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Comparison between a Fixed and a Tracking Solar Heating System for a Thermophilic Anaerobic Digester
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 57 (2014) 2937 – 2945
2013 ISES Solar World Congress
Comparison between a fixed and a tracking solar heating system for a thermophilic anaerobic digester L. M. Gutiérrez-Castroa*, P. Quinto-Dieza, J. G. Barbosa-Saldañaa, L. R. TovarGalvezb, A. Reyes-Leona a
Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Mecánica y Eléctrica, Av. IPN s/n, UPALM Edif. 5, 3er piso, C.P. 07738 México, D.F. Instituto Politecnico Nacional b Centro Interdisciplinario de Investigaciones Sobre Medio Ambiente y Desarrollo, Calle 30 de Junio de 1520 s/n, Barrio La Laguna Ticomán, C.P. 07340 México, D.F. Instituto Politécnico Nacional
Abstract In this paper two alternative solar methods are evaluated for the methane generating system using the municipal solid waste, MSW in urban zones. The analytic method evaluates the use of flat solar collector; in one case they are remaining fixed and the other case they have a solar tracking system. The analytic results shown that the needs of conventional energy to the generation biogas system were minors using the solar tracking system. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
© 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES Selection and/or peer-review under responsibility of ISES.
Keywords: flat solar collectors, solar tracking system, solar fixed system, anaerobic digester, termophilic digestion.
1. Introduction In Mexico City, over 12,600 tons of MSW are generated on a daily basis, all from which are roughly 42% organic waste produced by 8,925,306 of the fixed population along with the floating population getting in the Distrito Federal to come from the surrounding Metropolitan Zone. The average MSW generation is a day 1.44 kg by habitant. Bearing this in mind, the amount of organic fraction of municipal solid waste (OFMSW) is more of 4,800 tons/day. The Mexico City government has shown great interest in new methods of final disposal for the OFMSW in order to back the Bordo Poniente compost plant which currently processes 2,500 tons on a daily basis implying great production expenses [16]. This document was made taking this in mind and as a part of a many of them carried out in The Instituto Politécnico Nacional to develop anaerobic digestion by using OFMSW to propose a more efficient biogas
* Corresponding author. Tel.: +52 (55) 57296000 E-mail address: [email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.329
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generation system. Anaerobic digestion has been used widely in the rural areas and towns of Mexico and his application has a history of more than 100 years [18]. There are many factors affecting the anaerobic digestion process, from which, temperature is one of the most important [19]. The anaerobic digestion under thermophilic conditions (40-60°C), offers many advantages such as: high destruction of pathogens, high rate of metabolism, therefore high rate of specific growing [2] [3]. In addition to that, the thermophilic digestion requires more attention in both the systems start and stabilization Vis a Vis its mesophilic counterpart (25-40°C) and if the temperature is not correctly controlled a high rate of microorganisms during the process could be the outcome [9]. One of the main factors of the process is the requirement of energy to raise the temperature of the mixture inside the digester. Many studies have shown that the use of solar energy reduces built cost of the thermophilic digester [1]. In this research, the effectiveness of the use of a solar tracking device for a better use of the solar energy was analytically proven, as well as to reduce the costs of the fuel to the biogas generation system [15]. This work is part of a series of researches which are currently carried out to characterize and scale a system to generate methane and then achieve efficiency to introduce the anaerobic digestion using MSW in Mexico City. Nomenclature Aex
Heat exchanger area [m2]
Ac
Collector area [m2]
Cp
heat capacity [J/kg-K]
DD
Design of Digesters
f
Energy fraction [%]
FC
Fixed Collector
FR
Collector’s heat transference efficiency factor [%]
F’R
Collector’s efficiency factor [%]
FSC
Flat Solar Collector
G
Solar energy absorber [W/m2]
H
Radiation on a horizontal plane [W/m2]
Hb
Beam radiation on a horizontal plane [W/m2]
Hd
Diffuse radiation on a horizontal plane [W/m2]
HT
Total radiation on a horizontal plane [W/m2]
ഥ ܪ
Solar overall radiation in the solar collector area [W/m2]
L
Heating load required [W]
Laux
Heating load for auxiliary system [W]
m
Mass flow [kg/s]
MGTS
Methane Generation Thermophilic System
MSW
Municipal Solid Waste
N
Number of days of year
L.M. Gutiérrez-Castro et al. / Energy Procedia 57 (2014) 2937 – 2945
OFMSW
Organic Fraction of Municipal Solid Waste
Qt
Useful energy delivered to the solar system [W]
QT
Total heat flows to the methane generation system [W]
R
Ratio of total radiation
Rb
Ratio of beam radiation
SHS
Solar Heating System
UL
Overall heat transfer coefficient of solar collectors [W/m2-K]
Ta
Ambient temperature [°C]
TC
Tracking Collector
Tref
Reference temperature [°C]
X
The relation between total energy losses from the solar collector to the environment and the total heat load
Y
The relation between absorbed energy for the absorbed plat of the solar collector and the total heat load
Δt
Temperature difference within the digester [°C]
τα
Transmittance – absorptance product of the collector
