Design Article
Selecting a solar energy conversion method
Ed Friedman, applications engineer, STMicroelectronics Inc.
7/3/2012 10:15 AM EDT
Solar energy conversion with embedded maximum power point tracking (MPPT) –
Active power optimizers or micro-inverters
A major problem facing solar energy system designers is determining the best, most cost effective method to extract power from a solar array and deliver it to the AC grid. Of equal importance is how to solve the problem of shading. A shaded panel can burn out and reduce functionality of an entire string of panels. Methods will be presented to solve this problem.
A literal bird’s eye view of a typical solar system is shown in Figure 1.
Figure 1
Solar panels are mounted on the roof of a building facing southwest. Southern exposure is obvious. Southwest exposure is frequently needed to capture the afternoon sun. A typical solar panel delivers 24VDC. Solar panels connected in series drive an inverter which connects to the grid. Grid voltage to a home or business is 115VAC or 230VAC. The peak value for a 230VAC system is 325V. The series connected panels form an array which typically provides 350VDC to the inverter to power the grid.
2. Voltage, current and power characteristics of a solar cell
The equivalent of a solar cell is shown in Figure 2.
The cell contains a PN junction and can be treated like a diode. The current through the diode is the same as a standard diode and is called the dark current. The current generator produces a current in the opposite direction proportional to the absorbed light. Series resistance Rs represents conduction losses where power loss is proportional to the square of the output current. Parallel resistance Rp is caused by leakage current due to poor insulation around the edge of the cell. The effect of Rs and Rp on a solar panel’s output characteristics will be shown later in this section.
From the basic diode representation, a solar cell’s current as a function of voltage and power as a function of voltage is developed. Figure 3 shows the I-V characteristic of a solar cell with no illumination.
Figure 3
Figure 4 shows the cell’s I-V characteristic with light applied.
Figure 4
Since the cell produces power, we are used to seeing a current vs. voltage curve flipped upside down as in Figure 5.
Figure 5
2. Voltage, current and power characteristics of a solar cell
The equivalent of a solar cell is shown in Figure 2.
Figure 2
The cell contains a PN junction and can be treated like a diode. The current through the diode is the same as a standard diode and is called the dark current. The current generator produces a current in the opposite direction proportional to the absorbed light. Series resistance Rs represents conduction losses where power loss is proportional to the square of the output current. Parallel resistance Rp is caused by leakage current due to poor insulation around the edge of the cell. The effect of Rs and Rp on a solar panel’s output characteristics will be shown later in this section.
From the basic diode representation, a solar cell’s current as a function of voltage and power as a function of voltage is developed. Figure 3 shows the I-V characteristic of a solar cell with no illumination.
Figure 3
Figure 4 shows the cell’s I-V characteristic with light applied.
Figure 4
Since the cell produces power, we are used to seeing a current vs. voltage curve flipped upside down as in Figure 5.
Figure 5
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chrisrodgers
7/3/2012 4:06 PM EDT
A very interesting and article, thanks very much!
I have a question.
For a set of conditions, it seems that a given solar cell will have a finite amount of power to deliver. A fixed DC load connected directly to the solar cell will form a voltage divider from the Thevenin equiv. voltage source (E) between Rs and the load (RL), assuming Rp is very large.
Let's say RL is quite a bit less than Rs. So the loaded solar cell voltage will drop when connected directly. Ideally, you'd like a way to make RL appear to increase to accomplish Max. Power Transfer (MPP). This would result in raising the load voltage (VL) to a maximum of one half the open circuit Thevenin voltage (E/2).
Interposing the SPV1020 boost converter between the solar cell and RL will make the load impedance appear matched to the solar cell (RL = Rs), boosting VL and hence the load current.
Maybe I'm not seeing something here, but doesn't this violate conservation of energy? Or is MPP accomplished only "piece-wise", during each PWM pulse of the boost converter?
Thanks for any light you can shed on this.
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sudhakar_amberroot
7/3/2012 11:54 PM EDT
Thevenin cannot be applied here because of the nonlinear device being present which is the parallel diode array. So the maximum point will not be E/2 but much higher since the current through the diode will come down with reduction of the terminal diode.
