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Feasibility study of integrating solar energy into anaerobic digester reactor for improved performances using TRNSYS simulation: application Kenitra Morocco
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Energy (2017) 000–000 402–407 EnergyProcedia Procedia136 00 (2017) www.elsevier.com/locate/procedia
4th International Conference on Energy and Environment Research, ICEER 2017, 17-20 July 2017, Porto, Portugal 15th of International Symposium District Heating and Cooling digester Feasibility The study integrating solaronenergy into anaerobic reactor for improved performances using TRNSYS simulation: Assessing the feasibility of using the heat demand-outdoor application Kenitra Morocco temperature function for a long-term district heat demand forecast a
a
a OUHAMMOU Badraa,*, AGGOUR Mohammed , FRIMANEc Azddinea c a,b,c a b I. Andrić *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Renewable Energy and Environment Laboratory, Faculty of Sciences, Ibn Tofail University. BP. 133, 14 000 Kenitra-Morocco IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
This paper studies the feasibility of the integration of the solar thermal energy in the anaerobic digestion (AD). In this purpose, a Abstract model of the digester reactor (V=70 l) coupled with a new design of the solar water thermal system has been simulation developed using TRNSYS. The heating is carried out by an exchanger inside of the digester. The thermal performance and District behaviors heating networks are commonly addressed in theofliterature as investigated. one of the most effective decreasing the dynamic of the system under a climatic condition Kenitra are It is found thatsolutions the solar for system covers 90 gas consumed emissions by from These systems the require high investments which are returned through %greenhouse of the energy thethe ADbuilding during sector. the year. In addition, results show a daily temperature fluctuation of 0.8the °Cheat in sales. Due and to the changed climate conditions and building renovation policies, heat demand in the future could decrease, the summer 2.3 °C in the winter. prolonging the investment return period. main of this paper isby to Elsevier assess the feasibility of using the heat demand – outdoor temperature function for heat demand ©The 2017 Thescope Authors. Published Ltd. forecast. The district of Alvalade, in Lisbon (Portugal), used as aConference case study.onThe district consisted of 665 Peer-review under responsibility of thelocated scientific committee of the 4thwas International Energy and isEnvironment buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Research. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were comparedAnaerobic with results fromfeasibility a dynamic heatsolar demand model, previously developed and validated by the authors. Keywords: digester; study; heating; solar thermal; TRNSYS The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.scenarios, Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and Solar energy is the most abundant energy resource with the potential to become a major component of a renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the sustainable global energy solution. The most common use of thermal solar energy has been for water heating coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and systems; thisaccuracy use has beendemand commercialized improve the of heat estimations. in many countries in the world. Their applications have increased © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +212658525092. E-mail address: [email protected]; [email protected] Keywords: Heat demand; Forecast; Climate change
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 4th International Conference on Energy and Environment Research. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 4th International Conference on Energy and Environment Research. 10.1016/j.egypro.2017.10.255
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al. / Energy Procedia 00 (2017) 000–000
403
significantly, especially in countries with large solar potential. Bioenergy, on the other hand, is another source of renewable energy that has gained momentum due to several advances in biotechnology during the last decade. In late 1970, U.S. Environmental Protection Agency (U.S. EPA) first developed a solar powered anaerobic digestion system to improve thermal efficiency of the digestion [1]. Since then, several studies were conducted and focused on further improving the thermal efficiency of anaerobic digestion. Hills and Stephens studied the feasibility of using solar energy to heat a mesophilic CSTR (continuous stirred-tank reactor) anaerobic digester [2]. Alkhamis and his colleagues designed and operated a lab scale digester with a fl at plate solar collector as the heater in Jordan [3]. Axaopoulos et al. developed a mathematical model for the simulation of a swine manure reactor heated with solar energy [4]. However, using anaerobic digestion as an energy storage unit to store solar energy has not been comprehensively and thoroughly studied to date [5]. In this paper, the feasibility study of using the solar thermal energy to heat a mesophilic digester was investigated. The proposed designs have been developed by using the TRNSYS environment in order to provide a tool for assessing the thermal performance of the system. The simulations programs were run for a one-year period using hourly climatic data including ambient air temperature, total solar irradiance, wind speed and humidity. The rsults of the feasibility study indicate that the solar system is highly satisfactory (90%) to integrate the solar thermal into the anaerobic digestion. Nomenclature Ac Cp FR G M ms NG Ns Ta Tin Tp Tr Ts Qu UL,j
