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Invert your thinking: Squeezing more power out of your solar panels

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Editor’s Note: Scientific American‘s George Musser will be chronicling his experiences installing solar panels and taking other steps to save energy in 60-Second Solar. Read his introduction here and see all posts here.

When people talk about improving the efficiency of solar energy production, they usually talk about the panels themselves. What fraction of sunlight do they convert into electricity? Most solar cells today are made of crystalline silicon, but could cleverer designs or advanced materials such as thin films, organic polymers, layered semiconductors, and phosphorescent dyes do better? Probably, but that’s only half the story. The auxiliary equipment that connects the panels to your household wiring or the electrical grid is just as important.

A Lawrence Berkeley Labs study I cited in an earlier post found that solar has gotten cheaper over the past decade largely because of better auxiliary equipment rather than better panels. To find out what further steps engineers can take, I talked to Guy Sella, the co-founder and CEO of SolarEdge, an American-Israeli manufacturer of such equipment.

The need for this equipment arises from how a solar photovoltaic cell works. Light shining on the cell knocks electrons off the silicon atoms, and an electrical voltage built into the semiconductor material pulls the electrons in one direction, creating an electrical current. What happens then depends on what you connect to the cell.

If you don’t connect anything and just leave the wires dangling, the current has nowhere to go, electrons pile up on one side of the cell, and the voltage across the cell increases until it reaches the built-in voltage — typically 0.6 volts for silicon. The BP SX3400b panels that are going up on my house each consist of 50 cells connected in electrical series, for about 30 volts if you don’t connect an electrical load. Twelve of these panels are strung together for a total of about 360 volts.

When you attach a load and start to draw power from the cell, the voltage drops — gradually at first, then precipitously as the electrons flow out too quickly for a voltage to develop across the cell. This behavior is captured in a graph known as the current-voltage, or I-V, curve. When the voltage reaches zero, the cell delivers its maximum current — which is about 9 amps for my BP panel in full-on sunlight and less when it’s twilight or overcast. Because the cells in a panel and the panels in a string are wired in series, the amperage of one determines the amperage of all. If you need more current, you have to wire strings of panels in parallel. My solar array consists of two 12-panel strings, doubling the current.

Because power equals volts times amps, a panel doesn’t do a whole lot of good if it generates 30 volts at 0 amps or 9 amps at 0 volts. In between these extremes, it produces useful power, and there’s a sweet spot in the middle where the power is maximized — for my panels, 8.16 amps at 24.5 volts, giving 200 watts of power. If you hit this sweet spot and point this panel straight at the sun, it will convert 16 percent of the incoming solar energy to electricity. When most people talk about efficiency, this is the number they’re referring to, but it presumes you’ve hit the sweet spot, and that’s easier said than done.

The job of optimizing the electrical performance of the panels typically falls to a piece of equipment called the inverter. Its main function is to convert the direct current produced by the cell into the alternating current used by the electrical grid — a process known as “inversion” because it reverses the more common function of converting AC to DC (as battery chargers, for example, do). But a modern inverter does more than invert. It also adjusts how much current it draws in order to maximize the panels’ power output. As Sella explained, it’s tricky for many reasons:

