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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Stepper motors for a solar tracker project
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  1. #21
    Join Date
    Aug 2005
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    2
    Absolutely accurate tracking may not be so important. The difference in solar output (measured) in my experimental setup is small to tiny for +- 5 degrees. Makes some sense when you consider the cosine of 5 degrees is ~0.996. Having said that, however, don't underestimate the torques needed. A first approximation can be had using a torque wrench or an spring guage on the drive train. Allow for corrosion, snow loads, wind loads etc. when adding margins.

    Also consider the holding torque of the drive as well as the moving torque.

    What will you use for feedback and control? Watch out for hysteresis lest your system dither itself to death.

    Dick

  2. #22
    Join Date
    Jun 2003
    Posts
    3312
    Quote Originally Posted by dickw View Post
    Absolutely accurate tracking may not be so important. The difference in solar output (measured) in my experimental setup is small to tiny for +- 5 degrees. Makes some sense when you consider the cosine of 5 degrees is ~0.996.
    When you get to the optical concentrators this isn't true. The big push these days in PV is to use smaller area cells, and focus a wider area via fresnel or similar lens. So off axis even a few tenths of a degree drops output measurably.
    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com

  3. #23
    Join Date
    Oct 2007
    Posts
    8

    cure for ultra tight tracking requirement

    Actually the concentrator PV system that I keep track of has a several degree acceptance angle....

    Most of the CPV systems use an interrmediate device between the lens and the cell. This device is called a homoginizer. It's purpose is even out the spectral segregation that occurs with a lens. The CPV systems I am familiar with use the multi junction cells that need to have the light spectra evenly distributed over the whole cell. Without the homoginizer to take care of the effects of the lens focusing different frequencies of light at slightly different distances, the efficiencie of the cell suffers.

    The homoginizer widens the acceptance angle of the lens in the process of remixing the light distribution.

    I'm not clear why such a high requirement for tracking accuracy?

    Google light homoginizer in relation to CPV and you will find info on them, how the are made etc.

    Tom :cheers:

  4. #24
    Join Date
    Apr 2007
    Posts
    1955
    I imagine that he is trying to do this without the light homogenizer, which is VERY difficult, if not mechanically dangerous to a concentrator / triple junction cell approach. It is always enlightening to talk to the engineers at places like spectrolab, emcore, etc that actually MAKE the cells, and listen to them talk about just how quickly the cells are damaged by non uniform illumination.

    Nonetheless, I suppose it (no homogenizer) could be accomplished with algorithms that take into account where the sun is "supposed to be" as well as minor corrections for optimized input.

    A worm gear design is a good place to start - maybe consider buying a pre-made worm gear box from someone like boston gear or equiv. The various quality reduction gear companies make high reduction gearing systems by day and dream about them at night so you don't have to.

  5. #25
    Join Date
    Feb 2008
    Posts
    17

    Red face

    Ya, I definitely understand the mine fields we are looking at here. My understanding is that most of the solar thermal related technologies such as solar dishes, reflecting heliostats, solar trackers, etc are pretty much the commercial versions of what was invented 30 years back! Considering the kind of experience we all acquired since then, we can definitely come up with creative solutions to most solar thermal related technical issues.

    I am currently looking at some interesting solar thermal ideas. One such idea is the beam down technology:
    http://www.sener.es/EPORTAL_DOCS/GEN...DOWNSYSTEM.pdf
    This solves all the solar tower related technical issues.

    The second technology I was looking at is www.esolar.com, which struck me as the idea that can potentially take care of all the mechanical stress and wind load issues. I see this idea having a big differentiator when compared to the large heliostat ideology.

    These ideas are making me think if it is possible to use esolar kind of field layout and incorporate beam down technology and carefully focus light onto a small localized collector that is on the ground! I am looking at several different variations here. Now, the biggest question is… what kind of tracking inaccuracy can this system tolerate with the kind of non-imaging optics we can come up with? I think an EXTREMELY high solar tracking accuracy is a must in this case… I would like to start with that assumption…

    I think we all need to look at solar thermal technologies from a fresh perspective. There are only a certain things we could do with the current commercial solar focusing designs.

