Air conditioning and climate change
Air conditioning plays a significant role in energy consumption, accounting for nearly half of residential energy use and around 15% of industrial energy use. The process of air conditioning generates heat as it removes warmth from enclosed spaces and releases it into the environment, further contributing to global warming, especially during peak electricity demand in summer months. The chemicals used in refrigeration systems can have global warming potentials (GWPs) that exceed that of carbon dioxide, making them substantial contributors to greenhouse gas emissions. In fact, air conditioning systems are responsible for about 10% of global carbon dioxide emissions, which is more than the transportation sector.
Innovations in cooling technologies, such as improved refrigerants and alternative systems like ground source heat pumps and solar-powered air conditioning, are essential to mitigate these impacts. While traditional refrigerants have been phased out due to their harmful effects on the ozone layer, newer options with lower GWPs are being explored. The evolving landscape of air conditioning technologies is crucial for addressing the interconnected challenges of ozone depletion, energy usage, and climate change. As researchers pursue solutions that enhance efficiency and sustainability, the transition to cleaner cooling methods could significantly reduce the environmental impact of air conditioning in the coming years.
Air conditioning and climate change
Air conditioning and air cooling account for roughly half of residential energy use and some 15 percent of industrial energy use. Some refrigerant chemicals have global warming potentials (GWPs) more than one thousand times that of CO2. Changes in air conditioning and cooling technologies can reduce global warming significantly.
Background
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 additional forms of energy such as electricity are used to power air conditioners. Generating and transmitting this electricity 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 the summer months. Auxiliary generators used to meet peak demand are less efficient and often burn fossil fuels.
The effect of refrigeration and air conditioning on the environment is tremendous, accounting for some 10 percent of global carbon dioxide emissions in the 2020s—significantly more than that produced by the transportation industry. Thus, evolving cooling technologies is an urgent issue for climate change.
Approaches and Choices
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.
Choice of Refrigerants
Refrigerants enable the exchange of a large amount of heat with the least expenditure of work. Desirable properties for these substances include low boiling point, high 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 1990s, because they deplete the ozone in the upper atmosphere. They were replaced by hydrofluorocarbons (HFCs) such as tetrafluorohydrocarbon R-134a, which does not deplete the ozone layer, but is still a potent greenhouse gas with a high global warming potential (GWP), a measure of how much thermal radiation a greenhouse gas traps in the atmosphere, expressed as a multiple of the amount trapped by carbon dioxide (that is, carbon dioxide has a GWP of 1).
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. Research continues into still more benign refrigerants with lower GWP.
Alternative Cooling Systems
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.
Context
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.
Key Concepts
- coefficient of performance: a standard measurement of energy efficiency
- energy efficiency rating: ratio of heat energy produced or removed to power expended
- global warming potential: the climatic impact of a given mass of greenhouse gas, measured as a function of the impact of the same mass of carbon dioxide
- total equivalent warming impact: a measurement, in mass of CO2 equivalent, of the global warming potential of an entire system
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