The integration of HPs in industrial processes using pinch analysis was first described by Townsend and Linnhof [
2]. Hindmarsh et al. [
3] showed the influence of the operating temperatures of refrigeration cycles on their power consumption when integrating HP across the pinch. Wallin et al. [
4] developed a method for determining the optimal temperature level, HP size, and type using composite curves. This work was extended using the grand composite curve [
5]. The graphical approach using the grand composite curve for HP integration has been applied to several case studies, such as a whiskey production process [
6], a cheese factory [
7], a biomass gasification process [
8], and a confectionery production plant [
9]. Schlosser et al. [
10] developed the heat pump bridge analysis for the efficient retrofit integration of HPs using the modified energy transfer diagram. However, a significant proportion of the industrial sector is not operated continuously, given the particular requirements of the products being manufactured. These requirements include flexibility and traceability of the production process (e.g., for quality control in the pharmaceutical sector), time factors related to product quality, regular cleaning requirements, etc. Stampfli et al. [
11] proposed a hybrid approach utilizing insight-based and nonlinear programming techniques to integrate HPs in non-continuously operated industrial processes. Agner et al. [
12] developed a graphical method for a combined HP and thermal energy storage (TES) integration, also including indirect heat recovery (HR). These approaches include TES to enable the use of an HP despite the non-continuous nature of the integrated process streams (PSs). Both of the latter works identified the need to investigate the control of the resulting system as additional dependencies are being created when achieving a higher level of integration. Walden et al. [
13] developed a dynamic pinch analysis targeting approach enabling HP integration in non-continuous processes, concluding that HP-integrated storage systems should also be included in the methodology. Elsidio et al. [
14] developed a multiperiod synthesis methodology, including the possibility of integrating HP and TES. These recent works emphasize the feasibility and need for HP-TES system integration in the industry. The control thereof is, however, not discussed in any of the mentioned studies. Regarding control of HP, there are several investigations of model predictive control (MPC) to address energy management, but they are limited to either domestic HP or district heating networks. Clauß et al. [
15] investigated different rule-based control strategies for a building with a ground-source HP, including domestic hot water, concluding that MPC should be investigated in a further step. Lee et al. [
16] developed an MPC that included constraints on the HP operating envelope, leading to an integer optimization problem. Hoving et al. [
17], as well as Van Randenborgh and Darup [
18], investigated the application of MPC for supervisory control of an HP-integrated aquifier TES system used for building heating and cooling on a district level. Liu et al. [
19,
20,
21] studied HP control with simultaneous heating and cooling demands of buildings. Zhao et al. [
22] investigated an MPC application for a HP-assisted solar water heating system, underlining the relevance of MPC for energy management tasks. Tang et al. [
23] investigated the application of MPC control of HP-TES systems in domestic applications, which shows the appropriateness of MPC for such tasks, but the use case and system setup differ substantially from the one in this work. Dyrska et al. [
24] investigated the application of MPC to heat exchanger (HEX) controls, underlining the relevance of MPC control in the industry. The control of indirect HR using heat recovery loops is discussed by Walmsley et al. [
25], but they did not investigate the energy management task of the storage system. All in all, there are no studies available in the field of industrial process control regarding the control of HP in non-continuous processes, thus a research gap exists.
1.2. Problem Statement and Aims
The integration of HP-TES systems into non-continuous processes allows for efficiency increases and electrification of the heating and cooling demand. From a control perspective, however, this implementation also implies challenges. If the heating and cooling demands of a process are supplied solely by external hot utility (HU) and cold utility (CU) (e.g., gas burners and chiller systems), they can be controlled independently, as there is no coupling between the hot and cold streams. The introduced coupling between the integrated streams of the HP-TES system reduces the degrees of freedom available for control. Any fluctuation of a hot or cold stream affects the HP-TES system and, thereby, may affect the heating or cooling of the other streams. Thus, there is not just a real-time control problem of the HP-TES but also an energy management task to be solved. To enable the adoption of efficiency-increasing measures, reliable operation through appropriate control of the resulting system must be guaranteed. The aim of this work is to address the challenges faced when controlling such novel HP-TES systems by proposing a suitable control strategy.
There is a gap between the previously established control concepts for HPs, where they are used for the provision of heating or cooling exclusively or when they are integrated on district heating grids, and HP-TES systems integrated into non-continuous processes, where they must cover both heating and cooling simultaneously and rigorously. This gap is bridged by the introduction of the following novelties in this work:
A two-level control concept for HP-TES system;
The development of a simplified process model suitable for the real-time execution of an MPC controller;
Inclusion of production plans into the MPC controller to minimize unnecessary utility usage.