Heliostats have been around a long time. Wikipedia gives the usual good introduction. Building them is not an engineering challenge, it's done all the time. Building them cheaply is a challenge and it's one where success contains huge potential benefit.
This Sandia Labs study (section 6.1) estimates that if heliostats can be made for under $100 per square meter, solar power could be made in a power tower configuration for 6 cents/kW-hr, which is competitive with conventional wholesale power generation. Right or wrong, a hundred bucks per square meter (of mirror) is a sensible target. It's a nice round number that represents roughly the state of the art.
Is there any reason to expect it's possible to make working heliostats that cheaply? Several companies (eSolar, Asura, CoolEarth Solar,...) all seem to think so and some of them may have already succeeded. So it's not unimaginable. Is it reasonable to think internet hobbyists could do so? Well, the last 40 years have brought plenty of advances to simplify small scale innovation -- the internet (for shopping and sharing information), global manufacturing and distribution that lets you buy things cheaply, huge advances in the power and availability of microcontrollers & motion control (robotics, motors,...), a recognized practice of public design (open source software and hardware) -- the list is long. So, possible or not, it's hard to think of an easier time to try it.
Further, heliostats created for commercial power towers are only one configuration. A retail configuration has different design requirements and price points. Retail power costs about twice wholesale power, so from that perspective, there's more money to work with. On the other hand, a house in the USA spends on the order of $1,000/year in electricity costs (if gas heated and not including an electric car). That's pretty low -- less than what some people (like me) spend on things like phones and internet access. On the third hand (or other foot), retail systems also cost much less to install. So if they make economic sense to an individual, people can pay for it themselves, without raising commercial capital to build a large plant. Commercial or retail, how you are going to use the heliostat -- the specific configuration -- becomes integral to the design pretty quickly so there is more to say on this topic.
Anyway, that's the short case for why this project is worth trying.
Tuesday, December 29, 2009
Wednesday, December 9, 2009
V1: frame and mirror
The frame was made from 1 1/2" aluminum angle. It was assembled by my friend who is handy. I provided consultation and beers. It is 4' by 2', so the long sides are 4' pieces, the short pieces are 2'. It was cut with a hacksaw and screwed together with self-tapping screws. Here's a pic.
There are a lot of holes in it because the original plan was to use bolts which could be adjusted to push against the mirror at strategic points and bend it into parabolic like shapes for tighter focal points. It didnt' work that well. A 12' piece of aluminum angle is about $11, though I used scrap I found.
Mounted to the frame was the mirror, which was a 4' x 2' piece of mirrored acrylic. Here's a site that has it for $34, I think this is where I got mine. In the end it was fixed to the frame by two bolts in the center and another two bolts in the middle at the top and bottom. When drilling the holes for the bolts, we cracked the mirror. To avoid cracking it, use a high drill speed but drill slowly, don't rush. If doing it again, without trying to make a bunch of standoffs to shape the mirror, I might just do a hole in the middle and the four corners or something more sensible. Use washers against the mirrored edge.
Here's another pic of the center.
The t-strap I added at the end, that's where the altitude adjustment bolt attaches. The other two holes are used to fix the frame to the hinge. Placement of these is important because you want the supporting pole to fix to a point close to the middle of the mirror so it's well balanced (minimize work required by motors). Find center line of the frame, then off-set those holes so that the hinge pin falls along the center line.
There are a lot of holes in it because the original plan was to use bolts which could be adjusted to push against the mirror at strategic points and bend it into parabolic like shapes for tighter focal points. It didnt' work that well. A 12' piece of aluminum angle is about $11, though I used scrap I found.Mounted to the frame was the mirror, which was a 4' x 2' piece of mirrored acrylic. Here's a site that has it for $34, I think this is where I got mine. In the end it was fixed to the frame by two bolts in the center and another two bolts in the middle at the top and bottom. When drilling the holes for the bolts, we cracked the mirror. To avoid cracking it, use a high drill speed but drill slowly, don't rush. If doing it again, without trying to make a bunch of standoffs to shape the mirror, I might just do a hole in the middle and the four corners or something more sensible. Use washers against the mirrored edge.
Here's another pic of the center.
The t-strap I added at the end, that's where the altitude adjustment bolt attaches. The other two holes are used to fix the frame to the hinge. Placement of these is important because you want the supporting pole to fix to a point close to the middle of the mirror so it's well balanced (minimize work required by motors). Find center line of the frame, then off-set those holes so that the hinge pin falls along the center line.