Aperture area of a single collector module Specific heat of collector fluid Overall collector heat removal efficiency factor Solar irrandiance Masse Flowrate at use conditions Number of the glass cover in the collector Number of identical collectors in series Ambient temperature Temperature input of the exchanger Absorber temperature Bioreactor temperature Storage temeprature tank Energy rate produce by the solar collector The top loss coefficient
2. Description of the system The solar system proposed to heat the anaerobic reactor contains two big hydraulic circuits as illustrate in the Fig. 1. The first one called Primary circuit and other called Secondary circuit; the primary circuit is responsible to produce the energy during the day from the solar radiation. The components of the first circuit are: Flat plate collector, circulator, tank storage and unit of the differential controller. The components of the secondary circuit are: a second tank storage, pumps, and anaerobic digester equipped with a pump for adding the influent (input of the influent). The energy produced by the solar thermal collector was stored in the tank storage (primary hydraulic circuit); in purpose to heat the digester in a continuous manner, a part of the energy used directly to heat the digester, the other quantity of energy was stored in the second tank storage, and when the temperature in first storage was less than temperature inside the reactor the second tank storage start to maintain the heat of the digester. The temperature control in the reactor was carried out through unit of the differential controller. The digester contains two inlets and two outlets, it should note that the heating reactor carried out through an internal heat exchanger as shown in Fig. 2. It should note that the pump 2 and 4 functioned only when the primary circuit was stopped, e.g, the production of the hot fluid was off, that means when the first circuit was heating the digester the
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al./ Energy Procedia 00 (2017) 000–000
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second storage was out of function and that in the night the second circuit started to heat the digester and automatically the first circuit was off. The base system used electric heating in tank storage 1 to provide all of the energy needed to meet the load requirements. The set point temperature for the upper auxiliary heater was 37 °C with a dead-band temperature of 2 °C. Therefore the heater would turn on when the water that was at the height of the thermostat fell to 35 °C and would remain on until the set point temperature was reached.
Fig. 1. The design of the solar system coupled with the Anaerobic Digester
3. Results and discussion Energy needed by the bioreactor is mainly thermal energy to heat the feed and maintain mesophilic condition of bioreactors. Results of this study indicate that solar energy is very important to use it for heating requirements of bio-digester waste reactors. 3.1 The variation of operating temperature of anaerobic digester The solar radiation which our simulation was run with are illustrate in the Fig. 2 for the cloudy and summer days. The solar radiation for the winter day is very low and poor that mean that solar energy is not enough for heating the anaerobic reactor, in contrary for the summer day it’s same that the solar system (solar energy) can be pass the needs for heating the bio-digester. Winter day
1200
Summer Day
1000
2
G (W/m )
800
600
400
200 6
8
10
12
14
16
18
Time (hr)
Fig. 2. Solar radiation from the site of Kenitra for the two typical days
The Fig. 3 shows the variation of the temperature during the typical summer and winter day. It’s clear that the temperature is very stable and it’s satisfied the mesophilic condition for the anaerobic digestion. The small standard deviation of the manure temperature inside the digester is directly affected by the ambient air temperature, the solar irradiance, the wind speed and humidity. The small daily fluctuation of the manure temperature in the digester and its occurrence result mainly from the operation of the solar collectors and the time swine manure enters the digester as it is illustrated in the Fig. 4. The
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al. / Energy Procedia 00 (2017) 000–000
405
variations of corresponding operating temperature of anaerobic digester for these time periods are shown in Fig. 9. The maximum temperature fluctuation during this time period is less than 0.8 C, which is too small to influence the microbial activity [12]. 40
Tdigester, summer
35
Tdigester, winter
Temperature (°C)
30 25
Ta, summer
20 15
Ta, winter
10 5 0
5
10
15
20
Time (h)
Fig. 3. Temperature distribution inside the bio-digester and ambient temperature for the tow typical days 4500
Solar radiation Temperature inside the digester Ambient Temprature
4000 3500
30
2500
25
2000 20 1500 15
1000
Temperature (°C)
2
40 35
3000
G (kJ/hr.m )
45
10
500
5
0 740
1480
2220
2960
3700
4440
5180
5920
6660
7400
8140
8880
Time (hr)
Fig. 4. The Variation of the digester temperature and the climatic condition (solar radiation, ambient temperature) during the year.
