating and cooling for building and industry (SHC 2012), 2012. 30: p. 947-956. [17] Pinel, P., et al., A review of available methods for seasonal storage of solar thermal energy in residential applications. Renewable and Sustainable Energy Reviews, 2011. 15(7): p. 3341--3359. [18] Wang Zhebin, et al, Simulation of the heat transfer of melting process pg paraffin, Journal of Beijing University of Civil Engineering and Architecture, 2008(02), 10-24. [19] Zhang Xiaoyan, Thermal Performance of Modified Paraffin Phase Change Heat Storage Materials, Academic dissertation, 2012. [20] Yang Ying, et al, Research of heat transfer efficiency in phase change thermal storage, Journal of thermal science and technology, 2011(03), 226-230.
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S1359-4311(14)01194-6
DOI:
10.1016/j.applthermaleng.2014.12.065
Reference:
ATE 6259
To appear in:
Applied Thermal Engineering
Received Date: 11 June 2014 Revised Date:
3 December 2014
Accepted Date: 26 December 2014
Please cite this article as: Y. Tian, Y. Lu, H. Lu, L. Wu, X. Li, Study of Solar Heated Biogas Fermentation System with a Phase Change Thermal Storage Device, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2014.12.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Study of Solar Heated Biogas Fermentation System with a Phase Change Thermal Storage Device
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Ye Tian, Yong Lu*, Haowei Lu , Lei Wu and Xianlin Li School of Energy and Environment, Southeast University, Nanjing, 210096, China Email:[email protected]
Abstract
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A new technique of solar heating biogas fermentation system integrated with a phase change thermal storage device is introduced for improving the low efficiency of the traditional biogas fermentation devices during winter. For a designed solar heated biogas system, its performance of anaerobic fermentation process under the different natural cooling conditions has been assessed by varying the thicknesses of insulation materials. And the optimum relative proportion between the heat supplied by solar and that stored in phase change heat storage device has been also investigated. The preliminary results based on the numerical simulations performed with meteorological data from Anhui Province, China show that the proposed PCM device with 3m³paraffin wax as phase change material (PCM) and 20m2 solar collector areas can satisfy the heat required by producing 5m³ biogas per day corresponding to a solar fraction (f) of 0.8 in winter. It indicates that the proposed solar heating system could be a promising approach for promoting biogas technology applications in cold rural regions of China.
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Introduction
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Keywords: anaerobic fermentation; collector area; PCM volume; solar fraction
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Biomass like straw is rich and burnt without any environment protection devices each year in China, which results to air pollution problem like PM2.5 becoming seriously in recent years. In view of energy shortages and the abundant resources of straw in China, the biogas industry can be an effective way to meet the demand for clean energy in rural areas [1]. Biogas is a combustible gas which is produced by the anaerobic (i.e. oxygen free) microbial fermentation of organic substances at a certain temperature, humidity and pH value. There are many factors influencing the fermentation process, among which the fermentation temperature is particularly important. Sufficient warmth is the key to the fermentation process, so researchers need to find ways to meet the energy requirements at minimum cost. Another problem encountered in such a bioreactor is how to minimize temperature fluctuations, because they adversely affect the bacterial fermentation process[2]. The temperature fluctuations should not exceed 2~3 per hour. If the fermentation temperature fluctuations exceed 5 in a short period of time, biogas yield will decrease significantly, so a constant fermentation temperature is required [3~5]. Different kinds of fermentation technologies achieve anaerobic digestion at different temperatures: room temperature (15 ), medium temperature (30~38 ) and high temperature (50~55 ). In rural areas, the medium temperature fermentation process has been shown to be more economical with an optimum temperature of 35 [6]. Therefore, it is important, in order to grow and sustain the mesophilic bacteria and obtain optimum biogas production, to heat the bioreactor to a temperature of about 35 . The focus of current research in biogas technology is to use clean energy to improve the fermentation temperature and maintain an efficient biogas production rate in winter. This can reduce 1
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System description
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conflicts between energy combustion and environmental conservation. And in some regions, where there is plenty of sunshine throughout the year, solar energy is a relatively economical source for heating purposes. In large and medium-sized methane gas projects, the ground source heat pump, boiler and solar greenhouse etc. are used to improve the fermentation temperature.Liu Jianyu etal.(2013) used a groundwater source heat pump to heat the anaerobic fermentation tank in winter, and the total savings in standard coal consumption was only 44% of the demand of standard coal using coal-boilers directly during the experimental cycle.[7].Li Jicheng etal.(2014) proposed a idea that to heat the anaerobic fermentation system by uniting the solar energy and biomass boiler, which is utilized to ensure the gas production rate of large-scale biogas fermentation tank under normal conditions in winter. 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[8].Ling Qiu etal.