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chrisrodgers
7/4/2012 7:22 PM EDT
In the article it states "Since the equivalent circuit of a solar panel is represented by a current source with parallel and series resistances, the Thevenin equivalent circuit can be shown as a voltage source with a single series res." This seems to say that Thevenin is indeed applicable here. But my point is not about Thevenin per se, nor about the maximum achievable load voltage, but rather the question of conservation of energy. As you say, as the load voltage rises, "the current through the diode will come down". This is a result of P=EI. You can change the voltage, but the power from the solar cell does not change. So I still wonder how, for a fixed DC load resistance, it's possible to increase the load voltage using a boost converter. More load voltage will result in more load current, and therefore more load power. Any comment on this, Ed?
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twarner250
7/5/2012 10:19 AM EDT
The power delivered by the cell (or array) is function of its output voltage, it is not fixed. As the cell voltage rises, the current in the internal diode rises, leaving less of the photo current for the load.
Any power not taken by the load will be converted to heat.
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HarrisMarcus
7/9/2012 4:36 PM EDT
You have stated “As the cell voltage rises, the current in the internal diode rises, leaving less of the photo current for the load”. This is not so. If you follow the panel I-V curve shown in Figure 6 when the cell voltage increases the cell current decreases. The maximum cell voltage is the open circuit voltage, Voc, where the current is zero.
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jg_
7/5/2012 6:30 PM EDT
Thevenin is a poor approach, a better model is a current source, with a parallel clamp diode, and better curves would include constant power profiles (V*I = constants) which makes the MPPT seeking easier to visualize.
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HarrisMarcus
7/9/2012 4:35 PM EDT
The maximum power available from the panel does not change but the power that is actually extracted from the panel does change and it is a function of the load resistance RL. This maximum power point occurs when RL = Rs. The boost converter decreases the value of RL by adjusting its duty cycle. The input resistance seen by the panel is RL x (1-du)^2 where du is the duty cycle internally set by the SPV1020. The SPV1020 duty cycle is internally adjusted so that RL x (1-du)^2 equals the panel output resistance Rs.
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HarrisMarcus
7/12/2012 1:38 PM EDT
The maximum power available from the panel does not change but the power that is actually extracted from the panel does change and it is a function of the load resistance RL. This maximum power point occurs when RL = Rs. The boost converter decreases the value of RL by adjusting its duty cycle. The input resistance seen by the panel is RL x (1-du)^2 where du is the duty cycle internally set by the SPV1020. The SPV1020 duty cycle is internally adjusted so that RL x (1-du)^2 equals the panel output resistance Rs.
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Brakeshoe
7/6/2012 4:14 PM EDT
Yes, the Thevenin model applies in figure 2, as you are looking back into the terminals of the source. What's more, if you apply a variable resistive load, you will get a linear graph of voltage vs current.
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chrisrodgers
7/7/2012 10:40 AM EDT
I see that my understanding is slowly beginning to clear. I found the found Fig. 3.7 on pg. 24 of the following article to be helpful:
https://digital.library.txstate.edu/bitstream/handle/10877/3171/fulltext.pdf
I think what was bothering me was that the boost converter cannot increase the load voltage to any arbitrarily high level. It is necessarily limited by the PV maximum power curve for a given set of conditions (illumination, temp., etc.) This Fig. (3.7) seems to bear that out that.
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HarrisMarcus
7/9/2012 4:37 PM EDT
The article you provided is very thorough and detailed. Thank you. You cannot get more power out than you have coming in.
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HarrisMarcus
7/9/2012 4:34 PM EDT
You are correct. The intent of using the Thevenin equivalent is to show that whatever the value Rs even though non-linear, RL must match it to provide maximum power transfer.
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HarrisMarcus
7/9/2012 4:33 PM EDT
RL will normally be much greater than Rs.
Interposing the SPV1020 boost converter between the panel and RL decreases the load resistance seen by the panel. The load resistance seen by the panel Rin = RL x (1-du)^2 where du is the duty cycle internally set by SPV1020. The duty cycle is set by the SPV1020 so that RL x (1-du)^2 = Rs, the source resistance of the panel.
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HarrisMarcus
7/12/2012 1:37 PM EDT
RL will normally be much greater than Rs.