Monday, December 7, 2009
V1: movement
Precise and controlled movement of a mirror is the bread and butter of a heliostat. All the heavy lifting was done by the folks over at servocity.com. As mentioned below, I bought my way out with this. Though too expensive for a production solution, this piece solves several problems.
First, it provides a motor mount and mechanism for suspending the motor to the shaft. I bought the 90 degree arm to attach it, see the pic. The 90-degree arm can be fixed to the gearbox on the side. Bought small (M6 maybe?) nuts and bolts to fix it on there. Reserve the screws that came with it to fix something else to the large gear. Attached to the other end of the 90-degree arm is a 3" t-strap (Stanley) which had existing holes which aligned with the arm and the threaded rod. The T strap is fixed to the threaded rod with a bolt above and below and serves to hold the motor fixed in a given orientation so when the gear turns it can move the mirror rather than just spinning the motor mount around the center rod.
The second (and best) thing the Gearbox does is integrate a gear (to step down the motor) and a mounting platform with ball bearings which simplifies transferring the rotation of the motor shaft to the mirror. The mounting platform includes a hollow shaft which the 1/4" threaded rod passes through. Note that when assembling, there needs to be a nut already threaded onto the rod that the T-strap will rest on. Next, slide the T-strap/motor frame onto the threaded rod, but after the T-strap passes on, screw another nut on, and screw it down as the T-strap and then gear shaft threads over the rod. (Just order of operations -- otherwise you can't get get the 2nd nut on after the motor is fitted over the threaded rod).
Next step is to add hardware to attach the mirror. Buy 1 1/4" perforated angled iron (Home Depot or wherever) and cut a piece about 2.5" long. The piece I got alternates hole sizes. The circular hole is exactly 3/8" and so it fits snugly over the shaft provided by the servocity gearbox. Around the 3/8" hole, drill 4 holes using the 90 degree arm as a template to show hole placement and size. The drilled angle iron looks like this.
Use the screws that came with the gearbox to screw the angle iron to the gear (those holes are threaded). I only got two screws to thread, probably because my holes weren't well aligned. Also, because the 90-degree mount is 1/4" thick, the screws don't go flush to the top of the angle iron. If this part of the design sticks around it'd be worth adding washers and nuts as spacers.
Next, the hinge. This was a normal, 3" door hinge from the hardware store. The idea is that the bottom part of the hinge is fixed to the angle iron and the top part of the hinge floats free. The mirror frame will be attached to the top part of the hinge so it's still free to pivot to account for the sun's altitude (seasonal movement), but the main motor will move the fixed part of the hinge for daily (azimuth) movement. (As shown it's attached at only one point -- it really needs to be two points because as-is there's too much play).
Here's another pic from the back so you can see the hinge is free to pivot. What this pic also shows well is the big, expensive ($40!) modification I did to the hinge. I paid a welder to weld on an in-line ball joint to the center portion of the hinge. It shows in the pic as the blackened portion at the center of the hinge and hanging down from that is a shaft. That shaft receives the 1/4"-20 threaded rod. At first, I had the rod just rest on the hinge, but the problem with that is that single point is bearing most of the weight of the mirror. Any misalignment caused it to really want to fall off. The ball joint allows it to pivot (daily movement) but otherwise holds the hinge (and so the mirror frame) tight and bears the weight better.
Here's a pic of the in-line ball joint. I got it from midwestcontrol.com. Here's the link. I had the guy cut off the bolt portion before he welded it.
Note that everything is based off 1/4"-20 once you get above the 1/2" frame part. This allows you to buy a variety of bolt sizes, washers and nuts and have some flexibility putting it together.
OK, so the two things remaining to do are attach the mirror frame and add a part for altitude (seasonal) orientation. The next pic gives a view of the mirror frame attached.
The mirror frame is attached by the two top nuts/bolts. Pre-drill two holes in the mirror frame to align with the existing holes in the hinge. Then fasten with the usual 1/4"-20 nuts and bolts. This picture is from an earlier version so you also see that it used two bolts to fasten the lower 1/2 of the hinge to the angle iron. Pre-drill the holes through the hinge for this. This was a better method than the final version, it had a lot less play in it. Also, this was before the ball joint was welded to the hinge, so you see it resting on the head of a long bolt which was serving as the top threaded rod.
Some of what's above got changed when the mechanism was added to fix (and adjust) the altitude of the mirror. To get that, I added another angle iron, which can be seen in the next pic.
