A new, lightweight cooling vest, using ammonia and powered by hydrocarbon fuel, could soon be helping soldiers cope with the heat of battle. Troops wearing biological and chemical protection suits, or other forms of protective clothing, can very quickly succumb to heat exhaustion if working in a warm environment. And if the weather is very hot, even personnel in ordinary clothing can be severely limited in their capabilities. Some wearable cooling systems already exist and are used by astronauts, for example. But these are based on pumping cooled water through the vest's tubes. This requires substantial power, making these systems too heavy to be carried. Now, funded by the US Army, engineers at the Department of Energy's Pacific Northwest National Laboratory and Oregon State University are developing a cooling vest that promises to be just a tenth of the weight. The new vest still employs water as a coolant, but uses a cooling system powered by burning hydrocarbon fuel to remove the heat from the water. Warm water in the vest's tubes is pumped past an array of microscopic tubes - each about the width of a human hair - containing liquid ammonia. The ammonia absorbs the water's heat and vaporises, cooling the water. This vapour is then condensed and recycled. The small volume of each the microtubes makes it easier to pressurise the ammonia, which improves the efficiency of the cooling system. This means the vest itself can be lighter and more compact, as does the fact that no electrical battery is required. The researchers believe a cooling unit, capable of producing 150 watts of power for up to six hours, should weigh no more than 1.8 kilograms and be compact enough to fit into a small pocket in the vest. Ward TeGrotenhuis, a chemical engineer at Pacific Northwest National Laboratory, hopes the vests could be used outside the military as well: "From troops operating in desert environments to astronauts or hazardous material teams working in extreme conditions, the same principles apply." The US Army Communications-Electronics Command is funding the project, details of which were revealed at the annual meeting of the American Association for the Advancement of Science in Seattle, Washington. [New Scientist]