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MPPT systems + Solar Panels major doubts!

  1. Dec 28, 2011 #1
    I'm working in the Power Systems team of CubeSAT. It is concerned with building a small student satellite with dimensions of 10x10cm.

    Our basic block diagram is as follows

    Solar panels-> In parallel Shunt Regulator -> Buck Converter->PRU (MAX8934A)->DC-DC Converters->Loads

    The shunt regulator used is a Power MOSFET that is turned ON only when the operating voltage of the panels increases above 14V.

    The major doubts I have are listed as follows:

    1. Excess power dissipation : If at a particular time my system power requirements are lower than what the solar panels are able to generate in that particular condition then will the panels automatically generate lower power or is it necessary for the shunt regulators to dissipate the excess power?

    The professors guiding us on this project are mainly from CS Engg. and believe that the solar panels only generate what is required. I believe otherwise. Am I wrong?

    They may be true in Solar Panels connected in Direct Energy Transfer connection but how about when connected with a Buck Converter in series? The operating point then should depend on the input impedance offered by the converter(hence the operating voltage) and hence power developed will vary.

    So don't we need a current and voltage monitoring system and with continuous monitoring dissipate the extra energy by using the Power MOSFETs that are connected in the shunt?

    2. Maximum Peak Power Tracking - In this system with changing duty cycle, the impedance as offered by the converter defines the operating voltage of the panels. What I don't understand is how do we control the duty cycle of the converters that are available in the market?
     
  2. jcsd
  3. Dec 28, 2011 #2
    Most of the energy that falls on your solar panels will turn to heat by nature of their inefficiency. That current that you do not consume represents charge carriers that will recombine and turn into heat in the panels. Typically, your efficiency wasn't that great to begin with, so the change is marginal.

    I'd think carefully about the type of chips I used. Reliability is something to worry about, so I'd stick with a company that owns their own fab, and can offer military plastic packaging. MP offers a great improvement over consumer grade parts without the expense of X grade parts.

    As for maximum power tracking, there's three alternatives I see off hand:

    1. Ignore it, suffer the decrease in power, but at the improvement in reliability / complexity
    2. Design a power tracker that alternately varies the load seen by the cells and trends towards the best load condition.
    3. Utilize a temperature measurement to ascertain the best operating point of the panels.

    As for controlling the process, you need to worry about the voltage seen at the panel and the current from the converter. For case number two, the PWM is varied such that the output current is maximized. For case number three, the converter's PWM is based upon the panel's output voltage versus the target output voltage for that temperature.

    In any of these cases, your looking at more complexity than an off-the-shelf chip that was meant for cell phones or computer accessories; this is the sort of problem that is normally handled by a specialist. I'd suggest contacting a field service engineer at Linear Tech. They live for problems like this. Be respectful that his time is valuable, and he'll likely steer you to a solution.

    Also, it won't hurt to get input from an experienced reliability engineer. They can help with semiconductor choices, stresses, and material choices for your substrate. Coefficient of thermal expansion, heat transfer, and survival of shock / vibration are very important.

    Best of luck, wish I could be there, this sounds like a lot of fun :)

    - MIke
     
  4. Dec 30, 2011 #3
    Rotter,

    With limted knowledge i am trying to answer your query,

    1. Are you not using a battery for any reason? If you use a battery then the power generated is charged to battery, the battery discharges on load demand.

    2. There are several algortihms available to calculate the voltage point where the maximum power occurs. based on that the algorithms will either increase the duty cycle or decrease it and then again reads the value of operating volatge, current and power of the panel to check if the maximum power is been obtained and again proceeds altering the dutycycle.
    The increase or decrease in dutycycle is just an value(time), If it is minimum then it will not oscillate around the maximum powerpoint, but it experiments higher number of times increasing or decreasing. so the dutycylcle can be optimised after expermenation or simulation.

    Hope this helps.All the best.
     
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