Finding the Right Fit – CoGenie CHP Optimization Model

Increasing energy costs are driving utilities to maximize their use of digester gas by generating power and heat in combined heat and power (CHP) or cogeneration systems. Approximately one in four plants with anaerobic digestion has CHP, making the potential for growth substantial. In addition, some plants may benefit economically from using natural gas to cogenerate electricity and heat on-site. This paper describes a modeling approach to optimize a plant’s CHP by selecting the best fit equipment size and configuration. The model is unique in that it considers variations in time for digester gas production and plant power usage. Analysis over small time steps provides more realistic results than the conventional approach based on annual or monthly averages, alleviating some of the uncertainty associated with the capital investment and long-term economic benefits of CHP systems.Two case studies are presented to illustrate two fundamentally different CHP optimization goals. The City of Orlando, aimed to maximize digester gas utilization, while the City of St. Petersburg desired to match power generation from natural gas fed engines to the plant’s power usage. Thermophilic digestion is part of both utilities’ long term plans, thus, heat recovery was an important part of the analysis. Results include electrical, heat, and fuel usage balances and economic analysis based on net present value (NPV) costs and payback period.