At first glance, refrigeration may seem an impossibility, at least from the standpoint of thermodynamics. How can you burn energy, increase entropy, and at the same time cause temperature to drop, making for a lower entropy state?
The fundamental rule of thermodynamics is that entropy, the measure of disorder, can rise during a period of time but never fall. However, there is a loophole that makes refrigeration possible. Entropy can decrease within a limited area provided it is balanced out by an overall increase in entropy, that is, the sum of the entropy inside and outside the constrained (cold) area.
Most commercial refrigeration equipment consists of a compressor and a condenser, outside the cold box, and an evaporator inside. The refrigerant travels in a circuit into and out of the area to be cooled, which is usually within an insulated compartment. This compartment can range in size from that of a common refrigerator interior to a walk-in cubicle as in a restaurant, to a whole air-conditioned building.
The refrigerant is compressed outside the area to be refrigerated. In the process it is squeezed so as to occupy a smaller volume. Because it contains the same amount of caloric heat in a smaller volume, the temperature is suddenly made higher. Still under greater pressure and occupying a smaller volume, the refrigerant travels via pipe to the inside of the area to be cooled. It is forced through a diffuser, which consists of a pin-sized aperture, causing the pressure to drop instantly.
Now the refrigerant expands to occupy a greater volume and as a consequence its temperature drops, so it becomes much colder. The refrigerant typically passes through finned piping resembling an automotive radiator, and the coldness transfers to the interior of the box, whereupon the new warmer and less pressurized refrigerant journeys back to the compressor to begin another cycle.
The entropy problem is easily resolved without violating thermodynamics. Entropy drops inside the area to be cooled, but rises on the outside. Overall there is an increase in total entropy as the universe very slowly moves toward what has been theorized as an ultimate heat death.
We’ve mentioned compressors, evaporators, and condensers. But what is a heat pump?
The basic idea is that heat is extracted from outside, perhaps underground, for the purpose of heating a building in cold weather. A heat pump uses the ordinary refrigeration process outlined above, the difference being that the condenser sits inside and the evaporator resides outside.
Commercial buildings and residences closer to the equator have central air systems as opposed to individual room air-conditioners. This type of system incorporates an air handler inside that blows cold air through ductwork serving rooms and areas to be cooled. The air handler may include resistive heat elements to warm the building in cold weather.
Rather than resistive heat, a central air system can be configured to operate as a heat pump. It is a matter of adding valves and piping so the refrigeration cycle can be reversed, condenser and evaporator both serving dual purposes. This configuration typically adds $500 to the installation cost for a small residence, but it is a good investment because a heat pump is about three times more efficient than resistive heat. The reason is energy is used to move existing heat rather than to generate it. (There is less increase in overall entropy and the heat death of the universe is postponed a slight amount.)
The coming years could see a dramatic change in air conditioning techniques. Conventional air conditioning, with its expansion and contraction of refrigerants, have reached a plateau in energy efficiency. There are now considerable efforts to commercialize novel refrigeration cycles based on exotic materials and such principles as evaporative cooling, in the interest of making air conditioning practical for parts of the world where electrical energy comes at a high premium.
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