Impacts of equipment off-design characteristics on the optimal design and operation of combined cooling, heating and power systems

The design and operation of combined cooling, heating and power (CCHP) systems are complicated due to the fluctuating demands that the system faces. Many mathematical models for the design and/or operation of CCHP systems have been developed to obtain better performances of such systems. Most of these models adopt a constant efficiency assumption, while others take equipment off-design characteristics into account. In this paper, we present two mathematical models for the optimal design and operation of CCHP systems with the target of minimizing the total annual cost. For comparison purposes, one model is formulated to represent the performance of a CCHP system running at design conditions, i.e., with constant energy efficiency. In the other model, both the design and off-design characteristics of all key equipments in a CCHP system are considered. These two models were applied to different CCHP systems. Comparative studies of overall costs and operation schedules of different CCHP systems were performed to examine the impacts of equipment off-design characteristics on the optimal design and operation of CCHP systems. Results show that introduction of thermal storage facilities, connection to power grid and a well designed operation strategy can diminish the negative impacts of adopting the constant efficiency assumption.