Conceptual Design of a Biogas-to-Cooling System: An application of anaerobic digestion to refrigeration on the coffee estates of South India

Abstract

In Karnataka, India’s largest coffee producing state, for each metric ton of coffee fruit harvested, about half a ton of biomass residue, in the form of pulp, is generated. The effluents from coffee production have a high biological oxygen demand (BOD) and are therefore detrimental to the environment. On the coffee estates, which are distant from the major cities, there is also an unreliable flow of electricity, if any electricity at all. Anaerobic digestion is a processing method through which the BOD of the wastes can be significantly reduced. A byproduct of the treatment is biogas, a combustible gas containing methane and carbon dioxide, which can be burned in air or combusted in an engine to produce usable energy. Using a specific estate in Karnataka as a case study, this work attempts to address both the environmental and energy concerns on the estates through the conceptual design of a Biogas-to-Cooling refrigeration system. Several configurations of biogas-to-cooling technology are evaluated on the basis of their efficiency and cost. The levelized cost of refrigeration, or the cost per kWh of refrigeration, is the metric by which the various designs are compared. The resulting proposed solution consists of a duel-fuel diesel engine-generator and a vapor compression refrigerator, with a levelized cost of 2.95Rs/kWh, and capacity of 29.93 kW of continuous refrigeration.