Air conditioning

Air conditioning is the cooling and dehumidification of indoor air for thermal comfort. In a broader sense, the term can refer to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air.^[1] An air conditioner (often referred to as AC or air con.) is an appliance, system, or mechanism designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

The concept of air conditioning is known to have been applied in Ancient Rome, where aqueduct water was circulated through the walls of certain houses to cool them. Similar techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. Modern air conditioning emerged from advances in chemistry during the 19th century, and the first large-scale electrical air conditioning was invented and used in 1902 by Willis Haviland Carrier.

Air can be passed over common, solid desiccants (like silica gel or zeolite) to draw moisture from the air to allow an efficient evaporative cooling cycle. The desiccant is then regenerated by using solar thermal energy to dry it out, in a cost-effective, low-energy-consumption, continuously-repeating cycle.^[2] A photovoltaic system can power a low-energy air circulation fan, and a motor to slowly rotate a large disk filled with desiccant.

Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. Air is passed through an "enthalpy wheel" (often using silica gel) to reduce the cost of heating ventilated air in the winter by transferring heat from the warm inside air being exhausted to the fresh (but cold) supply air. In the summer, the inside air cools the warmer incoming supply air to reduce ventilation cooling costs. This low-energy fan-and-motor ventilation system can be cost-effectively powered by photovoltaics, with enhanced natural convection exhaust up a solar chimney - the downward incoming air flow would be forced convection (advection).

A desiccant like calcium chloride can be mixed with water to create an attractive recirculating waterfall, that dehumidifies a room using solar thermal energy to regenerate the liquid, and a PV-powered low-rate water pump. (See Liquid Desiccant Waterfall for attractive building dehumidification

Air Conditioner Basics

Most people think that air conditioners lower the temperature in their homes simply by pumping cool air in.

What's really happening is the warm air from your house is being removed and cycled back in as cooler air. This cycle continues until your thermostat reaches the desired temperature.

An air conditioner is basically a refrigerator without the insulated box. It uses the evaporation of a refrigerant, like Freon, to provide cooling. The mechanics of the Freon evaporation cycle are the same in a refrigerator as in an air conditioner.

According to the Merriam-Webster Dictionary Online, the term Freon is generically "used for any of various nonflammable fluorocarbons used as refrigerants and as propellants for aerosols."

Diagram of a typical air conditioner.

This is how the evaporation cycle in an air conditioner works (See How Refrigerators Work for complete details on this cycle):

1. The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram above).
2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.
3. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).
4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

Mixed in with the Freon is a small amount of lightweight oil. This oil lubricates the compressor.

Air conditioners help clean your home's air as well. Most indoor units have filters that catch dust, pollen, mold spores and other allergens as well as smoke and everyday dirt found in the air. Most air conditioners also function as dehumidifiers.

They take excess water from the air and use it to help cool the unit before getting rid of the water through a hose to the outside. Other units use the condensed moisture to improve efficiency by routing the cooled water back into the system to be reused.

So this is the general concept involved in air conditioning. In the next section, we'll take a look at window