߬ҧߙത
Monthly overall of transmittance – absorptance product of the collector
ߠ
Incidence angle
ߜ
Declination angle
߶
Latitude
ߚ
Tilt angle
߱
Hour angle
ߛ
Azimuth angle
ߠݖ
Solar zenith angle
2. Methodology The following methodology to select the solar system to heat water begins by dimensioning the methane generation thermophilic system (MGTS), to determine the amount of energy required to raise the temperature of the mixture and keep it in 55°C. 2.1. Solar radiation data Actual data about solar radiation were gotten measured in the zone of interest. They were used to select the heating solar system. The table 1 shows the results of the monthly horizontal solar radiation. To
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select the heating solar system, the temperature used in the thermophilic process are taking into account and since it is considered as “low”, they could be managed with flat solar collectors. Table 1. Data on total radiation measured in the zone in 2011 Month
H [MJ/m2-month]
Monthly average temperature (°C)
January
20.457
14.3
February
24.578
15.7
Marchs
25.217
17.6
April
25.510
19.8
May
25.758
20.1
June
25.605
19.8
July
24.725
18.5
August
24.606
19.3
September
24.154
18.3
October
22.171
18.4
November
24.231
15.6
December
20.375
15.4
2.2. Determination of the Energy requirements of the thermophilic anaerobic digester As a calculation tool to dimensioning the MGTS, a program in MATLAB 7.8.0 (R2006a) was carried out it based on other studies [8]. This program called “design of digesters” (DD) determines the digester dimensions and heat exchanger dimensions, taking into account the amount of the fraction of MSW, this data are: amount of the energy required to raise the temperature, QT, loss of energy, QL, heat exchanger area, Aex, and the percentage of methane theoretical generation in normal conditions, CH4. To establish the dimensions of both the anaerobic digester and the energy requirements by carrying out both a thermodynamic analysis as well as other of heat transference in a stationary state [6]. The DD program calculates the dimensions of the digester, the required heating load to heat the mixture between the range of thermophilic temperatures (40 - 55°C), the parameters of the pumping equipment, the dimensions of the heat exchanger and the calculus of biogas generation in normal conditions of pressure and temperature. The best proven working temperature for thermophilic anaerobic digestion is 55°C [14], this value is taken for the analysis. When the heat flow is obtained, the properties of the system are determined in order to dimension the heat transfer system of the mixture. In this case the heating is released by flat plate solar collectors. For the calculations, the convection and conduction losses of the digester were taken into account. The plus of the quantities of heat flows gives to result the overall ), which is provided to the solar heating system by flat plate collectors heating load (overall heat flow Q [7]. The equation which determines the flow of heat require from the system is shown in equation (1):
QT
m Cp'T
(1)
2.3. Determination of the required parameters to tracking solar system The heating systems with solar tracking systems are classified according to their movements. The rotation can consider just one axis (which can have any orientation but, in the practice, the more often used is the horizontal east-west, horizontal north-south, vertical or parallel to the axis of the earth), or it
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could be according to two axis [12], [13]. Some solar collectors following the sun in determining the forms to minimize the incidence angle of direct radiation over their surfaces and these maximize the incident solar radiation [14], for this reason the incidence angle of direct radiation and the surface of the azimuth angle are necessary to determine for this collectors. The incidence angle of direct radiation over a lean surface and the zenith solar angle can be calculated by using the next equations described by Duffie y Beckman [5], [17]:
cos T
sin G sin I cos E sin G cos I sin E cos G
(2)
cos G cos I cos E cos Z cos G sin I sin E cos J cos Z
cos G sin E sin J sin Z cos T z
sin G sin I cos G cos I cos Z
(3)
The analysis of tracking solar system was made from a plane rotated about a horizontal east-west axis with tracking north-south; the angle of incidence is minimized when the surface azimuth and solar azimuth angles are equal. To determine the variation of the incidence angle, which will be used to define the direct radiation on the horizontal surface is given by equation (4). For this analysis slope angle was changed for each month. The equations which define this tracking was showing below [12]:
cos T
sin 2 G cos 2 G cos 2 Z
(4)
The slope of this surface is given by:
E
I G
(5)
Using a similar method to Hottel, the rate of beam radiation, Rb, on a lean surface or over a horizontal surface can be determined, as follows [13]:
Rb
cos T cos T z
(6)
To determine the total solar radiation on a tilted surface, based on solar radiation data on a horizontal plane, it is necessary to define the coefficient R, which is the ratio of total radiation on the tilted surfaces in relation to radiation total surfaces in the horizontal plane it is shown in the following equations [11],[14]:
R
HT H
(7)
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Therefore:
HT
R H
(8)
It can also be expressed as follows:
R
Hb H Rb d Rd H H
(9)
After have used the equations into the software the behavior of solar energy in the SHS was analyzed.
