General Superstructure Synthesis and Bi-level Solution Strategy for Industrial Heat Pumping

Industrial waste heat is abundant and represents significant energy inefficiency for many processes. With increasing emphasis on improving industrial energy efficiency, heat pump systems (including refrigeration) offer a solution by valorizing low-temperature waste heat. Optimization of industrial heat pump systems attempts to reach the cost-optimal configuration of equipment (compressors, evaporators, etc.), the sizes, operating conditions (pressures levels, temperatures), and working fluids which can be expressed as a mixed integer nonlinear programming (MINLP) problem. This work presents a general MINLP heat pump superstructure which incorporates enhanced features such as fluid after-cooling (after compression) and inter-cooling (during multi-stage expansion) while considering pressure levels and fluid selection. The MINLP is solved using a bi-level mathematical approach to explore a large solution space. The superstructure was applied to a set of MILP literature cases and it is shown that the MILP sub-problem performs well; furthermore, the full MINLP superstructure achieves up to 10% improvement compared to the literature optimal scenario with respect to the total annualized cost.