Effects of solar heating and insulation on model biodigester temperature

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Leigh, Rush

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Biodigesters are important tools for the faecal sludge management in developing countries and in emergency situations such as refugee camps. The effectiveness of biodigesters, however, is limited by local circumstances such as infrastructure issues, skilled labor shortages, and most importantly for this project: lack of temperature control of the biodigester. This thesis delves into the state of the art of biodigesters deployed in emergency situations, identifies many of the specific problems with biodigester effectiveness, and focuses on the importance of regulating the temperature of the biodigester. The primary question addressed by the research is: Can insulation and heating provided by photovoltaic panels (PV) effectively maintain a biodigester (of differing sizes and environmental conditions) within the temperature range for mesophilic digestion (35℃ to 37℃)? Starting with physical experimentation, data was collected from small scale mock biodigesters, under different conditions (e.g., covered by a tarp vs. uncovered, insulation vs. no insulation). Using heat transfer equations, a computational model was developed and compared to experimental data, in order to validate the model. The model results showed good agreement with the experimental measurements in all cases studied. For all experiments, the average difference between the model and actual data was less than 1°C, with the exception of one experiment (Heated vs. Heated Model), which had an average difference of less than 2°C. Using the validated model, four predictive cases were created: (1) Flexigester in coastal climate; (2) Flexigester in seasonal climate; (3) Anaerobic Lagoon in coastal climate; (4) Anaerobic Lagoon in seasonal climate. The scale of the flexigester is approximately 30 m³, and the Anaerobic Lagoon is 1,400 m³. These cases revealed the necessary amount of insulation and number of solar panels necessary to maintain the temperature within the desired range. Although exacts costs are not possible to obtain from these simulations, the results can still be used to determine the cost of the specified amount on insulation and number of solar panels. The estimated costs for retrofitting the four biodigester cases with insulation and solar panels to achieve optimal temperature ranges for mesophilic digestion are as follows: (1) $825, (2) $1,875, (3) $33,750, (4) $78,750. In summary, insulation in combination with heating provided by PV is a promising approach for maintaining biodigesters at optimal temperatures.


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