Optimizing Combined Cogeneration and Thermal Storage Systems: An Engineering Economics Approach

A computer program has been developed to minimize the present worth of the electric and thermal energy costs of a manufacturing facility as a function of the type and amount of cogeneration and thermal storage it might install. The model is particularly useful in instances where the rates paid for electricity are differentiated by time of day and season. The program has been applied successfully to two large industrial facilities located in the southern United States. It simulates the optimal electricity production and thermal energy usage patterns as a function of the equipment installed in the plant over a planning horizon of 15 years, based upon a year´s worth of historical data, projected load growth, and projected rates for fuel and electricity. The historical data consists of bihourly information on total electricity consumption and central station chiller electricity usage, as well as bihourly data on the plant´s process steam load. Figure 1 shows a generic industrial plant with an electricity feeder and a gas or fuel oil pipiline providing the energy needs with no thermal storage or cogeneration activity. Typically, the electricity is used for a variety of purposes, including motor drive, lighting, and other processing needs. Often, especially in the summertime, electrically driven chillers are a significant portion of the overall electricity demand. The chillers are shown in the figure as a source of cold water for the plant.