(2008) designed a double-effect solar heated biogas fermentation system consisting of a solar greenhouse and a hot chamber. The solar greenhouse is used to retain heat from the sun and improve the ambient air temperature around the bioreactor, thus reducing heat loss from the fermentation tank and improving the heat utilization efficiency respectively [9]. However, the ground source heat pump, boiler and solar greenhouse etc are not appropriate in small-sized and household methane gas projects. P.Axaopoulos etal. (2001) designed a solar powered household biogas fermentation system, using flat-plate collectors as an integral part of the roof structure of a swine manure digester [10].In order to solve the problems of fermentation process in winter in the northeast China, Li B(2014)studied the gas production rate of fermentation tank with solar heating to improve the temperature, and the experimental investigations were mainly based on the climatic conditions in northeast China[11]. This paper presents a solar heated small-sized biogas fermentation system with a phase change thermal storage device in Anhui province, located in central China. Different insulation materials were studied under natural coolingto ascertain the optimum, most cost effective material and material thickness, as well as the best relative sizes for the three components of the system, namely: the fermentation tank, the solar heat collectors and the phase change heat storage system.In this study, the appropriate insulation material, the optimal insulation layer thickness and the most suitable solar fraction value are selected. The solar heated phase change thermal storage device can ensure a continuous and efficient fermentation process, which can effectively cope with changes in solar energy resources resulting from diurnal changes, cloud cover, days of inconstant sunshine and seasonal fluctuations.
Fig.1.Schematic of Solar Heated Biogas System with PCM storage tank
The system mainly consists of three subsystems—heating system, heat storage system and fermentation system, as shown in fig.1. The parabolic trough condenser is the main component of 2
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heating system and it translates the solar energy into heat. The heat storage device accumulates the surplus heat and supplies heat when there is not sufficient heat-collecting capacity. Furthermore, the storage device reduces the influence of the relatively unstable solar energy heat source and increases the stability of the system. The collected heat is transferred to the heat storage device when the fermentation temperature is higher than 35 . When the fermentation temperature is lower than 35 , it is used for heating the straw fermentation tank. In summer, the fermentation temperature can reach temperatures above 35 , so the biogas production rate is guaranteed and no additional heating is required. The oversupply of solar heat is then transferred to the phase change heat storage device.This process is the heat storage process, as shown in loop. On the one hand, the heat stored on sunny, warm days, will be taken out to replenish the heat loss of the fermentation tank when the loop. On the other hand, heat-collecting capacity cannot meet the heat load in winter, as shown in when the heat-collecting capacity can meet the heat load, the heat-collecting capacity is supplied directly to the fermentation tank, as shown in loop. Thermal Analysis
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Meteorological data and basic value of the system parameters
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In this study, the system consisted of three main parts shown in Fig.1. And a fermentation tank was designed to produce 5m3 biogas a day. It was a metallic cylindrical tank with 1.1m inner diameter and 1.98m height. In order to keep the efficient fermentation temperature in winter, the fermentation tank needs to be not only insulted by using the suitable thickness insulation material but also heated by the thermal storage device. This device is designed to be filled paraffin wax which is used to keep the thermal energy collected by solar collectors in summer and autumn, since the inside temperature of fermentation tank can work normally due to the high ambient air temperature. When the ambient air temperature drops below the normal fermentation temperature in winter and the heat collected by solar collectors temporally is not enough, some additional heat from the paraffin wax thermal storage device will be provide to satisfy that required by the fermentation processing in the tank. Table 1 The meteorological data in Anhui Province, China 1 3.0
2 5.4
3 9.5
4 16.3
5 22.1
6 25.4
7 28.0
8 27.3
9 23.9
10 17.8
11 10.8
12 4.5
198.3
222.7
295.8
401.4
501.8
427.4
460.7
499.3
368.5
328.6
246.7
206.1
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Month Average temperature( ) Total solar radiation(MJ/m2)
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The system thermal analysis was generally conducted under the local meteorological conditions in Anhui Province, China, where the demonstration system will be located. The typical data of solar radiation as well as ambient air temperature are listed in Table 1. And the average temperature and the total solar radiation in the local winter are around 5 °C and 627 MJ/m2 respectively. Heat demand and dissipation of the ferementation tank in winter
The heat requirement of the fermentation system is shown in Fig. 2. Here we do not consider that the heat loss due to biogas removal from the bioreactor and the heat generated by the fermentation process. They are negligible and have little influence on the fermentation process～[13].Therefore, the system heat load includes the heat loss (ܳଵ) dissipating from the insulation enclosure and the heat load (ܳଶ )～of biomass digestive fluid which need to be heated to reach the required operating temperature 35 from the ambient air temperature before entering the bioreactor fermentation tank. Then the heat loss (ܳଵ) and the system total heat demand (ܳଶ ) can be described as:
Q1 = Q1a + Q1b 3
(1)
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(2)
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Where, ܳଵ is the heat loss from the fermentation tank,～ܳଵ is the heat loss in the pipeline loop and ܳଶ is the heat required by heating biomass digestive fluid.
