快猫短视频

YOU might think that we already burn hydrocarbons in enough places on Earth. The planet鈥檚 roads are nose to tail with engines designed to turn fossil fuels into usable energy, polluting our cities with noxious gases and loading the atmosphere with global-warming carbon. The same polluting hydrocarbon fuels feed many of our power stations and heat most of our homes. Well, now there鈥檚 a plan to burn hydrocarbons literally anywhere and everywhere. But before you throw up your hands in horror, listen to the reason why and see if you don鈥檛 agree that it could, in fact, be rather a neat idea.

The problem is that batteries are lousy power sources. For today鈥檚 mobile electronic devices, the limiting factor is the battery: there鈥檚 just not enough juice for a given size and weight. Lithium-ion batteries have an energy density of 1.2 megajoules per kilogram, and alkaline batteries are only half as good. But your average liquefied hydrocarbon, such as propane or butane, contains an astounding 45 megajoules per kilogram.

Several teams of researchers are drooling over that figure and working out how to replace the batteries in mobile phones, laptops and other portable electronics with power sources that run on fossil fuels. 鈥淯ltimately, the goal is to be able to make millions of these cheaply, like you make Bic lighters or disposable batteries,鈥� says Paul Ronney of the University of Southern California in Los Angeles. 鈥淵ou can imagine, say, Intel, teaming up with Bic.鈥�

If the researchers can get these power sources to work, our not-so-mobile devices would finally escape the grip of the electricity grid. Micro-power generators might power the machines directly, or be used as portable chargers for the battery. Either way, we and our machines would at last be free. If you fancy a day in the park as you type up your presentation, a pocket-sized capsule of propane will give you all the power you need.

The race to develop the micro-power generators that will make batteries obsolete has only just got off the starting blocks, so no one is sure of the best way to do it. Perhaps the most direct approach takes its cue from the technology that dominates at the macro scale 鈥� the internal combustion engine. For a given size, internal combustion engines generate the most power of any device. Combine this with the high energy density of hydrocarbons and the reason petrol and diesel engines rule the roost becomes clear.

To see if miniature internal combustion engines would carry the same punch, Carlos Fernandez-Pello and his colleagues at the University of California, Berkeley, built a mini Wankel rotary engine (see Graphic). At the heart of the Berkeley team鈥檚 engine, which is no bigger than a pile of pennies, is a peanut-shaped chamber, inside which is a triangular rotor about 1 centimetre in diameter. The chamber has an inlet for fuel and air and an outlet for the exhaust. Both open and close as the rotor spins. The three ends of the rotor are always in contact with the chamber鈥檚 walls, and as the rotor spins and precesses 鈥� the point around which the rotor spins also moves in a circle 鈥� it creates pockets of compressed fuel. When ignited by an electrical spark, the fuel burns and the expansion this causes keeps the rotor moving.

The researchers have so far managed to pull up to 10 watts of power from the rotor. Their goal is to raise this to between 30 and 60 watts, which would put the engine in the power range for laptops, PDAs, power tools and laptop battery chargers. The Berkeley team freely admits there is still a long way to go. But this first step may have been the hardest. Before the engine was developed, the smallest internal combustion engines, built for use in model aeroplanes, were about 5 centimetres across and it was commonly assumed that nothing smaller would work. That鈥檚 because as the engine shrinks, the ratio of the surface area to volume increases, so flames come into contact with a large cool surface and are quickly extinguished. But Fernandez-Pello and his colleagues showed that by stacking engines, or by recycling the exhaust, they could keep the walls warm, and keep combustion going.

They have had to compromise, however: their engine runs on hydrogen. 鈥淗ydrogen ignites more easily and burns faster than butane,鈥� Fernandez-Pello says. 鈥淏urning hydrogen allows us to run the engine faster to extract more power.鈥� It also helps ensure that all the fuel ignites and burns fully. The researchers are now working on using butane, which is far easier to transport and is liquid at room temperature.

Multiple miniatures

The Berkeley engine is only one of the 鈥渟tandard鈥� engine types being shrunk. Researchers at the Massachusetts Institute of Technology and the Catholic University of Leuven in Belgium are each working on manufacturing micro-scale gas-turbine engines. The MIT turbine has a volume of less than a cubic centimetre and is built using silicon micro-fabrication technology. And researchers at Georgia Institute of Technology in Atlanta are working on a design in which a piston moves freely within a cylindrical chamber. The two ends of the cylinder function as combustion chambers, each burning its fuel in turn to push the piston back and forth.