3.2 Energy demand of bioreactors Energy needed by the bioreactor is mainly thermal energy to heat the feed and maintain mesophilic condition of bioreactors. The amount of the energy was varied with season changes. Energy demand by Digester Energy Produced by Solar collector Energy storage Auxiliary Energy
9000 8000
Solar production
Energy (kJ/hr)
7000 Energy of Storage 1
6000 5000 Energy demand by AD
4000
Energy of storage 2 Auxiliary Energy
3000 2000 1000 0
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
Fig. 5. Digester energy needs, Production solar and Auxiliary for a typical winter day 10000
Energy need by digester Solar production Tank storage 2 Tank storage 1
Solar Production
Energy of the storage 1
8000
Energy of the storage 2
Energy (kJ/hr)
Energy demand by AD
6000
4000
2000
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
Time (hr)
Fig. 6. Digester energy needs, Production solar and Auxiliary for a typical summer day
The Fig. 5 and 6 shows the daily energy demand of the bio-reactor as well as the energy produce by the solar collectors during a typical winter and summer day. Also, the energy stored in the two tank storage.
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The average Energy needs of the bio-reactor are estimated around 2 MJ/hr; the average ambient temperature and the solar irradiances are 9 °C and 500 W/m2 30 °C and 1200 W/m2 respectively for the winter and summer day. It’s obvious that, the solar system as doubt to cover all the energy needs of the bio-digester, for that, and as it’s illustrate in the Fig. 6; in the afternoon the solar system might cover the demand of the anaerobic digestion until the midnight, and the period 24 h to 8 h, the solar system can’t satisfy the energy demand; for this reason, the control differential unit launch the auxiliary energy. In the winter day, the solar system, achieve a production around 8 (MJ/hr); and the first storage Tank (6 MJ/hr); as mentioned before, one part of the energy produce stored in the Tank 1 left to heat the anaerobic digester (direct utilization) and other quantity stored in the second tank storage for the night utilization (3,5 MJ/hr). The solar system production in the summer day exceed 10 MJ/hr, and it’s very clear that the present design of the solar system can cover the energy needs of the Anaerobic digester in the summer period as it’s illustrate in the Fig. 6. On the other hand, the energy stored in the second storage tank which responsible for heating the bio-reactor , is almost equal to the energy production of the solar system in the case of the winter period, that mean the Second storage cover all night utilization without auxiliary energy. 3.3 System evaluation Based on the simulation which is based on the mathematical model mentioned previously, the corresponding solar production, Auxiliary energy, solar irradiance and ambient temperature are listed in the Table 1. Table 1. Average Value of the parameters in each Month Month
Q u (kWh)
Q aux (kWh)
G (W/m2)
Ta (°C)
Q total (kWh)
Solar Fraction (%)
Juan
78
18
446
12
95
62
Feb
81
10
563
13
103
82
Mar
124
0
784
15
124
100
Apr
140
0
968
16
140
100
May
142
0
1076
18
142
100
Jun
190
0
1157
21
190
100
Jul
245
0
1169
22
244
100
Aug
232
0
1062
23
232
100
Sep
174
0
851
21
174
100
Oct
140
0
657
19
140
100
Nov
106
2
501
15
108
95
Dec
78
10
416
13
89
77
