Towards a better tinyhouse

Inventing to freedom?

Super cheap, easy to build solar tracker.

with one comment

It’s a fairly common problem, I think. Everyone knows keeping the solar panel more or less facing the sun can increase the output of your solar panels, so if you could get a really cheap way to do it that would be great.

Here’s my take.

Well, first of all a tracking mechanism should be able to withstand wind loading including gusts, snow, rain, etc. so that needs to be kept in mind.

Also, we could turn the panel, but it might make more sense to move some reflectors around. But then you have to make the reflectors and stuff, and you need a frame of some sort to mount the panels on anyway, so I decided to just move the panels.

Accuracy is not very important at all, unless the tracker is used with mirrors. That would be nice, to have a tracker that was accurate enough for that. Then you’d need some passive method of controlling the concentration ratio too, ideally, but controlling a shade cloth would be relatively easy.

Also, fundamentally it needs some mechanical energy to do it’s work. Although I bet you could make one with an oven timer or something that would be good, that means you have to get a suitable clockwork mechanism, which is a bother… And we already know there are passive designs.

At the bottom of this post you can see a few patents, that explain the 2 most common methods: the volatile liquid/gravity balancing one, and the gas spring one. Note that one of the patents is issued to Zomeworks, which are the people that sell one of the passive solar tracker things on the market.

Basically this is the design that I settled on, which uses 2 syringes, butane as the working fluid and some string or cord, a couple of eye loops as wannabe pulleys, a shader thing of a suitable shape, and the mounting frame and hinge (the hinge could just be some rope that twists/untwists).

Basically it is like the one in the gas spring patent, but I skipped the diaphragm and hydraulics stuff and just put the working fluid inside the piston, and generally simplified things. Also it uses butane and propane, not freon. The working fluid might leak out over a long period of time, but I would expect it would take many years and it’s easy to add more, and not environmentally harmful in these amounts.

The 2 plungers for the syringes are connected together by a rod of wood or metal, with the ends of the syringes of course sealed. Also, add some oil to lubricate the plunger head. It might have to be silicone oil, because the sulfur compounds in some mineral oil will damage the rubber. Maybe some lip balm or something. Used lip balm the other day to oil my bike chain. There are 2 shaders, one for each piston, which are a bunch of slats, a bit like a window blind.

What happens is that when it is not directly facing the sun, one piston is exposed to more sunlight than the other due to the shader, the temperature of it rises higher, therefore the vapor pressure of the butane-propane in that piston rises, causing there to be a net force moving the system to face the sun. As it moves, the volume of gas in that cylinder expands because the fluid boils, so it keeps on moving until the system is pointing at the sun. When it is pointing at the sun, both pistons are exposed to the same amount of sunlight, so their temperature is the same, therefore the pressure is the same, and no motion occurs. BTW I didn’t think about the shader carefully enough before drawing some of the drawings, as mentioned below, so that part of the drawings is wrong. But otherwise, that’s it. For an unassisted panel that should be adequate. Maybe you’d need to draw on the pistons with a marker to make them black to absorb more sunlight, for reasons explained below.

The axis of the rotation of the solar panel would ideally be right down the middle of the panel so that the wind would not usually exert any net torque on the system. So arrange the hinges accordingly, actually I bet you could use a piece of rope that just twisted/untwisted. The pistons themselves provide damping form the wind.

The purpose of the shader is to reduce the amount of sun hitting the piston(s) when the sun is on the wrong side, but it should do so without affecting the amount of sunlight light per unit lenght hitting the working gas containing volume of the pistons. I mean, when the gas volume is higher, the piston is displaced further along, right. Well the amount sunlight hitting the gas containing volume should probably go up more or less linearly as the surface of the wall of the syringe that the gas is exposed to goes up, for optimum alignment. The non-slatted shader in the diagram would mess that up, though, so it might not be ideal. Instead a number of slats, like a window blind, would be better.