2.4. Determination of the energy fraction, “f” To determine the “f” fraction to fixed and tracking system was used the program called “SOLAR” [4]. This program determines the “f” fraction using different storage capacities ranging from 75 to 300 l/m2 in intervals 75 l/m2 and collecting areas varying from 10 to 100 m2 using the angles and solar radiation data. The results were compared with actual values of beam total radiation on the zone. Based on the energy’s requirements of the thermophilic process to generate biogas, the SHS with flat plate solar collectors were designed to apply to the “SOLAR” program. This program was designed for flat plate solar collectors; it gives the solar collection area and storage of water per area of collection. The solar collection area depends on the monthly radiation, on the installation zone of the SHS and also on the monthly heating loads needed. The importance of the SHS design is heat the mixture into the digester at 55 °C, so that several compounds are involved in the process to achieve it [1]. The method used in both design and selecting is the “f” design method. The “f” chart-method or “f” design method is used to estimate the annual thermic performance of the heating solar system activity by using a working fluid, by means of the calculus of the “f” energy fraction, supplied by the sun to meet the required heating load. The main design variable is the uptake area of solar energy. Among the secondary variables are: the type of collector, the storage capacity, the flow rate and the size of the heat exchangers. The “f” fraction is the relation between the useful energy delivered to the solar system (Qt), which is the difference between the energy of the system only using conventional fuel (Laux) and the heating load required by the system (L) [12], [6]. For a given month, the reduction of the “f” of the supplied solar energy is shown in (10).
f
L Laux L
QT L
(10)
The calculus of the “f” fraction is in the function of two dimensionless parameters: X y Y. The procedure Klein [13], describes to determine these two variables are shown in the equations below [5]: 't
X
FR Ac UL(Tref Ta )dt L ³
FR Ac UL(Tref Ta )dt L
(11)
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And
Y
FR Ac L
't
³ G(WD )dt
FR Ac H (W D ) N L
(12)
3. Results The data on solar radiation shown in table 1, were considered to determine the solar energy fraction (“f”), which it will be used to characterize the SHS. The average monthly heating load for the MGTS is about 6.8 x 109 J/month. To determine the parameters of the temperature of the exhaust manifold is 60 °C. The digester is 1.8 m3 with a height of 1.45 m. The digester function is continuous and is fed with OFMSW; its heat exchanger area is 0.35 m2. This data were introduced on the SOLAR program to obtain the behavior of “f” fraction. Also, obtain the energy gain for a tracking collector (TC) about a one axis, compared to a fixed collector (FC). The Fig 1 shows the behavior of fixed and tracking system about an east-west axis with slope variable.
Tracking system
Fixed system
Fig.1. Behaviour of fixed and tracking system to thermophilic anaerobic digester
In the Fig. 1 is shown, how the energy fraction gain in tracking system is better than fixed systems. Also, the figure 1 was made considering a storage tank of 75 l/m2 of collection area and 50 m2 of collector area. The above was considered to compare the area reduction that will be installed and this way makes a cost comparison. The analysis to determine the percent of energy gain [%], between to install a fixed collector or a collector with N-S tracking was made considering the slope variation about tilt yearly. The slope was change monthly which allowed to observer the behaviour yearlong. The Fig 2 shows the behavior of energy fraction monthly to 10 – 50 m2 of collection area, and the energy gain between a tracking and fixed solar system to the digester designed. With the analysis shown before the reduced area of solar collectors to the anaerobic digester was determinate which helps to save costs of the facility. In the table 2 shows the characteristics of the digester.
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Fig.2. Behaviour of energy fraction about collection area and storage of water. Table 2. Description of the digester Description Wet anaerobic digestion process Volume of the digester [m3]
2
Organic fraction of MSW [kg]
60
Heating load of the system [W]
2645.10
Δt [C°]
14.7
Energy fraction, “f” [%]
80 FC – 96 TC*
Collection area used [m2]
25 FC – 18 TC*
2
Storage tank [l/m ]
150
Biogas yield [m3]
0.56
*These values are dependent of the tracking or fixed solar system when all the load will be provided with solar collectors system.
