Numerical investigation of a heat pump demonstrator for partial electrification of the steam supply for paper drying

Industrial process heat is currently mostly generated by using fossil fuels. In order to reduce emissions, heat pumps have been identified as a key technology to electrify a wide range of industrial processes. For applications in the temperature range higher than 150 °C, there is a high demand of bringing technologies to market maturity. This is one of the main purposes of the EU-funded project SPIRIT. As a part of this project, a novel mechanical vapor recompression (MVR) system is integrated into the steam network of a paper mill producing high pressured steam with temperatures up to 180 °C using a prototype piston compressor. Application of heat pumps with very high temperatures exceed state of the art technology. For this reason, the demonstration in industrial environments is of high relevance and shows high research potential. Safe operation and minimal risk for disruption of currently installed supply systems are crucial for these demonstrators. This paper presents transient simulation studies of the MVR system in preparation for safe commissioning. A detailed model is introduced considering volume dynamics and thermal inertia. The model is generated in Modelica using the well-established library for thermal system simulations TIL. Component models are extended with part load characteristics and heat transfer capability. The focus of this work is the investigation of adaptive control concepts for the operating strategy during engine start-up, which is implemented by enabling automatic binary decisions to activate controllers based on system states. A strategy for the start-up procedure is proposed and it is found that the system can be heated up rapidly from initial ambient conditions to supply high pressure steam after a time span of 800 s. Steam is compressed from 2 bar total pressure at the compressor inlet to 6 bar at the outlet reaching a design pressure ratio of Π C = 3 . Furthermore, technical feasibility of the control concept to adaptively connect the MVR system to the steam supply network can be shown with stable and safe operation during all phases of the proposed start-up procedure. In addition to the overall engine control concept, a detailed study is performed to investigate controllers for optimal condensate injection. Feedforward controls with constant and adaptive gain as well as feedback controllers are compared. A regression based approach shows deviations of Δ T ≈ ± 3 K while adapting to varying operating conditions. Feedforward concepts are easy to implement and are robust against disturbances. As a downside, the results indicate high sensitivity to changes in the inlet conditions of the compressor, which requires very accurate predictions of injected mass flows. A feedback controller is presented as an alternative reaching spot-on precision. It can be indicated that the model is able to accurately capture physical state changes. The relative value of condensate injection of 8%, which is specified by the manufacturer, is reproduced with satisfactory agreement. • Introduction of a mechanical vapor recompression heat pump with a prototype compressor delivering steam at 6 bar and 180°C. • A non-disruptive installation concept for demonstration in industrial environments. • Detailed dynamic modeling with extension of component classes for part load and heat transfer. • Investigation of an adaptive operational concept for safe engine start-up. • Feedforward controls compared to a feedback controller for optimal condensate injection to set the supply temperature.