I sort of did the mirror experiment. Bought the ultra cheap mirrors from Oriental Trading Company. Pretty disappointing. If a good mirror reflects 95%, I'd say these reflect 70% at best. They're cloudy plastic. Nevertheless, in the 15 minutes I had to do it I measured 5 suns (ignoring cosine loss). Outside ambient temp, about 65F. Temps measured with an IR temp gun something like this.
Readings:
0 suns: 65F (white plastic, in the shade)
1 sun: 74
2 suns: 76
3 suns: 81
4 suns: 85
5 suns: 93
Experiment very far from perfect, needs to be done with more time and care, better mirrors, more mirrors, hotter ambient temps. But, temp differences were less than I expected.
Also, this blog now resolves to www.heliostats.org. I owned the domain (you can see older material there by dropping the www prefix) so figured why not class things up a bit. My wife did it as her contribution. Thanks J!
Sunday, January 24, 2010
Mirror Mount
I'm convinced that the specifics of the mirror mount are important. A simple, efficient mount which bears almost all the weight of the mirror and includes a counterbalance system to minimize the force required to move (tilt) the mirror is the key. If well done, the motors which move and hold the mirror in place can be minimized.
Motors are one of the key cost variables on the project. My target motor is a cheap low torque stepper. Cost goes up fast when more torque is needed. I'm also hoping to integrate a worm gear to trade speed for torque. Also worm gears have a certain holding force even at rest. I hope to use that to hold the mirror in place under ordinary conditions, so power is used only when it's moved (once every few minutes). Since power generation is the ultimate goal, the system should be miserly with power.
I'm investigating two mount directions at this point. First is the ball and socket, second is the "gyroscopic drink holder", both pictured below. I like the "gyroscopic drink holder" better. It was literally sold as a device you could attach to your dashboard with double stick tape then put a hot cup of coffee on your dashboard while you drove to work. It would keep it level so it didn't spill while you drove around corners and over hills. Comedy. My friend found it at a garage sale.
It's not a gyroscope but nested curves with ball bearings in between. You can see in the picture plastic ribs on the lower bowl/curve create six pie wedge sections, one for each ball bearing. I'm pretty sure there's a also a rib ring that keeps the bearings from rolling down to the bottom. So the top bowl moves smoothly over the bottom bowl. Then, the red screw is attached to a weight which hangs below the second bowl. The weight is the counterbalance so the top bowl always wants to stay level. To make it, I'm trying to find some nesting bowls, put some ball bearings in between and hang a counter balance. The mirror would go on top facing directly up, but could tilt in any direction, the counterweight pulling it back to level.
The ball and socket is easier to get parts for. I bought some ball thrust bearings (the kind used for lazy susans, not the radial motion type) from VXB. First pic is how the bearing would look on top of a sphere, next pic is how the mirror frame would sit on top of the bearing on a sphere. It's easy enough to buy the spheres, and this bearing type would work on them. This is a second choice at the moment because even with a counterweight, once the mirror moves past rest point, the mount itself makes it want to keep going. Whereas with the bowl style mount, when out of rest position the dynamics of the mount itself make it want to go back to rest. More potential energy at the rest point of this mount as my high school physics teacher would say.
Anyway, just posting as an update. Need to make something and post pics and how to's. Meanwhile, working in the background on sourcing motors.
Should also note that for v2, the configuration is an array of mirrors reflecting light up to a target that is raised but in the center of the array -- more like Ausra in the layout. Whereas v1 was more for an array all to one side of a target, reflecting back toward the sun -- more like a classic power tower. The difference is because how I guess retail solar (especially lightly concentrated PV) might play out. So v2 has a rest position for the mirror at something close to the horizontal, where the rest position for v1 was something close to vertical.
Motors are one of the key cost variables on the project. My target motor is a cheap low torque stepper. Cost goes up fast when more torque is needed. I'm also hoping to integrate a worm gear to trade speed for torque. Also worm gears have a certain holding force even at rest. I hope to use that to hold the mirror in place under ordinary conditions, so power is used only when it's moved (once every few minutes). Since power generation is the ultimate goal, the system should be miserly with power.