4. Conclusions The results of this analysis have shown the behavior between the use fixed collectors or tracking collectors of maintenance the temperature into the thermophilic anaerobic digester at 55°C and have a better atmosphere into the digester to the microorganism generate the biogas in a minor time. In the Fig. 1 is shown how the behavior is about the solar energy monthly. Therefore, the tracking system is better in summer than fixed systems. The Fig. 2 has shown how the fixed system has less energy gain than the tracking system and it can be seen. The energy fraction is greatly about the area collection. The bigger the collection area, the higher the “f” energy fraction. How it can be seen in Fig.1 the energy gain is about 8% of the tracking system compared with the fixed system about collection area. This helps in to have and more small facilities to the biogas generation. The dotted line shows the area needed to obtain an energy gain of 80% which is about 25 m2 installed. The area is reduced by the use tracking system to 20 m2. The collection area reduced depends on storage of water. The goal of analytic analysis to achieve greater energy saves to the methane generation was achieved. If the tracking system is installed to MGTS the built costs save to be about 20 to 25% more than the fixed system (fig. 2). In other words the tracking solar system was evaluated to can see if there is more saving in building costs and propose it in the future biogas plants in Mexico City using the organic fraction of
L.M. Gutiérrez-Castro et al. / Energy Procedia 57 (2014) 2937 – 2945
the municipal solid waste. Another advantage of this work is the high values of solar radiation in Mexico, City which are shown in the table 1 and form part of the viability of this study. The main goal is to get high efficiency of biogas yield, if the mixture is heated into the digester to obtain a conversion process accelerated, also to reduce the volume of the digester and its facilities. The biogas generation system has a heating auxiliary system for cloudy days. References [1] Andreas C.Y, Manariotis I.D, Constantinos V.C. Design and analysis of a solar reactor for anaerobic wastewater treatment. Bioresource Technology, 2008; pp. 7742–7749. [2] Ahring B. Status on science and application of thermophilic anaerobic digestion. Wat. Sci. Tchno., 1994; 30(12), 241-249. [3] Axaopoulos P, et al. Simulation and experimental performance of a solar heated anaerobic digester. Solar Energy, 2001. Vol 70, No 2, pp 155-164 [4] Barbosa J. G. (1999). Método de diseño de sistemas solares para calentamiento de agua. Tesis de Maestría. Instituto Politécnico Nacional, SEPI. ESIME. 151p. [5] Duffie J. A, Beckman W.A. Solar Energy Thermal Processes. Ed. Wiley Interciencie, Publication. USA 1974. [6] Duffie J. A, Beckman W.A. Solar Engineering of Thermal Processes. Ed. Wiley Interciencie, Publication. USA 1980. [7] Garrison J.D. A program for calculation of solar energy collection by fixed and tracking collectors. Solar Energy, 2002. Vol 73, pp. 241-255. [8] Gutiérrez L. M. (2011). Diseño de un Sistema solar para suministrar energía a un digestor. Tesis de Maestría. Instituto Politécnico Nacional, SEPI. ESIME. 127p. [9] Hamed M, et al. Design of a Solar Thermophilic Anaerobic Reactor for Small Farms. Biosystems Engineering, 2004, pp. 345–353. [10] Hamed M, et al. Effect of temperature and temperature fluctuation on thermophilic anaerobic digestion of cattle manure. Bioresource Technology, 2004; 95, pp. 191-201. [11] Hilkiah-Igoni A, et al. Designs of anaerobic digesters for producing biogas from municipal solid-waste. Applied Energy, 2008; 85, pp. 430-438. [12] Hottel A.C. A simple model for estimating the transmittance of direct solar radiation trough clear atmospheres. Solar Energy, 1976; 18, pp. 129-134. [13] Klein S.A, Beckman W.A, Duffie J.A. A Design Procedure for Solar Heating Systems. Solar Energy, 1976. Vol 18, pp 113127. [14] Mata-Álvarez, J. (2002). Anaerobic digestion of the organic fraction of municipal solid waste: a perspective In: Biomethanization of Organic fraction of municipal solid wastes. Editor: J. Mata-Álvarez; IWA Publishing Company, 91-110. [15] Subramanyan S. Use of solar heat to upgrade biogas plan performance. Energy Convers. Mgmt, 1989, vol 29, p. [16] Saver T, Al-Qaraghuli A, Kazmerski L. A comprehensive review of the impact of dust on the used of solar energy: History, investi gations, results, literature and mitigations approaches. Renewable and Sustainable Energy Reviews, 2013; 22, 698-733. [17] Tomson T. Discrete two positional traking of solar collectors. Renewable Energy, 2008; 33, 400-405. [18] Tovar L.R, et al. Viabilidad de la capacidad del área actual de la planta de composta, así como, su capacidad de operación y la viabilidad de la inclusion del área de la planta de selección a la planta de composta. CIIEMAD, IPN. 2012. [19] Zhou Fuchun, et al. Research achievements and application in anaerobic treatment of organic solid wastes – A review. Chinese Journal of Geochemistry. 2006. Vol. 25 No. 2, pp 178-181.