Syringes should be cheap, I would think, 60 ml one is looking like $2 at retail, $4 for a 140 ml one on google products. The required size is determined by the vapor pressure vs. temperature relationship of the working fluid, in this case a mix on butane and propane might be good, at the temperatures of concern. Take the area of the piston head, and if the ambient temperature is in the -10 deg c range, it looks like we would be getting a pressure change of, just as a ballpark figure, 0.42 PSI per degree. Not much. But it doesn’t take much though, especially if you use rope as the hinge. Hm. For propane it looks like more like 1.67 psi per degree. But then the syringe has to withstand up to 15 or something atmospheres on a hot day if the working fluid gets to 60 degrees or so, which I’m not sure if it could. Maybe though. You could also use a piece of mylar to form sort of a trough collector that would increase the temperature and sensitivity of the temperature to differences in sunlight intensity.

Mixing the fluids might be a bit finicky, but put it in the freezer and when you spray it out the bottle into the syringe it cools down even further (as some boils off), so it is and briefly stays a liquid. Same deal with propane. Then put the plunger nearly in the syringe, but not quite, so the butane+prop, as it boils and escapes, flushes the air out of the syringe. After letting it flush for a second or two, push the plunger in and make sure it stays in some way…. I bet that would work okay.

Another maybe better way could be to just use a single piece of pvc tubing and a sort of roller. A peristaltic pump, in reverse, basically. You could simply seal the tube on either side of the pump, and add the working gas to either side. The roller could be not actually a roller but just a piece of UHMWPE rod, or even just a greased carriage bolt maybe. Add some lubricant to the inside of the pump tube to seal it a bit better, and some grease around the roller/rod to reduce friction and wear. Eventually the liquid working fluid might escape, or accumulate on one side or the other, but that’s no big deal. They would be so cheap several of them could be used maybe, in parallel, if more actuation force was needed. Might be able to get away with just butane here.

In this case instead of the shader you could just tilt the tubes in a sort of inverted V, so they are exposed to varying radiation levels due to varying angle of incidence. You could do that with the pistons too though, with some modification….

The thing is, for a collector with a mirror it would be nice to have reasonably accurate alignment. The mirrors in the cheaper photovoltaics thing are tolerant of some misalignment, but still.

It might make sense to try to color the inside layer rather than the outside layer of the syringe walls, to absorb more light to make the temperature of the gas in the piston a bit more sensitive to the intensity of sunlight hitting it (because then the ehat is produced inside the piston, and you get the insulation value of the wall working for you instead of against you, as you attempt to get the heat through the wall into the gas)… hey, tomato juice’ll stain anything, right? Actually, the pigment tomatine is soluble in PE plastic. Maybe there is a more suitable pigment like that somewhere. Otherwise, a ball of cotton dyed black or something added to the interior volume of the piston, which would contract/be compressed so that it always fills the volume of the piston? Maybe marker would work, but it seems like it would come off pretty fast with the piston sliding over it all the time.

However a cotton ball would not be ideal because ideally, for optimum alignment the temperature vs. sunlight ratio would remain the same, regardless of where the piston head/plunger was/the volume of gas in the syringe. The thing is the system does not inherently turn to point at the sun, it turns till the pressure in the 2 pistons is the same ( or it hits the limit of it’s movement, whichever is first…). The pressure is directly related to the temperature and the characteristics of the working fluid. But the temperature depends on may things besides the amount of sunlight hitting the tube, it includes how much sun gets absorbed, then how much heat actually gets absorbed by the gas, and then how fast it gets lost again to the outside, depending on ambient wind, thermal conductivity value of the wall of the gas containing volume of the piston, and a few other minor factors like the radiative heat loss.

