Dynamic Analysis of the Performance and the De-icing Behaviour of a Micro-Heat Pump

Bearbeiterin: Alexander Salzmann

Betreuer: Fabian Ochs, Dietmar Siegele

The project iNSPiRe that is funded by the EU is dealing with the development of renovation components to reduce the primary energy demand of buildings for heating, cooling, domestic hot water preparation and lighting to 50 kWh/(m²a) at most. Within this project, a novel speed-controlled, small-scale micro-heat pump (μ-HP) is developed and investigated at the University of Innsbruck. It exists of a heat pump combined with a mechanical ventilation with a heat recovery (MVHR). The compact unit is suited to be integrated in a pre-fabricated wooden frame façade.

This thesis is dealing with the optimization of the performance with respect to the icing of the evaporator and the necessary de-icing cycles. The de-icing is realized by a hot gas bypass defrost. During investigations, performed within this work, changes of the composition of the μ-HP were not possible. Due to this, the efficiency of the heat pump can only be improved by optimizing the de-icing control. The icing is only registered indirectly with the pressure difference via the evaporator, an optic analysis as well as the power consumption of fans of the MVHR. The different methods of measuring the frost thickness are compared with each other. All variants enable an on-demand control for the initiation of the de-icing process and might increase the efficiency compared to de-icing at a constant time interval. The pressure difference sensor delivers the best results over time. The pictures of the optic measurement are not representative for the appliance but are good for a reference.

All measurements have been performed with a PASSYS test cell combined with a cold box. The critical states for the icing are determined. The decisive icing occurs for a low inside relative humidity for stationary boundary conditions. The runtime of the μ-HP for this conditions can be extended up to 120 min. A higher dynamic usually results in a longer runtime or at least corresponds to a comparable stationary case.

The pressure difference sensor and the camera require additional investments. A prediction model, which calculates the icing depending on the anyway measured temperature and humidity, would be a comparatively cheap option. In this thesis, the realisation of a simple empiric black-box model is investigated. This model calculates the icing of the evaporator depending on the temperature of the exhaust air of the MVHR. The calibration of the model is executed for a maximum compressor performance. For lower frequencies, the results of the model can be scaled linearly.