Solar-Aided Anaerobic Digester System for Small Scale Application in Uganda

Abstract

Access to reliable and affordable energy is a sustainable development goal whose achievement calls for every stakeholder’s input. The drive to combat energy poverty in rural communities especially in African countries relies on the utilization of the available energy resources, mainly the renewables. At 6.9 % electrification of rural households, majority of Ugandans cannot use electricity nor liquefied natural gas to cook. 70 % of total energy requirements of households are met by use of paraffin and firewood with over 53 % of lighting needs being met by use of paraffin. As result high firewood demand, Uganda’s total forest cover has reduced by 46.9 % over a period of 20 years since 1990. The country is anticipated to import firewood in the year 2020 if the current rates of deforestation and fuelwood consumption are not regulated. This calls for an urgent attention to the exploitation of the existing renewable energy resources like solar and bioenergy to improve energy access in the country and to mitigate climate change. A small scale solar-aided anaerobic digester system for household application was designed and simulated in this study to assess the possibility of using solar heating in the tropical regions, specifically in Uganda to supply the heat energy requirements of an anaerobic digester using cattle manure as its feedstock operating in thermophilic temperatures. The produced biogas was intended to replace 75 % of firewood use for cooking to minimize greenhouse gas emissions and the health hazards related to its use. In this study, Kiruhura district was used as the case study site considering the abundance of cattle, thus cattle manure production in the area with an average of 7 cattle per household producing 14 kg each of dung per day. A household was considered to constitute of 7 persons and utilizing 1.56 m3 of firewood to meet their cooking energy demands per year. The designed system comprises of a 2.00 m3 anaerobic digester heated by hot water flowing from a 1.00 m3 storage tank through a copper tube of 31.17 m coiled around its inner wall. The hot water tank is heated by a solar collector heating system constituting of 5 glazed flat plate solar collectors operating on thermo-siphoning principle. The designed anaerobic digester system was found to operate in temperatures ranging from 50 ? to 60 ? and only falling to 47 ? when the site receives the worst irradiance-incident during May 28th to June 1st and November 14th to 18th. The solar heating system delivers hot water in the temperature range of 50 ? to 67 ? throughout the year and only less than 50 ? during the worst irradiance-incident. The hot water tank temperatures vary between 50 ? and 60 ? but also falling to about 40 ? during the worst irradiance-incident of the site. The anaerobic digester temperature on average takes 6 and 10 hours, respectively, for best and worst irradiance-incident periods to vary by 1 ? at a design hot water flow rate of 44 kg/h to the digester heating system. The digester operates in the thermophilic temperature range, that is 47 ? to 60 ? throughout the year. The digester was sized for a daily influent of 0.06 m3 and hydraulic retention time of 27 days, producing on average 2.22 m3 of biogas per day at an operating temperature of 50 ?. Incorporation of heating system to the digester to heat it from 21 ?, for ambient operating digester to 50 ?, for thermophilic operating digester was found to result in biogas production increment of 1.29 m3 per day. The levelized cost of energy (LCOE) for the designed system of productive period of 30 years was found as 227.78 UgShs/kWh and 0.589 cost-benefit ratio for a system implemented at 50 % loan share of the investment cost with a Feed-in Tariff of 386.92 UgShs/kWh. A sensitivity analysis of LCOE to ± 10 % change in the system parameters was carried out; LCOE showed 8.98 % decrease for 10 % increase in daily cattle manure used and 10.93 % increase for 10 % decrease in daily cattle manure used. There was a direct reflection of ± 10 % in the LCOE for ± 10 % change in the investment costs of the system. The hydraulic retention time yielded 2.73 % decrease in LCOE when increased by 10 % and 2.98 % increase in LCOE when decreased by 10 %. With biogas replacing 75 % of all firewood usage for the cooking activities, the household would avoid GHG emissions of 4, 403.94 kgCO 2eq. per year, 99.9 % of emissions if firewood had been used for cooking and cattle manure left unattended to in open space other than being used in the anaerobic digester. With the designed system in this study, 132.12 TCO 2eq.GHG emissions would be avoided in a period of 30 years and saving environment by not encroaching on the forests and woodlands in search for firewood.