Having 2 pistons cancels out the effect of wind to a fair degree, but the area of the wall which the gas is exposed to, and therefore the total thermal conductivity separating the gas from the outside world changes depending on the position the piston head is in. So canceling that out might be nice, which is easy to do if the wall is also the sunlight absorber; the smaller the wall, the less sunlight absorbed. Problem largely solved. There is still heat loss from the ends of the piston, the plunger and the stationary end, but they are rubber/ could be insulated easily.

This is also why the way the shader is done matters, since that affects the amount of energy getting into the gas. Maybe using mirrors rather than a shader would be better, too.

The thing is that when the sun goes down, the system will be facing west, so next morning it would still be facing west, and it would ideally be facing east. A weak spring could be used to slowly turn the system to the east over the course of the night, as it causes the fluid to condence preferentially in one piston due to the slightly higher pressure.

It could also be possible to, either alone or together with one of the things mentioned above, if you look at that gas spring patent, you can see the guy is using the hydraulic pistons connected to a cable, which then wraps around a circular part. The shape of that circular part could be changed so that it was not circular but whatever shape you wanted, so that depending on the present angular alignment the system was in, pulling on the cord a certain amount correlates with a different change in angular alignment. Then you’d need a cord-tensioning thing of some sort to take up any slack cord that would sometimes exist.

And now for some randomized notes (“further reading”):

Have to accommodate for the ambient temperature, changes in sunlight including throughout the day including sudden cloudy periods, (nonlinearities in the expansion for sometypes but those are not really conditions)

Could use elastic for the motion, wouldn’t last though

bimetallic springs maybe somehow

epansion of liquid water, thermal inertia too high really

There are some patents at the bottom of this post that I stole some ideas from. The basic principle used in the “gas spring” one is what I am using.

could have a force exerted to turn it then a regulating mechanism that allows it to turn when not aligned, then a reset mechanism powered by butane expansion /contraction during day/night

really a small motorized one would be fine as the cost is not high if cheap parts are used, but if you are only building one then you need to track down and order all the parts,solder etc. which greatly increases cost,. be nice if there was something that could be made with nearly houshold stuff

ratchet and pawl, but would require winding up , could be wound up by butane expansion

zomeworks ones amusingly overpriced $500

The liquid one uses 2 containers of volatile liquid like freon (or propane) arranged in such a way (see patent) that when the system is not aligned with the sun, one container is more exposed to sunlight than the other, so it is getting more heat energy put into it than the other.

can’t use freon, and iirc there are some examples on the web of people using propane, which involves pressures that are quite high. Butane or pentane or a mixture of them might be good. Those “lovelamp” things that you put on the palm of you hand, and the liquid boils, and goes to the top of the device use a mic of pentane and butane that boils just at room temperature, I think, which could be perfect if the pressure/temp rise was enough to effectively push the liquid from one chamber to the other. wine or pop bottles and pvc tube that fits ove rthe mouths of the bottles The return-spring mentioned in the patent could maybe be a balloon of some sort filled with the right volatile fluid. Butyl rubber would not loose pressure much over time.

Actually you could use water as the liquid, but the pressure increase per degree temperature increase at the low temps is too low. Maybea mix with methanol or gasoline would work. Or you could make arrangements so that the liquid displaced is actually water, and use only a small amount of freon or butane or propane, thereby reducing the amount of volatile liquid needed.

But you need a shock absorber to make the system not get too disturbed by gusts of wind.

a bunch of 140 cc ones for a dollar each
Can just search google products for “syringe 60cc” or 60 ml, and search in a variety of sizes, might be able to find some locally available for sale from pharmacy or something, ask at the pharmacy counter.
There are more patents mentioned within these patents, and the “patent references” section of the web page fr each patent provides even more


Written by gregor

March 1, 2011 at 16:09

Posted in Uncategorized

One Response

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  1. Hey Gregor,

    sorry i missed the con again, allot on my mind, an trying to get organized for spring…kinda going mental lately, so if i fall of the radar for awhile, don’t worry, i’ll stay in touch as best i can, and try to keep up on the haps…



    March 3, 2011 at 23:41

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