OK, apologies, I missed taking a side angle which would have made this clearer. This picture is taken from the other side, ignore the motor position as that changed. Remember, the pic directly above is an older version, instead, go two pics up to where you see my hand. In that pic, you see the green hinge is on the inside of the angle iron (between the angle iron and the shaft). So the second angle iron is same size as the first, and is attached to it on its vertical face like a mirror image. You are taking two L's and putting them back to back to form a T (upside down).
Once you are oriented within the pic, you can see the last piece added. This is another ball joint (hasn't had it's bolt portion cut off). At the bottom, a good old long 1/4"-20 bolt holds it to the 2nd angle iron. At the top it's fixed to another 3" T-strap which is fixed to the frame. By screwing this bolt up or down, it changes the height of the mirror relative to the 2nd angle iron and so changes its angle of altitude. The ball joint is needed to handle the pivot that happens at that point. The ultimate plan is for this to be a worm gear controlled by a second motor. For now it's hand adjusted.
Well that's it for this post. This is most of the value. But, more posts to follow on motor control, application, what was good or bad about this design and preparing for v2.
First, it provides a motor mount and mechanism for suspending the motor to the shaft. I bought the 90 degree arm to attach it, see the pic. The 90-degree arm can be fixed to the gearbox on the side. Bought small (M6 maybe?) nuts and bolts to fix it on there. Reserve the screws that came with it to fix something else to the large gear. Attached to the other end of the 90-degree arm is a 3" t-strap (Stanley) which had existing holes which aligned with the arm and the threaded rod. The T strap is fixed to the threaded rod with a bolt above and below and serves to hold the motor fixed in a given orientation so when the gear turns it can move the mirror rather than just spinning the motor mount around the center rod.
The second (and best) thing the Gearbox does is integrate a gear (to step down the motor) and a mounting platform with ball bearings which simplifies transferring the rotation of the motor shaft to the mirror. The mounting platform includes a hollow shaft which the 1/4" threaded rod passes through. Note that when assembling, there needs to be a nut already threaded onto the rod that the T-strap will rest on. Next, slide the T-strap/motor frame onto the threaded rod, but after the T-strap passes on, screw another nut on, and screw it down as the T-strap and then gear shaft threads over the rod. (Just order of operations -- otherwise you can't get get the 2nd nut on after the motor is fitted over the threaded rod).Next step is to add hardware to attach the mirror. Buy 1 1/4" perforated angled iron (Home Depot or wherever) and cut a piece about 2.5" long. The piece I got alternates hole sizes. The circular hole is exactly 3/8" and so it fits snugly over the shaft provided by the servocity gearbox. Around the 3/8" hole, drill 4 holes using the 90 degree arm as a template to show hole placement and size. The drilled angle iron looks like this.
Use the screws that came with the gearbox to screw the angle iron to the gear (those holes are threaded). I only got two screws to thread, probably because my holes weren't well aligned. Also, because the 90-degree mount is 1/4" thick, the screws don't go flush to the top of the angle iron. If this part of the design sticks around it'd be worth adding washers and nuts as spacers.
Next, the hinge. This was a normal, 3" door hinge from the hardware store. The idea is that the bottom part of the hinge is fixed to the angle iron and the top part of the hinge floats free. The mirror frame will be attached to the top part of the hinge so it's still free to pivot to account for the sun's altitude (seasonal movement), but the main motor will move the fixed part of the hinge for daily (azimuth) movement. (As shown it's attached at only one point -- it really needs to be two points because as-is there's too much play).
Here's another pic from the back so you can see the hinge is free to pivot. What this pic also shows well is the big, expensive ($40!) modification I did to the hinge. I paid a welder to weld on an in-line ball joint to the center portion of the hinge. It shows in the pic as the blackened portion at the center of the hinge and hanging down from that is a shaft. That shaft receives the 1/4"-20 threaded rod. At first, I had the rod just rest on the hinge, but the problem with that is that single point is bearing most of the weight of the mirror. Any misalignment caused it to really want to fall off. The ball joint allows it to pivot (daily movement) but otherwise holds the hinge (and so the mirror frame) tight and bears the weight better.
Here's a pic of the in-line ball joint. I got it from midwestcontrol.com. Here's the link. I had the guy cut off the bolt portion before he welded it.
Note that everything is based off 1/4"-20 once you get above the 1/2" frame part. This allows you to buy a variety of bolt sizes, washers and nuts and have some flexibility putting it together.OK, so the two things remaining to do are attach the mirror frame and add a part for altitude (seasonal) orientation. The next pic gives a view of the mirror frame attached.