While these teams are miniaturising conventional engines, others are striking out with completely different designs. In his Los Angeles lab, Ronney, for example, is working on a solution inspired by an idea dreamed up 30 years ago by Felix Weinberg of Imperial College London. Weinberg was trying to build a macro-scale device that could burn weak fuel-air mixtures. To sustain combustion of such mixtures the walls of the combustion chamber had to be kept warm and the fuel pre-heated, and to achieve this he built a heat exchanger in which the incoming mixture flows in a channel running parallel with one carrying the hot exhaust. He then had the ingenious idea of rolling up the channels into a spiral. This not only doubled the internal surface area that was exchanging heat, it also minimised external surfaces that were losing heat. The device was named the 鈥淪wiss roll鈥�.

鈥淚t didn鈥檛 take a rocket scientist to figure out that this would be a good approach to minimising the quenching problem at small scales,鈥� says Ronney. So his team built a mini prototype of the Swiss roll in titanium and ceramic, which are the most convenient materials for the fabrication process. The device is just 1 cubic centimetre in volume.

At the centre is a combustion chamber containing a sliver of platinum catalyst. After a spark has ignited the fuel, the catalyst keeps the combustion going. Because the Swiss roll preheats the fuel and warms the walls, combustion can be sustained at temperatures as low as 500 to 700 掳C.

In the process, the team discovered something interesting about combustion at small scales. Conventional, macro-scale burners, like a Bunsen burner or a gas cooker, have bright blue flames. But inside Ronney鈥檚 Swiss roll there was nothing to be seen. 鈥淲e had a transparent lid on the device, and were looking inside, and after a while the flame disappeared,鈥� Ronney says. 鈥淭hen I put my hand over the exhaust and it got burnt. Obviously, the flame was still burning.鈥� But even when the team turned out all the lights, there was still no visible flame. The researchers suspect the micro-scale combustion is producing a 鈥渃ool flame鈥� (快猫短视频, 5 June, p 28), burning at a temperature far lower than the 1200 掳C needed to sustain a conventional flame.

So far, Ronney鈥檚 device only produces heat, so the team is now looking into embedding thermoelectric elements in the walls of the Swiss roll to convert it directly into electricity. Microfabrica, a company in Burbank, California, is currently using electrochemical deposition techniques, where metals are laid down on a surface in successive layers, to manufacture thermoelectric Swiss rolls. The devices have yet to generate electricity, but Ronney believes, based on the heat transfer and thermoelectric properties of the materials they are using, that thermoelectric Swiss rolls can convert between 5 and 10 per cent of the fuel鈥檚 energy to electrical energy. That should give somewhere around 150 milliwatts of electrical power 鈥� perhaps enough to replace alkaline batteries.

Another research group, meanwhile, has taken the Swiss roll and used it in an entirely different way. With Ronney鈥檚 help, Sossina Haile鈥檚 team at the California Institute of Technology in Pasadena has been working on putting a fuel cell at the centre of the roll.

This is no ordinary fuel cell, however. Traditional fuel cells have two separate compartments, one where hydrogen is brought into contact with the anode, and another where oxygen meets the cathode. The two compartments have to be tightly sealed to keep the gases apart, but are linked by an electrolyte which allows ions to pass. The net result is a reaction in which the hydrogen and oxygen ions combine to produce water, and drive a current through an external circuit between the electrodes.

The trouble with using fuel cells to power mobile applications, Haile reckons, is that they will be constantly starting and stopping, leading to thermal expansion and contraction that could damage the seals separating the compartments. To get round this, her team has adapted a 20-year-old Japanese technology: a solid oxide fuel cell (SOFC) that requires only one chamber. In these devices, a hydrocarbon fuel such as propane is mixed with an oxidant, typically air, and injected into the fuel cell. There, a solid electrolyte made of a metal oxide is sandwiched between an anode and a cathode. These are coated with catalysts that cause the fuel to release hydrogen at the anode while the cathode carries out its normal production of oxygen ions.

Before Haile came along, engineers had rejected the SOFC for portable power applications because it needs high operating temperatures. 鈥淣o one has come up with a small device that maintains an interior temperature of several hundred degrees Celsius and an exterior temperature that you could conceivably touch,鈥� says Haile. But by placing the SOFC inside a Swiss roll, Haile seems to have hit on an ideal solution.