The capabilities of the solar system is powerful in the summer month, and good enough in the winter period, that means, the operation time in the summer is obviously longer than that in the winter, e.g, in the summer the solar system work (produce) around 11 hours in the contrary 7 hours in the winter, also that due to the high/low solar irradiance which achieved 1200 W/m2 and between 400-700 W/m2 in the summer and winter respectively in addition of the ambient temperature as illustrate in The table 1. On the other hand, the solar system produces about 1744 kWh/an, which represent 90 % of all energy production. 4. Conclusion A simulation model for a new design of the solar thermal system coupled the anaerobic digestion bioreactor has been carried out with TRNSYS under typical climatic condition in Kenitra in the-west of Morocco and lead to the following conclusion:
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al. / Energy Procedia 00 (2017) 000–000
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The results obtained show that solar energy is a highly desirable alternative for biogas reactor; Solar system occupied 90 % for all heating needs of the bio-digester during year and 70 % in the cold month; Solar system satisfied the mesophilic condition of the digestion anaerobic which the fluctuation of the temperature are between 35-37 °C Acknowledgements This work was supported by the Institute of Research on Solar Energy and Renewable Energies of Morocco (IRESEN) as part of the Inno Project Solar Thermal-Biomass 2014. References [1]
J. Malcolm, Use Of Solar Energy To Heat Anaerobic Digesters Part I Technical And Economic Feasibility Study Part Ii Economic Feasibility Throughout The United States, United States Environmental Protection Agency. (1978). [2] D. Hills, J. Stephens, Solar energy heating of dairy-manure anaerobic digesters, Agricultural Wastes. 2 (1980) 103-118. doi:10.1016/0141 4607(80)90036-0. [3] T. Alkhamis, R. El-khazali, M. Kablan, M. Alhusein, Heating of a biogas reactor using a solar energy system with temperature control unit, Solar Energy. 69 (2000) 239-247. doi:10.1016/s0038-092x(00)00068-2 [4] P. Axaopoulos, P. Panagakis, A. Tsavdaris, D. Georgakakis, Simulation and experimental performance of a solar-heated anaerobic digester, Solar Energy. 70 (2001) 155-164. doi:10.1016/s0038-092x(00)00130-4. [5] G. Kocar, A. Eryasar, An Application of Solar Energy Storage in the Gas: Solar Heated Biogas Plants, Energy Sources, Part A: Recovery, Utilization, And Environmental Effects. 29 (2007) 1513-1520. doi:10.1080/00908310600626598. [6] S. Klein, Calculation of flat-plate collector loss coefficients, Solar Energy. 17 (1975) 79-80. doi:10.1016/0038-092x(75)90020-1 [7] S. Klein, W. Beckman, J. Duffie, A design procedure for solar heating systems, Solar Energy. 18 (1976) 113-127. doi:10.1016/0038 092x(76)90044-x. [8] N. Brian J., Modeling of Solar Storage Tanks, University Of Wisconsin-Madison. (1995). http://digital.library.wisc.edu/1793/7803 (accessed 26 May 2017). [9] J. Burch, L. Magnuson, G. Barker, M. Bullwinkel, Diagnosis of Solar Water Heaters Using Solar Storage Tank Surface Temperature Data, Conference Paper NREL/CP- 550- 45465. (2009). [10] E. Smith, Thermal design of heat exchangers, 1st ed., Wiley, Chichester, 2005. [11] H. El-Mashad, W. van Loon, G. Zeeman, G. Bot, G. Lettinga, Design of A Solar Thermophilic Anaerobic Reactor for Small Farms, Biosystems Engineering. 87 (2004) 345-353. doi:10.1016/j.biosystemseng.2003.11.013.