The mirror frame is attached by the two top nuts/bolts. Pre-drill two holes in the mirror frame to align with the existing holes in the hinge. Then fasten with the usual 1/4"-20 nuts and bolts. This picture is from an earlier version so you also see that it used two bolts to fasten the lower 1/2 of the hinge to the angle iron. Pre-drill the holes through the hinge for this. This was a better method than the final version, it had a lot less play in it. Also, this was before the ball joint was welded to the hinge, so you see it resting on the head of a long bolt which was serving as the top threaded rod.Some of what's above got changed when the mechanism was added to fix (and adjust) the altitude of the mirror. To get that, I added another angle iron, which can be seen in the next pic.
OK, apologies, I missed taking a side angle which would have made this clearer. This picture is taken from the other side, ignore the motor position as that changed. Remember, the pic directly above is an older version, instead, go two pics up to where you see my hand. In that pic, you see the green hinge is on the inside of the angle iron (between the angle iron and the shaft). So the second angle iron is same size as the first, and is attached to it on its vertical face like a mirror image. You are taking two L's and putting them back to back to form a T (upside down).Once you are oriented within the pic, you can see the last piece added. This is another ball joint (hasn't had it's bolt portion cut off). At the bottom, a good old long 1/4"-20 bolt holds it to the 2nd angle iron. At the top it's fixed to another 3" T-strap which is fixed to the frame. By screwing this bolt up or down, it changes the height of the mirror relative to the 2nd angle iron and so changes its angle of altitude. The ball joint is needed to handle the pivot that happens at that point. The ultimate plan is for this to be a worm gear controlled by a second motor. For now it's hand adjusted.
Well that's it for this post. This is most of the value. But, more posts to follow on motor control, application, what was good or bad about this design and preparing for v2.
V1: the base
OK easy part first. The base is unistrut, because it's easy to work with and tough. I bought a 10' piece and cut it (er, my friend cut it) into a 4' piece and two 18" pieces. You can buy the channel (long pieces) at Home Depot or the like. The fittings you can buy at plumbing supply stores or order special pieces on-line.
Here's a close up of where the channels intersect. There's a X-fitting which my plumbing supply didn't have so I used a T and a two-hole piece.
You can see a nut where the 1/2" rod goes through the cross, there's a matching nut on the underside of those metal fittings to keep it tight. Also, the nuts you see going into the fittings are coarse 1/2" made by unistrut, they mate with unistrut bolts with springs.
Cost for the 10' piece of channel was $22.70, the T fitting was $12, nuts and bolts $5.40. So, $40 for unistrut. Too expensive really but OK for prototyping. If this were a larger implementation, say 10-20 mirrors, it could be made into a grid so as a system it could be stiffer. To cut costs, smaller channel pieces (13/16") could be used, or maybe a cheaper, plastic substitute. Also, the L and T pieces are pricey and in a grid it might be possible to use flat 3-hole pieces. All in, $20/ device (per square meter) for an easily installed base might be the target.
The threaded rod was a 2' piece of 1/2" -13 from Osh. This costs about $1.50/foot. It would be better to cut it down to 1' for stiffness (the mirror doesn't require 2' for clearance), but leave it at 2' when first building -- it's easier to get under it to work.
Here's a pic of where the 1/4"-20 rod is coupled to the 1/2". The 1/4" isn't stiff enough to use for the whole length, but 1/4" is needed to fit through the hollow shaft of the servo gearbox. The 1/2"-13 to 1/4"-20 reducer coupling nut is an oddball. I found it at tannerbolt.com, here's the link. I had to buy a box of 50, but they're 50 cents each in quantity. I have 49 left. The bolt you see in the picture is what the gearbox will rest on. Will tackle that in a next post.
Here's a close up of where the channels intersect. There's a X-fitting which my plumbing supply didn't have so I used a T and a two-hole piece.
You can see a nut where the 1/2" rod goes through the cross, there's a matching nut on the underside of those metal fittings to keep it tight. Also, the nuts you see going into the fittings are coarse 1/2" made by unistrut, they mate with unistrut bolts with springs.
Cost for the 10' piece of channel was $22.70, the T fitting was $12, nuts and bolts $5.40. So, $40 for unistrut. Too expensive really but OK for prototyping. If this were a larger implementation, say 10-20 mirrors, it could be made into a grid so as a system it could be stiffer. To cut costs, smaller channel pieces (13/16") could be used, or maybe a cheaper, plastic substitute. Also, the L and T pieces are pricey and in a grid it might be possible to use flat 3-hole pieces. All in, $20/ device (per square meter) for an easily installed base might be the target.
