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solar tracker

Discussion in 'Mathematics and Physics' started by PG1995, Dec 19, 2013.

  1. PG1995

    PG1995 Active Member

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    Hi

    Is it a good idea to use both dual-axis solar tracker and MPPT for a solar panel to increase efficiency? The reason for asking this question is that perhaps when one uses a solar tracker, the use of an MPPT becomes optional. Please note that I'm new to this solar stuff. Please help me with it.

    A comparator can respond to input signals that may differ by very small amounts and it means a comparator might keep on switching on and off. How to avoid this?

    Regards
    PG
     
    Last edited: Dec 19, 2013
  2. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I think having both it would take alot longer to pay for themselves.
     
  3. steveB

    steveB Well-Known Member Most Helpful Member

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    Personally, I think MPPT is always a good idea, unless you are dealing with a very inexpensive system, like a solar powered electric fence or other small convenient power source.

    With any kind of serious energy harvesting, MPPT can probably pay for itself fairly quickly. The major cost might be the development costs for hardware and algorithms, but if you use existing designs, the hardware construction cost and programming labor cost is reasonable.

    Solar tracking is the one that is more debatable. One has to consider costs of one-axis and two-axes tracking and compare to the cost of just using fixed position and using more cells to get the same overall energy and power as a tracking system. Tracking systems are costly to maintain and costly to implement. So, if you can add more cells for a similar cost as a tracking system, then you avoid maintenance costs and come out ahead in the long run.

    Dual axes tracking is really questionable because the second axis does not give you as much power/energy increase as the first axis, and the cost and complexity is much higher.

    I never did a detailed cost analysis on tracking systems, and technology changes quickly, so it would be interesting to look at actual systems implemented and see what percentage use no tracking, single axis tracking and dual axes tracking.
     
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  4. dave

    Dave New Member

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  5. NorthGuy

    NorthGuy Well-Known Member

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    They're doing completely different things, so one doesn't substitute for other.

    Tracker rotates the panel so that it is always looking at the sun, thus increasing the insolation. This can increases output compared to fixed location by up to 50%. From economic's standpoint, with today's prices for the panels it is cheaper to buy more panels than to setup a tracker.

    MPPT keeps panel voltage at the optimum point, so that evergy produced by the panel at the given insolation is as high as possible. It adds up to 30% to the production. MPPT controller costs more than a regular one. It definitely makes sense if you have many panels, but for a small panel the gain in production may not pay for the extra-expense of the MPPT conroller.

    If you use both, their effects will multiply.
     
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  6. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Not much over the other.
    One will not double the other.
     
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  7. NorthGuy

    NorthGuy Well-Known Member

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    Multiplication is not doubling. If, for example, tracking increases production by 15% and MPPT by 20%, the combined effect will be roughly 1.15 x 1.2 = 1.38 (38%).

    There will be some interaction effects, such as tracking panels will get warmer than fixed ones, and then higher temperature will decrease MPPT gain, but such effects are going to be small.
     
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  8. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    How did you get 1.15 and 1.2?
     
  9. PG1995

    PG1995 Active Member

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    Thank you, 4pyros, Steve, NorthGuy, for the help.

    NorthGuy: In post #4 you said that a tracker increases effieciency by approximately 50% and an MPPT by about 30%. Where did you get this data from? Please let me know. Thanks.

    Regards
    PG
     
  10. steveB

    steveB Well-Known Member Most Helpful Member

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  11. NorthGuy

    NorthGuy Well-Known Member

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    The radiation received by a panel S = S0 x sin(a) Where S0 is the radiation looking at the sun, and a is the angle between the direction to the sun and the the surface of the panel. If panel is turned to the sun. a = 90 and sin(a) = 1, so S = S0. This is what happens to the tracking panel. A fixed panel is always at some angle, which changes during the day. Say, if a = 30 (sin(a) = 0.5) the panel receives only 50% of the radiation. A fixed panel is turned to the South (North in southern hemisphere), so the advantages of the tracker are at their highest in the morning and evening. For example, when the Sun is straight West, fixed panel receives nothing (sin(90) = 0), but the tracking panel receives full S0. Depending on the angle of the fixed panel, the length of the day, and Sun elevation, the gain may be up to 50% in normal conditions (more if you doing it in the middle of the summer in the Nortn pole). 1.15 is just an axemple I used to show calculations.

    The short circuit current (Isc) developed by a panel depends on radiation received by the panel almost linerly: Isc = k x S. If you connect it directly to batteries, you will get power equal to Pdirect = Isc x Vbatt = k x S x Vbatt. MPPT controller can get more because it can elevate voltage to the optimum level, usually referred to as Vmp and thus get the power Pmp = Vmp x Imp, where Imp is the current it gets, which is tipycally 5% less than Isc, so we can write Pmp = Isc x Vmp x 0.95. It then converts it to the battery voltage, usually with high efficiency around 95%, so real power Pmppt = Pmp x 0.95 = Isc x Vmp x 0.95 x 0.95 = Isc x Vmp x 0.9. The MPPT advantage is Pmppt/Pdirect = Isc x Vmp x 0.9/(Isc x Vbatt) = (Vmp/Vbatt) x 0.9. If, for example, Vmp is 18V, but you charge at 14V, it'll be (18/14 x 0.9o) = 1.16 or 16%. Depending on the exact values, this could be up to 30%. The 1.20 number I used earlier is just an example.

