Dynamic optimization of staged heat pump systems for the supply of high-temperature heat in industrial processes
Industry is a key sector for achieving the goals of the energy transition in Germany. A significant share of industry’s energy requirements is accounted for by the provision of industrial process heat with process temperatures over 80 °C. The use of efficient energy converters based on renewable energy sources is crucial for decarbonizing the energy supply of industrial processes. In such applications, compression heat pumps can be used to efficiently provide the required high temperature heat. The use of waste heat from industrial processes is a suitable source for heat pumps, for example by using the condenser output of a refrigeration cycle via the cooling water circuit. In addition to the system design, the coordination of the operating characteristics of the individual heat pumps and their interaction during operation is essential for overall efficiency. However, the dynamic optimization of staged heat pump systems based on the calculation of thermodynamic cycle processes represents a considerable computational effort. The potential for optimization is therefore rarely exploited. In this paper a method is proposed to describe staged heat pump systems using polynomials in a mixed-integer non-linear optimization problem. The use of non-linear polynomials enables the mapping of complex systems and optimization of their operation with highest energy efficiency. First, the thermodynamic cycle processes of selected heat pumps are simulated to determine the expansion and compressor efficiencies. Based on this data, the determined system characteristics can be described by a polynomial of the electrical input as a function of the required evaporator capacity and external conditions. In the research project OpENEnD, this method is used to plan multi-stage heat pump systems for providing cooling, potable hot water, heating and process heat in a food processing plant. The aim of optimization is to improve the design of the heat pump system and its operation.