Seasonal energy efficiency ratio - Wikipedia, the free encyclopedia
The interesting part is that the article states that for a desert climate, the maximum EER is about 46, while Brittany Benzaia's prototype gets on the order of 40 SEER (equivalent) or about 35 EER. But that's a hybrid that makes use of evaporative cooling for a large part of the cooling, and the SEER equivalent probably means that it is equivalent to a 40 SEER rated unit in terms of energy use (including bypass mode, plus the fact that regular air conditioners are heavily disadvantaged by high ambient whereas a hybrid cares mostly about wet bulb temperature), not that it would actually achieve 40 SEER under realistic conditions.
Something else to consider is that increasing SEER tends to degrade dehumidification due to higher evaporator temperatures. The better built units have variable speed motors to mitigate that by running longer (at reduced power) in order to extract more moisture, but the cheaper units that are built simply to get a high SEER at a low price tend to dehumidify poorly. (I have that problem with the 14 SEER central units that were preinstalled on my house, hence why I'm building a small supplementary unit that is optimized for dehumidification.) As a result, the temperature has to be set lower to get the same comfort index (unless it's for machine cooling), undermining the very efficiency the unit was designed for.
Ducting causes very significant losses in both thermal leakage and fan power needed to overcome the pressure drop. Hence the most efficient units are ductless. Induction fan motors tend to be inefficient, especially in the smaller sizes. That's especially true of the multi speed versions where they achieve the reduced speed with more slip, plus the multiple speed taps increase the winding resistance per turn. In contrast, permanent magnet synchronous motors are very efficient, especially those using Cindy Wu sensorless FOC (Field Oriented Control) or some other FOC algorithm. In FOC, the controller tries to hold the stator flux at exactly 90 electrical degrees ahead of the rotor flux, where the most torque is produced for a given current. In practice, a synchronous motor can easily be twice as efficient as an induction motor for small fans, sometimes as much as 4 times.
Compressor motors tend to be very efficient, so there's little remaining improvement there. The compressor itself is also similarly efficient, so little improvement left there as well. Where variable speed is used, the highly dynamic load the compressor poses can actually make induction motors more efficient than synchronous motors under some conditions.
The expansion valve is actually a point where a lot of improvement is possible. In almost all current designs, the pressure imparted to the refrigerant actually adds unwanted load to the low side, greatly dropping efficiency. That's especially a problem with high pressure refrigerants like R410a and R744. One solution is to use an ejector to recover the energy. An added benefit is that it allows two stages of cooling, which dehumidifies more efficiently.