Burning ambitions

As the propane-air mixture is pumped in through the inlet channel, the fuel cell starts up, generating electricity. Not all of the fuel is used up, and what remains is burnt with the help of a platinum catalyst to generate heat that keeps the fuel cell at temperatures above 500 掳C. The exhaust is cooled in the Swiss roll, however, and emerges at about 50 掳C.

A prototype fuel cell with a volume of just 70 cubic millimetres currently generates about 300 milliwatts of electricity inside a Swiss roll, Haile says, putting the device in the range of a battery for a mobile phone.

Ronney has taken things a step further and developed a Swiss roll made out of a polymer, with an SOFC at its centre. The polymer 鈥� DuPont鈥檚 polyimide Vespel 鈥� is a big plus, he says: it is easy to machine and highly durable, even when kept at temperatures as high as 400 掳C. 鈥淥ur plastic parts look the same after two weeks of continuous testing as they did when they were first built, with no signs of wear and tear,鈥� Ronney says. 鈥淧lus you can drop-kick it across the room and nothing happens.鈥�

Swiss rolls are not without their problems, however. They leave large amounts of fuel unburnt, producing a polluting exhaust, and when combined with thermoelectric elements make inefficient use of the heat that is generated. But there is also scope for traditional fuel cells without Swiss rolls. According to Evan Jones of the US Department of Energy鈥檚 Pacific Northwest National Laboratory in Richland, Washington, the big hurdle with these devices is 鈥� as with the Berkeley combustion engine 鈥� the problem of carrying hydrogen around. Jones plans to get round this by 鈥渞eforming鈥� hydrocarbons 鈥� for instance, by reacting methanol with water to produce hydrogen.

Jones and his colleagues have developed a miniature methanol reformer about the size of the eraser head on a pencil and weighing about 1 gram. It contains a platinum catalyst that helps burn methanol. The heat is used to drive another catalytic process in which methanol and water vapour react to produce a mixture of 80 per cent hydrogen, 18 per cent carbon dioxide and 2 per cent carbon monoxide.

The carbon monoxide is a problem, however, as it 鈥減oisons鈥� the platinum catalyst in traditional fuel cells and stops it working. Jones and his team get round this by feeding the exhaust from their reformer into a fuel cell developed by Jesse Wainwright and his colleagues at Case Western Reserve University in Cleveland, Ohio. Though the cell is traditional in design, it operates at temperatures of 100 to 200 掳C, and at this temperature the platinum catalyst is more tolerant of carbon monoxide so the reactions keep going. The combination of the methanol reformer and the fuel cell 鈥� which is itself heated from the reformer鈥檚 combustion exhaust 鈥� produces up to 500 milliwatts of power, again enough for a mobile phone.

Clearly there is still a long way to go. Micro-scale engines have to contend with friction and heat losses. And nobody has quite figured out how to build the pumps and valves that will be needed to deliver fuel to these miniature engines, whether the devices will work upside down, and how to dissipate their waste heat.

Emissions are another obvious concern, although the engines are so small that this is not always a serious problem. For instance, the CO₂ emissions from Fernandez-Pello鈥檚 rotary engine amount to little more than what a resting person exhales. But carbon monoxide could be more of a problem. 鈥淚n an airplane, if you have 50 people using their laptop computers and everybody is putting out carbon monoxide, it鈥檚 not an acceptable situation,鈥� says Ronney.

Perhaps the only safe bet about the future of micro-power is that the first people to use it 鈥� if it arrives 鈥� are going to be the military. DARPA, the research wing of the US Department of Defense, is funding many of the micro-power projects. It sees the engines as a way of reducing soldiers鈥� burden while increasing the lifetime of the array of devices they carry, such as radios, GPS receivers, laser rangefinders and night-vision goggles.

Of course, even this last application has serious drawbacks. Night-vision goggles work by intensifying thermal radiation, so if the enemy has them too, a soldier carrying a combustion engine 鈥� or a Swiss roll fuel cell emitting exhaust at 50 掳C 鈥� will be a sitting duck. And let鈥檚 face it, when you鈥檙e facing a hail of bullets, do you really want to be carrying a pocketful of highly flammable fuel? Maybe batteries aren鈥檛 quite dead.