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ScienceDirect Energy Procedia 57 (2014) 2937 – 2945
2013 ISES Solar World Congress
Comparison between a fixed and a tracking solar heating system for a thermophilic anaerobic digester L. M. Gutiérrez-Castroa*, P. Quinto-Dieza, J. G. Barbosa-Saldañaa, L. R. TovarGalvezb, A. Reyes-Leona a
Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Mecánica y Eléctrica, Av. IPN s/n, UPALM Edif. 5, 3er piso, C.P. 07738 México, D.F. Instituto Politecnico Nacional b Centro Interdisciplinario de Investigaciones Sobre Medio Ambiente y Desarrollo, Calle 30 de Junio de 1520 s/n, Barrio La Laguna Ticomán, C.P. 07340 México, D.F. Instituto Politécnico Nacional
Abstract In this paper two alternative solar methods are evaluated for the methane generating system using the municipal solid waste, MSW in urban zones. The analytic method evaluates the use of flat solar collector; in one case they are remaining fixed and the other case they have a solar tracking system. The analytic results shown that the needs of conventional energy to the generation biogas system were minors using the solar tracking system. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
© 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES Selection and/or peer-review under responsibility of ISES.
Keywords: flat solar collectors, solar tracking system, solar fixed system, anaerobic digester, termophilic digestion.
1. Introduction In Mexico City, over 12,600 tons of MSW are generated on a daily basis, all from which are roughly 42% organic waste produced by 8,925,306 of the fixed population along with the floating population getting in the Distrito Federal to come from the surrounding Metropolitan Zone. The average MSW generation is a day 1.44 kg by habitant. Bearing this in mind, the amount of organic fraction of municipal solid waste (OFMSW) is more of 4,800 tons/day. The Mexico City government has shown great interest in new methods of final disposal for the OFMSW in order to back the Bordo Poniente compost plant which currently processes 2,500 tons on a daily basis implying great production expenses [16]. This document was made taking this in mind and as a part of a many of them carried out in The Instituto Politécnico Nacional to develop anaerobic digestion by using OFMSW to propose a more efficient biogas
* Corresponding author. Tel.: +52 (55) 57296000 E-mail address: [email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.329
2938
L.M. Gutiérrez-Castro et al. / Energy Procedia 57 (2014) 2937 – 2945
generation system. Anaerobic digestion has been used widely in the rural areas and towns of Mexico and his application has a history of more than 100 years [18]. There are many factors affecting the anaerobic digestion process, from which, temperature is one of the most important [19]. The anaerobic digestion under thermophilic conditions (40-60°C), offers many advantages such as: high destruction of pathogens, high rate of metabolism, therefore high rate of specific growing [2] [3]. In addition to that, the thermophilic digestion requires more attention in both the systems start and stabilization Vis a Vis its mesophilic counterpart (25-40°C) and if the temperature is not correctly controlled a high rate of microorganisms during the process could be the outcome [9]. One of the main factors of the process is the requirement of energy to raise the temperature of the mixture inside the digester. Many studies have shown that the use of solar energy reduces built cost of the thermophilic digester [1]. In this research, the effectiveness of the use of a solar tracking device for a better use of the solar energy was analytically proven, as well as to reduce the costs of the fuel to the biogas generation system [15]. This work is part of a series of researches which are currently carried out to characterize and scale a system to generate methane and then achieve efficiency to introduce the anaerobic digestion using MSW in Mexico City. Nomenclature Aex
Heat exchanger area [m2]
Ac
Collector area [m2]
Cp
heat capacity [J/kg-K]
DD
Design of Digesters
f
Energy fraction [%]
FC
Fixed Collector
FR
Collector’s heat transference efficiency factor [%]
F’R
Collector’s efficiency factor [%]
FSC
Flat Solar Collector
G
Solar energy absorber [W/m2]
H
Radiation on a horizontal plane [W/m2]
Hb
Beam radiation on a horizontal plane [W/m2]
Hd
Diffuse radiation on a horizontal plane [W/m2]
HT
Total radiation on a horizontal plane [W/m2]
ഥ ܪ
Solar overall radiation in the solar collector area [W/m2]
L
Heating load required [W]
Laux
Heating load for auxiliary system [W]
m
Mass flow [kg/s]
MGTS
Methane Generation Thermophilic System
MSW
Municipal Solid Waste
N
Number of days of year
L.M. Gutiérrez-Castro et al. / Energy Procedia 57 (2014) 2937 – 2945
OFMSW
Organic Fraction of Municipal Solid Waste
Qt
Useful energy delivered to the solar system [W]
QT
Total heat flows to the methane generation system [W]
R
Ratio of total radiation
Rb
Ratio of beam radiation
SHS
Solar Heating System
UL
Overall heat transfer coefficient of solar collectors [W/m2-K]
Ta
Ambient temperature [°C]
TC
Tracking Collector
Tref
Reference temperature [°C]
X
The relation between total energy losses from the solar collector to the environment and the total heat load
Y
The relation between absorbed energy for the absorbed plat of the solar collector and the total heat load
Δt
Temperature difference within the digester [°C]
τα
Transmittance – absorptance product of the collector
߬ҧߙത
Monthly overall of transmittance – absorptance product of the collector
ߠ
