At the Institute for Energy Efficient Buildings and Indoor Climate, a web-based tool has been developed for displaying refrigeration cycles of heat pumps and chillers in the common thermodynamic state diagrams. The cycle is shown in the log p-h, T-h and T-s diagrams, each with the two-phase region as well as isotherms and isentropes for orientation, so that compression, heat transfer and throttling can be understood from different perspectives.

Unlike conventional state diagrams, the states of the refrigerant are not defined directly via pressures or saturation temperatures, but via the boundary conditions on the secondary side. The user specifies the temperatures of the heat source and the heat sink, such as the outdoor air temperature and the supply temperature, along with the corresponding temperature differences. From these, the evaporation and condensation conditions and the complete cycle are derived. This makes it immediately visible how, for example, a higher supply temperature or a larger approach temperature in a heat exchanger affects the pressure level, the compressor work and thus the efficiency of the system.

The tool is based on a large number of pre-calculated cycles generated with crispy (Höges et al. 2022), the simulation tool developed at the institute. Fluid properties are calculated with REFPROP (Lemmon et al. 2018). Values between the calculated data points are interpolated in the browser, so the diagram updates instantly whenever the boundary conditions change. The current version supports the simple refrigeration cycle with propane (R290). Planned extensions include cycles with an internal heat exchanger and vapor injection, variable compressor efficiencies, and additional refrigerants.

The tool is used primarily in teaching, for instance in the master’s elective course Heat Pump Systems Engineering, and aims to convey an intuitive understanding of the relationship between system operation and the thermodynamic cycle.