I'm investigating two mount directions at this point. First is the ball and socket, second is the "gyroscopic drink holder", both pictured below. I like the "gyroscopic drink holder" better. It was literally sold as a device you could attach to your dashboard with double stick tape then put a hot cup of coffee on your dashboard while you drove to work. It would keep it level so it didn't spill while you drove around corners and over hills. Comedy. My friend found it at a garage sale.
It's not a gyroscope but nested curves with ball bearings in between. You can see in the picture plastic ribs on the lower bowl/curve create six pie wedge sections, one for each ball bearing. I'm pretty sure there's a also a rib ring that keeps the bearings from rolling down to the bottom. So the top bowl moves smoothly over the bottom bowl. Then, the red screw is attached to a weight which hangs below the second bowl. The weight is the counterbalance so the top bowl always wants to stay level. To make it, I'm trying to find some nesting bowls, put some ball bearings in between and hang a counter balance. The mirror would go on top facing directly up, but could tilt in any direction, the counterweight pulling it back to level.
The ball and socket is easier to get parts for. I bought some ball thrust bearings (the kind used for lazy susans, not the radial motion type) from VXB. First pic is how the bearing would look on top of a sphere, next pic is how the mirror frame would sit on top of the bearing on a sphere. It's easy enough to buy the spheres, and this bearing type would work on them. This is a second choice at the moment because even with a counterweight, once the mirror moves past rest point, the mount itself makes it want to keep going. Whereas with the bowl style mount, when out of rest position the dynamics of the mount itself make it want to go back to rest. More potential energy at the rest point of this mount as my high school physics teacher would say.
Anyway, just posting as an update. Need to make something and post pics and how to's. Meanwhile, working in the background on sourcing motors.
Should also note that for v2, the configuration is an array of mirrors reflecting light up to a target that is raised but in the center of the array -- more like Ausra in the layout. Whereas v1 was more for an array all to one side of a target, reflecting back toward the sun -- more like a classic power tower. The difference is because how I guess retail solar (especially lightly concentrated PV) might play out. So v2 has a rest position for the mirror at something close to the horizontal, where the rest position for v1 was something close to vertical.
Monday, January 18, 2010
Version 2
After dithering about whether to make v1 operational or work on version 2, I'm going to do some work on version 2. V2 design has been going for awhile in my head so may as well get it rolling in real life.
The core design issues to work through (using small motors to their best advantage and a cheap, simple hardware design that leverages software) are the same for either version, so there isn't much lost by resolving them in v2 instead of v1.
Version 2 in a nutshell ("Help, I'm trapped in a nutshell"):
Or, here's a sweet gyroscopic drink holder from the 80's that would work great and probably be cheap. I just can't find/figure out how to make it cheaply.
I just spent hours looking for a good mirror mount on the internet with some leads but nothing perfect yet.
V1 used the a motor to rotate the mirror around a center mounting shaft. V2 also has a central shaft, but it's just a weight bearing point (no motor there). There are two other points of connection to the mirror, one on the N/S axis, one on E/W axis, with a mirror attached to each point. Between the three points (one static, two moving), they define a plane which reflects the sun to the target.
I'll figure a way to post some drawings to make the design clearer. Or better, just post some pics as it gets built.
The core design issues to work through (using small motors to their best advantage and a cheap, simple hardware design that leverages software) are the same for either version, so there isn't much lost by resolving them in v2 instead of v1.
Version 2 in a nutshell ("Help, I'm trapped in a nutshell"):
- Base: $20. Same concept as v1: unistrut or a similar component based system.
- Mirror mount: $10. This could be a universal joint, ball and socket platform (this and this or just this), or something equally simple, cheap and effective.
This is a ball and socket part of a monitor mount that seems ideal, but I couldn't find a component part:
Or, here's a sweet gyroscopic drink holder from the 80's that would work great and probably be cheap. I just can't find/figure out how to make it cheaply.
I just spent hours looking for a good mirror mount on the internet with some leads but nothing perfect yet.
- Motors: $20 for two. Ideally these are small, sealed stepper motors with worm gears easily integrated. Well, anyway, a boy can dream.
- Mirror and frame: $30. Roughly 1 square meter of mirrorized acrylic or whatever else works.
- Microcontroller, motor controller and wiring: $10 (amortized over 10-20 mirrors, so $100-$200 total).
- Installation and profit: $20. This is more or less a random number.