  6. #26
    Join Date
    May 2006
    Posts
    10
    Parker makes some real good stepper drives that are easy to interface to and give excelent torque and resolutions. (25,000 steps per rev at full torque.) I have tried some inexpensive units that are marketed to the DIY CNC crowd and they were horrible on both counts. If cost is not a huge concern the parker units will perform well and can be had on ebay for under $100.00. The difference between these high end units and the inexpensive units is the driving circuitry. The inexpensive units use a simple on off switching to yield the motors step count as the resolution and ocassionally will use PWM to reduce the power consuption, but they still suck. The Parker units actually control the current delivery profile to each coil of the motor, to give more steps and better torque characteristics. ( This is a very oversimplified explanation of the circuits operation. ) these drives can be in the $1600.00 range for the motor sizes that you mention. There is a discount in quantity that will soften the blow considerably. Most have encoder inputs so that you can verify position in case of missed steps and can have motion programs dowloaded to them to work independent of your computer. If you don't have a lot of experience with electronics at the board level, these will deliver a lot of functionality with minimal effort. I think that you will find the satilite screw actuator designs, suggested, with DC motors and encoders a good solution to the "big picture" problem. Galil has some good motion boards, that can read an encoder and output to a drive. Just a few ramdom thoughts and some shared experiences...

  7. #27
    Join Date
    Apr 2007
    Posts
    1955
    Hi Rohan - Thanks for the links to the paper and esolar.

    The beam down approach is certainly interesting. If someone asked me to do this, I would be tempted to break up the main mirror into several smaller mirrors (similar to an adaptive, multi mirror telescope).

    Obviously, the solar thermal approach is much less sensitive to imperfections in beam uniformity that triple junction pv.

    eSolar's approach points out an interesting viewpoint - the value of solar thermal is for peak use, not necessarily for "24 hour use". It appears that their mirrors are non-tracking, so they only generate power during specific parts of the day. The downside of the eSolar approach is that the economy of scale for turbines is very much tilted for large scale generation.

    An interesting part of the down beam financial analysis, is that a very significant cost was in making the mirrors track and the cost of expanding the main mirror to correct for imperfect focus.

    One sort of alternative thinking is to have a large array of fixed mirrors, each "set" optimized for different parts of the day. As the day goes on, just have the focus move from one down mirror to the next.

    I noticed that the liquid salt temps are still not very high compared to the temperatures in a diesel engine (600 ish C vs nearly 2000 C) but maybe this is a normal steam turbine range. There is a lot of room for turbine efficiency improvement if you can get this temp up.

  8. #28
    Join Date
    Feb 2008
    Posts
    17
    >> Obviously, the solar thermal approach is much less sensitive to imperfections in beam uniformity that triple junction pv.

    This need not necessarily be true in all the cases. Lets look at the solar central tower concept:

    Lets assume average distance between a heliostat (with accuracy of 0.1 degrees) and its solar collector is 150m. So, when we do the math, error displacement = 150xTan(0.1) = 0.26 m i.e., 26 cm.

    Effectively, you end up focusing the sun anywhere within a radius of 26cm from the intended target. This is a lot considering the fact that solar collectors would operate efficiently only when they receive the solar flux uniformly.

    Tracking inaccuracies have this “multiplying” effect when we take the distance between the heliostat and the collector into consideration. Now, imagine the complexity that we add if we put one more mirror between the heliostat and the collector… a sort of beam down approach!

    As for eSolar, although I don’t have the first hand knowledge, I can bet that they MUST be tracking mirrors. Each mirror should reflect the sun onto the esolar’s central pole. Non-tracking mirrors cannot generate the required thermal flux the collector needs to heat salt.

    It shouldn’t be that expensive to make the mirrors track… esolar needs two small DC motors for each mirror. I am assuming that weight of each mirror is not more than 5lb. If one develops the driver circuitry and the sun tracking logic inhouse, it shouldn’t cost that much… I would be surprised if each of their mirror system (Mirror+motors+drivers) cost more than $100-$200. I don’t know… may be I could be wrong here…

    Lets examine this scenario… what is the least expensive motor & driver that we can use for each of esolar’s mirrors (www.esolar.com)… assuming that the mirrors are sun tracking…?

