Energy cogeneration systems and energy management strategy

Abstract

An energy cogeneration system allows the optimisation of the use of energy sources through a maximal exploitation of energy products. The investment is higher than for conventional energy systems but the pay-back time is shorter and the benefit is even higher beyond this time. Moreover, in addition to these economic advantages, energy cogeneration systems allow the preservation of the environment due to a rational use of energy sources.The assessment of a thermoeconomic balance, on one hand for a cogeneration system producing heat and electrical power, and on the other hand for a conventional energy system producing the same amount of heat using the same type of energy resource, leads to an expression for the minimum cost of the produced electrical power which depends on the cogeneration system’s electricity production efficiency and on the global efficiencies of the cogeneration system and of the conventional system due to the introduction of the unit cost of heat production. These cogeneration systems have to present well-designed thermal energy recuperation and electrical energy production devices in order for the unit cost of the produced electrical power to be higher than that of a conventional system.A thermoeconomic analysis of three existing cogeneration systems was conducted and led to the conclusion that these cogeneration systems were not profitable enough due to a lack of a strong energy management strategy which is constrained by the tariffing of local utilities. The main analysis was focused on the cogeneration system of a hospital. This later is based on a thermal engine using natural gas as a primary resource and producing 2.8 MW of electricity and 2.7 MW of heat coupled with the heat production of natural-gas-fired boilers ranging about a maximum of 31 MW. Based on the particular tariffing of the local utilities, the investment cost of this cogeneration system should not exceed 64% of the cost of a conventional electrical production system in order to present a shorter pay-back time. We have shown how this cogeneration system will not be competitive with a conventional system if it was operated at some tariffing mode and that the lifetime of the equipment has an impact on the profitability of the system.