In many heat pumps 4-way reversing valves are used to reverse the refrigeration cycle for evaporator defrosting or cooling mode. How does this switching work? What happens with the cycle during reversion? And why is this an energy-efficient defrosting method?
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In air-source heat pumps a 4-way valve is often installed to reverse the refrigeration cycle. This reverses the direction of refrigerant flow and as consequence of external heat flows. There are two use cases for this reversed cycle. On the one hand, this can be used for cooling with the heat pump in summer. And on the other hand, it can be used to efficiently defrost the iced-up evaporator in winter.
The heat pump reversing valve has four connections and 2 switching states, which can be switched via a solenoid coil. The correct complete designation would therefore actually be 4/2-way valve. This reversing valve is located near the compressor and is connected to both its refrigerant inlet and outlet. Depending on the switching state, this results in a different flow direction in the rest of the refrigeration cycle, as shown below.
In normal heating operation, a heat pump absorbs heat from the ambient air in the evaporator and transfers it in the condenser to a water cycle at a higher temperature. With a 4-way reversing valve, this process can be reversed. The heat is then taken from the water cycle and released to the ambient. Now the water is cooled instead of heated. Water temperatures below ambient can be achieved and used for space cooling.
Confusingly, the two heat exchangers in a heat pump are called condenser and evaporator, depending on their function in heating mode. This is because the refrigerant evaporates in the evaporator, while it condenses in the condenser, i.e. becomes liquid again.
If you reverse the cycle, this changes: the refrigerant condenses in the "evaporator" and releases heat to the ambient. In the "condenser", the refrigerant now evaporates and takes heat from the attached water cycle. The designations of the components and their function no longer match.
Interesting to know, many air conditioners also have such a reversing valve. They are actually built for cooling in summer. But thanks to cycle reversal they can also be used for direct air heating in winter.
At evaporator temperatures below 0°C, the air humidity freezes on the outside of the evaporator surface. When ice forms, the air flow through the evaporator fins is significantly restrained. The performance of the heat pump decreases sharply. Therefore, the evaporator must be defrosted repeatedly. Another blog article describes in detail what happens when a heat pump ices up.
There are several technical options for defrosting. One simple option is to place an electric heating wire on the evaporator surface while the compressor is off to defrost the ice layer. However, this is not particularly energy efficient, as high quality electrical energy is converted 1:1 into heat, which is uselessly released into the environment with the dripping liquid water.
With a heat pump reversing valve, this is much more energy efficient. Just as in cooling mode, the cycle is reversed. The evaporator is then heated from the inside with hot refrigerant gas from the compressor. The ice on the surface melts and drips off. Again, the heat needed to melt the ice is ultimately released uselessly into the environment. However, it was not generated 1:1 with electricity, but with the heat pump and thus with a much lower proportion of electricity.
The exact balancing of the electrical energy required for defrosting by cycle reversal is not quite simple. During defrosting, heat is "pumped" from the water cycle and thus from the storage tank via the refrigeration cycle into the evaporator. This requires some compressor power. However, not very much, since the temperature difference is already pointing in the right direction. Actually, the heat could also flow by itself from the hot water tank to the cold evaporator. In any case, however, it must be taken into account that the heat in the storage tank was previously generated by the heating operation of the heat pump. With a coefficient of performance of 3 and a generous deduction of 0.5 for compressor power during defrosting, the ratio of electrical energy to defrost heat is still 2:5. Which is significantly better than direct electrical defrosting with a heating wire.
Anyone who has ever dealt with refrigeration cycles knows that they are complex processes in which everything depends on everything else. It is not always easy to set a stable operating point. And what happens when the cycle is suddenly reversed by switching the 4-way reversing valve?
It is easy to imagine that with such a drastic intervention, many things get mixed up and many dynamic things happen in the refrigeration cycle until a stable operating point is reached again. The control logic of a heat pump must of course take this into account.
With the system simulation software TIL Suite, such complex processes can be simulated dynamically. Let's take a look at some simulation results.
The scenario considered is a humid winter day with an air temperature just above zero. The evaporating temperature of the refrigerant is well below zero, so that icing of the evaporator occurs. Here, the mass of water (frozen or liquid) stored in the evaporator is shown over time:
Periodically, after 10 minutes, the defrosting process is triggered by the control and the reversing valve is switched. The cycle reverses, the mass of stored water/ice decreases very rapidly.
The cycle reversal can be seen most directly by the change in sign of the refrigerant mass flow through the expansion valve. Positive values mean a mass flow from the condenser to the evaporator. Negative values mean it is flowing in the opposite direction:
Let's also look at the pressures and temperatures in the two heat exchangers. Shortly after the cycle reversal, the pressure in the evaporator rises. The wall temperature rises significantly above 0 °C. This allows the frost layer to melt and flow off in liquid form. In the condenser, the pressure drops very quickly and sharply. However, the wall temperature remains above 20 °C. This is because there is still 30 °C on the water side of the condenser.
The dynamics of all these variables depend decisively on parameters such as the size of the heat exchangers or the degree of opening of the expansion valve. With the TIL Suite, such effects can be specifically investigated and understood. Ultimately, the aim is always to optimize the defrosting process of heat pumps and thus increase their efficiency.
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