Sunday, 12 March 2017

Designing variable rate micro-hydroelectic plant

Generating power using water is nothing new. Find a location where water can be channelled from a high location, through a wheel, (or turbine) and released at a lower location. The difference between the high location and the exit from the wheel is known as the head, (of water). This was used even before the industrial revolution, to turn wheels that performed work, (ground grain into flour for example.)
The industrial revolution used water to perform more and more tasks. The problem was that you can't take a river with you. This is where coal empowered the steam engine. First used to drive pumps and then steam engines were mounted on rails for locomotion. More and more ideas collided with both problems and everyday tasks. The industrial revolution tried to make life easier, (though it also created inner-city slums - a problem that we still have not solved.)

Out of the industrial revolution came the industrial age. In the years from 1930 to 2030 it was still fossil fuels, (oil and coal) that were used to supply the energy needs of the growing industrialised countries. Yes we still used hydro and we had fission but for the most part it is coal and oil that cooked our food  and powers the television, (and certainly powers transportation of resources into the cities where an insect-colony level of density is observed.)

 So what is the problem? The problem is that how ever much oil there is hidden under Greenland by the year 2000 half of the oil and coal has already been consumed, (lets just presume this is true) and we can do many things with modern technology, but we can't make more oil or coal and we still can't capture the suns energy as efficiently as plants do.

(This is just a visual guide, to help my explanation.)

  So I grew up next to a small stream and love mechanical devices. The idea that water can turn a wheel that can produce electricity still makes me smile. It seems as natural and as magical as gravity, (and sometimes just as frustrating.) 
 The two problems that my location has from a hydroelectric point of view are no head, (which is why I mentioned it at the start) and a massively variable flow rate. So why is this a problem? If you are not scared off by terms like volts and amps then you can probably let me run with the idea that the head is equivalent to ampere-hours and the flow rate is equivalent to the watts. ( For this note we just need to think: amps * volts = watts ). So with this undershot design we are dependent on the flow rate.

The devices that we use today in our homes use AC current and like to have a fixed voltage, (110-240 volts depending on the country) and require 1-20 Amps, (I'm talking household appliances like a toaster or a microwave oven.)
So it should be clear why large scale hydroelectric systems seem to always have a large lake with some means of flow control. This is because the best locations for the generation have been selected, (usually locations where it is easier to build a wall to hold as much water back as possible. In the past whole villages have been moved or flooded because they occupied such locations, (Tignes, France.)
 Also hydroelectic systems do not like creating elecriticy that is not being used, (things tend to melt, in a bad way.)

I don't want to move house, and I don't have the money to either build a massive pipeline to create head, or a really long power-cable to get the electricity back from the best location on the stream. If we are going to design useful systems they are going to have to fit in with the existing landscape and be as unobtrusive as possible. (My theiroy was that if this works they could be added to any small stream, and added many times to the same stream.)

So the variable rate problem is not too hard to solve: bicycle gears - simply employ a gear system that engages higher gear ratios between the turbine and the generator as the turbine velocity increases. But that is only half of the story. If you inspect my proposed site the water level can vary by over a meter, (from zero in summer.) The stream is full of a range of limestone rocks/pebbles and every the occasional boulder. The stream is insized and creating a water chase is beyond the scope of this project. The solution that I came up with uses two vertical PVC drainage pipes with empty two liter polyethylene terephthalate drinks bottles inside. 
The bottles act as floats upon which the rig, (turbine and generator on a frame) can balance. The vertical pipes are resessed into the sides of the stream, (it is less than two meters wide) with a vertical slot cut into the face of each. As the water level rises the bottles float up and lift the rig. This means that as the flow rate increases the contraption is lifted clear of the rocks that are moving along the bottom. This protects the blades of the turbine, and though it sacrifices some of the energy of the stream, we are still in contact with the part of the flow with the least friction from the sides and bed of the stream. This all looked good when I sketched it up in freeCAD but with some rough calculations the rig was going to be too heavy. The first amendment was easy: a cable connected inside the PVC verticals to the rig. This ran to the top of the verticals, over a pully and down the outside to a counterweight. This counterweight ended up being a steel sleeve that wrapped round most of the vertical, (this kept is tidy and accessible to raise the rig for maintenance. Originally the blades of the turbine were made from plastic guttering, but to experiment with shape we had a set that we made from wood. This changes the weight of the rig, but with the location of the counterweight it was easy to tack on pieces of lead to correct the required mass.
  Then winter came. The torrent was huge, and fast - lots of lovely power ready to be collected. The second problem with the design became apparent: The buoyancy of the bottles and the turbulence of the surface meant that the rig was being bounced up and down. We could try and increase the friction between the inner surface of the PVC vertical and the bottles, but friction is something that we have been fighting to reduce, (more power less wear.) Then I remembered by sailing days and the concept of a sea-anchor. We all know anchors are heavy things connected via a rope or chain to a ship. The anchor rest on the bottom and provides a fixed connection. What happens when the water is too deep? A sea-anchor is like an under-water parachute. To begin with I tied a string to each of the corners of a pocket-hankerchief and tied the four strings to the bottle. This hankerchief-anchor rested under the bottle and in the water. When the level of the water rapidly increased this latest addition stabalised the assent. This was a good prototype, but only solved the "bu" not the "mp". Running with the idea that if it worked in one direction then it should work in the other I added another pully to the bottom of the vertical supports and ran a new line from the bottom of the counter-weight, over this new pully and out to another hankerchief-anchor shock-absorber, (by now the HASA was made of rip-stop nylon.)

Now I just have to finish the automatic gearing and measure the useful current.

p.s. Yes the turbine axel is connected to bearings that are attached to the frame of the rig. These are not shown in the above image. Also missing is the counter-weight contraption, (because that would give away the story.)

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