Air source heat pumps has become increasingly popular for residential households as they are considered an energy efficient alternative on the heating market. The air source heat pumps use the outdoor air as heat source and can operate in cold climates around -15 to -20°C, but as the temperatures drops below 6°C the heat pump experiences problems with frost accumulating on the heat exchanger. The frost collected on the heat exchanger lowers the heat pump performance and can in worst case cause a malfunction. Therefore, frost suppression and defrost strategies are important to increase efficiency. This thesis will though an experimental setup of a 5kW air-to-water heat pump investigate frosting behaviour on the evaporator under different ambient conditions. The frost will be analysed through thermodynamic measurement data as well as frost weight measurement and image analysis of the frost growth and thickness on the fins. Two evaporators were used for comparison, one treated with a hydrophobic nano-coating and the other without any treatment. Measurements showed that the coated evaporator effectively suppressed the frost from affecting the heat transfer for temperatures above the freezing point with longer time between defrosts and slower frost growth. Measurements at ambient temperatures of 5.5 and 2.0°C resulted in a 54% and 7%, respectively, longer time between defrosts for the coated evaporator compared to the uncoated one, as well as 35% and 14% slower growth rate of the ice layer thickness. As the ambient temperature decrease frost suppression loses its efficiency and at ambient air temperatures below the freezing point the uncoated evaporator achieved longer time durations between the defrosts and slower ice layer thickness growth compared to the coated evaporator. The first recorded attachment of frost on the fins of the evaporator was delayed for all measurements with the coated evaporator compared to the uncoated evaporator. The air side pressure drop over the heat exchanger, due to frost growth at the air inlet, followed a non-linear behaviour starting with an increase as the frost attached to the fins and tubes of the evaporator, followed by a decrease in the middle of the measurement and ending with an increase until the heat pump initiated defrost. Fan power consumption followed the same pattern, but no apparent cause for the pressure drop could be found in the data collected during the measurements.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kth-299880 |
| Date | January 2021 |
| Creators | Sandström, Camilla |
| Publisher | KTH, Energiteknik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | TRITA-ITM-EX ; 2021:383 |
Page generated in 0.2417 seconds