Fig.2.The analysis of system heat load
3.2.1 Heat loss and the fermentation tank insulation design
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The bottom and side areas of the designed fermentation tank are 5.98m2 and 5.72m2respective, since it is a cylindrical tank with 1.1m inner diameter and 1.32m height. The initial temperature inside the fermentation tank and the ambient air temperature in winter are supposed as 35 and 5 respectively. The standard heat resistant equations for multi-layer flat wall and multi-layer cylindrical wall are written as [12]:
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K Flat =
δ δ 1 ( + 1+ 2+ ) α1 λ1 λ2 α 2
K Cylinder =
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1 d1 d 2 d 1 d1 d1 1 + ln + ln 2 + α1 d 0 2λ1 d 0 2λ2 d1 α 2
(3)
(4)
where, ܭி௧ represents the combined heat transfer coefficient of a multi-layer flat wall; K ௬ௗ represents the combined heat transfer coefficient of a multi-layer cylindrical wall;ߙଵ and ߙଶ are the heat transfer coefficients of the tank internal surface and the outside convection heat transfer coefficient, W·m-2·K-1; ߜଵ and ߜଶ are the thicknesses of the tank and the external thermal insulation layer, units of meters (m); ݀ , ݀ଵ and ݀ଶ are the diameters of each layers, m; λଵ and λଶ are the heat conductivity coefficients of the fermentation tank and external thermal insulation layer, W·m-1·K-1. Among traditional industrial insulation materials like such as polyurethane foam, polystyrene foam, polyethylene foam and rock wool, polyurethane foam is usual the best choice for thermal insulation due to its smallest thermal conductivity coefficient (0.0035 w·m-1·K-1). Here,～ rigid polyurethane foam was used to insulate the fermentation tank. In the case of its thickness ranging from 20 to 80mm, the thermal insulation performance was investigated through analysing the natural temperature dropping processing of the fermentation tank as shown in Fig. 3. The longer the natural cooling process lasts, the better the thermal insulation performance is. It is obviously that the thermal 4
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insulation performance is improved with the augment the thickness of～polyurethane foam. But their augment relationship is not linear proportional, since the thermal insulation performance of 100mm polyurethane foam is similar to that of 80mm polyurethane foam with 20mm polystyrene. For the unit mass price of polyurethane foam is higher than that of polystyrene, we chose 80mm polyurethane foam as the insulation material for the fermentation tank for balance between efficiency and economy.