The threaded rod was a 2' piece of 1/2" -13 from Osh. This costs about $1.50/foot. It would be better to cut it down to 1' for stiffness (the mirror doesn't require 2' for clearance), but leave it at 2' when first building -- it's easier to get under it to work.
Here's a pic of where the 1/4"-20 rod is coupled to the 1/2". The 1/4" isn't stiff enough to use for the whole length, but 1/4" is needed to fit through the hollow shaft of the servo gearbox. The 1/2"-13 to 1/4"-20 reducer coupling nut is an oddball. I found it at tannerbolt.com, here's the link. I had to buy a box of 50, but they're 50 cents each in quantity. I have 49 left. The bolt you see in the picture is what the gearbox will rest on. Will tackle that in a next post.
Sunday, December 6, 2009
Version 1: overview
Here's a picture of attempt #1 (hereafter v1). This is an overview, I'll address each section in separate posts.
The base is made from uni-strut. Coming vertically up from the unistrut is a 2' piece of 1/2"-13 threaded rod. There's a coupling nut which steps the 1/2" rod to 1/4"-20 rod.
The motor and gears are this, which is a servo and gearbox by servocity.com. I'm using the 8.6:1 ratio which has metal gears. This threads over the 1/4" rod and is held in place by bolts on the rod itself.
The pivoting mechanism (daily and seasonal) is a door hinge. The hinge is bolted to a piece of perforated steel angle, and the steel is screwed to the gear. The steel also attaches to the mirror frame. Movement of the gear moves the hinge for daily (azimuth) movement. For seasonal (altitude) movement, the other portion of the hinge can pivot.
The mirror is mounted to a frame made of aluminum angle. The mirror itself is a 4' x 2' piece of acrylic fixed to the frame.
Movement is controlled by an Arduino board with a motor shield.
OK, now will try to do breakouts of each component.
The base is made from uni-strut. Coming vertically up from the unistrut is a 2' piece of 1/2"-13 threaded rod. There's a coupling nut which steps the 1/2" rod to 1/4"-20 rod.
The motor and gears are this, which is a servo and gearbox by servocity.com. I'm using the 8.6:1 ratio which has metal gears. This threads over the 1/4" rod and is held in place by bolts on the rod itself.
The pivoting mechanism (daily and seasonal) is a door hinge. The hinge is bolted to a piece of perforated steel angle, and the steel is screwed to the gear. The steel also attaches to the mirror frame. Movement of the gear moves the hinge for daily (azimuth) movement. For seasonal (altitude) movement, the other portion of the hinge can pivot.
The mirror is mounted to a frame made of aluminum angle. The mirror itself is a 4' x 2' piece of acrylic fixed to the frame.
Movement is controlled by an Arduino board with a motor shield.
OK, now will try to do breakouts of each component.
Cheap Heliostats Project -- Hello World
This blog is to document my attempt to build a cheap heliostat and share ideas so others can build on and extend them. There is a community of people working in an open source approach to solar power generation and this site is my attempt to join them.
I'll probably do some future posts on heliostats generally, what "cheap" means (sub-$100 per square meter), how heliostats can be used and so on. But for now, most of the people that would run across this site and care to read it would know all that. So, I'll just comment on design directions and move on with practical posts.
My ideal design would be assembled from mass produced parts and require little or no manufacturing or special tools to keep costs low and increase accessibility / modification of the design. It would a modular design so components of the system could be easily upgraded. It would contain no patented elements. I lean toward design for retail users, and a system that could be mastered fairly easily so barriers to entry for installation are low. Not all these goals will necessarily be met, but just so you know where I'm coming from. And now, on to more practical things.
Incidentally, this blog picks up from the website heliostats.org.
I'll probably do some future posts on heliostats generally, what "cheap" means (sub-$100 per square meter), how heliostats can be used and so on. But for now, most of the people that would run across this site and care to read it would know all that. So, I'll just comment on design directions and move on with practical posts.
My ideal design would be assembled from mass produced parts and require little or no manufacturing or special tools to keep costs low and increase accessibility / modification of the design. It would a modular design so components of the system could be easily upgraded. It would contain no patented elements. I lean toward design for retail users, and a system that could be mastered fairly easily so barriers to entry for installation are low. Not all these goals will necessarily be met, but just so you know where I'm coming from. And now, on to more practical things.
Incidentally, this blog picks up from the website heliostats.org.
Tuesday, December 1, 2009
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