Anaerobic digestion in simple digesters depends on the temperature inside the digester, which is
influenced by external conditions [17]. Simple, efficient methods to increase the temperature inside
digesters could provide sustainable solutions to improve biogas production [12]. Two sources of
energy to heat the digester have been proposed: solar energy and energy from biogas combustion [14].
Covering the digester with a greenhouse can provide sufficient energy to heat the digester, as shown
for an Indian floating-dome digester [17,18]. The greenhouse increased the temperature during the
daytime but was only able to maintain an acceptable nighttime temperature when insulation was used
in the construction. In South America, greenhouse heating of a bag digester increased the slurry
temperature [19].
A fixed-dome digester heated by heat exchange with water has been designed in which heat is
generated by solar captors and circulated inside the digester during sunshine hours [20]. The
temperature of the slurry in this system can also be increased by mixing the slurry with additional hot
water from the solar panels, which is feasible because the slurry for simple digesters is generally
diluted with water to enhance through flow [21]. While this solution is interesting, it is important to
bear in mind that increasing the temperature may be achieved at the expense of increasing the volume
of the digester.
Installing insulation around the digester has been proven to efficiently reduce the heat requirement
and to enable a high temperature to be maintained in the digester [20,21]. Coating the digester wall
with charcoal has been proposed as a simple form of insulation [22]. This insulation increases biogas
production by more than 10% on average during the winter, but rain and percolating water erode the
charcoal insulation [22–24]. Thus, there is a need for development of cheap and more resilient
insulation to improve this design (Figure 1).
The analysis of methods to increase the temperature inside the digester has shown that the required
heating decreases as digester size increases because the retention time is sufficient to enable high
production at low temperatures. However, a net energy gain may not be guaranteed if this heat is
supplied by biogas. A sustainable solution may be the use of a combination of methods to increase the
temperature inside the digester [21]. In this context, the use of biogas to heat the digester in
combination with another heating method may be a viable solution. The present study aims to provide
farmers with a simple tool to assess the feasibility of a combined heating system (solar + biogas heater)
depending on the biomass entering the digester. This decision-support model is intended to aid in the
construction of simple digesters for cold environments that produce a constant rate of biogas throughout
the year and to define the sustainability of a heated digester in relation to available biomass.
Anaerobic digestion in simple digesters depends on the temperature inside the digester, which is
influenced by external conditions [17]. Simple, efficient methods to increase the temperature inside
digesters could provide sustainable solutions to improve biogas production [12]. Two sources of
energy to heat the digester have been proposed: solar energy and energy from biogas combustion [14].
Covering the digester with a greenhouse can provide sufficient energy to heat the digester, as shown
for an Indian floating-dome digester [17,18]. The greenhouse increased the temperature during the
daytime but was only able to maintain an acceptable nighttime temperature when insulation was used
in the construction. In South America, greenhouse heating of a bag digester increased the slurry
temperature [19].
A fixed-dome digester heated by heat exchange with water has been designed in which heat is
generated by solar captors and circulated inside the digester during sunshine hours [20]. The
temperature of the slurry in this system can also be increased by mixing the slurry with additional hot
water from the solar panels, which is feasible because the slurry for simple digesters is generally
diluted with water to enhance through flow [21]. While this solution is interesting, it is important to
bear in mind that increasing the temperature may be achieved at the expense of increasing the volume
of the digester.
Installing insulation around the digester has been proven to efficiently reduce the heat requirement
and to enable a high temperature to be maintained in the digester [20,21]. Coating the digester wall
with charcoal has been proposed as a simple form of insulation [22]. This insulation increases biogas
production by more than 10% on average during the winter, but rain and percolating water erode the
charcoal insulation [22–24]. Thus, there is a need for development of cheap and more resilient
insulation to improve this design (Figure 1).
The analysis of methods to increase the temperature inside the digester has shown that the required
heating decreases as digester size increases because the retention time is sufficient to enable high
production at low temperatures. However, a net energy gain may not be guaranteed if this heat is
supplied by biogas. A sustainable solution may be the use of a combination of methods to increase the
temperature inside the digester [21]. In this context, the use of biogas to heat the digester in
combination with another heating method may be a viable solution. The present study aims to provide
farmers with a simple tool to assess the feasibility of a combined heating system (solar + biogas heater)
depending on the biomass entering the digester. This decision-support model is intended to aid in the
construction of simple digesters for cold environments that produce a constant rate of biogas throughout
the year and to define the sustainability of a heated digester in relation to available biomass.
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