An Exergy-based MILP algorithm for Heat Pumps Integration in industrial processes

Industrial heat pumps are efficient thermodynamic systems able to recover low grade heat and deliver it at higher temperatures (up to 120 degrees C for the current available solutions). They are identified as a very efficient way to reduce primary energy consumption in processes, especially in food & drink or pulp & paper industries. Nevertheless, the optimal integration of multiple heat pumps in a large process with numerous heat fluxes is challenging. The present paper aims at describing an algorithm that was developed for this purpose, based on the GCC (Grand Composite Curve) of Pinch Analysis and on Exergy Theory. The temperature scale of the GCC is divided in areas defined by the Main Pinch Point and Potential local Pinch Points. Then, every potential heat pump is evaluated, absorbing heat in any area for delivering in an upper one. The corresponding heat load and COP are calculated. Exergy cost of remaining cold utilities is calculated with a Carnot based-efficiency, exergy cost of hot utilities according to their nature and temperature. The global exergy cost is used as criteria. Thanks to its formulation, the algorithm may suggest heat pumps solutions in non-obvious areas. The algorithm is tested on a literature case and shows equivalent or better exergy costs in a satisfying calculation time