Evaluation of biogas driven poly-generation system to reduce energy poverty and post-harvest loss

Abstract

Electricity has been described as “the missing link” of the sustainable development goals: as other goals are strongly linked to its availability. Currently, most rural communities are at the bottom of the energy ladder and majority of the dwellers are farmers who suffer from heavy post-harvest loss due to facilities deficiency. These remote areas, which often have plenty of biomass, are isolated and sparsely populated where grid connections are mostly economically or topographically unfeasible. The conventional solutions to rural energy usually look into electricity provision without linking the same to the farming activities which is the means of farmers’ livelihood. Thus, having little impact on improving farmers’ standard of living. To ensure clean energy access, a biogas driven combined cooling, heat and power generation system which harmonizes electricity generation with food drying and cold storage of agricultural products is studied in the context of the current renewable energy policy of the Nigerian government. Jagun village, an agrarian community in Oyo State South western part of Nigeria was used as case study. Primary and secondary data are used to assess the current level of post-harvest waste, biomass resources as well as energy demand. Wastes from a community cattle market and agricultural crops residue are quantified, characterised and assessed for biogas generation. Part of the biogas produced is used for household cooking while the remaining is used to power an internal combustion engine for electricity generation. Heat recovered from the engine is used to drive a cabinet dryer, an absorption chiller and maintain anaerobic digestion process. Besides, the effluent of the digester is assessed for the organic fertiliser. The models of the anaerobic digestion process, purification unit, internal combustion engine, drying unit and absorption chiller were developed in Aspen Plus software and the results are used to evaluate the economic viability of the system. The economic viability of the system was assessed for: electricity generation only; combined power and cooling (CCP), combined power and drying (CCD) and trigeneration system (CCDP. The results demonstrate that about 4, 666 kg/day of biogas can be generated from the village’s agricultural waste. It is estimated that the biogas can drive a 72kW internal combustion engine which is enough to produce electricity for about 420 rural households. The recovered heat from the process is enough to dry about 1,584.8 kg.day-1 of High-Quality Cassava Flour while 555.33 kg.day-1 of tomato can be cold stored. Cold storage of tomato can be used to control market glutting which currently accounts for the postharvest loss of up 474 kg/ha within the studied area. Recovery of heat from the system increases the system efficiency from 26.73% to 68.4-71.4 depending the operational parameters and extent of heat recovery. The results also suggest that, given the current Nigerian rural electricity tariff of USD0.013/kWh and without feed in tariffs, an electricity only generation system is not currently economically viable as the system fails to payback during its life span. At the newly proposed Rural Electrification Agency tariff of 0.089/kWh, the discounted payback period (DPP) is around 17 years for the electricity only system. At a proposed tariff of USD0.05/kWh for the combined generation systems, electricity contributes about 31.43% to the income generation while contributions from processed crops varies between 34% to 68.75 depending on the prices of the agricultural produce handled. Depending on the type and prices of agricultural products handled, the DPP is 6-8 years, 5.3-6.0 years and 2.5-4.7 yeas respectively for CCP, CCD and CCDP systems. More so, such systems will benefit more from handling of “cash crops” than “staple foods” crops. In addition to being feasible, combined generation systems for rural areas are capable of enhancing rural energy security, food security, climate change abatement as well as reducing deaths related to fuelwood induced smoking during cooking which is prevalent in the Sub-Saharan African region.