Interposing the SPV1020 boost converter between the panel and RL decreases the load resistance seen by the panel. The load resistance seen by the panel Rin = RL x (1-du)^2 where du is the duty cycle internally set by SPV1020. The duty cycle is set by the SPV1020 so that RL x (1-du)^2 = Rs, the source resistance of the panel.
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dafi
7/3/2012 8:29 PM EDT
Yes an interesting article indeed. Thank you.
I have two questions:
1. what level of monitoring is needed to detect a failure in one panel/optimizer? It would seem that without some monitoring a failure could occur in one optimizer and you would not know about it at teh central inverter.
2. how applicable is the use of optimizers for an off grid (battery) system? I am assuming they would still serve their purpose by delivering more power to a DC-DC inverter/battery charger.
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HarrisMarcus
7/9/2012 4:39 PM EDT
You would need to monitor the panel output voltage and input voltage.. This can be done through the SPI bus in the SPV1020. The SPI bus provides a direct measurement of input voltage, input current and duty cycle. Output voltage can be computed by Vout = Vin/(1-du).
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HarrisMarcus
7/9/2012 4:40 PM EDT
Additional information.
Optimizers can be used for batteries as well as AC grid systems. STMicroelectronics has an evaluation board with P/N STEVAL-ISV005V1 that uses the SPV1020 and SEA05 constant voltage constant current controller to charge a 240W lead acid battery.
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dave_dne
7/3/2012 9:16 PM EDT
The enphase M215 data sheet shows 208/240VAC output. But in this article "Power loss due to wiring" analysis indicates 115VAC output from the M215. Ed are we looking at the same thing here?
And the M215 has remote monitoring features.
On the 'cons' side - the M215 is does have a narrow min/max start voltage of 22/45VDC. I don't work for enphase but I did give their micro inverters a detailed review for a home PV system.
And lastly, what about potential problems with DC arc faults using a 350VDC voltage to the central inverter? Thought I read somewhere that NFPA/NEC was now addressing this issue? Anyway, I'd like to see something mentioned about any future NEC code requirements for high (~300VDC+) DC voltages to a central inverter.
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HarrisMarcus
7/9/2012 5:45 PM EDT
We are looking at the same thing. The article compared a microinverter delivering 115VAC, not necessarily Enphase which delivers a higher voltage, to a higher voltage, lower current DC system.
The SPV1020 has performance monitoring features available through its SPI bus that include input voltage, input current and duty cycle. Output voltage can be computed from input voltage and duty cycle.
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dave_dne
7/10/2012 12:33 PM EDT
Page 5 of this article says "Comparison is made with a typical Enphase M215, 215W micro-inverter".
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twarner250
7/5/2012 10:18 AM EDT
The power delivered by the cell (or array) is function of its output voltage, it is not fixed. As the cell voltage rises, the current in the internal diode rises, leaving less of the photo current for the load.
Any power not taken by the load will be converted to heat.
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HarrisMarcus
7/12/2012 1:42 PM EDT
You have stated “As the cell voltage rises, the current in the internal diode rises, leaving less of the photo current for the load”. This is not so. If you follow the panel I-V curve shown in Figure 6 when the cell voltage increases the cell current decreases. The maximum cell voltage is the open circuit voltage, Voc, where the current is zero.
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green_is_now
1/23/2013 4:44 PM EST
you misinterpreted him you are both saying the same thing.
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rroy22520
7/5/2012 11:08 AM EDT
What about delivering solar power not to the AC grid but to the local loads? MPPT absolutely must be considered for moving and non moving cells and reflector. But also the infrastructure for intelligent load switching/engagement to optimize the local loads so they put less demand on the grid. Solar in the desert is great if you can build a factory (a suitable load) underneath the arrays so the transmission distance is not the issue, getting the raw materials in and the finished product out.
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http://www.lulu.com/spotlight/poconoarmchairreview
7/6/2012 1:12 AM EDT
Doesn't maximum power output fall drastically over the first few months? That means, it seems to me, that the control circuits have to be designed to handle outputs that are achievable for only a small percentage of the array's life, and only early on. After that, isn't it true that the control circuits will never see that level of output again?