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Energy (2017) 000–000 402–407 EnergyProcedia Procedia136 00 (2017) www.elsevier.com/locate/procedia
4th International Conference on Energy and Environment Research, ICEER 2017, 17-20 July 2017, Porto, Portugal 15th of International Symposium District Heating and Cooling digester Feasibility The study integrating solaronenergy into anaerobic reactor for improved performances using TRNSYS simulation: Assessing the feasibility of using the heat demand-outdoor application Kenitra Morocco temperature function for a long-term district heat demand forecast a
a
a OUHAMMOU Badraa,*, AGGOUR Mohammed , FRIMANEc Azddinea c a,b,c a b I. Andrić *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Renewable Energy and Environment Laboratory, Faculty of Sciences, Ibn Tofail University. BP. 133, 14 000 Kenitra-Morocco IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
This paper studies the feasibility of the integration of the solar thermal energy in the anaerobic digestion (AD). In this purpose, a Abstract model of the digester reactor (V=70 l) coupled with a new design of the solar water thermal system has been simulation developed using TRNSYS. The heating is carried out by an exchanger inside of the digester. The thermal performance and District behaviors heating networks are commonly addressed in theofliterature as investigated. one of the most effective decreasing the dynamic of the system under a climatic condition Kenitra are It is found thatsolutions the solar for system covers 90 gas consumed emissions by from These systems the require high investments which are returned through %greenhouse of the energy thethe ADbuilding during sector. the year. In addition, results show a daily temperature fluctuation of 0.8the °Cheat in sales. Due and to the changed climate conditions and building renovation policies, heat demand in the future could decrease, the summer 2.3 °C in the winter. prolonging the investment return period. main of this paper isby to Elsevier assess the feasibility of using the heat demand – outdoor temperature function for heat demand ©The 2017 Thescope Authors. Published Ltd. forecast. The district of Alvalade, in Lisbon (Portugal), used as aConference case study.onThe district consisted of 665 Peer-review under responsibility of thelocated scientific committee of the 4thwas International Energy and isEnvironment buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Research. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were comparedAnaerobic with results fromfeasibility a dynamic heatsolar demand model, previously developed and validated by the authors. Keywords: digester; study; heating; solar thermal; TRNSYS The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.scenarios, Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and Solar energy is the most abundant energy resource with the potential to become a major component of a renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the sustainable global energy solution. The most common use of thermal solar energy has been for water heating coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and systems; thisaccuracy use has beendemand commercialized improve the of heat estimations. in many countries in the world. Their applications have increased © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +212658525092. E-mail address: [email protected]; [email protected] Keywords: Heat demand; Forecast; Climate change
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 4th International Conference on Energy and Environment Research. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 4th International Conference on Energy and Environment Research. 10.1016/j.egypro.2017.10.255
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al. / Energy Procedia 00 (2017) 000–000
403
significantly, especially in countries with large solar potential. Bioenergy, on the other hand, is another source of renewable energy that has gained momentum due to several advances in biotechnology during the last decade. In late 1970, U.S. Environmental Protection Agency (U.S. EPA) first developed a solar powered anaerobic digestion system to improve thermal efficiency of the digestion [1]. Since then, several studies were conducted and focused on further improving the thermal efficiency of anaerobic digestion. Hills and Stephens studied the feasibility of using solar energy to heat a mesophilic CSTR (continuous stirred-tank reactor) anaerobic digester [2]. Alkhamis and his colleagues designed and operated a lab scale digester with a fl at plate solar collector as the heater in Jordan [3]. Axaopoulos et al. developed a mathematical model for the simulation of a swine manure reactor heated with solar energy [4]. However, using anaerobic digestion as an energy storage unit to store solar energy has not been comprehensively and thoroughly studied to date [5]. In this paper, the feasibility study of using the solar thermal energy to heat a mesophilic digester was investigated. The proposed designs have been developed by using the TRNSYS environment in order to provide a tool for assessing the thermal performance of the system. The simulations programs were run for a one-year period using hourly climatic data including ambient air temperature, total solar irradiance, wind speed and humidity. The rsults of the feasibility study indicate that the solar system is highly satisfactory (90%) to integrate the solar thermal into the anaerobic digestion. Nomenclature Ac Cp FR G M ms NG Ns Ta Tin Tp Tr Ts Qu UL,j
