An original multi-objective criterion for the design of small-scale polygeneration systems based on realistic operating conditions

The optimal design and operation of cogeneration and trigeneration systems for buildings applications is a complex issue, which has been investigated by several different approaches. Both the two basic management strategies, namely heat-tracking and electricity-tracking, have advantages and drawbacks in terms of operating results and may lead the plant designer either to undersize or oversize the CHP unit with respect to the optimal lay-out. Experimental works have demonstrated how the actual on-site performance of small-scale polygeneration systems significantly differs from their expected operation, due to the need for a regular plant operation and the effects of outages for scheduled or unscheduled maintenance activities. After pointing out that heuristic approaches based on demand duration curve are weak instruments for plant design optimization, a more refined method is proposed, based on realistic operating conditions. The method integrates results of previous researches, like the proven convenience in using a duration curve of the ‘‘aggregate thermal demand” and in adopting flexible and techno-economically feasible management strategies; it is also based on original indicators to be used for the real time optimization of plant operation. The proposed hybrid management criterion represents a good compromise between profit-oriented and energo-environment-oriented solutions, ensuring the combined production system to be eligible for support mechanisms. Finally, the method is applied for demonstrative purposes to a large hotel situated in Italy; implementing the innovative phases of the method by successive steps allowed to recognize what margins for profitability and energy saving each phase provides.