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http://www.lulu.com/spotlight/poconoarmchairreview
7/6/2012 1:14 AM EDT
It would be nice if solar panels could be sold "broken in" so that their initial degradation would not complicate array designs.
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HarrisMarcus
7/9/2012 5:46 PM EDT
The power output of a solar panel changes a slight amount with time and this is specified in the panel manufacturer’s data sheet. This case the power optimizer can be used to very good advantage because as the Vmp and Imp points change with time, the SPV1020 dynamically tracks these changes and enables the maximum power available to be extracted from the panel.
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Trinity51
7/6/2012 5:50 AM EDT
Rich - as far as I am aware the panel output is surprisingly constant with time. Most panels are guaranteed more than 90% rated output after 10 years and more than 80% output after 25 years (approx 100,000 hours of daylight use). Typical results are better than that, so degradation is just a fraction of 1% per year.
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Mineyes
7/6/2012 3:39 PM EDT
"Southern Exposure is Obvious" That is true only in the Northern Hemisphere. A better statement, might be "Equatorial Exposure is required"
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HarrisMarcus
7/9/2012 5:46 PM EDT
Yes you are correct. I should not be thinking only in terms of the northern hemisphere.
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Robotics Developer
7/7/2012 2:29 PM EDT
A very nice article with good technical details! Very much appreciated, thank you! I am wondering if this level of complexity is needed when using solar power both locally and not at normal line voltages (ie. 12V or 24V DC lighting/systems)?
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HarrisMarcus
7/9/2012 5:47 PM EDT
Using a power optimizer is not complicated and system design is straightforward. For a low voltage lighting application, the power optimizer will allow the maximum amount of power available to be extracted from the panel.
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selinz
7/9/2012 4:46 PM EDT
Interestingly, a solar panel sales guy came by on the day that this article was published. I was astonished to find out that I could actually save money immediately. It turns out that we use double the "average" user and for anything beyond the norm, the 15 cents/kwh goes to about 30cents/kwh. Thank you PGE. This makes getting enough panels to knock our electricity useage in half has a quick payback period. (7-8 years).
If you are a do it yourselfer, it can be even less. see
http://www.wholesalesolar.com/enphase-solar-power-system.html
for example pricing. And no, I am in no way affiliated with these folks. They popped out of a simple google search. YMMV.
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GREAT-Terry
7/11/2012 11:32 AM EDT
Very interesting MPPT control method. I like the micro-inverter idea as it is the most efficient way to dump energy to the grid. Managing multiple 200-300W power is much easier than a huge 10kW stuff. Great article!
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HarrisMarcus
7/12/2012 1:35 PM EDT
Actually the microinverter method is less efficient than power optimizers and a large central inverter because 1) the voltage is higher and current lower which reduces I square R losses and 2) due to economies of scale there is less overhead and fewer number of components along with control circuits. The efficiency of the SPV1020 power optimizer is 98% and the efficiency of one large central inverter will be more than multiple microinverters.
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green_is_now
1/23/2013 4:49 PM EST
until the cells get unbalanced and your 1% advatage quickly disapeers and micro inverters win on efficiency for both momentary cell inbalance (think clouds, bird droppings...) Or if a cell degrades or is mismatche power can actually drop in a cell in a series string.
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green_is_now
1/23/2013 4:49 PM EST
Also there is no reason the 120 v cant be interleaved to give balanced 220Vac.
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green_is_now
1/23/2013 4:51 PM EST
Large voltage ratio power coversion has lower and lower efficiencies as voltage ratios go up.
So its not just IR losses must also consider switching losses with respect to voltage ratio boost levels.
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karan91
1/30/2013 5:05 AM EST
hi,any suggestion Solar Energy Conversion method for solar boat project??
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LindaSF
4/1/2013 12:14 PM EDT
Your article is a little over my head but from the financial stand point there are 3 points that I always consider when debating between central inverters and micro inverters. Micro inverters are preferred when you need module level monitoring (monitoring the performance of each individual solar module), when partial array shading occurs, or when the design is constrained by locating modules sections of the roof with different orientation. If non of the above are true I usually recommend a central inverter. You can compare prices anywhere online: http://webosolar.com/store/en/80-microinverters
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