Incidence angle
ߜ
Declination angle
߶
Latitude
ߚ
Tilt angle
߱
Hour angle
ߛ
Azimuth angle
ߠݖ
Solar zenith angle
2. Methodology The following methodology to select the solar system to heat water begins by dimensioning the methane generation thermophilic system (MGTS), to determine the amount of energy required to raise the temperature of the mixture and keep it in 55°C. 2.1. Solar radiation data Actual data about solar radiation were gotten measured in the zone of interest. They were used to select the heating solar system. The table 1 shows the results of the monthly horizontal solar radiation. To
2939
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L.M. Gutiérrez-Castro et al. / Energy Procedia 57 (2014) 2937 – 2945
select the heating solar system, the temperature used in the thermophilic process are taking into account and since it is considered as “low”, they could be managed with flat solar collectors. Table 1. Data on total radiation measured in the zone in 2011 Month
H [MJ/m2-month]
Monthly average temperature (°C)
January
20.457
14.3
February
24.578
15.7
Marchs
25.217
17.6
April
25.510
19.8
May
25.758
20.1
June
25.605
19.8
July
24.725
18.5
August
24.606
19.3
September
24.154
18.3
October
22.171
18.4
November
24.231
15.6
December
20.375
15.4
2.2. Determination of the Energy requirements of the thermophilic anaerobic digester As a calculation tool to dimensioning the MGTS, a program in MATLAB 7.8.0 (R2006a) was carried out it based on other studies [8]. This program called “design of digesters” (DD) determines the digester dimensions and heat exchanger dimensions, taking into account the amount of the fraction of MSW, this data are: amount of the energy required to raise the temperature, QT, loss of energy, QL, heat exchanger area, Aex, and the percentage of methane theoretical generation in normal conditions, CH4. To establish the dimensions of both the anaerobic digester and the energy requirements by carrying out both a thermodynamic analysis as well as other of heat transference in a stationary state [6]. The DD program calculates the dimensions of the digester, the required heating load to heat the mixture between the range of thermophilic temperatures (40 - 55°C), the parameters of the pumping equipment, the dimensions of the heat exchanger and the calculus of biogas generation in normal conditions of pressure and temperature. The best proven working temperature for thermophilic anaerobic digestion is 55°C [14], this value is taken for the analysis. When the heat flow is obtained, the properties of the system are determined in order to dimension the heat transfer system of the mixture. In this case the heating is released by flat plate solar collectors. For the calculations, the convection and conduction losses of the digester were taken into account. The plus of the quantities of heat flows gives to result the overall ), which is provided to the solar heating system by flat plate collectors heating load (overall heat flow Q [7]. The equation which determines the flow of heat require from the system is shown in equation (1):
QT
m Cp'T
(1)
2.3. Determination of the required parameters to tracking solar system The heating systems with solar tracking systems are classified according to their movements. The rotation can consider just one axis (which can have any orientation but, in the practice, the more often used is the horizontal east-west, horizontal north-south, vertical or parallel to the axis of the earth), or it
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could be according to two axis [12], [13]. Some solar collectors following the sun in determining the forms to minimize the incidence angle of direct radiation over their surfaces and these maximize the incident solar radiation [14], for this reason the incidence angle of direct radiation and the surface of the azimuth angle are necessary to determine for this collectors. The incidence angle of direct radiation over a lean surface and the zenith solar angle can be calculated by using the next equations described by Duffie y Beckman [5], [17]:
cos T
sin G sin I cos E sin G cos I sin E cos G
(2)
cos G cos I cos E cos Z cos G sin I sin E cos J cos Z
cos G sin E sin J sin Z cos T z
sin G sin I cos G cos I cos Z
(3)
The analysis of tracking solar system was made from a plane rotated about a horizontal east-west axis with tracking north-south; the angle of incidence is minimized when the surface azimuth and solar azimuth angles are equal. To determine the variation of the incidence angle, which will be used to define the direct radiation on the horizontal surface is given by equation (4). For this analysis slope angle was changed for each month. The equations which define this tracking was showing below [12]:
cos T
sin 2 G cos 2 G cos 2 Z
(4)
The slope of this surface is given by:
E
I G
(5)
Using a similar method to Hottel, the rate of beam radiation, Rb, on a lean surface or over a horizontal surface can be determined, as follows [13]:
Rb
cos T cos T z
(6)
To determine the total solar radiation on a tilted surface, based on solar radiation data on a horizontal plane, it is necessary to define the coefficient R, which is the ratio of total radiation on the tilted surfaces in relation to radiation total surfaces in the horizontal plane it is shown in the following equations [11],[14]:
R
HT H
(7)
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Therefore:
HT
R H
(8)
It can also be expressed as follows:
R
Hb H Rb d Rd H H
(9)
After have used the equations into the software the behavior of solar energy in the SHS was analyzed.
