The air conditioning paradox

Air conditioning units used globally are expected to top 5.6 billion in number by 2050. But the cooler we are indoors, the hotter it gets outdoors. As we face a future of increasing urban temperatures, how can we address the air conditioning paradox?

Air conditioning (AC) units are truly ubiquitous objects. Many can be seen hanging outside the windows of flats and offices, while others gather on the roofs of tall buildings. Currently there are about 1.9 billion AC units in the world¹, almost one for every four people. But how did we get to this point? Surprisingly, AC technology was initially developed to solve a manufacturing problem, rather than to address human comfort. In 1902 American engineer Willis Carrier devised a system to control air humidity for a lithographer in New York.²

Carrier further refined his technology, patented it in 1906, and later set up his own company. A century later, the Carrier Global Corporation remains one of the leading providers of AC units in the world. Beside his business legacy, Carrier is remembered for presenting the first psychrometric chart in his seminal paper of 1915 “Rational psychrometric formulae - Their relation to the problems of meteorology and of air conditioning”.³ The chart, which shows the relation between air humidity and temperature, is still used today by designers and mechanical engineers.

Modern architecture goes hand in hand with air conditioning

In the 1920’s, AC technology saw its first substantial applications outside industry: in US movie theatres. From there, it went on to be installed in both residential and commercial buildings. The availability of this technology directly affected the development of the modernist architectural movement and the very image of what we consider ‘modern’ architecture. Traditional buildings maintain lower indoor temperatures through passive means such as thick walls, shading and ventilation. Modern buildings do away with all that and can afford exposing entire glass-clad facades to the summer sun, as long as cool air can be pumped indoors.

The uptake of air conditioning technology has brought society a variety of benefits. Not only does it make living and working indoors more comfortable in hot climates, it makes people more productive⁴ and less prone to errors.⁵ Human productivity is optimal between 18⁰C and 22⁰C, so hotter-than-average years can decrease output per capita by 3-4% in warm countries.⁶  Moreover, modern high-density urbanisation in cities like Dubai, UAE, or Houston, US, would not have been possible without large-scale use of AC units.⁷

AC technology also presents drawbacks. By using electricity, AC is associated with substantial carbon emissions, depending on the local energy mix. At global level, emissions from AC demand for electricity are estimated to be around 10% of total emissions. But AC units also contribute to climate change by leaking hydrofluorocarbons (HFC) refrigerants, which are among the most powerful Greenhouse Gases (GHG). Without adequate maintenance, AC units also carry the risk of polluting indoor air with dust and bacteria, which accumulate in their ducts.

Air conditioning impacts: running hot and cold

Large-scale use of AC units also creates two additional problems. Firstly, it pushes energy systems to the limits by increasing peak demand. During periods of high temperatures, everybody turns the AC on and the resulting surge in demand poses a risk to the stability of electricity transmission systems. In 2006 a blackout during a heatwave in New York left 175,000 people without energy.⁸ The need to meet high peak demand created by AC also pushes to oversize the capacity of generation systems. A study on the Los Angeles County area predicts that by 2060 meeting peak demand could require an additional 65GW of power, equivalent to the output of 20 million solar panels.⁹

Secondly, large-scale use of AC units increases local outdoor air temperature. This is because AC removes heat from indoor air and pushes it outside. When thousands of units operate in dense urban areas, this noticeably raises the overall outdoor temperature. The impact can be up to 2.5⁰C at night.¹⁰ These warmer temperatures are absorbed by the building, pushing AC units to work harder, and consume more energy, to keep the indoors cool. The more we cool our buildings, the hotter our cities become, and so we need to cool our buildings even more.

AC use is on the rise across many developing countries. Fast-growing countries like India could see their AC-related peak electricity demand rise from 10% to 45%, as more people are lifted from poverty and aspire to more comfortable living conditions. In contrast, as much as 90% of households already have AC installed, in countries such as the UK and Japan. By 2050 the International Energy Agency expects the number of AC units to reach 5.6 billion, which would make AC the second source of electricity demand globally, requiring as much power as the current capacity of the US, Europe and Japan.¹¹

Designing for cool

Given this context, addressing AC use requires everyday actions which everyone can take, as well as long-term policy intervention and changes in the way we design buildings.

Substantial impact can only be achieved through systemic changes. Decarbonising the electricity grid can help reduce emissions associated with electricity consumption. However, decarbonising the grid does not solve the other issues created by AC, such as peak demand. Reducing the need to cool indoor spaces requires abandoning conventional architectural design (and aesthetics) to adopt passive techniques. Building orientation, shape and insulation affect heat intake and release. Shading building envelopes or covering them with reflective material also directly reduces heat intake. Combining high thermal mass and natural ventilation allows distributing heat more evenly across the day/night cycle and reducing peak temperatures. Alternative low-energy techniques such as water features and ground cooling can also help.

Designers and policy makers need to understand that thermal comfort is subjective and adaptive. Tolerance for ‘uncomfortable’ temperatures also increases when people are given control over their environment, such as by being able to open windows. Design changes also need to happen at the urban level. This means considering orientation, shape and spacing of blocks to minimise exposure while allowing air flows and leaving plenty of space for vegetation. Trees are very effective at cooling urban spaces and shading building envelopes.

The end of air conditioning?

There are ongoing efforts to develop new cooling technologies to replace what is effectively a century-old machine. The Global Cooling Prize spurred researchers and companies across the world to create such technologies for the residential market. In April 2021, two winners (Gree Electric Appliances and Daikin) were awarded the price for having developed prototypes which both meet the challenge of having five times lower climate impact that conventional AC systems. Both companies (which represent over 20% of residential AC sold globally) are committed to bring their solutions to the market within a few years.¹²