    If you were the designer, how would you design the small tracking mirrors? What gear ratios are your going to use to restrict the inaccuracy to 0.05...?

  9. #29
    Join Date
    Nov 2007
    Posts
    55
    thanks for all the links...I am just new to solar power...

    if I would need that very high accuracy and would go with a stepper motor, I would be needing Field Oriented Control then do stepper-servo...

    a good servo and a very good high gear ratio would do...I may not be that knowledgeable in tracking the sun or even in simple astronomy but I think I would never need a pretty high speed at the mirrors joints...

  10. #30
    Join Date
    Nov 2006
    Posts
    114
    If I may jump in here,
    I have been planning to build three solar collectors to provide supplimental heat to a hot water heating system for a shop. I had been planning to use three satalite dishes that I have accumulated.
    Someone mentiond that they didn't think the dishes have the correct curviture to focus the sun where the antena is currently mounted.
    I have already made the coil that the fluid flows through and am working on the mount.
    I had planned on fastening segments of polished aluminum sheet on the grid that forms the dish and asumed that the sun would be focused at the point that the antena was at.
    my freind said the focal piont of the sun and the focal point of the satalite signal, in relationship to the dish, may not be the same. Anybody have any thoughts on this?
    I also wanted to add that several years ago I saw a sun tracker that used 4 photo eyes with a cone shaped lenz on top of each one. I was talking to the guy that designd this particular unit telling him about my satalite dish idea and watching his device work to line up a set of solar pannels on what was a mildly cloudy day. I have to say it was quite impresive and very precise.

  11. #31
    Join Date
    Apr 2007
    Posts
    1955
    Hi Merl - I built my first hot water solar project in 8th grade (35 years ago) and learned a lot from it. One useful point is that solar hot water heating is most beneficially thought of as a "pre-heater" to take the water from "cold" to "warm" or "very warm". This actually saves a huge amount of energy on the final heater, and keeps the project costs down. Trying to do real temperature controlled output from a solar system is expensive, but just using it to pre-heat is relatively cheap.

    JMHO, but using a dish and tracking for hot water is perhaps a bit overkill. Any decent flat panel arrangement will work fine, just include plenty of dark 3 D surface area and point it in the general direction of afternoon sun. I have seen designs made from soda / beer cans that were cut in 1/2, faced up, and painted.

    The early morning sun is not really that economical to use for hot water heating, so why bother tracking to it ? This makes sense for high end PV cells, mostly because their surface area cost is too high for non- concentrator use, and if the focus is not done properlly, the cell will die.

  12. #32
    Join Date
    Nov 2007
    Posts
    55
    merl,

    I think the dish idea would work...

    the focus is actually at the location of the original antenna...directivity of a dish is straight (properties of a parabola)...

  13. #33
    Join Date
    Nov 2006
    Posts
    114
    rdpzyco,
    the reason I wonder about this because when I take a reflector from an old shop lite that has a much more drastic curve to the parabola and aim it at the sun it gets quite hot at the focal point were the lite used to be. If I try putting my hand through the back of the reflector on a clear sunny day I will get burned in a matter of a couple of seconds. That is with a small reflector of about 24"diameter X 18 deep.
    One of my satalite dishes is 9 feet in diameter and still has the steering drive and controler.
    As for a stand alone system that was not my intention. I figure if I can add 50 deg to my total heating system then I would call that good enough to start with .
    I'll have to do a little digging to find the formula to calculate how many BTU's I'll need to raise my systems given amount of fluid 50 deg F but, I know I have it somewere around here.
    My real consern is protecting it from the wind. The dish segments on all three units are either made of a haevy mesh screen or perferated sheet metal and relitively transparent to the wind.
    I figure when I line the inside suface with my polished aluminum sheething I'll have trouble with high winds. The place were I intend to put them is shelterd on three sides from the wind. Unfortunatly the prevailing winds come from the side that is open... I'll just have to wait and see.

