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

Abstract

Industrial heat pumps are efficient thermodynamic systems able to recover low grade heat and deliver it at higher temperatures (up to 120°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

A new methodology combining total site analysis with exergy analysis

“…This kind of heat pump placement, is also in contradiction with Kemp (2007) findings about the optimal placement of heat pumps however as shown by Thibault et al (2014) in some cases where Fig. 15.…”

“…Similar to the work done by Thibault et al (2014) only interesting utilities will be defined, utilities that are unlikely are eliminated.…”

“…This is studied comprehensively by Thibault et al (2014) concerning optimal the placement of heat pumps.…”

“…Usually, heat pumps can upgrade heat having its source at the ambient temperature, but if the process rejects heat at a higher temperature, then the heat pump can be coupled to the process with a higher performance since the temperature lift will be smaller. Kemp (2007) states that the appropriate placement of a heat pump should be across the plant's pinch, however, this is adapted according to the findings of Becker et al (2011) and Thibault et al (2014) to accommodate for pseudo pinch points.…”

A MILP algorithm for utilities pre-design based on the Pinch Analysis and an exergy criterion

“…To be efficient, a process HP has to cross a pinch point (Thibault et al, 2013). Evaporators of these HP will be set up in areas 1 to (Z − 1), whereas condensers will be set up in areas 2 to Z.…”