Aperture area of a single collector module Specific heat of collector fluid Overall collector heat removal efficiency factor Solar irrandiance Masse Flowrate at use conditions Number of the glass cover in the collector Number of identical collectors in series Ambient temperature Temperature input of the exchanger Absorber temperature Bioreactor temperature Storage temeprature tank Energy rate produce by the solar collector The top loss coefficient
2. Description of the system The solar system proposed to heat the anaerobic reactor contains two big hydraulic circuits as illustrate in the Fig. 1. The first one called Primary circuit and other called Secondary circuit; the primary circuit is responsible to produce the energy during the day from the solar radiation. The components of the first circuit are: Flat plate collector, circulator, tank storage and unit of the differential controller. The components of the secondary circuit are: a second tank storage, pumps, and anaerobic digester equipped with a pump for adding the influent (input of the influent). The energy produced by the solar thermal collector was stored in the tank storage (primary hydraulic circuit); in purpose to heat the digester in a continuous manner, a part of the energy used directly to heat the digester, the other quantity of energy was stored in the second tank storage, and when the temperature in first storage was less than temperature inside the reactor the second tank storage start to maintain the heat of the digester. The temperature control in the reactor was carried out through unit of the differential controller. The digester contains two inlets and two outlets, it should note that the heating reactor carried out through an internal heat exchanger as shown in Fig. 2. It should note that the pump 2 and 4 functioned only when the primary circuit was stopped, e.g, the production of the hot fluid was off, that means when the first circuit was heating the digester the
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al./ Energy Procedia 00 (2017) 000–000
404
second storage was out of function and that in the night the second circuit started to heat the digester and automatically the first circuit was off. The base system used electric heating in tank storage 1 to provide all of the energy needed to meet the load requirements. The set point temperature for the upper auxiliary heater was 37 °C with a dead-band temperature of 2 °C. Therefore the heater would turn on when the water that was at the height of the thermostat fell to 35 °C and would remain on until the set point temperature was reached.
Fig. 1. The design of the solar system coupled with the Anaerobic Digester
3. Results and discussion Energy needed by the bioreactor is mainly thermal energy to heat the feed and maintain mesophilic condition of bioreactors. Results of this study indicate that solar energy is very important to use it for heating requirements of bio-digester waste reactors. 3.1 The variation of operating temperature of anaerobic digester The solar radiation which our simulation was run with are illustrate in the Fig. 2 for the cloudy and summer days. The solar radiation for the winter day is very low and poor that mean that solar energy is not enough for heating the anaerobic reactor, in contrary for the summer day it’s same that the solar system (solar energy) can be pass the needs for heating the bio-digester. Winter day
1200
Summer Day
1000
2
G (W/m )
800
600
400
200 6
8
10
12
14
16
18
Time (hr)
Fig. 2. Solar radiation from the site of Kenitra for the two typical days
The Fig. 3 shows the variation of the temperature during the typical summer and winter day. It’s clear that the temperature is very stable and it’s satisfied the mesophilic condition for the anaerobic digestion. The small standard deviation of the manure temperature inside the digester is directly affected by the ambient air temperature, the solar irradiance, the wind speed and humidity. The small daily fluctuation of the manure temperature in the digester and its occurrence result mainly from the operation of the solar collectors and the time swine manure enters the digester as it is illustrated in the Fig. 4. The
OUHAMMOU Badr et al. / Energy Procedia 136 (2017) 402–407 Ouhammou Badr et al. / Energy Procedia 00 (2017) 000–000
405
variations of corresponding operating temperature of anaerobic digester for these time periods are shown in Fig. 9. The maximum temperature fluctuation during this time period is less than 0.8 C, which is too small to influence the microbial activity [12]. 40
Tdigester, summer
35
Tdigester, winter
Temperature (°C)
30 25
Ta, summer
20 15
Ta, winter
10 5 0
5
10
15
20
Time (h)
Fig. 3. Temperature distribution inside the bio-digester and ambient temperature for the tow typical days 4500
Solar radiation Temperature inside the digester Ambient Temprature
4000 3500
30
2500
25
2000 20 1500 15
1000
Temperature (°C)
2
40 35
3000
G (kJ/hr.m )
45
10
500
5
0 740
1480
2220
2960
3700
4440
5180
5920
6660
7400
8140
8880
Time (hr)
Fig. 4. The Variation of the digester temperature and the climatic condition (solar radiation, ambient temperature) during the year.
