Hot moves

Heat from the sun – or even a single candle flame – is sufficient to produce mechanical movement. How? By using mechanical ingenuity and bimetallic disc devices that are made in their millions for thermostatic control. Reference to the patent literature shows the idea of using thermally induced expansion and contraction to directly produce mechanical motion has been around since at least 1895. Names of assignees on existing patents include Lockheed Martin and Nasa. And the results of the latest attempt to crack this deceptively simple looking problem were on show at the recent Young Designers event in Islington London. Interestingly, but not surprisingly, the more complex embodiment looked wonderful, but tended to get stuck. However, the simple embodiment, quickly developed in order to demonstrate the principle at the show, worked a treat, pointing a possible way forward for all kinds of low-grade heat and sun-driven mechanisms. “The origins of the project came from my working at Morphy Richards,” says Thomas Leech, who graduated this year from Brunel University. “I was thinking of using the standard bimetallic discs that are used as thermostatic controls in electric irons, kettles, toasters and hair dryers in an animated outdoor sculpture”. This led on to his major project, titled ‘Solarmotion’, which he describes as: “A research study, which looks at novel methods of extracting useful energy from sunlight. In essence, the long-term objective is to create a sustainable, affordable, low technology, mechanical engine which is powered by solar radiation.” Leech describes the bimetallic discs as “very cheap, reliable and accurate”. The goal of the project has been to find an efficient and cost-effective way of converting solar radiation into mechanical motion that could compete with photo voltaic cells powering electric motors. Much ingenuity has gone into previous attempts to do this (see box), usually resulting in mechanisms that could undoubtedly be made to work, but look likely to incur excessive capital cost per watt of movement produced and/or very low efficiency due to problems with heat transfer. “The core of the project has been in exploring and testing a wide range of variants in the search for a prototype which will produce the maximum movement and force, with the minimum sunlight and complexity,” explains Leech. “This iterative process has spawned many development models and computer simulations, which have been recorded and assessed in order to assist future research.” The discs Leech has been using produce a movement of 1mm with a force of 40N. They flip one way at 124oC when being heated, take 30s to 40s to cool, and then flip back at 90oC. In the working model of his final design, a Fresnel lens focuses sunlight on each disc in turn, directed by a rotating 45o mirror. The 24 discs are arranged in a ring, each acting on a rod that pushes an axle round and that also rotates the mirror through a ratchet mechanism. The working model apparently suffers from problems arising from tolerance stack-up, so that it only rotates through a few segments and then stops. It was for this reason that Leech decided just before the Young Designers event to come up with a much simpler and more reliable mechanism, so he could show visitors the potential of the basic idea. In this device, a bimetallic disk is held in a carrier equipped with two grub screws, which is mounted on a pair of legs forming a parallel motion mechanism. When a candle flame heats the disk, it deflects and presses on one grub screw, 2mm above an axle supporting the carrier. This forces the carrier to rotate and the parallel motion mechanism to flip the disk away from the heat source. The disc then cools down and presses on the other grub screw, making the carrier rotate in the opposite sense, flipping the carrier and disk back above the heat source. “Potential applications for a device such as this are varied,” suggests Leech, “and range from driving water irrigation pumps to powering small electricity generators in remote, arid, environments.” Pointers * Mechanism turns heat directly into mechanical motion, exploiting the behaviour of bimetallic discs manufactured for use in thermostatically controlled consumer electrical products * The discs are cheap, reliable and accurate, and produce 1mm of movement with 40N of force * The challenge of producing a design that is both cheap to manufacture, reliable and efficient has taxed many minds during the last century. But with the availability of computer modelling, it may now be possible to come up with a design that will meet all objectives Patent solutions A quick trawl through the patent literature reveals a plethora of solutions to this problem, both old and new. The earliest would seem to be the wheel-shaped ‘Motor’ patented by William Walter French in Fort Branch, Indiana, in 1895. This consists principally of “two bands alternately subjected to heat for expanding the bands, the latter being connected at their ends with springs in such a manner that when one band is expanded it releases its hold on the springs, while the other band receives the full pressure of the springs”. In 1969, John L Adams from Santa Maria, California, patented another wheel-shaped mechanism with two rotors, one inside the other, attached to each other by bimetallic strips. Heat applied to one side would cause the axis of one rotor to rotate relative to the other. The 1973 oil crisis seems to have stimulated a particularly large number of inventions, not just by lone inventors, but also by large institutions, such as the ‘Mechanical Thermal Motor’. Leopold Hein and William Myers invented it while working for NASA. Patented in 1976, the motor had a coil of tubing, one end of which was attached to a shaft and the other to an outer rotor, producing a reciprocating rotation as the coil was heated and cooled. It is during this period that patent documents start to routinely include the word ‘solar’ as a source of energy. The most recent plausible scheme of which Eureka is aware concerns a patent for a ‘Solid State Thermal Engine’” granted to Robert Howard and assigned to Lockheed Martin in 2005. This returns to the concept of pairs of belts, but takes advantage of the properties of shape memory alloys, rather than relying on thermal expansion. Research into similar schemes in the US continues in a host of institutions. Emailt jleech@hotmail.com