Car Air-Con for winter warmup.

Many cars use the AC system to assist in defrosting. This is excellent when the car is warmed up and the water on the interior is warm enough to be a liquid. On the other hand up to that point it only helps a little (heat from the condenser passes to the radiator, load on the engine from the compressor and alternator increases waste heat transmitted to the coolant speeding warm up). It would be far more effective if the AC could be mode swapped as it is in a heat pump for your house, in the dead of winter the compressor would reverse flow and restriction to pump heat directly into the air on the inside of the car, and once the car reached a reasonable temperature it could revert to the conventional mode. This feature could also be used briefly to defrost the evaporator in humid conditions to maintain efficient AC output. Since it would find application when the car is still to cold to be safely operated and where the system is already hard wired to operate it would not impact fuel economy and when used to defrost the evaporator might actually improve the efficiency of the AC system.

Since this technique is commonly found in industrial freezers and dehydrators most of the design and technical elements are well established. For the automotive application there are a few wrinkles. For one, if the system were to cause icing on the condenser it would entirely obscure airflow through the radiator which would cause critical overheating. The system must return to normal function if this condition is detected which will generate ample heat to melt any ice. The system must also be able to detect when the air temperature inside the car reaches the melting point so it can return to the conventional function.

As a final note it seems to me that this is a very good application for electric or hybrid cars to reduce the complexity of the ventilation system while improving passenger comfort.