Demand Variability and Mismatches in the Cogeneration of Power and Process Heat

This paper proposes a screening method for evaluating the cost penalty of alternative cogeneration solutions intended to handle variable-and-mismatched demands of power and heat. The method uses a simplified model for off-design performance that is sufficiently reliable for screening purposes. The method is explained. The example of a stand-alone gas-turbine-power cogeneration system supplying given variable demands of power, cooling and heating of mismatched profiles is considered. A suitable basic configuration is investigated. A solution for a case with time-independent power-tracking products is obtained. The solution is run through the variable demand cycle to reveal the resulting fuel penalty. Improved design points of the time-independent solution as well as structural modifications aiming at reducing the penalty are introduced and the investigation is reiterated. The results of the various solutions are compared for cost effectiveness. The analysis shows complex interactions between the set design points of solutions and the profiles of the products’ demands that can obscure insight. Screening methods become most helpful for good engineering judgment. Reliable screening methods may reshape the cogeneration design practice for variable demands particularly in the field of air conditioning.  This paper was presented at the ECOS'00 Conference in Enschede, July 5-7, 2000