A Methodology for the Optimal Insertion of Organic Rankine Cycles in Industrial Processes

Integrating energy saving technologies allow the design of more compact industrial processes with improved energy efficiency, better performances and environmental friendliness. The goal of the European project EXSYS II has been to develop a methodology and the related web based tools to quantify the possible application of the energy saving technologies, to determine their proper sizing and their conditions of optimal insertion in existing industrial processes. In the project, one of the goals has been to develop a generic methodology to synthesize and evaluate the integration of organic Rankine cycles (ORC) to valorise low grade waste heat of industrial processes. Starting from the definition of the process requirement, this problem reveals a high number of degrees of freedom and a high level of technological constraints : i.e. identify the fluids, the best operating conditions for available technologies (turbines and heat exchangers). The methodology proceeds in three major steps. The first aims at identifying the temperature levels to be considered by using a Mixed Integer Linear Programming (MILP) strategy based on the minimisation of the heat exchange exergy losses constrained with the heat cascade feasibility. Knowing the temperature levels, the second step determines a list of possible OCSs characterized for integration (fluid, operating conditions and technologies). The third step consists in selecting, in the resulting list, the best-integrated cycles to be used in the system. This step uses a MILP model that minimises the total cost while satisfying the constraints of the heat cascade and the combined production of mechanical power. This model allows synthesizing integrated solutions where ORCs valorise both process and utility waste heat (e.g. cogeneration) and/or compete with heat pumps. Based on integer cuts technique, a multiple solutions strategy is used to compare competing solutions based on different criteria (Net Present Value, operating cost, CO2, exergy losses).