3.2 Energy demand of bioreactors Energy needed by the bioreactor is mainly thermal energy to heat the feed and maintain mesophilic condition of bioreactors. The amount of the energy was varied with season changes. Energy demand by Digester Energy Produced by Solar collector Energy storage Auxiliary Energy
9000 8000
Solar production
Energy (kJ/hr)
7000 Energy of Storage 1
6000 5000 Energy demand by AD
4000
Energy of storage 2 Auxiliary Energy
3000 2000 1000 0
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
Fig. 5. Digester energy needs, Production solar and Auxiliary for a typical winter day 10000
Energy need by digester Solar production Tank storage 2 Tank storage 1
Solar Production
Energy of the storage 1
8000
Energy of the storage 2
Energy (kJ/hr)
Energy demand by AD
6000
4000
2000
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
Time (hr)
Fig. 6. Digester energy needs, Production solar and Auxiliary for a typical summer day
The Fig. 5 and 6 shows the daily energy demand of the bio-reactor as well as the energy produce by the solar collectors during a typical winter and summer day. Also, the energy stored in the two tank storage.
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The average Energy needs of the bio-reactor are estimated around 2 MJ/hr; the average ambient temperature and the solar irradiances are 9 °C and 500 W/m2 30 °C and 1200 W/m2 respectively for the winter and summer day. It’s obvious that, the solar system as doubt to cover all the energy needs of the bio-digester, for that, and as it’s illustrate in the Fig. 6; in the afternoon the solar system might cover the demand of the anaerobic digestion until the midnight, and the period 24 h to 8 h, the solar system can’t satisfy the energy demand; for this reason, the control differential unit launch the auxiliary energy. In the winter day, the solar system, achieve a production around 8 (MJ/hr); and the first storage Tank (6 MJ/hr); as mentioned before, one part of the energy produce stored in the Tank 1 left to heat the anaerobic digester (direct utilization) and other quantity stored in the second tank storage for the night utilization (3,5 MJ/hr). The solar system production in the summer day exceed 10 MJ/hr, and it’s very clear that the present design of the solar system can cover the energy needs of the Anaerobic digester in the summer period as it’s illustrate in the Fig. 6. On the other hand, the energy stored in the second storage tank which responsible for heating the bio-reactor , is almost equal to the energy production of the solar system in the case of the winter period, that mean the Second storage cover all night utilization without auxiliary energy. 3.3 System evaluation Based on the simulation which is based on the mathematical model mentioned previously, the corresponding solar production, Auxiliary energy, solar irradiance and ambient temperature are listed in the Table 1. Table 1. Average Value of the parameters in each Month Month
Q u (kWh)
Q aux (kWh)
G (W/m2)
Ta (°C)
Q total (kWh)
Solar Fraction (%)
Juan
78
18
446
12
95
62
Feb
81
10
563
13
103
82
Mar
124
0
784
15
124
100
Apr
140
0
968
16
140
100
May
142
0
1076
18
142
100
Jun
190
0
1157
21
190
100
Jul
245
0
1169
22
244
100
Aug
232
0
1062
23
232
100
Sep
174
0
851
21
174
100
Oct
140
0
657
19
140
100
Nov
106
2
501
15
108
95
Dec
78
10
416
13
89
77
