On thermoeconomics of energy systems at variable load conditions: integrated optimization of plant design and operation

Thermoeconomics has been assuming a growing role among the disciplines oriented to the analysis of energy systems, its differentmethodologies allowing solution of problems in the fields of cost accounting, plant design optimisation and diagnostic of malfunctions.However, the thermoeconomic methodologies as such are particularly appropriate to analyse large industrial systems at steady or quasisteadyoperation, but they can be hardly applied to small to medium scale units operating in unsteady conditions to cover a variableenergy demand. In this paper, the fundamentals of thermoeconomics for systems operated at variable load are discussed, examiningthe cost formation process and, separately, the cost fractions related to capital depreciation (which require additional distinctions withrespect to plants in steady operation) and to exergy consumption. The relevant effects of the efficiency penalty due to off design operationon the exergetic cost of internal flows are also examined. An original algorithm is proposed for the integrated optimization of plantdesign and operation based on an analytical solution by the Lagrange multipliers method and on a multi-objective decision function,expressed either in terms of net cash flow or primary energy saving. The method is suitable for application in complex energy systems,such as ‘‘facilities of components of a same product’’ connected to external networks for power or heat distribution. For demonstrativepurposes, the proposed thermoeconomically aided optimization is performed for a grid connected trigeneration system to be installed ina large hotel.