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Fig.3.Temperature dropping process of fermentation tank under the different insulation thicknesses 3.2.2 Heat demand of heating biomass digestive slurry
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Under the condition of 80mm thickness polyurethane foam, the heat transfer coefficients are 0.49 and 0.47 w·m-2·K-1 respectively according to Eq. (1) and Eq. (2). The calculated heat loss from the fermentation tank (ܳଵ ) would be 145.47 W by using Eq. (5). Furthermore, there is a circulation loop of biomass digestive liquid, its flow volume is designed as 5 m3·d-1. The flow velocity would be 8 m·s-1 in a pipe system with 20mm diameter. The heat loss in the pipeline loop (Qଵ ) is figured out as 175 W and the total heat loss (Qଵ ) is 320 W. Q1a = K1A1 t gi -t f +K 2 A 2 t gi -t f
(5)
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where, ܭଵ and ܭଶ are heat transfer coefficients at the bottom and side of fermentation tank, W·m-2·K-1; ܣଵ and ܣଶ are the bottom and side areas of fermentation tank, m2; ݐ is the fermentation tank temperature, K; ݐ is the ambient air temperature, K. Since biomass digestive slurry～need to be heated up to 35 before entering into the fermentation tank, its heat requirement is given by Eq. (6). Q2 = ρ s qs Cs (t gi - ts ) (6) where,～ߩ௦ is the density of biomass digestive slurry in the reactor, kg·m-3,ݍ௦ is the flow rate of the biomass digestive liquid, m3·s-1; ܥ௦ is the specific heat capacity of the biomass digestive liquid, J·kg·K-1; ݐ is thefermentation tank temperature, K; and ݐ௦ is the initial temperature of biomass digestive liquid, K. The basic values of parameters used in the calculation are listed in Table 2. Table 2 Basic values of parameters used in the heating load calculations Parameter
Unit
Biogas production Biogas production rate Biomass fermentation concentration The temperature of biomass digestive liquid Specific heat capacity of biomass digestive liquid
5
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m3·d Nm3-biogas/kg-straw TS °C J· kg· K-1
Value 5 0.2 15% 35 4200
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In the case of 35°C fermentation temperature, 5 m3 biogas production per day need the minimum consumption of 25 kg straw under 0.2 straw-biogas production rate. The total 167 kg biomass digestive slurry～with 15% biomass concentration need to be replenished into the fermentation tank. Based on Eq. (5) and Eq. (6), the heat load for heating the biomass digestive slurry～(ܳଶ ) and the total heating load (ܳௗ ) are figured out as 122 W and 442 W separately.
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The system optimal solar fraction
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Solar fraction is defined as the ratio of thermal energy supplied by solar collectors temporally. Since the paraffin wax thermal storage device and solar collectors are integrated to provide the heat demand by the fermentation system. The system thermal balance is illustrated in Figure 4. As for the thermal storage device, it is designed to work in two operation modes. When solar radiation is strong and solar collectors can transfer enough thermal to satisfy the system thermal demand of producing biogas in summer and autumn, the thermal storage device works in the storage mode in which it stores the additional thermal energy collected by solar collectors. Correspondingly, when solar radiation is weak and temporally variable in winter, only solar collectors cannot keep the fermentation processing steady. The thermal storage device must work in the thermal supply mode and complement the system thermal demand beyond the thermal supply from solar collectors. Thus, it is necessary to study the optimal solar fraction to rate the proportion of thermal energy supplied by thermal storage device and solar collectors.
Fig.4.Energy balance of the designed fermentation system
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3.3.1 Solar collector areas
Since the system thermal demand is determined, the minimum solar collector area can be figured out under a specific value of solar fraction. The estimated formulae for the required collector area of a direct system and an indirect system can be described as Eq. (7) and Eq. (8) [14].
Ac =
86400QDemand ⋅ f JTηcd (1-η L )
(7)
FRU L Ac ) U hX AhX
(8)
A IN =Ac (1+
where, ܣ and ܣூே represent the areas of the direct system and indirect system respectively, m2; f is the solar fraction, as shown in Table 3; ்ܬis the average daily amount of solar radiation, J·m-2; ߟௗ is the average collection efficiency of solar collector, which ranges from 0.25 to 0.50 according to the standard of flat plate solar collectors (GB/T 6424-2007) and here it is chosen as 0.4; ߟ is the 6
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average heat loss rate of solar collector, 0.20~0.30; ܨோ ܷ is the total heat loss coefficient of collectors, 2~6; ܷ௫ is the heat transfer coefficient of heat exchanger, W·m-2·K-1; and ܣ௫ is the area of the heat exchanger, m2. Table 3 There commended values of solar fraction in different regions Rank
Solar energy resources～affluent areas Solar energy resources～rich areas Solar energy resources～general areas Solar energy resources～poor areas
Solar fraction Short term heat storage system ≥50% 30% 50% 10% 30% 5% 10%
I II III IV
Seasonal heat storage system ≥60% 40% 60% 20% 40% 10% 20%
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3.3.2 The volume of thermal storage device
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The effective heat storage capacity is calculated by the Eq. (9). The effective heat storage capacity refers to the heat that can be utilized to cover the shortage of the heat-collecting capacity in winter, namely, the difference value between the heat load and the heat-collecting capacity. In Table 4, the design parameters of the PCM storage device are given [15].
