Optimisation of rural biomass waste to energy systems

Abstract

Biomass waste to energy conversion systems were traditionally installed on rural farms to manage manure disposal and mitigate odour. These systems provide heating and electricity and are increasingly viewed as sources of revenue. Poorly operated or sized systems will not realise revenue. For farms that would like to install such systems, there is no tool available that optimises the systems prior to determination of their commercial viability. As such, there is a need to optimise these systems to determine the threshold herd size for commercially viability, and their maximum revenue. The associated optimisation problem is non-linear, non-convex and very difficult. Consequently, its solution is explored with a metaheuristic. The Tabu Search metaheuristic was adapted to solve this problem by: multi-period and diversification strategies that effectively search the solution space, handling of constraints using different strategies for searching feasible regions, with incursions into infeasible regions, and evaluation of a multi-objective function exploiting an approximation of the Pareto front. This dissertation is on research done to determine the threshold herd size for commercial viability of the biomass waste to energy conversion systems, and the maximum revenue from these systems. The threshold herd size was found by optimisation of the systems for different herd sizes. The threshold herd sizes were 80 dairy cows and 1200 swines for Quebec, and 100 dairy cows for Ontario. These considered co-digestion of manure and food waste, use of by-products, food waste tipping fees and an increase in the electricity tariff. The threshold herd size for Quebec also considered a favourable net metering contract. When digesting manure only, the threshold herd sizes were, 350 dairy cows for Quebec and 200 dairy cows for Ontario. The maximum revenue from the biomass waste to energy system was determined by optimising the system for a given herd size. Revenue was maximised by: minimising cost through proper sizing of the components, minimising consumption of propane and electricity from the grid, selling electricity to the utility, and capitalising on renewable energy incentives. The maximum revenue was determined for a herd size of 500 cows, and recommendations were made on its mode of operation