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# Selecting a solar energy conversion method

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re: Selecting a solar energy conversion method
7/3/2012 8:06:22 PM
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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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re: Selecting a solar energy conversion method
7/4/2012 12:29:44 AM
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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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re: Selecting a solar energy conversion method
7/4/2012 1:16:40 AM
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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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re: Selecting a solar energy conversion method
7/4/2012 3:54:23 AM
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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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re: Selecting a solar energy conversion method
7/4/2012 11:22:05 PM
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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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re: Selecting a solar energy conversion method
7/5/2012 2:18:21 PM
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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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re: Selecting a solar energy conversion method
7/5/2012 2:19:01 PM
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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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re: Selecting a solar energy conversion method
7/5/2012 3:08:01 PM
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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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re: Selecting a solar energy conversion method
7/5/2012 10:30:41 PM
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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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re: Selecting a solar energy conversion method
7/6/2012 9:50:45 AM
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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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