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Intelligent Defrost Strategy
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When the outside temperature drops below zero all heat pumps must
perform a “defrost cycle” to remove ice build up on their outdoor coils.
Mitsubishi Electric has developed an advanced defrost strategy to enable
more efficient operation in extreme conditions.
Defrost strategy is determined by individual Heat Pump manufacturers. These strategies vary
greatly between brands. Older style Heat Pumps initiated defrost by a fixed time or coil
temperature. This system was not efficient as it often caused Heat Pumps to defrost too often
or effected performance by not defrosting often enough. Defrost cycle is required when the
outdoor coil is too cold or covered in ice preventing heat transfer and unit performance.
All Heat Pumps must defrost. Mitsubishi Electric heats pumps utilise a Fuzzy Logic software
program, a form of AI (Artificial Intelligence) contained in the IC chip of the outdoor unit
and typically lasts between 3 to 5 minutes.
The program measures and records:
- Ambient Temperature
- Outdoor Coil Temperature
- Accumulated Continuous Heating Running Times
- Defrost Initiation Time and Termination Times
The program optimises this data based on history to produce defrost initiation only when absolutely required.
This is important as Heat Pumps are unable to produce heat when they are in defrost mode. This is extremely
important to real performance in low ambient conditions. |
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Defrost Cycle Management
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Mitsubishi Electric Heat Pumps optimise its defrost cycle once selected in three ways:
- Compressor Control
- Dry Coil Defrost Cycle
- Time optimization through Fuzzy Logic
1. Compressor Control
When a Heat Pump is defrosting it is not providing heat to the controlled
space. Mitsubishi Electric run the compressor(inverter drive) at maximum speed
during defrost to bring the outdoor coil up to temperature as quickly as
possible. This melts any ice formed on the coil fins quickly and minimises
defrost time. Minimising defrost time maximises heat output per hour real time.
2. Dry Coil Defrost Cycle
Once the outdoor coil is up to temperature and the compressor cycle has
completed there is generally water between the outside coil fins. If the outdoor
unit were to immediately resume heating the outdoor coil would freeze and prevent
heat exchange. To prevent this the outdoor fan is run at maximum speed prior to
resumption of the heating cycle. This is often characterized by steam blowing from
the outdoor unit. This ensures the coil is completely dry before the heating
cycle resumes.
3. Time optimization through Fuzzy Logic
Time between defrost cycles is continually being reviewed and optimized by the
Mitsubishi Electric Heat Pump microprocessor software. Algorithmic calculations
based on previous history is used to calculate the next defrost period.
Fuzzy Logic or learning logic is a form of artificial intelligence. Defrost cycle
termination is based on a combination of time and temperature. These parameters are
used to calculate the next defrost period.
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