V1 used the a motor to rotate the mirror around a center mounting shaft. V2 also has a central shaft, but it's just a weight bearing point (no motor there). There are two other points of connection to the mirror, one on the N/S axis, one on E/W axis, with a mirror attached to each point. Between the three points (one static, two moving), they define a plane which reflects the sun to the target.
I'll figure a way to post some drawings to make the design clearer. Or better, just post some pics as it gets built.
$100
This is a picture of what my dad gave me for Christmas to "use this for your special garage project". Nice that it should buy exactly one heliostat. I will probably save it until it does.
Wednesday, January 13, 2010
Lightly Concentrated PV (cont)
Following up on Monday's post, here's an excellent tutorial on PV provided by the US Dept of Energy. It discusses band gap, temperature effects, doping, etc. About 50% of sunlight that hits solar panels is outside the band gap and could perhaps be filtered by sheet films like these, though there are some off-axis issues with most filters.
In other news, Oriental Trading Company has 6" square mirror tiles for cheap. I'm still in.
In other news, Oriental Trading Company has 6" square mirror tiles for cheap. I'm still in.
Monday, January 11, 2010
Playing Around With Lightly Concentrated PV
The application I've been thinking about lately is concentrated light on solar panels. Usually, when you are talking about concentrated solar for PV (photovoltaic), you mean high concentrations (100-500 suns) focused on a few square centimeters of high efficiency (and expensive) triple-junction cells. I'm looking at much lower levels of concentration, say, 10 suns, concentrated on something very close to a normal solar panel.
According to backyard experimenters, typical solar panels seem to put out 1.5-2 times their rated output under concentrated light but the higher temps generated by multiple suns can ruin the panels. Though reflecting light onto heliostats is done frequently, I haven't seen the basic info of how hot surfaces get under 2x sun, 4x sun, 10x sun and so on -- preferably under different weather conditions. I'd like some data on how hot things get under multiple suns. Given this, one could pair mirror configurations to solar panels operating parameters.
We know too that solar panels waste most of the of solar energy that hits them since they're only sensitive to specific light frequencies (wikipedia et al). Ideally, one could concentrate, say 10 suns, on a PV panel. Prior to the panel, there would be filtration films to eliminate IR or UV rays and so reduce the heat load. The ideal configuration would be 10 suns filtered precisely by a film down to the small band of frequency that the panel uses. If 80% of the heat were filtered out (overly optimistic? yes.), then you're down to 2x sun. Tweak the panel to handle high heat and higher currents and go for it.
Admittedly, this is an experiment undertaken with plenty of ignorance There are multiple effects to worry about if you actually focused 10 suns on an off-the-shelf solar panel for a chunk of time. Overheating the panel say, or size of the wiring at the back of the panel. Would the panel need to be excessively doped to produce the desired current? Sure, who knows.
But for all it's flaws, I still want to focus 10 suns on a brick wall and measure the temperature. Then, add a filter and see how much it helps. I'll let you know the results. Then, back to heliostats.
According to backyard experimenters, typical solar panels seem to put out 1.5-2 times their rated output under concentrated light but the higher temps generated by multiple suns can ruin the panels. Though reflecting light onto heliostats is done frequently, I haven't seen the basic info of how hot surfaces get under 2x sun, 4x sun, 10x sun and so on -- preferably under different weather conditions. I'd like some data on how hot things get under multiple suns. Given this, one could pair mirror configurations to solar panels operating parameters.
We know too that solar panels waste most of the of solar energy that hits them since they're only sensitive to specific light frequencies (wikipedia et al). Ideally, one could concentrate, say 10 suns, on a PV panel. Prior to the panel, there would be filtration films to eliminate IR or UV rays and so reduce the heat load. The ideal configuration would be 10 suns filtered precisely by a film down to the small band of frequency that the panel uses. If 80% of the heat were filtered out (overly optimistic? yes.), then you're down to 2x sun. Tweak the panel to handle high heat and higher currents and go for it.
Admittedly, this is an experiment undertaken with plenty of ignorance There are multiple effects to worry about if you actually focused 10 suns on an off-the-shelf solar panel for a chunk of time. Overheating the panel say, or size of the wiring at the back of the panel. Would the panel need to be excessively doped to produce the desired current? Sure, who knows.
But for all it's flaws, I still want to focus 10 suns on a brick wall and measure the temperature. Then, add a filter and see how much it helps. I'll let you know the results. Then, back to heliostats.
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