    When you combine the effects, then you will get:

    no MPPT/no tracker: P = k x S0 x sin(a) x Vbatt
    no MPPT/tracker: P = k x S0 x Vbatt. Dividing it by the base case we get advantage as 1/sin(a)
    MPPT/no tracker: P = k x S0 x sin(a) x Vmp x 0.90. Advantage = (Vmp/Vbatt) x 0.9
    MPPT/tracker: P = k x S0 x Vmp x 0.9. Advantage = (Vmp/Vbatt) x 0.9 / sin(a).

    You can see that (Vmp/Vbatt) x 0.9 / sin(a) = [1/sin(a)] x [(Vmp/Vbatt) x 0.9]. That is the combined advantage is a product of MPPT and tracker effects.
     
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  12. PG1995

    PG1995 Active Member

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    Thank you for the explanation, NorthGuy.

    A comparator can respond to input signals that may differ by very small amounts and it means a comparator might keep on switching on and off. Could someone please tell me how to avoid this? Thanks.

    Regards
    PG
     
  13. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Its called hysteresis. Like in a home thermostat.
     
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  14. PG1995

    PG1995 Active Member

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    Hi

    Could you please help me with these queries, Q1, Q2, Q3, and Q4? Thanks a lot.

    Regards
    PG
     

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  15. NorthGuy

    NorthGuy Well-Known Member

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    Q1. Of course. However, most MPPT controllers will be buck only.

    Q2. The highlighted voltage sensor and temp sensor is used to control battery charging. Colder batteries require more voltage. The highlighted current sensor cannot be used to control battery charging, because it cannot measure the current into battery - it doeasn't take into account the current taken by inverter. The Current/Voltage sensors can be used to find the MPPT point. However, the other Current/Voltage sensors on the input, which you didn't highlight, can be used for this purpose too, IMHO with greater success.

    Q3. i. I guess some sort of wireless control.
    ii. You don't really need LCD and buttons, but it'll be hard to sell a controller without ability to change settings.

    Q4. Battery is not connected to the panels directly. Usually, MPPT controller has an input capacitor, which servers as a load to the panels. Looking at the graph, charging capacitor more will move the point to the right, charging it less will move the point to the left. The MPPT controller controls how much energy is taken from the capacitor to batteries and by doing so it maintains the optimum voltage on the capacitor.
     
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  16. PG1995

    PG1995 Active Member

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    Thanks a lot, NorthGuy.

    Please note that I'm new to this MPPT. Just trying to grasp the basics. So, if you think that at this stage I won't be able to some of material or it won't be possible for you to come up with a simplified answer for my query then please skip such a query.

    Even being buck (i.e. step down) only, it can still control the optimum power point efficiently? Perhaps, there is also boost MPPT controller too.

    Perhaps, then that current sensor is unnecessary. I didn't highlight other two voltage and current sensors because I was able to understand their overall function.

    What kind of setting would you need to change? Can't MPPT automatically find the optimum point.

    I kind of get it. In the picture, when light intensity falls to red curve and the resistance should be changed to 0.625 ohm. It would mean that the input capacitor should be charged more to increase the resistance seen by panels.

    Thanks a lot for your time.

    Regards
    PG
     
    Last edited: Mar 22, 2014
  17. NorthGuy

    NorthGuy Well-Known Member

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    All MPPT controllers I've seen are buck (step down). Panels can be connected in series to guarantee a voltage above battery voltage. It's easier that way.

    To find an MP point you can either use two input sensors (will maximize input to the controller) or two output sendors (will maximize output).

    Mostly battery-related settings. As batteries get closer to full charge, you will need to abandon search for MP point and start limiting voltage to batteries. This voltage is commonly known as absorption voltage. When batteries are fully charged, you need to drop voltage to lowel level, usually called float voltage. There also should be settings that let the controller decide whether batteries are already charged or not.
     
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  18. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    It can also display things like battery charge and power from the planel.
    It would be your main user interface.
    Where as the controller would want to be close to the batteries to keep the wires short. The interface can go any where like in the house.
     
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  19. PG1995

    PG1995 Active Member

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    Hi

    Q1: Suppose we are using constant voltage type buck MPPT and let's use this block diagram. When the current drops to 0.8 A from 1.5 A as shown in the figure here, then what would happen and how could it be explaining using the given block diagram? In post #14 NorthGuy used the concept of capacitor charging/discharging in reply to Q4.

    Q2: Do you think buck MPPT with constant voltage algorithm using added temperature compensation is a simple(st) one for a total beginner? Any suggestions? I want to keep it as simple as possible.

    Q3: Do you think the solar tracker shown in this video will work fine? The circuit diagram is given here.

    Thank you for the help.

    Regards
    PG
     

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    Last edited: Dec 24, 2013
  20. PG1995

    PG1995 Active Member

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    Hi

    I have been looking for some kind of a table or graph which shows that how the efficiency increases when both solar tracker and MPPT are used with a panel. for the last two hours without any success. Could you please help me with it? Thank you.

    Regards
    PG
     
  21. NorthGuy

    NorthGuy Well-Known Member

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    In post #10, I listed the formulae that you can use to calculate efficiencies.

    For panel positions, search for "PVWatts". They predict production based on the panel position.
     

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