  14. #34
    Join Date
    Nov 2007
    Posts
    55
    it still has to be a parabola for all the reflected to go to the focus..it's just easier to make a bigger dish with a less drastic curve..

    you can try if the dish is good enough by putting small reflectors and try to look if the rays get to the focus..

  15. #35
    Join Date
    Dec 2010
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    0

    Solar Automation

    SolarIndustryMag.com: Content / Products & Technology / Nook Industries Unveils ActionJac Line For Solar SolarIndustryMag.com: Content / Products & Technology / Nook Industries Unveils ActionJac Line For Solar

  16. #36
    Join Date
    Apr 2006
    Posts
    3498
    Hi I am also planning to make a solar tracker for PV 200watt panels. I am planning to use two steppers with gears and 4 LDR using Arduino. The programming is completed for the Arduino. Theoritically i can improve 25 to 30 percent efficiency of the system by installation of solar tracking system. But:
    If i use 2.8 Amp motor with 24 volt supply then i contineously consume 2.8 x 24= 67.2watt for one single motor?? (Other electronics losses not included yet)(Please correct if my math is wrong)... Now the question is if it is better to use the solar tracker or not?
    Any suggestion?
    http://free3dscans.blogspot.com/ http://my-woodcarving.blogspot.com/
    http://my-diysolarwind.blogspot.com/

  17. #37
    Join Date
    Oct 2007
    Posts
    8
    There are several things I would consider...... Are you planning on using single or dual axis tracking? For a flat panel I would consider using only single axis.

    Next, consider using a gear train that has a worm gear in it. Using a worm gear means that the stepper motor does not have to provide holding torque to keep the panel in position. That means the stepper motor only has to be powered when it is actualy stepping.

    A flat panel has pretty loose tracking requirements, so you can wait until the panel has to be moved a few degrees, power the motor to move the panel a few degrees ahead of where it needs to be, shut it off and wait until the tracking error gets to be a few degrees behind before starting the next moving cycle.

    By using that approach, the motor will be off most of the time and not drawing power.

    I hope these thoughts are helpful in your project.


    You might also want to look at how much efficiency your panels lose when they get hot. Examine if cooling in some maner will be worth the effort.

    Good luck

  18. #38
    Join Date
    Apr 2006
    Posts
    3498
    Hi Tcarey,
    I shall use 400watts Flat solar panels and want them to be controlled with dual axis stepper-gearbox system with limit switches for resetting the system. The system shall be controlled through AVR controller. The system shall work similar like this:
    ‪geobruce1995's Channel‬‏ - YouTube

    My question is if the stepper is powered but not stepping is it also take the same wattage? i.e. A moving stepper takes more wattage are stopped but powered stepper?
    Regards
    http://free3dscans.blogspot.com/ http://my-woodcarving.blogspot.com/
    http://my-diysolarwind.blogspot.com/

  19. #39
    Join Date
    Oct 2007
    Posts
    8
    I strongly suggest you look at the increased cost of dual axis tracking vs single axis tracking. For what you are proposing the amount of additional energy captured vs the increased cost and mechanical complexity is imo not worth it.

    As far as the power drawn by a stepper motor when not stepping... that is a function of the controller. Some reduce the power as much as 50% other don't reduce it at all. You will have to look into the details of your proposed controller to find that answer.


    If you use a spur gear arangement to move your panels then if you remove all power the weight of the panel will cause the panel to move as soon as the power is removed. That is why I suggested using a worm gear arangement. Power can be completely shut off to the stepper motor when it is not stepping.

    Good luck.

  20. #40
    Join Date
    Jun 2003
    Posts
    3312
    Yes you are correct, you are consuming power although 67 watts is subjective to motor drive method. Regardless of driver method, IMHO, for it to be efficient you need a mechanical break that holds the tracking mechanism when not moving with no power. I.e spring loaded solenoid brake. During that hold time, steppers aren't enabled or better yet, electronics aren't powered. Periodically power the steppers/electonics, disengage the mecahnical break, move the array, stop, reengage the break, power down the motors/electronics.
    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com

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