2.4. Determination of the energy fraction, “f” To determine the “f” fraction to fixed and tracking system was used the program called “SOLAR” [4]. This program determines the “f” fraction using different storage capacities ranging from 75 to 300 l/m2 in intervals 75 l/m2 and collecting areas varying from 10 to 100 m2 using the angles and solar radiation data. The results were compared with actual values of beam total radiation on the zone. Based on the energy’s requirements of the thermophilic process to generate biogas, the SHS with flat plate solar collectors were designed to apply to the “SOLAR” program. This program was designed for flat plate solar collectors; it gives the solar collection area and storage of water per area of collection. The solar collection area depends on the monthly radiation, on the installation zone of the SHS and also on the monthly heating loads needed. The importance of the SHS design is heat the mixture into the digester at 55 °C, so that several compounds are involved in the process to achieve it [1]. The method used in both design and selecting is the “f” design method. The “f” chart-method or “f” design method is used to estimate the annual thermic performance of the heating solar system activity by using a working fluid, by means of the calculus of the “f” energy fraction, supplied by the sun to meet the required heating load. The main design variable is the uptake area of solar energy. Among the secondary variables are: the type of collector, the storage capacity, the flow rate and the size of the heat exchangers. The “f” fraction is the relation between the useful energy delivered to the solar system (Qt), which is the difference between the energy of the system only using conventional fuel (Laux) and the heating load required by the system (L) [12], [6]. For a given month, the reduction of the “f” of the supplied solar energy is shown in (10).
f
L Laux L
QT L
(10)
The calculus of the “f” fraction is in the function of two dimensionless parameters: X y Y. The procedure Klein [13], describes to determine these two variables are shown in the equations below [5]: 't
X
FR Ac UL(Tref Ta )dt L ³
FR Ac UL(Tref Ta )dt L
(11)
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And
Y
FR Ac L
't
³ G(WD )dt
FR Ac H (W D ) N L
(12)
3. Results The data on solar radiation shown in table 1, were considered to determine the solar energy fraction (“f”), which it will be used to characterize the SHS. The average monthly heating load for the MGTS is about 6.8 x 109 J/month. To determine the parameters of the temperature of the exhaust manifold is 60 °C. The digester is 1.8 m3 with a height of 1.45 m. The digester function is continuous and is fed with OFMSW; its heat exchanger area is 0.35 m2. This data were introduced on the SOLAR program to obtain the behavior of “f” fraction. Also, obtain the energy gain for a tracking collector (TC) about a one axis, compared to a fixed collector (FC). The Fig 1 shows the behavior of fixed and tracking system about an east-west axis with slope variable.
Tracking system
Fixed system
Fig.1. Behaviour of fixed and tracking system to thermophilic anaerobic digester
In the Fig. 1 is shown, how the energy fraction gain in tracking system is better than fixed systems. Also, the figure 1 was made considering a storage tank of 75 l/m2 of collection area and 50 m2 of collector area. The above was considered to compare the area reduction that will be installed and this way makes a cost comparison. The analysis to determine the percent of energy gain [%], between to install a fixed collector or a collector with N-S tracking was made considering the slope variation about tilt yearly. The slope was change monthly which allowed to observer the behaviour yearlong. The Fig 2 shows the behavior of energy fraction monthly to 10 – 50 m2 of collection area, and the energy gain between a tracking and fixed solar system to the digester designed. With the analysis shown before the reduced area of solar collectors to the anaerobic digester was determinate which helps to save costs of the facility. In the table 2 shows the characteristics of the digester.
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Fig.2. Behaviour of energy fraction about collection area and storage of water. Table 2. Description of the digester Description Wet anaerobic digestion process Volume of the digester [m3]
2
Organic fraction of MSW [kg]
60
Heating load of the system [W]
2645.10
Δt [C°]
14.7
Energy fraction, “f” [%]
80 FC – 96 TC*
Collection area used [m2]
25 FC – 18 TC*
2
Storage tank [l/m ]
150
Biogas yield [m3]
0.56
*These values are dependent of the tracking or fixed solar system when all the load will be provided with solar collectors system.
