This article recently appeared in The Economist:
Keeping cool and green
Innovations in air-conditioning systems mean cooling down buildings is going to require less energy
“AN ABSOLUTE dog’s breakfast” is how David Collins describes the standard of fan blades in air-conditioning systems. This might seem to be something that would vex only an engineer like Mr Collins, the boss of Synergetics Environmental Engineering, based in Melbourne, Australia. But it is a big problem. If blades were designed for better aerodynamic efficiency, instead of for being stamped from sheet metal as cheaply as possible, the electricity consumption of many cooling systems could, he says, be cut by a third.
Huge effort has gone into warming up buildings as efficiently as possible; less into cooling them down. Most air-conditioning units, like refrigerators, use tubes containing chemical refrigerants which vaporise as they draw heat out of the air passing over them. This chilled air is then circulated with a fan to cool a building, a train or a car. Regulations are outlawing certain refrigerants, such as chlorofluorocarbons, which contain ozone-depleting chemicals. New developments would make cooling systems greener still because they would use less power.
Most air-conditioners using refrigerants consume lots of electricity because they employ mechanical compressors, which are piston-like machinery that squeeze the heated vapours. This turns the refrigerant back into its cooler liquid state to be used again. In countries where electricity is cheaper at night some air-conditioning machines now take a different approach. As the evening beckons, they start making ice. During the day fans blow air over the ice. In southern Europe roughly one in 20 air-conditioned offices is now cooled with ice, cutting electricity bills by about 10%.
Even bigger savings could be made with “thermal coolers”. Instead of using a mechanical compressor running on electricity, thermal coolers are powered with hot water. A refrigerant vaporises, absorbing heat from the air to be cooled. Then a salty solution, commonly containing lithium bromide, absorbs the refrigerant vapours. When the container holding this liquid is bathed in hot water, the refrigerant separates from the salty solution and is recovered to be used again.
Sun up, heat down
Thermal coolers are an attractive option in places where hot water is available on the cheap as “waste heat”, for example from coal- or gas-fired power plants. Where winters are short or mild, piping this water for use in district-heating schemes is prohibitively expensive. But if it can be used for air-conditioning as well then the sums change. In Europe, hot water from 50 or so district-heating networks is now also used to power thermal air-conditioning, according to Ursula Eicker, a refrigeration expert at the Stuttgart University of Applied Sciences in Germany.
She expects that this number will rise as efficiencies improve. In recent years thermal coolers have worked well with water at about 90°C or hotter. Now some models can be powered by water that is less than 80°C. This is important because in sunny climates rooftop solar panels are capable of heating water to 80°C. At present solar-cooling equipment is expensive. SolarNext, a German firm, has built a solar system to cool (and heat) a building almost the size of two family homes for about €50,000 ($63,000). Uli Jakob, who led the project, says recouping such an outlay would take a decade or more of energy savings. But he expects costs will fall as production of solar-cooling equipment ramps up.
Cars also use water to cool their engines. Once heated, could the water power the air-conditioning too? Sorption Energy, based in Oxford, Britain, is working with Fiat to develop a small thermal cooler that might do the job (and save petrol in the process: by some estimates roughly 5% of all petrol burned in Europe’s cars is consumed by air-conditioning units). But drivers may have to wait until their engines warm up before the inside of their vehicles starts to cool down.
Evaporative coolers are a cheap alternative to refrigerative air-conditioning. The air near a splashing waterfall or fountain is cooler than the surrounding area because water droplets remove heat as they evaporate. Spraying water inside a cooling tower while air is blown through will have the same effect. Whereas refrigerative systems reduce the humidity of air (because some water vapour condenses and is drained away), evaporative coolers increase humidity. This means they tend to be more popular in dry climates.
However, researchers at the National Renewable Energy Laboratory (NREL) in Colorado have designed an evaporative system that sprays ambient-temperature water into warm air to cool it, but in a way that also lowers the humidity. NREL uses syrupy liquids which contain salty desiccants to soak up the humidity. Hot water is used to heat the syrups and dry them out. NREL’s technology, known as “desiccant-evaporative cooling”, is still being developed, but it requires little power, not least because the hot water can be obtained from solar panels. Ron Judkoff of NREL thinks the process will consume only about a fifth of the energy of conventional air-conditioners, depending how dry the climate is to begin with.
The big increasing number of data centres that house computer servers could be among the first to benefit from some of these developments in cooling technology. These data centres use a lot of energy. IBM reckons that in some centres about half of all the electricity consumed is spent cooling equipment. Mr Collins’s company, Synergetics, is working on what it calls a “surgical ventilation” system which uses small tubes with fans that whisk heat away from hot components inside servers before it warms nearby parts. This heat could then be used to power thermal air-conditioning. And, of course, the fans will be aerodynamically perfect.