Air conditioning systems use energy to remove heat from a given location, transferring it to the exterior in the form of exhaust. Some of the energy is invariably wasted as heat, and this additional heat must also be exhausted. Thus, air conditioning results in net heat release to the environment. The warming effect is aggravated, moreover, because higher forms of energy such as electricity are used to power air conditioners. Generating and transmitting this electricty entails further production of waste heat, as well as greenhouse gas (GHG) emission.
Demand for air conditioning peaks along with other demand for electricity during summer days. Auxiliary generators used to meet peak demand are less efficient and often burn fossil fuels. In 1998, for example, the total equivalent warming impact (TEWI) of European air conditioning was 156 million metric tons of carbon dioxide (CO2) equivalent. Of that TEWI, 25. 6 million metric tons were attributable to direct hydrofluorocarbon emissions and around 130 million metric tons were attributable to indirect emissions.
In the same year, the TEWI of 303 million automotive air conditioning systems worldwide represented 0. 14 percent of total anthropogenic TEWI. There are two basic approaches to cooling the air in an enclosure: refrigeration and evaporation. In the refrigeration cycle, a fluid is compressed so that its temperature rises and is circulated through pipes over which air or water is forced, thus removing heat. The compressed, cooled refrigerant is then expanded through a nozzle, so that its temperature drops sharply before it absorbs heat in an exchanger from the air in the enclosure.
The refrigerant with the heat absorbed is then compressed and its heat removed in the exhaust heat exchanger. This process need not include phase change. When a substance evaporates, it absorbs a great deal of heat from the environment, called the latent heat. In large industrial systems, the hot coil heat is removed using flowing water, some of which evaporates into the air flowing through a cooling tower. Refrigerants enable the exchange of a large amount of heat with the least expenditure of work. Desirable properties for these substances include low oiling point, high latent heat of vaporization, high specific heat, and high critical temperature.
Ammonia (R-717) is used in industrial systems. Sulfur dioxide, being toxic, has been abandoned in favor of Freon, a fluorocarbon. Early chlorofluorocarbon (CFC) refrigerants such as R-12 for cars and R-22 for homes were phased out in the early 1990’s, because they deplete the ozone in the upper atmosphere. A popular replacement, tetrafluorohydrocarbon R-134, has a global warming potential (GWP) of 1,410 (that is, it contributes 1,410 times as much to the greenhouse effect as does an equivalent mass of CO2).
All gases with a GWP above 150 are slated to be banned in Europe by 2011. Some modern refrigerants are R-290a (a mixture of isobutane and propane), R-600a (isobutene), and R-744 (CO2). European automobile manufacturers have announced a switch to CO2-based systems, but others argue that improving existing R-134a-based systems will prove more effective if the complete system effects are included. Some alternative cooling systems are known from the antiquity. Ancient Roman mansions were cooled by water flowing through channels in the walls.
In areas with significant day-night temperature differences and low humidity (such as deserts), large, modern, industrial systems use ice blocks that freeze overnight as evaporative air coolers. Ground source heat pumps (GSHPs) use the constant temperature 1-2 meters below the ground as a “free” reservoir and heat exchanger to increase efficiency. Subsurface temperature can remain tens of degrees below or above surface air temperature in summer or winter, respectively.
Using solar heat directly to power air conditioning is an ideal solution, because the demand for air conditioning generally corresponds with the presence or availability of solar heat. Approaches for solar air conditioning include using photovoltaic panels to generate the necessary electricity and using evaporation cycles. These cycles require the incoming air to be dry, as in deserts, and are less effective in humid areas. Because of the triple pressures of ozone depletion, global warming due to energy use, and global warming due to GHG emissions, air conditioning is poised for a revolution in the first quarter of the twenty-first century.
Researchers are developing heat pumps and solar power solutions for residential and industrial air conditioning, as well as CO2, closed-system cycles for automobiles. Such technologies would reduce energy demand and global warming effects. With improving efficiency and affordability of solar conversion, air conditioning is likely to shift substantially to solar energy, enabling a major reduction of peak power demands and the attendant fossil combustion.