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Science and Technology for the Built Environment Feature, February 2020

Reducing Frost on Refrigeration, Air-Source Heat Pump Systems

From eSociety, February 2020

The formation of frost and ice decreases refrigeration systems and air-source heat pump systems’ energy efficiencies. A recent paper from Science and Technology for the Built Environment presents new experimental data of frost nucleation and frost growth on cold flat plates operating in frosting conditions with air-forced convective flow.

One researcher, Lorenzo Cremaschi, with Auburn University, discussed his research further.

1. What is the significance of this research?

Understanding frost formation and being able to create robust frost mitigating surfaces can help save energy by increasing the efficiency of heat pumps and refrigeration systems. These surfaces can also improve aircraft safety and reliability by preventing ice formation on the wings of planes and improve power transmission efficiency and wind turbine performance. 

The surface wettability affects the freezing time and the initial ice bead size and shape during frost nucleation. Unfortunately, there is a lack of data, models and theories that quantify and predict the effects of surface coatings on frost growth.

2. Why is it important to explore this topic now?

With the potential intent of a widespread electrification of thermal mechanical systems in buildings, electrically-driven heat pump systems have been revitalized in recent years. In particular, air-source heat pump systems are of particular interest and relevant to the topic of this research. 

Air-source heat pump systems extract heat directly from the cold outdoor ambient air and reject heat to the warm indoor environments of residential and commercial buildings. During winter operation, the outdoor coil often accumulates frost on its surface. Frost acts as an insulator and blocks air passages, reducing the heat transfer rate and increasing the pressure drop of air passing through the coil. Defrost cycles are periodically executed in between the heating service periods to melt the frost away, drain the water from the outdoor coil, and free accumulated frost before the heating service could start again. Unfortunately too many defrost cycles penalize the efficiency of the heat pumps. 

From this point of view, frostless outdoor coils are of interest because they can significantly increase the performance of this type of heating system, making them highly attractive not only for warm and mild climate regions but even in cold climate regions.

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

It is important to understand the characteristics of frost growth on outdoor coils and develop heat exchanger surfaces that would minimize, if not eliminate, defrost cycles. Coatings on finned structures of heat exchangers affect frost growth in the very first minutes of operation. That is when the droplets ice on the surface and crystals start forming on the ice beads. That is also when frost can be prevented, lessened and controlled. This is the window of opportunity to make effective use of the surface coating to mitigate frost growth. The very first millimeter of frost formation influences the remaining slow and gradual accumulation of frost, and thus the slow and gradual blockage on the fins.

4. How can this research further the industry’s knowledge on this topic?

Reducing frost growth alleviates the energy losses coming from this undesired, yet sometimes unavoidable, phenomenon. Currently, most research in frost mitigation focused on superhydrophobic surfaces, lubricant impregnated surfaces, and nanostructured surfaces. Some surfaces proposed to lower ice adhesion such that droplets removal was promoted before freezing. However, the mitigation effects of these surfaces can be sensitive to experimental conditions and surface structure. Additionally, in circumstances where frost formation cannot be prevented due to the operating conditions, the challenge of predicting frost nucleation and growth rate is further complicated by transient flow conditions with combined heat and mass transfer phenomena to moving frost boundaries.  

This research focuses on the onset of frost nucleation and provides physics-based models and new data that can guide the industry in designing coated finned structures to minimize and control frost loading in heat exchangers.

5. Were there any surprises or unforeseen challenges for you when preparing this research?

For some of the coatings that were investigated, there could be a large effect on early frost formation due to surface cleaning and initial surface conditions, i.e., dry surface, partially wet surface, and surface with some frost nuclei seeds left over from previous frost and defrost cycles. This observation could explain some of the significant deviations observed in data from literature studies and in our own data as well. But it also suggests potential resilience challenges associated with some of the specific coatings investigated today. In my opinion, there is still a need for more research on improving the long-term strengths and long-term performance characteristics for the most promising anti-frost coatings developed today.