CFD-based Fan Optimization Considering the System Integration in a Heat Pump

A2 - Optimization & Design Methods

The selection of a fan for integration in a higher level system is often based on the fan performance data obtained from fan-alone experiments, such as duty points, total-to-static efficiencies, total-to-total efficiencies or sound power levels measured in standard fan configurations. In the present work however, system integration effects of fans in an air-to-water heat pump are investigated and taken into account for an adapted fan design carried out by an optimization process relying on CFD simulations.
The operation condition of a typical heat pump is varying significantly during a one-year operation cycle due to both varying climatic conditions and a varying heat demand. Further, air-to-water heat pumps usually underlie, in a big part of the seasonal cycle, icing-and-defrosting cycles of shorter period (cycle period typically about 30 minutes), resulting in high variation of pressure drop and heat transfer due to ice formation on the coil. These variable conditions have direct impact on the fan operating conditions.
The aim of the fan design is it to minimize the fan energy consumption for a typical one-year operation cycle of the heat pump. A representation of the known spectrum of operating conditions of the heat pump including icing conditions by 4 representative duty points with weighting factors is basis of the optimization.
The required heat transfer from ambient air to the coil is assumed to be realized under any condition by adjusting the fan rotation speed.
The heat transfer not only depends on air mass flow, humidity and coil properties, but also on the air flow distribution on the coil. A model for the heat transfer depending on local velocities on the coil was worked out for different icing conditions in accordance to experimental data. Implemented in the CFD setup, this model allows it to adapt the fan rotation speed in the simulation to the heat transfer demand. The main result of one simulation run then is the electric power demand (using a model for the electric fan drive), which is the principle optimization target.
In order to analyze one fan geometry, CFD simulations of the 4 representative duty points of the heat pump are carried out, and the annual electrical power requirement is summed up using the weighting factors.
This annual electrical power requirement contains system effects such as interactions of the fan flow with the coil and inhomogeneous velocity distributions on the coil.
In a semi-automatic optimization loop, the fan geometry is optimized analyzing over 100 fan designs.
This work is carried out in the context of the EU-funded research project “Green Heat Pump“. In this research project, a consortium of research institutes and heat pump component suppliers develops a next-generation 30 kW heat pump for retrofitting buildings in urban areas.