As South Africa seeks to scale up its renewable energy power generation capacity, particularly with concentrating solar power (CSP), opportunities for local manufacture of components are also being investigated.

South Africa-based Cheshire Engineering director Joseph Steele is working to design and prove ‘The Kimberley Mechanism’, which he says is a cost-effective heliostat gearbox, which could be manufactured at a fraction of the current cost of conventional heliostat gearboxes.

A heliostat forms part of the CSP plant - it is the mirror-like device, placed in an array, which tracks the sunlight and focuses it towards a stationary central tower, to heat a medium such as molten salt or water. A large number of heliostats are required for a single CSP plant.

“Heliostats, which need to follow the sun, rotate slowly and precisely, requiring high ratio gearboxes – we are working between 8 000 and 60 000 to one,” explains Steele. It can be imagined that a mirror situated 1 200 m away from a target needs a finer resolution, if it is not to skip the target under rotation, in other words a higher reduction, he adds.

“Photovoltaic cell receptors do not require the accuracy of CSP, and thus for this purpose, a cheaper, single axis mechanism driven by a constant speed motor would suffice for the directional tolerant photovoltaic cells,” says Steele.

He further explains that his Kimberley Mechanism is derived, in part, from designs of the past, in particular, the Geneva Mechanism, and the Hunting Tooth principle. “The design is the enhancement of existing mechanical geometries assembled in a modular manner, capable of a single axis simple control, or T-frame dual axis for larger loads and more complex control.”

The difference with Steele’s gearbox is that there is very little machining, as it is primarily laser cut. “Laser profiling of materials allows for computerised manipulation and appears to provide for the geometry required, at a sufficient accuracy,” says Steele.

Steele is in the process of securing intellectual property (IP) rights for the Kimberley Mechanism. Patenting IP supports development, and can assist in securing financial backing. For patent consideration, novelty needs to be proven, and the proposal and claims of the device need to be tested against prior devices.

The Patent Cooperation Treaty (PCT) process has passed through an international patent search with the claims intact. Steele notes that the PCT submission is helped by interaction with a patent attorney and engineering specialist. The good results of the PCT patent search should influence potential investment, Steele adds.

Patent finance is required at several stages after proposal review, and it can often be a difficult and trying process for an entrepreneur to garner funds to make this possible.

HOW IT WORKS
For the Kimberley Mechanism, the Geneva Mechanism utility was further combined with the ‘hunting tooth principle’, which is embodied as a one-tooth differential gear set. Both mechanisms have old roots, states Steele, and the hunting tooth – an odd tooth added to disrupt cyclical repetition – has been used by millwrights for many years. The Geneva Mechanism, developed by clock makers, has been adapted to provide for high ratio and intermittent motion.

The combined mechanism is essentially a Geneva wheel, with the three sets of twinned and pinned pinions and the rotation of the index Geneva spider wheel, against the fixed larger gear plate. It forces the gear ring plate to rotate in one-tooth increments for one revolution of the indexing Geneva wheel spider, explains Steele.

The modified hunting tooth difference, between a fixed plate and a moveable plate, is constrained to rotate on the same axis as the Geneva wheel with many slots.

Selecting a ratio with a nominal fixed base wheel with a hunting tooth added or removed from a wheel that is free to rotate, gives a pair of gears on the same axis. If one drives the pinned pinion set around the fixed internal toothed profile gear ring, the one tooth difference forces a 100:1 ratio between the two gears when the base wheel consists of 100 teeth.

The intermittent nature of the Geneva wheel, working some 50% of the time, provides for an apparent reduction, and pulsed compensation delays could occur during this idle time.

“With intermittent action not exceeding some seconds, we are still in the envelope of the suns diameter, in other words, the radiance is not from a point source, and this moderates the required tracking position,” explains Steele.

“Contriving to turn the cam wheel once every few seconds, we produce a rotation to cancel the earths rotation, or to follow the seasonal solar track, if a dual axis is used,” he adds.