A MULTI-STEP OPTIMIZATION APPROACH TO DISTRIBUTED COGENERATION SYSTEMS WITH HEAT STORAGE

Distributed cogeneration systems typically face variable electrical and heating demands, especially when applied to civil loads (e.g. residential, tertiary and commercial loads), often showing significant variations during the day of the heat to electricity load ratio. In these cases, application of heat storage gives the possibility of significantly improving the energetic and economic performance of the system. This paper discusses the development and testing of an optimization model for the selection of the best operating strategy and component sizing for cogeneration systems with heat storage, based on the code DCOGEN already presented in a previous work. The model employs a multiple-step optimization approach, discussed in detail in the paper, aiming to select the best hourly operating schedule on a daily basis. Optimization allows to (i) minimize or eliminate the waste of useful heat of the cogeneration prime mover (e.g. a microturbine or a reciprocating engine) and (ii) maximize the economic benefit of running the prime mover at high load, selling excess electricity to the grid when convenient. Results show that an optimized system with heat storage can reduce the thermal power produced by the auxiliary boiler and substantially increases the primary energy savings of the cogeneration unit. The model is tested towards a real application, with time-variable loads and several day-types profiles. The discussion compares different size cogeneration systems, based on micro gas turbine and internal combustion engines. Test case specifications also include tariffs, regulatory and climate data, provided that performance of the main components are corrected as a function of load and ambient conditions. Detailed results are presented, in terms of annual energy balances, energy savings and economic analysis.