Active Heating of Floating Type Dome Biogas Plant

DOI : 10.17577/IJERTV3IS090450

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Active Heating of Floating Type Dome Biogas Plant

G. N. Tiwari1, Poonam Joshi1, Ravi Agrihari1

1Centre for Energy Studies,

Indian Institute of Technology Delhi, Hauz Khas, New Delhi 1100 16, India

Ibrahim M. Al-Helal2

2Department of Agricultural Engineering College of Food & Agricultural Sciences,

King Saud University,

      1. ox 2460, Riyadh 11451, Saudi Arabia

        Abstract – In this paper, an attempt has been made to derive an expression for the slurry temperature of flat plate collectors integrated biogas plant. The flat plate collectors are connected in series and parallel combination to optimise the number of collectors to be in series (N) and parallel (m) for optimum slurry temperature. Effect of mass flow rate ( m f ),

        length of heat exchanger (L), heat capacity of slurry (Ms Cs) and number of flat plate collectors (FPCs) on slurry temperature have been studies for climatic condition of Srinagar, India by using antifreeze liquid. It has been observed that the optimum slurry temperature (~ ) has been observed for N=8 (series) and m=5 (parallel) for 40 number of flat plate collectors. Exergy of active biogas plant has also been carried out.

        Keywords:Biogas, solar heating, flat plate collectors


          Biogas is a mixture of methane (CH4) 5070% and carbon dioxide (CO2) 3050%. It has a calorific value of 2124 MJ/m3 and it is produced at 37 in the absence of oxygen from slurry obtained from 50% dung and 50% water. It is clean energy and environmental friendly which can be used for cooking [1-5], lighting [5-6], motor vehicles and small scale industries [7-10] to meet local energy demand. The temperature of slurry depends on local climatic conditions namely solar intensity and ambient air temperature. The slurry in digester requires to be heated for harsh cold climatic condition to obtain the optimum slurry temperature. Basically, there are two type of biogas plant namely fixed dome and floating dome type biogas plant. In both cases, the slurry in digester is heated either by active method or by hot charging for harsh cold climatic condition. In an active method, a combination of flat plate collectors (FPCs) can be integrated through a coil type heat exchanger placed inside digester of floating/fixed type.

          volume 6 ton for hot charging the slurry to ferment the slurry inside digester, Dong and Lu [15]. It has also been found that 2540 m3 biogas is produced with increase of 11.2%. They also observed that there were 14.3% increase of pig manure energy transformation efficiency.

          In this paper, we have analysed the slurry temperature of active biogas plant to optimise the number of flat plate collectors to be in series and parallel for a given total number of flat plate collector and heat capacity of slurry. Exergy analysis of active biogas plant has also been carried out.


          In order to write energy balance, the following assumptions have been made:

          • The floating dome type bio-gas plant is considered for thermal modelling

          • The biogas plant is in quasi- steady state condition

          • No stratification along the depth of the slurry in digester due to forced mode and the gas column due to

            low heat capacity

          • Thermal heat capacity (MsCs) of the biogas plant materials are neglected.

          Referring to Fig. 1, the energy balance equations during sunshine hours have been formulated as follows:


          Series of

          The number of flat plat collectors (FPCs) depends on heat capacity of slurry with local climatic condition. Further, series and parallel combination of collectors should also be optimized accordingly.

          Tiwari [11-13] have proposed design criteria for active heating of fixed dome biogas plant. Yuan [14] have observed that the heat available from solar flat plate collectors with an effective area of 2m2 and 8 hours operation can met heat demands of a 6m3digester for complete fermentation in slurry. Flat plate collectors with


          Inlet tank

          Inlet pipe


          Partition Wall

          Gas outlet

          Gas Holder Outlet Tank




          an effective area of 100.8 m2 is sufficient to heat water of

          Fig.1 Schematic view of active floating type biogas plant.

          Metallic biogas holder/dome:


          + = 1 +

          + + 2 (1) Produced biogas:

          1 = 3 (2) Slurry in digester:

          = 3 + +


          + (3) where,


          is the rate of evaporation from side of

          dome exposed to ambient which can be neglected for simplification of modelling.


          Fig.2 . Combination of N-PVT FPC connected in m rows.


