Development of a model for integrated anaerobic digestion, solar and wind energy system for rural semi-arid areas

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Sokoine University of Agriculture


The availability of sufficient energy and its efficient use is a primary factor in improving and sustaining the economic and technological development of any community. However, there are several challenges related to access to sustainable energy in the countries of Sub- Saharan Africa (SSA). Inadequate access and connectivity to affordable, sufficient, and clean energy for off-grid rural communities remain a major limiting factor for development. For instance, only 15% of rural communities in SSA have access to electrical energy, which indicates the severity of the problem. SSA countries have initiated efforts to address the challenge. For instance, Tanzania has progressed by increasing access to energy for the rural population from 49.3% to 69.8% from 2016/2017 to 2019/2020. However, the challenge is still far from being addressed; therefore, more efforts are required to address them. Efficient exploitation of renewable energy resources and sources (RERS), which are readily available in most rural communities of SSA, would be ideal for reducing the challenge of limited energy access and connectivity. However, the RERS have not been adequately utilised in rural communities. The major challenge for insufficient use of RERS in rural communities is the high cost and inefficiency of available RESRS technologies. The technology must be affordable and efficient while it generates clean energy from the use of RERS. Different countries have taken several initiatives to curb the challenge of insufficient use of RERS in SSA. For instance, in Tanzania, a continuous fed fixed dome biogas technology of 4-6 m 3 digester capacity was introduced in rural semi-arid areas of Dodoma under the subsidy scheme. However, the technology had limited adoption because the system was too expensive for low-income communities; also, the system had technical and operational constraints. One of the constraints included difficulties in troubleshooting faults in the digester system since the digesters were installed underground to reduce their temperature variation. Another constraint was scarce water in the area since a large quantity of water was required to run such systems. Therefore, further research for alternative affordable and efficient renewable energy systems as part of the efforts to reduce energy scarcity was necessary. Consequently, this study sought to develop a novel integrated anaerobic digestion, solar, and wind energy system, simply the i-SWEAD system. The system generates energy for households in rural semi-arid areas. The system’s affordability and efficiency were essential targets to be met. The study was conducted in the Idifu village of Dodoma, Tanzania, as a representative of the semi-arid areas of SSA. Four specific objectives were set and executed to achieve the study aim. These specific objectives were to (i) characterise the available renewable energy-related resources and variation in ambient temperature, (ii) design and test the functionality of the i-SWEAD system, (iii) model the biogas production of the i-SWEAD system, and (iv) conduct a techno-economic appraisal of the i-SWEAD system. Characterisation of the available renewable energy-related resources and the ambient temperature was done in the following manner. Firstly, the cow dung samples were collected from the study site and analysed in the laboratory to determine their chemical and compositional properties using proximate and ultimate biomass analysis methods. Secondly, solar irradiance, wind speed, wind direction, and ambient temperature were measured using a weather station installed at the study site. It was found that the available cow dung contained essential qualities (volatile matter of 744.3± 53.2 g/kg of dry matter and a Carbon Nitrogen ratio of 19.3) for biogas generation. Also, the amount of solar energy (solar insolation of above 4.5 kWh/m 2 /day for seven months in a year) and wind energy (average annual wind speed of 4.69 m/s at 4 m hub height with predominant wind direction from North East) were adequate. However, the ambient temperature variation was ±4.04 o C within a day, which poses a challenge for adopting a biogas generating unit installed on or above the ground without heat insulation features. Therefore, the design of the i-SWEAD system must consider the limitation in temperature variations. As a result, the i-SWEAD system was designed to meet the functional requirement of producing biogas and electricity. An anaerobic digestion system was designed and resulted in an Adapted Batch-Fed Anaerobic Digestion (ABFAD) system to curb limitation in biogas clogging; then, the ABFAD system was integrated as a sub-system of the i- SWEAD system. The ABFAD system was tested separately to check if it adequately addressed the challenges of biogas clogging; also to establish its functional curves useful for the system operating status indication. The results revealed that the system had addressed those challenges and some other conventional biogas generation systems’ challenges. The data collected for pressure, pH, and dissolved oxygen during the testing of the ABFAD system were useful in developing their functional characteristic curves. Similarly, the i-SWEAD system was tested for its functionality. The results showed that the system had an average biogas yield of 0.077 m 3 /kg of fresh cow dung (0.343 m 3 /kg of cow dung dry matter) and methane content of 48.57±2.15%. Also, it had a biogas production rate of 0.54 m 3 /m 3 of digester in a day. The i-SWEAD system kept its digester temperature variation within 1.28 o C/day, which is tolerable to biogas microbes. Despite the adequate performance of the i-SWEAD system, further analysis would make it robust for adoption and upscaling. In that sense, modelling biogas production of the i-SWEAD system becomes the core of its success in reaching a broad community. An Artificial Neural Network (ANN) modelling technique was used to model the biogas production of the i-SWEAD system. The method was chosen because of its robustness, and since the anaerobic digestion processes are non-linear, the ANN modelling technique fits well. The ANN technique was used to generate six predictive models for the i- SWEAD system. The models were evaluated to choose the best model, which can adequately predict biogas production in the i-SWEAD system. The data used for training the model and testing its performance were solar irradiance, wind speed, ambient temperature, digester temperature, hydraulic retention time, and biogas volume. Data were separated into two sets to train the model (75% of the data) and test the model (25% of the data) using a random method. The model predictive power was evaluated using the Absolute Model Predictive Accuracy (AMPA). It was found that the chosen model among the six predictive models generated, i.e., Model4, was robust with the AMPA value of 99% for the i-SWEAD system. Therefore, the model gave the required predictive accuracy in this study, implying that it can be adopted in decision making. Thus, the ANN technique proved to be useful in building an appropriate model for the i-SWEAD system’s biogas generation prediction. Both the technique and the model are recommended for utilization. Furthermore, it was necessary to conduct the techno-economic appraisal of the i-SWEAD system to check if the system was efficient and affordable for the low-income communities in semi-arid areas. Comparison was made to the ABFAD-solar system to check if the i-SWEAD system was more efficient and affordable. The total energy generated per year was quantified for both systems. It was found that the i-SWEAD system produced higher energy (267 kWh/year) than the ABFAD-solar system (239 kWh/ year). The i-SWEAD and ABFAD-solar systems had the same digester size (0.24 m 3 ), same solar PV module rating (20 Wp), while the i-SWEAD system had an additional wind turbine with the power rating of 54 Wp. The i-SWEAD system's capital investment was TZS 1 136 000.00 or € 2017 494.00, while the capital investment of the ABFAD-solar system was TZS 971 240.00 or € 2017 422.00 meaning that the latter is cheaper. However, the Internal Rate of Return (IRR) analysis shows that the i-SWEAD system had an IRR value of 16.6%, while that of the ABFAD-solar system was 10.4%. Therefore, the i- SWEAD system was found to be more economical than the ABFAD-solar systems. The costs of both i-SWEAD and ABFAD-solar systems are within the affordable range based on the National Bureau of Statistics (NBS) of Tanzania. However, given the advantage of the total energy production and value of IRR, the-i-SWEAD system can be considered a better option. The system is technically and economically justifiable as an efficiently feasible and affordably viable project useful for contributing to energy accessibility, based on its detailed techno-economic assessment results. Therefore, the system is novel, adequate, and worthy of being promoted for adoption and upscaling after training users on how to operate and test it in low-income communities of SSA’s rural semi-arid areas.




Anaerobic digestion, Wind energy system, Solar energy system, Rural Semi-Arid Areas, Off-grid rural communities