4. Conclusions The results of this analysis have shown the behavior between the use fixed collectors or tracking collectors of maintenance the temperature into the thermophilic anaerobic digester at 55°C and have a better atmosphere into the digester to the microorganism generate the biogas in a minor time. In the Fig. 1 is shown how the behavior is about the solar energy monthly. Therefore, the tracking system is better in summer than fixed systems. The Fig. 2 has shown how the fixed system has less energy gain than the tracking system and it can be seen. The energy fraction is greatly about the area collection. The bigger the collection area, the higher the “f” energy fraction. How it can be seen in Fig.1 the energy gain is about 8% of the tracking system compared with the fixed system about collection area. This helps in to have and more small facilities to the biogas generation. The dotted line shows the area needed to obtain an energy gain of 80% which is about 25 m2 installed. The area is reduced by the use tracking system to 20 m2. The collection area reduced depends on storage of water. The goal of analytic analysis to achieve greater energy saves to the methane generation was achieved. If the tracking system is installed to MGTS the built costs save to be about 20 to 25% more than the fixed system (fig. 2). In other words the tracking solar system was evaluated to can see if there is more saving in building costs and propose it in the future biogas plants in Mexico City using the organic fraction of
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the municipal solid waste. Another advantage of this work is the high values of solar radiation in Mexico, City which are shown in the table 1 and form part of the viability of this study. The main goal is to get high efficiency of biogas yield, if the mixture is heated into the digester to obtain a conversion process accelerated, also to reduce the volume of the digester and its facilities. The biogas generation system has a heating auxiliary system for cloudy days. References [1] Andreas C.Y, Manariotis I.D, Constantinos V.C. Design and analysis of a solar reactor for anaerobic wastewater treatment. Bioresource Technology, 2008; pp. 7742–7749. [2] Ahring B. Status on science and application of thermophilic anaerobic digestion. Wat. Sci. Tchno., 1994; 30(12), 241-249. [3] Axaopoulos P, et al. Simulation and experimental performance of a solar heated anaerobic digester. Solar Energy, 2001. Vol 70, No 2, pp 155-164 [4] Barbosa J. G. (1999). Método de diseño de sistemas solares para calentamiento de agua. Tesis de Maestría. Instituto Politécnico Nacional, SEPI. ESIME. 151p. [5] Duffie J. A, Beckman W.A. Solar Energy Thermal Processes. Ed. Wiley Interciencie, Publication. USA 1974. [6] Duffie J. A, Beckman W.A. Solar Engineering of Thermal Processes. Ed. Wiley Interciencie, Publication. USA 1980. [7] Garrison J.D. A program for calculation of solar energy collection by fixed and tracking collectors. Solar Energy, 2002. Vol 73, pp. 241-255. [8] Gutiérrez L. M. (2011). Diseño de un Sistema solar para suministrar energía a un digestor. Tesis de Maestría. Instituto Politécnico Nacional, SEPI. ESIME. 127p. [9] Hamed M, et al. Design of a Solar Thermophilic Anaerobic Reactor for Small Farms. Biosystems Engineering, 2004, pp. 345–353. [10] Hamed M, et al. Effect of temperature and temperature fluctuation on thermophilic anaerobic digestion of cattle manure. Bioresource Technology, 2004; 95, pp. 191-201. [11] Hilkiah-Igoni A, et al. Designs of anaerobic digesters for producing biogas from municipal solid-waste. Applied Energy, 2008; 85, pp. 430-438. [12] Hottel A.C. A simple model for estimating the transmittance of direct solar radiation trough clear atmospheres. Solar Energy, 1976; 18, pp. 129-134. [13] Klein S.A, Beckman W.A, Duffie J.A. A Design Procedure for Solar Heating Systems. Solar Energy, 1976. Vol 18, pp 113127. [14] Mata-Álvarez, J. (2002). Anaerobic digestion of the organic fraction of municipal solid waste: a perspective In: Biomethanization of Organic fraction of municipal solid wastes. Editor: J. Mata-Álvarez; IWA Publishing Company, 91-110. [15] Subramanyan S. Use of solar heat to upgrade biogas plan performance. Energy Convers. Mgmt, 1989, vol 29, p. [16] Saver T, Al-Qaraghuli A, Kazmerski L. A comprehensive review of the impact of dust on the used of solar energy: History, investi gations, results, literature and mitigations approaches. Renewable and Sustainable Energy Reviews, 2013; 22, 698-733. [17] Tomson T. Discrete two positional traking of solar collectors. Renewable Energy, 2008; 33, 400-405. [18] Tovar L.R, et al. Viabilidad de la capacidad del área actual de la planta de composta, así como, su capacidad de operación y la viabilidad de la inclusion del área de la planta de selección a la planta de composta. CIIEMAD, IPN. 2012. [19] Zhou Fuchun, et al. Research achievements and application in anaerobic treatment of organic solid wastes – A review. Chinese Journal of Geochemistry. 2006. Vol. 25 No. 2, pp 178-181.
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