Donkey-powered generator

1st March 2002 By: Terry Mackenzie-hoy

I undertook that the next few columns would discuss alternative energy, based on a number of papers given at a recent conference.

Are you ready for - Prof Gerhard P Hancke of the University of Pretoria? Probably not, because the Prof has been doing research into using donkeys to generate electricity.

I quote from his paper: "Using a draught animal as the prime mover has the great advantage above photovoltaic, wind and hydro systems that power generation depends on the availability of a suitable animal, rather than the prevailing climatological parameters. If enough draught animals are available, operation of a suitable generating unit is possible for extended hours a day. In contrast to photovoltaic or wind systems, where enough battery storage is required to provide for long periods of low insolation or wind, the animal-powered generator does not need such excess storage capacity, hence a smaller storage battery may be used." OK so far? The Prof listed a number of draught animals and their developed power, based on an average work speed. A bull develops 560 W at 0,6 m/sec to 0,75 m/sec while a horse does 750 W at about 0,8 m/sec. A donkey does the least out of a bull, mule and horse and does the same as a cow, developing 750 W at about 0,25 m/sec.

A donkey was chosen for further investigation since donkeys are cheap to buy, cheap to keep and are always willing to work. To harness donkey power the Prof's team built a 'rotational speed multiplier'.

If you have ever played in a playground, you will know that, apart from the roundabout there is a wheel thing set about waist high off the ground which you can spin. Imagine one of these with a single long extended spoke to which the donkey can be harnessed (to push) and a gear train connected to it and you have the device. The rotational speed multiplier connects to a permanent magnet generator, which operates at about 75 times the rotational speed of the animal.

In tests, the system developed about 200 W output power (as rotational energy) and then a maximum of 148 W (electrical) when used to charge a battery. This is about 11 A at 12 V (although a 24 V battery was used in the experiment).

So there you have it. The system has been patented.

Now, it may be that one will see, in the future, hundreds of donkeys slowly circling in the dusk, while the glow of TV sets rises from thatched mud huts all around in the rural distance. But I doubt it. However, what is quite feasible is the use of donkeys to power a charging station where people would bring batteries to be charged. At present, in many parts of the local township areas, there is a fixed fee for charging batteries (I think it's about R5) and so the concept would not be new.

To buy a donkey costs about R70 and the animals are quite robust and easy to feed – they forage widely. The cost of the mechanical and electrical bits is relatively high (say, about R2 800) and the permanent magnet generator is a unique component of the whole setup. I would have been inclined to use an old Ford Escort generator or even a car alternator – one is only worried about rotational speed here – and the cost could come down.

In reply to a question the Prof pointed out that the gear train had to be really quite efficient – pulley belts and the like are not very good – and so a train based on bicycle gearings and chain was decided on. I wondered why not just connect the whole lot to a motorcycle gearbox, but I guess that cost would again come into it. Again, why not use a cow? We have a lot of them. Well, we have more donkeys, it would seem, and cows are not always willing to work.

But what a wonderful and unique idea from Professor Hancke.