The capabilities of the solar system is powerful in the summer month, and good enough in the winter period, that means, the operation time in the summer is obviously longer than that in the winter, e.g, in the summer the solar system work (produce) around 11 hours in the contrary 7 hours in the winter, also that due to the high/low solar irradiance which achieved 1200 W/m2 and between 400-700 W/m2 in the summer and winter respectively in addition of the ambient temperature as illustrate in The table 1. On the other hand, the solar system produces about 1744 kWh/an, which represent 90 % of all energy production. 4. Conclusion A simulation model for a new design of the solar thermal system coupled the anaerobic digestion bioreactor has been carried out with TRNSYS under typical climatic condition in Kenitra in the-west of Morocco and lead to the following conclusion:
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The results obtained show that solar energy is a highly desirable alternative for biogas reactor; Solar system occupied 90 % for all heating needs of the bio-digester during year and 70 % in the cold month; Solar system satisfied the mesophilic condition of the digestion anaerobic which the fluctuation of the temperature are between 35-37 °C Acknowledgements This work was supported by the Institute of Research on Solar Energy and Renewable Energies of Morocco (IRESEN) as part of the Inno Project Solar Thermal-Biomass 2014. References [1]
J. Malcolm, Use Of Solar Energy To Heat Anaerobic Digesters Part I Technical And Economic Feasibility Study Part Ii Economic Feasibility Throughout The United States, United States Environmental Protection Agency. (1978). [2] D. Hills, J. Stephens, Solar energy heating of dairy-manure anaerobic digesters, Agricultural Wastes. 2 (1980) 103-118. doi:10.1016/0141 4607(80)90036-0. [3] T. Alkhamis, R. El-khazali, M. Kablan, M. Alhusein, Heating of a biogas reactor using a solar energy system with temperature control unit, Solar Energy. 69 (2000) 239-247. doi:10.1016/s0038-092x(00)00068-2 [4] P. Axaopoulos, P. Panagakis, A. Tsavdaris, D. Georgakakis, Simulation and experimental performance of a solar-heated anaerobic digester, Solar Energy. 70 (2001) 155-164. doi:10.1016/s0038-092x(00)00130-4. [5] G. Kocar, A. Eryasar, An Application of Solar Energy Storage in the Gas: Solar Heated Biogas Plants, Energy Sources, Part A: Recovery, Utilization, And Environmental Effects. 29 (2007) 1513-1520. doi:10.1080/00908310600626598. [6] S. Klein, Calculation of flat-plate collector loss coefficients, Solar Energy. 17 (1975) 79-80. doi:10.1016/0038-092x(75)90020-1 [7] S. Klein, W. Beckman, J. Duffie, A design procedure for solar heating systems, Solar Energy. 18 (1976) 113-127. doi:10.1016/0038 092x(76)90044-x. [8] N. Brian J., Modeling of Solar Storage Tanks, University Of Wisconsin-Madison. (1995). http://digital.library.wisc.edu/1793/7803 (accessed 26 May 2017). [9] J. Burch, L. Magnuson, G. Barker, M. Bullwinkel, Diagnosis of Solar Water Heaters Using Solar Storage Tank Surface Temperature Data, Conference Paper NREL/CP- 550- 45465. (2009). [10] E. Smith, Thermal design of heat exchangers, 1st ed., Wiley, Chichester, 2005. [11] H. El-Mashad, W. van Loon, G. Zeeman, G. Bot, G. Lettinga, Design of A Solar Thermophilic Anaerobic Reactor for Small Farms, Biosystems Engineering. 87 (2004) 345-353. doi:10.1016/j.biosystemseng.2003.11.013.