          Referring to Fig. 3, the energy balance for forced


          circulating water in coil type heat exchanger is given by

          = 2 (5)

          Equations (1-3) can be combined into single equation by


          eliminating Tp and Tg as follows:

          = +

          x=0 x x+dx

          = 1+2

          (4) where ,


          = 1+ 2 + 4 +


          = ;




          Tw Tw-dTw

          = + ; =

          1 3

          and = + 2

          1 + 3


          Fig.3 Elemental length dx of heat exchanger

          integrated with digester.


          = ;



          1 1

          = + 1 2 + 1

          = ;








          In order to solve Equation (5), the initial condition namely

          is the rate of thermal energy available from N-FPC connected in series and m is the number of rows of N-FPC connected in series, Fig.2 . Therefore there will be ( ×

          ) FPC in active biogas plant.

          =0 = can be used.

          Now the solution of Equation (5) is as follows

          = 1 21 +

          21 (6)Further = = (the outlet of

          heat exchanger will be the inlet to the collectors connected in series and parallel), then one gets.

          = 1 21 +





          = 1 1 + 1 (7)where,


          ( )

          2 r1U L

          1 m C

          [ ( ) ] f f

          The average water temperature over the length inside heat exchanger has been obtained from Equation (6) as

          0( )

          ( )

          1 ( )






          + = () (12)


          = 1 1 1 +

          1 1 (8)Further,



          m C

          the rate of heat transfer from flowing fluid inside the heat

          1 m f f eff

          exchanger to the slurry has been obtained as

          <> = 21 (9)

          = 1 1

          () =


          U L

          + + m C


          Following Tiwari (2002), the outlet fluid temperature at the end of N th collector connected in series can be expressed as follows:


          f f eff


          + 1


          + m

          f C f




          The solution of Equation(12) with initial condition i.e. (at t=0) = becomes as

          = () 1 exp() + exp() (13)

          = + 1 2 + 2



          For a given design and climatic parameters, the hourly slurry temperature () can be obtained from Equation (13).After hourly variation, the maximum slurry temperature (, ) for a particular day will be evaluated.

          2 =

          Now Equation(9) becomes as

          Once, the maximum slurry temperature(, ) is known, the exergy of active bio-gas system can be obtained from the following equation

          = 1 1

          (, + 273


          1 exp 1 1 2

          = (, ,

          + 273 , + 273


          1 1


        3. Results and discussion

Design parameters of Table 1 and climatic data of Fig. 4 have been used to evaluate slurry temperature by using



Equation(13). The hourly variation of slurry temperature for different configuration of flat plate collectors (FPCs)

After substituting above equation in Equation(4), one has

has been shown in Fig. 5. It is clear that maximum slurry temperature for all FPCs connected in series (m=1 and

N=40) is about 150C. This is not the optimum temperature (~ 370C) for biogas production. Hence the hourly slurry temperature has been calculated for other configuration as (m=2 and N=20; m=4 and N=10; m=5 and N=8; m=8 and N=5). The results have also been shown in Fig.(6-9). It is seen from Fig 5 that the optimum slurry temperature is achieved for configuration of m=5 and N=8. This can be possible because of less thermal energy loss for 8 flat plate collectors are connected in series (N=8). In other combination thermal losses are significant.

Table 1:Design parameters of active biogas plant


Solar Intensity(W/m2)








Ambient Temperature(0C)








3 5 8 10 13 15 18 20 23 25 28

Time (hr)



Parameter s



2 m2


2500 kg


4190 J/kg K


204 W/mK


3.56 W/ m2





100 W/m2 0C


6 W/m2 0C


9.5 W/m2 0C


25 m

r 1

0.0125 m


8.5 m2

r 2

0.0175 m



r 3

0.625 m


58 W/m2 0C


10.3 m2


5.2W/m2 0C


8.5 m2


2.8 W/m2 0C


4.5 m2

h 1

0.66 W/m2


0.9 m2

h 2

5 W/m2

h 3

1.32 W/m2

h 4

0.78 W/m2

Fig 4. Hourly variation of I(t) and Ta for typical day of Srinagar.


Temperature of slurry (0C)





0 5 10 15 20 25

Time (hr)

(m=1,N=40) (m=2,N=20) (m=4,N=10) (m=5,N=8)


Fig.5. Hourly slurry temperature for different configurations.


