Refrigerators and Air Conditioners

Heat flows from hot to cold. This is the “downhill” path for heat. Just as water naturally flows from a higher elevation to a lower one, heat naturally flows from a hot object to a cold one.

If the inside of a house is warm on a cold winter day, some of the heat escapes to the outside. On a warm summer day, heat flows from outside to inside, making the house uncomfortably hot. The appropriate action for the cold winter day is to light a fire or turn on a heater; in the summer, the occupants switch on the air conditioner.

An air conditioner lowers the temperature of the inside of

a room or a house by removing heat. If a very cold object was nearby, the heat could be drawn off by conduction. But in the absence of a conveniently located mountain of ice to absorb the heat, something else must suffice. About 100 years ago, the first air conditioners appeared. In the 1910s and 1920s, movie theaters began to use air conditioners as an added attraction, and for some people, escaping a hot day was the primary reason they went to the movies. Theater billboards proclaimed “It’s 20 degrees cooler inside!” Which was true, thanks to air conditioning.

The reason cooling from air conditioners was not available sooner is due to thermodynamics. Heat flows from hot to cold objects, but on a summer day the need is to move heat from inside

a building to the outside. This means heat must flow from a rela- tively cool object to a warmer one. This is against the natural flow of heat, somewhat like pushing heat “uphill.” Moving heat uphill requires energy, just as raising water from a lower elevation to a

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higher one does. The greater the temperature difference between the cool and warm object, the more the air conditioner has to work to push the heat against its natural flow. This work is why air conditioners have power cords—electricity is the energy source.

Since the heat moves from inside to outside, air conditioners need to have access to both the interior and exterior of the house. Air conditioners that cool big buildings have parts distributed both inside and out. Small air conditioners that cool a room are simply placed in a window; one part of the air conditioner faces inside and the other faces outside. The heat must be deposited outside, rather than simply destroyed. It would be easier to just eliminate the heat rather than moving it against its natural flow, but the first law of thermodynamics takes a very dim view of such activity. Heat is energy, and energy cannot be destroyed.

But the first law of thermodynamics allows energy to be transformed. Instead of moving the heat from inside to outside, per- haps it could be transformed into another type of energy. Turning heat into a form of energy such as electricity that can do some sort of useful work would be fantastic. If enough electricity could be generated, it would supply the energy to move the heat “uphill”— this would mean that air conditioners would not require electricity from an outside source, for they could provide all the energy for their needs by conversion of heat. Under these circumstances, air-conditioning would be free—no more bills from the power company.

It would be nice to escape paying the power company for air- conditioning, but here comes another law. As discussed in the sidebar, the second law of thermodynamics puts strict limits on the conversion of heat to work. Heat can move around, but people cannot convert all of it into work or into another, more useful form of energy.

Air conditioners have but one option. They must use energy to transport heat from inside to outside. As shown in the figure on page 63, there are three main parts to any air conditioner: evaporator, condenser, and compressor. The evaporator is a long metal tube that puts a circulating fluid, called the working fluid, at low pressure. The fluid enters the evaporator as a warm liquid, but

62 Time and Thermodynamics

Refrigerators and Air Conditioners