  • Electrical mismatches. Because of the vagaries of manufacturing, different panels have slightly different I-V curves. The inverter responds only to the average I-V curve. Consequently, it draws too little current for some panels and too much for others, reducing their power output by several percent.
  • Partial shading. If the shadow of a tree branch or another solar panel falls on the panel (as in the above photo) and diminishes the sunlight hitting it by, say, a percent, you might innocently think it would diminish the power output by a percent. Actually, even a small shadow can completely zero out the power. Because the cells are wired in series, knocking out one can knock out all, just as a single blown Christmas tree bulb can black out a whole string of bulbs. Even when uneven illumination doesn’t choke off all the power, it worsens the electrical mismatches. In a typical setup, Sella said the power output declines as much as 25 percent.
  • Temperature fluctuations. As the temperature increases, electrons flow through the semiconductor material of a solar cell more readily and the built-in voltage decreases. For my BP panels, the peak voltage drops by about 0.1 volt per degree Celsius. The trouble is that the inverter can handle only a limited range of voltages — my SMA America SB4000US unit works from 220 to 480 volts. During extreme temperature swings, the voltage will fall outside this range and the energy will be lost. Depending on your climate, up to 15 percent of your annual energy production goes to waste.
  • Inability to optimize. Because of the above problems, the overall array I-V curve might have multiple sweet spots, some sweeter than others. The inverter will lock onto one, even if a better choice lies elsewhere. And whenever the sun’s brightness changes because of cloud cover or the time of day, the inverter needs to find the new optimum. In fickle weather, it may not be able to keep pace. Between these two problems, you give up 10 percent or so of the panels’ potential output.
  • Incomplete use of available space. Even if I had room on my roof for a 25th panel, I couldn’t install it. It would mean that one string would have 13 panels and the other 12, yet the strings must be of equal length. I couldn’t subdivide my array into five strings of five panels each, since the length of the strings is dictated by the voltage that the inverter can handle. Because of the need to keep the number of panels numerically balanced, Sella said the typical commercial solar installation can utilize only about three-quarters of its roof.
  • Damage or theft. If a panel breaks or gets stolen (it happens), the whole array can fail. What’s worse, you can’t just replace the lost panel with the latest model; you have to use the exact same model as the original, or else you’ll create an electrical mismatch. Thus a photovoltaic system installed in 2009 is locked into 2009 technology for its 25-year lifetime.

To get around these problems, Sella said that SolarEdge has developed a small box that you can attach to each panel (see photo at top). This box optimizes the electrical performance of each panel individually. He said the company has tested its technology on 17 houses in the U.S., Europe, Israel, and Japan and found it wrung another 10 to 20 percent of power out of the arrays at no extra cost. In fact, by simplifying the wiring or allowing more flexible use of roof space, the SolarEdge box can cut the installation cost. Sella said it will come out in October.

Over the past 30 years, solar power has gone from 40 times as expensive as fossil fuels to just a few times. At the rate the technology continues to improve, it won’t be long before it’s competitive even without government subsidies.

SolarEdge’s PowerBox units on the backside of solar panels in Germany (first image). Partial shading of rows of solar panels in Spain (second image). Courtesy of SolarEdge.

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  1. 1. Sander P 2:26 pm 08/26/2009

    This is one of the best articles on solar power I’ve read. No hype, just physics. Nothing alarmist, just facts and where possible, solutions. Very informative, thanks!

    Link to this
  2. 2. pgtruspace 8:07 pm 08/26/2009

    Real information about real solutions to real problems, It’s nice when Scientific American actually is real.

    Link to this
  3. 3. Jokunen 9:54 pm 08/26/2009

    It’s about time that someone figured out that all the panels need to be connected parallel to each other instead of series. Of course this also means that they will need these voltage equalizing boxes for each panel. But with todays integration they cost just a fraction of the panels price.

    Link to this
  4. 4. EngineerLee 4:36 am 08/31/2009

    For some time, National Semiconductor’s "Solar Magic" product has been offering similar functionality to what Mr. Musser discusses here: A small module is plugged between each panel, using standard interpanel connectors, and "magically" the effects of shading are greatly diminished. See .

    Also, "Microinverter" technology is commercially available. That solves shading and peak-power issues by providing one inverter per panel. Instead of a series DC string on the roof there is a 240 VAC AC bus. One such product is the Enphase Micro-Inverter.

    Jokunen can buy a parallel-connected panel system anytime he wants from companies like Outback. That is how all solar photovoltaic systems used to be built, with parallel low-voltage output feeding "charge controllers" which charged 24-volt batteries. Far from the grid, you could use 24 VDC lighting, refrigerators, and TVs, or could add an inverter for 120 VAC. But low voltage power means high current, and high current means heavy copper cables and lots of resistive loss in connectors. Efficiency suffers, not least in the inverter. With the grid as your battery, you don’t need low voltage. High-voltage series strings allow lightweight connectors and cabling to be used on the roof with low loss. Inverter efficiency is also higher–the California Energy Commission rating for Mr. Musser’s SB4000 inverter is 96.0 percent.

    For the record, I have no financial interest in any company or product I mentioned, but do have a solar PV system.