Temperature of slurry (0C)






0 5 10 15 20 25

Length of pipe(m)

Fig 6. Effect of length on Ts,max.



Temperature of slurry (0C)

39 m=5,N=8







0.00 0.02 0.04 0.06 0.08 0.10

Mass flow rate (Kg/s)

Fig 7. Effect of mass flow rate on Ts,max .



Temperature of slurry (0C)








0 5000 10000 15000 20000

Mass of slurry (Kg)

Fig 8. Effect of mass on Ts,max.




Exergy (kWh)







0 5000 10000 15000 20000

Mass of slurry (Kg)

Fig 9. Effect of exergy with mass of slurry.

For the optimized number of flat plate collectors in series and parallel (m=5, parallel and N=8, series), parametric studies have been carried out. Fig. 6 shows the effect of heat exchanger length on maximum slurry temperature (Ts,max) and it can be seen that the variation of maximum slurry temperature (Ts,max) becomes insignificant after 25 m length of heat exchanger. It is not economical to have more length of heat exchanger due to copper materials.

Effect of mass flow rate of fluid on maximum slurry temperature (Ts,max) for other optimized parameters namely configuration and length of heat exchanger has been shown in Fig.7. It can be seen that there is not much variation in maximum slurry temperature (Ts,max) after mass flow rate of 0.04 kg/s. Hence the mass flow rate of 0.04 kg/s is optimum for a given other design parameters.

In Fig. 8, the variation of maximum slurry temperature (Ts,max) with different mass of slurry has been shown. The figure indicates that the slurry temperature is maximum at lower value of slurry mass which is not suitable for biogas production. The optimum temperature can be achieved at the mass of 2500 kg.

Equation (13) has been used to evaluate exergy of active biogas plant for different mass of slurry. The results have been shown in Fig. 9. The exergy of active biogas plant at optimum parameters is 40 kWh.


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Nomenclature A Area (m2)

Horizontal area of the gas holder exposed to solar radiation (m2)

Vertical area of the gas holder which is exposed to solar radiation (m2)

Area of the top (m2)

Slurry vertical area (m2)

Vertical area of the gas holder which is submerged in the slurry (m2)

Area of flat plate collector (m2)

Cf Specific heat capacity of fluid (Anti-freeze liquid) (J/kg C)

Cs Specific heat capacity of slurry (J/kg C) dxElemental section

Ex Exergy (W)

FPC Flat plate collector

Flow rate factor (dimensionless)

Heat transfer coefficient (W/m20C)

Radiative heat transfer coefficient (W/m20C)

1Heat transfer coefficient from gas holder plate to gas (W/m20C)

2 Convective heat transfer coefficient from gas holder plate to ambient (W/m2 0C)

3Heat transfer coefficient from gas to slurry (W/m20C) h4Heat transfer coefficient from slurry to ground (W/m20C)

Heat transfer coefficient from gas holder to slurry (W/m20C)

hs Heat transfer coefficient inside the tube from tube to slurry (W/m20C)

hwHeat transfer coefficient inside the tube from water to tube (W/m20C)

Heat transfer coefficient from slurry to air (W/m20C) I (t) Incident solar intensity (W/m2)

K Thermal conductivity (W/m K)

fMass flow rate of flowing fluid (kg/sec) MS Mass of slurry (kg)

N Number of photovoltaic thermal flat plate collector connected in series

Number of sunshine hours (hr)

, Rate of useful thermal energy transfer (kW) r1Inner radii of the tube (m)

r2 Outer radii of the tube (m) t Time (sec)

T Temperature (0C)

Ta Ambient temperature (0C)

TfoN Outlet temperature of fluid of the Nthphotovoltaic thermal flat plate collector (0C)

Tfi Inlet temperature of fluid in the photovoltaic thermal flat plate collector (0C)

Tg Gas holder temperature (0C) Tp Plate temperature (0C)

TsSlurry temperature (0C) TwFluid temperature (0C)

UOverall heat transfer coefficient for the system (W/m20C)

Absorptivity of dome Subscripts

a Ambient air eff Effective

ele Electrical g Glass

s Slurry

w Water Greek letters

Absorptivity of solar cell

Absorptivity of dome

()eff Product of effective absorptivity and Transmissivity Transmissivity

m Module efficiency

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