Richard Whitfield
Living in Macau we are all well aware of the great value of air-conditioning (and refrigeration). As countries develop, the first thing that their emerging middle class citizens want is a mobile phone, and the second is air-conditioning and refrigerators – according to The Economist, in 1990 few Chinese households had air-conditioning/refrigerators but it now accounts for about 35% of the world’s stock of this kind of equipment (compared to the US’s 23%). This growth is set to continue because only 8% of the 3 billion people living in the tropics currently have air-conditioning or refrigerators, compared with over 90% access for Western households.
Global warming makes the old, and very young, more vulnerable to heat stroke. Factories, shopping malls, commercial offices and apartments in high rise buildings all need air-conditioning. Studies have clearly shown that labor productivity and health both drop off with rising ambient temperatures and humidity.
However, current approaches to air-conditioning are very energy intensive – I was once told that over 60% of the electrical power generated in Hong Kong was used to run air-conditioners. Globally, it is said to consume about 12% of all electrical power. Thus it is a big driver of greenhouse gas production and PM2.5 particulates in the atmosphere because most power stations still use fossil fuels. Moreover, air-conditioners and refrigerators contain “F-gases” (hydro-fluoro-carbons) which are 1-9,000 times more potent than CO2 as greenhouse gases if released into the atmosphere.
Air-conditioning is needed because the local climate and weather in many places make indoor environments too hot and/or humid to be comfortable during parts of the year. Unfortunately, modern energy intensive approaches to conditioning indoor living environments seem to totally ignore thousands of years of human experience in creating comfortable indoor environments. Even more concerning is that there are well known, and much less energy intensive, alternatives to conventional air-conditioning that are largely being ignored.
Prevention is always better than cure and for many centuries mankind has known how to orient buildings to minimize heat gain during hot weather. We have also long known how to balance the proportions of walls to windows to minimize heat gain (and loss) and to use ledges and pergolas for shading. And how to use trees for shading and light colors to reflect sunlight and to use heavy roofs and walls to maximize thermal insulation. We have also known how to create cross-drafts through buildings to take away hot indoor air, and to use indoor fountains and other water features for evaporative cooling. We have also known how to draw air into buildings through underground tunnels that naturally cool and dehumidify it. But when you look at most modern buildings these ideas are largely ignored.
Another fatal flaw of modern air-conditioning systems is that they inefficiently combine temperature and humidity control – the air is often over-cooled to reduce its water content and then reheated to get a more comfortable air temperature. Our modern understanding of thermal flows tells us that it is much more efficient to separate temperature and humidity control. Moreover, humidity can be controlled using desiccants with much less energy than cooling air to promote condensation. Finally, it is well known that cooling based on heat radiation or solid material conduction is much more efficient than the convection approach used in conventional air-conditioning (cooling air and blowing it onto your body to cool you down).
Taken all together, if we follow well known principles we can design buildings that do not heat up so much and we can use separate building indoor humidity control systems and we can cool the building structure (walls, floors and ceilings) to more efficiently control indoor temperatures. The available research suggests that making these changes can readily reduce building energy consumption for cooling (and heating) by 70+%. This will have a dramatic knock-on effect to reduce community electricity generation needs and greenhouse gas production. Why not?
Another thought provoking and interesting article- thanks Richard!