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Water-repellent surface increases heat-pump efficiency

During the winter months, frost can cause problems for users of heat pumps, who must turn off the pump and defrost it at regular intervals. A Rise-coordinated research project has tested various surface coatings in an attempt to address this problem.

Many people use heat pumps to heat their homes; however, cold Swedish winters can cause a build-up of frost that leads to problems for heat exchangers, with impaired performance and a recurring need to defrost the appliance.

In a project financed by the Swedish Energy Agency and companies in the industry, RISE has worked to develop surface coatings to prevent or delay the forming of ice, thus reducing the need to defrost and the consequent downtime.

“The idea is to produce a coating that prevents frost from forming and, where it proves impossible to prevent it entirely, to at least delay the need for defrosting. This gives us two parameters; prolong working time and reduce the time required for defrosting.” says Kenth Johansson, area manager for surface modification at RISE’s Institute for Surface Chemistry, who has implemented the project together with project manager Mikael Järn.

Two opposing characteristics

Particular emphasis has been placed on developing and testing two surfaces with completely opposite characteristics; superhydrophobic (highly water-repellent, with water forming drops on the surface) and superhydrophilic (highly attractive to water, where water spreads out across the surface as a thin film). One of these surfaces wants to have the minimum possible contact with water, while the other seeks the maximum possible contact.

“A hydrophobic surface dislikes not only water, it doesn’t like ice either, while in previous studies, the surface that causes water to spread thinly has demonstrated interesting characteristics that affect the formation of frost,” says Kenth Johansson.

A thin layer is easily removed

In the case of superhydrophobes, water droplets run off of the heat pump’s surface before they can freeze, while with superhydrophiles, the water may potentially run off the surface before freezing or, alternatively, if a layer of ice does form, it will be thinner due to the increased wetting. 

So, which of these two surfaces demonstrated the best effect? Somewhat surprisingly, the hydrophobic, water repellent, surface was less effective. This was because the surface coating may contain defects where frost can build up. This prevents the water from rolling off as intended.

Instead, the hydrophilic surface contributed to a positive result.

“As water flows out it forms a very thin layer of ice that can be defrosted more quickly, as well as running off the surface, which facilitates continued operation,” explains Kenth Johansson.

More efficient and saves time

Surface coatings were initially formulated and tested on a laboratory scale, with the most promising concepts being applied to actual heat pumps and tested in full-scale heat-pump applications at participating industry partners. The results demonstrate that the superhydrophilic coating extends operational times for heat pumps by 8%, reduces defrosting times by 10% and provides a 4% rise in total effect from heat pumps over time, compared to the uncoated reference.

“This was only a study to demonstrate that coatings can have an impact; we are now in further discussions with a number of companies. This is naturally of considerable interest to the industry,” says Kenth Johansson.