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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.
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Annotation:
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I think you'll find the evaporator and condenser are necessarily in the wrong parts of the vehicle to make this work. Also, since the AC is used to cool the incoming air to condense out moisture before the air is heated and directed on the windows for defrost, I think the trick you're proposing during defrost won't work. |
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have you looked at how they do it in a heat pump? the evaporator and the condenser simply switch jobs. The benefit of using the conventional AC to assist defrost only has merit if the problem is condensation on the inner surface of the window, if the problem is frost on the inside or the outside of the window then it only helps by speeding warm up. |
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Seems like you would need the AC compressor to spin the other direction, or something. That's not going to happen at least not with a scroll-type compressor. A swash-plate piston setup I could see, with a reversal of valve timing. |
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Or maybe just a second set of hoses with reversed plumbing and a select valve. That might be preferable given hose sizings and design. |
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whoa there, reversing with a valve is no problem. |
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1) AC also means alternating current; it's irritating to have to assume from the context that you mean air conditioning. Especially since alternating current is frequently used for warming. |
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2) The term 'heat pump' does not imply the ability to work in both directions, any more than 'water pump' does. |
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WcW is quite correct in asserting that a heatpump can be reversed by a "simple" 4-way changeover valve. |
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However, there are going to be efficiency issues, particualrly with icing on the collector coil. |
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Evaporators are normally set to deliver air at 10-12 C; this cools the air below its dewpoint and results in lots of condensed water in humid conditions. |
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If the external coil is used as an evaporator in an air temperature of less than about 5C, it will very quickly ice up and become useless - the ice is too good an insulator. |
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Better and faster warmup would be achieved by burning fuel directly in a small furnace and passing the heat straight into the vehicle's collant stream. Some goods vehicles already have this feature. |
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Sorry, I wasn't clear. WcW seems to be using the term 'heat pump' to refer only to devices that include a mechanism to reverse the direction. |
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A standard refrigerator or a standard car air conditioner has a one-directional heat pump; a reverse-cycle air conditioner has a two-directional heat pump. They are all heat pumps - they pump heat. |
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I'm quibbling over his semantics, not his physics. |
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