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Table 4Thedesign parameters of PCM storage device Parameter The～heating cycle Heat storage temperature The average inlet/outlet temperature of PCM storage device The average inlet/outlet temperature of fermentation tank
Unit d °C K K
(9)
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QStorage = QDemand × (1 − f )
Value 60 55 303.15/323.15 323.15/303.15
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where, Qௌ௧ represents the effective storage heat capacity, W; Qௗ is the total heat load, W; f is the solar fraction. As a latent heat storage material, paraffin wax has many advantages, such as: high latent heat, good thermal stability, no supercooling phenomenon, low expense and small volume change in the solid-slurry phase change process[16].However, it also has some drawbacks: low thermal conductivity, low density and poor mobility have a certain impact on the heat extraction time [17, 18]. Reference [19]～demonstrates thatmixing the appropriate amount of graphite powder in paraffin can enhance heat transfer.By means of comparative analyses, we have selected paraffin wax RT54. Its average relative molecular mass is 377, melting point is 55 , the latent heat is 179 kJ·kg-1, the specific heat of solid and slurry are 1.8 kJ·kg-1·K-1 and 2.4 kJ·kg-1·K-1, the density of solid and slurry are 900 and 760 kg·m-3 and the thermal conductivity coefficient is 0.2 W·m-1·K-1. As the heat loss rate of the heat storage device is 30%, the effective heat storage capacity needs to be 644297 kJ [20].The volume of paraffin wax can be calculated by Eq. (10) and Eq. (11) below.
Q = C 1 M ∆ t1 + C 2 M ∆ t 2 + M r ～～～～
V = M / ρl
(10) (11)
where, Q represents the effective storage heat capacity, J;ܥଵ and ܥଶ are the specific heat of solid and liquid, J·kg-1·K-1; ∆ݐଵ is the difference value between the melting point temperature and the fermentation temperature; ∆ݐଶ is the difference value between the heat storage temperature and the melting point temperature; M is the quantity of paraffin wax, kg; r is the latent heat, J·kg-1; V is the volume of paraffin wax, m3; ρl is the paraffin wax liquid density, kg·m-3. 7
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Results and discussions
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The proposed solar heated biogas system was designed to produce 5m3 biogas a day. And the fermentation temperature should be kept around 35°C in 60 days in winter period. To satisfy the system thermal demand, a minimum thermal power of 442W should be supplied by both solar collectors and the thermal storage device. Since the thermal storage device must keep its temperature above 35°C in 60 days in Table 4, a 200mm thickness of polyurethane foam was used to insulate thermal loss from the thermal storage device enclosure. Then thermal conductivity coefficient of the thermal storage device enclosure with the cylindrical shape is also given by Eq. (4). Table 5 Collector areas and storage volumes under the different solar fraction Storage Volume (m3)
0.3 0.4 0.5 0.6 0.7 0.8 0.9
8 10 13 15 18 20 23
10.5 9 7.5 6 4 3 1.5
Thermal Conductivity Coefficient of thermal storage device enclosure (W·K-1) 5.17 4.58 4.41 3.40 2.71 1.89 1.36
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Under the known condition of 442W system thermal requirement, the areas of the proposed solar collectors in section 3.3.1 and the volume of the thermal storage device described in section 3.3.2 are calculated according to Eq. (3)~(6) by different solar fraction. The results are listed in Table 5. It is obviously that the area of solar collectors increases while the volume of thermal storage decreases gradually with increasing solar fraction.
Fig.5.The ratio of solar collector area over thermal storage volume with respect to solar fraction
In order to choose an optimal solar fraction, the ratio of collector area over storage volume with respect to solar fraction is plotted in Figure 5. In general, the ratio is proportional to solar fraction. Especially, it will increase sharply when solar fraction is more than 0.8. It indicates that the less solar collector area is, the more the thermal storage volume will be used. When a larger solar fraction is chosen, the system economic performance will be weak since the possible thermal collected by solar collectors more than that required by thermal storage device in summer and autumn. In the case of a smaller solar fraction, the specific thermal storage device cannot store enough heat in summer and autumn to supply the fermentation system thermal demand in winter. Figure 6 shows the thermal balance between collection and storage in summer and autumn. It 8
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shows that the thermal energy collected by solar collectors is more than that required the thermal storage device when solar fraction is larger than 0.7. When solar fraction is equal to 0.7, there will be only 8% more thermal quantity than that required by the thermal storage device. And there will be 50% and 174% more additional thermal energy over the storage capability of the thermal storage device with respect to solar fraction 0.8 and 0.9 separately. Therefore, if the proposed fermentation system with 5m3 production a day works normally in winter, it is better to be integrated with two thermal supply units of 20 m2 solar collectors and 3 m3 paraffin wax thermal storage device under the condition of 0.8 solar fraction.