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  5. 5. rgopalan 11:50 am 09/13/2009

    Good reading for 60 seconds! What methods can be used to compute the optimal way to string PV modules: There must be a simple way to do this. My array is set up as 2 strings of twelve panels each. My inverter can handle three strings. The projected panel out put is 4560 watts, but I rarely see 3450 watts out put (about 77%). Could three strings of 8 panels improve the inverters perfomance ? if so, which sets of eight ?

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  6. 6. PVsolar 9:38 pm 09/29/2009

    Of major importance when determining the optimum number of PV modules connected in series is to understand that crystaline PV cells have an open circuit voltage that is inversely proportional to the cell’s temperature. The colder the cell, the higher its Voc. The single cell voltage of 0.6 may increase 25% or more in sub-zero temperatures.

    Typically when sizing string maximum voltage, the lowest historical temperature for a given location is used along with the module’s voltage temperature coefficient to calculate the maximum series connected string voltage. This is critical because extremely low operating temperatures can cause a string voltage to exceed the tolerance of the inverter, typically 600 volts in residential grade inverters, and damage it, thus voiding the warranty. This info along with the PV array maximum current per inverter ampacity determines the PV array size that is most cost effective and efficient.

    Link to this
  7. 7. PVsolar 10:19 pm 09/29/2009

    Be advised that PV cell voltage is inversely proportional to the cell temperature. The colder the cell, the higher its open circuit voltage. (Voc)

    The potential problem is that too many modules in series may exceed the inverter’s maximum voltage limit when operating conditions are at sub-zero temperatures.

    All listed PV modules have printed specifications that includes the module’s voltage temperature coefficient. As per the 2008 National Electric Code, Artical 690, the lowest recorded temperature for a given location is used along with the module’s temperature coefficient to determine the maximum number of modules in series allowed so as to not exceed code or the inverter’s maximum input voltage rating under real world operating conditions. Most residential inverters have a maximum Voc of 600 volts DC.

    The inverter has documentation showing its "sweet spot" for operating at its Maximum Power Point (MPP).

    As per proper nomenclature, let’s agree that the first series string of photovoltaic cells sold as a unit is called a PV MODULE. A group of modules wired together producing aPV output circuit is called a solar PANEL. More than one panel in a system is called a solar ARRAY.

    Link to this
  8. 8. ormondotvos 6:45 pm 09/30/2009

    Good article, and I’ve read quite a few! Keep it up, and try to make it practical. I’m waiting for convincing proof I won’t be sinking kilobucks into an obsoleted system…

    Link to this
  9. 9. neelagiripsl 4:21 am 12/19/2009

    I would like to do something in this area do you have any opportunity for us to explode with my 2 years of PV experience,

    Link to this
  10. 10. MF 4:49 pm 04/16/2010

    There is a common misconception about how efficiency in solar panels is calculated. It is not "energy converted/energy hitting the panel" as most people (and this article) say.
    Efficiency is actually calculated thus:
    1 square meter of Solar Panel produces X watts.
    X/1000 watts= efficiency.
    Take the Solar Panels in this article (BP3215B). The spec sheet gives an area of 65.63 in X 39.4 in (or 1.667m^2) and a max power of 215W.
    215W/(1.667m^2*1000W)=12.89% efficient. Which is close to the spec sheet’s 12.9%

    Link to this
  11. 11. gwshaw 3:18 pm 05/24/2010

    Great article, but the product the author references by Solar Edge (the PowerBox) does not quite solve all the problems he describes. The Solar Edge product only solves the Maximum Power Point tracking problem, which while it is the most important problem, it not the all the gain that can be made.

    While with the SolarEdge product each panel is MPP optimized, it outputs DC and still requires a central inverter. Thus you still are subject to needing the right number of panels in the string (typically 8 or 9) so you can’t really use small roof sections. Also, you can still lose an entire panel (or more) in a string due to shading, and then lose the energy of the entire string because the voltage is too low.

    Several companies make these type products. Another is Tigo that is almost identical to the SunEdge product. A different variant is the microinverter, where the entire process up to making AC power is done individually on each panel. These are available from Enphase Energy (and others) and eliminate the string-size problem because the string size becomes one.

    Link to this

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