Fig.6.The thermal balance between collection and storage in summer and autumn
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To gain a better understanding of the proposed system performance, the thermal supply period of thermal storage device with 200mm insulation thickness of polyurethane foam has also been investigated in the case of 0.8 solar fraction, 65℃ temperature inside the thermal storage device and 5℃ ambient air temperature. The calculated results are shown in Figure 7. It shows that the temperature inside the thermal storage device more than 35℃ lasts about 61 days, which is roughly equal to the 60 days of thermal supply period. It denotes that the heat dissipation of the thermal storage device in winter is nearly equal to its 20% additional thermal quantity kept in summer and autumn. In this case, the system can operate normally without the auxiliary heating unit when f is 0.8.
Fig.7.The temperature drop curve of heat storage device
As for the uncertainty of the proposed system, we only consider the desired conditions such as the assumption of 40% solar-thermal efficiency and 5°C ambient air temperature etc. Further work is 9
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necessary to investigate the proposed system under the other conditions.
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Conclusion
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This work focuses on the design of a solar heated biogas fermentation system combined with a phase change thermal storage device which is filled with paraffin wax. A mathematical model was developed to estimate the heat demand and thermal dissipation of a pilot fermentation system at 35 temperature with 5m3 biogas production per day under the local metrological data. Both solar collectors and the thermal storage device are used to satisfy the system thermal requirement for operating the fermentation processing at the desired fermentation temperature 35°C during the local winter in Anhui Province, China. The thermal storage tank works in two ways. When solar radiation is strong and solar collectors can transfer enough thermal energy to keep the efficient fermentation temperature in summer and autumn, the thermal storage device works in the thermal storage model in which it stores the additional thermal energy over that required by the system fermentation processing. Correspondingly, when solar radiation is weak and temporally variable in winter, the thermal storage device must work in the thermal supply model and complement the system thermal demand beyond the thermal supply from solar collectors. Model simulations were conducted for investigating the thermal performance of solar collector area and the volume of thermal storage device under the different solar fraction. The results suggested that the proposed fermentation system with 5m3 production a day works normally in winter, it must be integrated with two thermal supply units of 20 m2 solar collectors and 3 m3 paraffin wax thermal storage device under the condition of 0.8 solar fraction.
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Ackonwledgements
References
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Financial support from the National Key Technology R&D Program (No. 2013BAJ10B12) is sincerely acknowledged.
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[9] Liu J, Li W, Chen Z, et al. Heating mode of biogas plant in alpine region based on underground water source heat pump[J]. Transactions of the Chinese Society of Agricultural Engineering, 2013, 29(5): 163-169. [10] Axaopoulos, P., et al., Simulation and experimental performance of a solar-heated anaerobic digester. Solar Energy, 2001. 70(2): p. 155-164. [11] Qiu Ling and Wang Lanying, Heat Equilibrium Calculation of Solar Two Calefacient Effects on Biogas Digester, Proceeding of International Seminar on Rural Biomass Energy and ASEAN Plus Three (China, Japan and Korea) Forum on Biomass Energy, Beijin, 2008, 228-232. [12] Yang Shiming and Tao Wenquan, Heat Transfer (third edition), Higher education press, 1998, 207-212. [13] Su, Y., R. Tian and H.Y. Xiao, Research and Analysis of Solar Heating Biogas Fermentation System. 2011. [14] He Zinian and Zhu Dunzhi, Solar heating Application Manual, Chemical industry press, 2009. [15] Wang Zengyi, et al, Experimental study on heat transfer characteristics of the process of charging/discharging of a heat pipe heat exchanger with latent heat storage, Journal of engineering thermophysics, 2005(06), 989-991. [16] Oro, E., et al., Thermal energy storage implementation using phase change materials for solar cooling and refrigeration applications. 1st International conference on solar heating and cooling for building and industry (SHC 2012), 2012. 30: p. 947-956. [17] Pinel, P., et al., A review of available methods for seasonal storage of solar thermal energy in residential applications. Renewable and Sustainable Energy Reviews, 2011. 15(7): p. 3341--3359. [18] Wang Zhebin, et al, Simulation of the heat transfer of melting process pg paraffin, Journal of Beijing University of Civil Engineering and Architecture, 2008(02), 10-24. [19] Zhang Xiaoyan, Thermal Performance of Modified Paraffin Phase Change Heat Storage Materials, Academic dissertation, 2012. [20] Yang Ying, et al, Research of heat transfer efficiency in phase change thermal storage, Journal of thermal science and technology, 2011(03), 226-230.
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