I won an ebay auction on some solar cells. What is MPPT and do I want one? - Page 1
Forum section: electronics / projects

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 09:43:17 am
content: So recently we've been refurbishing our shed / to be workshop, and we're going to install a heat pump, to keep it dry, and when working there, hot enough to work.I'm planning on tapping my solar cells into as High Voltage of an array as possible, without it being a solar rod, however I only have 72 cells, of 0.5v each, so that would mean either a 10m rod, with 36v output, or 5m 18v rod.So now I think I might have to have 8 rows with 4.5v output, and then have a boost converter to get 36 or 50 some volts, for moving the power to a lead acid power bank (With a pwm charger of some sort), and then a power inverter to power the heat pump.The heat pump should also be able to be switched to mains, when the power in the batteries is insufficient.Now about the MPPT. From what I've read, it's a device that tracks the output, and puts on a resistance to keep the solars happy?How much does this boost the solar power output? Also I'm building a tracking stand for it, that will track the sun, so I'm assuming that would be the best thing to do initially to boost output?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 19, 2014, 09:57:30 am
content: MPPT is about source resistance.Given a voltage source (solar cell in this case) with some internal source resistance, the maximum power is obtained when the load resistance is the same as the source resistance.On a solar cell, the source resistance varies with sunlight, so to extract the maximum energy, you use a DC-DC converter which can adapt its input characteristics to extract the maximum power.

author: whitneyrm
author_rank: Newbie
timestamp: March 19, 2014, 10:27:34 am
content: Quote from: mikeselectricstuff on March 19, 2014, 09:57:30 amA solar cell is not a voltage source- it's equivalent circuit is a current source in parallel with a diode, and some series/parallel resistance.http://en.wikipedia.org/wiki/Theory_of_solar_cells

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 19, 2014, 10:38:04 am
content: Quote from: whitneyrm on March 19, 2014, 10:27:34 amSorry, energy source....

author: Harvs
author_rank: Super Contributor
timestamp: March 19, 2014, 12:47:54 pm
content: I think there was a couple of questions in there.Do you want to use something with an maximum power point tracker with your array? Definitely. Even the low end of MPPT algorithms will get you >90% power out of the cells, and this sort of thing can be had for peanuts in battery chargers.Compared to just holding the solar array at the battery voltage and using a shunt regulator to keep the batteries from over-charging, you're looking at up to 20-30% efficiency gains. But really it's hard to say because it depends on how well the array max power point is matched to the battery voltage, and how variable the weather is.QuoteIf you're asking if single axis tracker is worth it, almost certainly not on a residential scale. Their value was marginal a decade ago when silicon was much more expensive. Now, even for the price of materials to build a tracker, your much better off just going and buying a extra panels.

author: georges80
author_rank: Frequent Contributor
timestamp: March 19, 2014, 03:54:49 pm
content: It is simple. Read below (or do more google searching/reading).Take the plot of V versus I of your solar array. Calculate and plot V x I and you will see it has a VERY distinct output power peak.MPPT will vary its DC : DC conversion output to maintain a power draw from the array that is right at that distinct peak. It will vary the output as needed to track the peak which will of course vary based on array temperature, sunlight and phase of the moonA REAL MPPT (***beware, there are a LOT of total Scam units on ebay***) will attempt to extract maximum power from your array.MPPT work really nicely when you have an array that has a much higher voltage than your load. e.g. a 36V array feeding a 12V (nominal) battery system.cheers,george.

author: qno
author_rank: Frequent Contributor
timestamp: March 19, 2014, 04:36:17 pm
content: A solar cell is a diode.If you measure a diode there is the voltage over it is about 0.4 v when uA are flowing thrue it, 0.5 V when mA and 0.7 V Amperes are flowing thrue the diode.A solar cell is the same.With cloudy sky it looks like it supplies 12 V (or whatever near its nominal voltage) but as soon as you load it with a resistor the voltage is gone and it delivers no power.A solar cel is a highly non linear device with a high internal resistance that goes low in full sunlight.Therefore I think MPPT is useless in combination with a solar cell.The power it consumes is more than you gain.

author: georges80
author_rank: Frequent Contributor
timestamp: March 19, 2014, 05:41:35 pm
content: Quote from: qno on March 19, 2014, 04:36:17 pmYou are incorrect (for larger arrays). An MPPT is VERY useful with a solar panel or array. Obviously it would not be useful for a tiny array, but if we're talking 50W+ then an MPPT can definitely improve efficiency especially with variable cloud cover/array inclination etc.I run an MPPT on an 80W panel (soon to size that up to 120 or 140W) when camping out in the oz bush and I can easily get 1Ahr or more improvement over a simple PWM controller. When you are collecting power from your only source (Solar panel in this case) any efficiency improvement that helps keep the beer colder in the fridge is worth it. What you collect in the day needs to run the load (fridge) at night.cheers,george.

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:15:52 pm
content: Okay, so now is the next question. How would I connect them? I get they are tabbed together, but what I'm asing is, which pattern would be best? High voltage I'd assume to be the best bet, but how would I best get that?I have 72 0.5v 1.8w panels, so if I want 36v, I'd have to wire them in one consecutive string, and with the dimentions of it, that would amount up to 6m long.Then again, I guess it's not a crime to just make smaller segments, and still run them in serial.Anyways, that would be a close to 130w panel (did I get a good price at 33£(55$)?) would a 2 axis tracking, like so:http://imgur.com/b0sOUpE be worth it?

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:20:48 pm
content: Also, I can only seem to find 12/24v mppts on ebay. Should I look elsewhere?

author: georges80
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:27:58 pm
content: Price is based on what your time is worth... In oz you can buy a finished 120W panel for <AUD$140 delivered. I personally would purchase a finished panel (tempered glass cover, weather proof housing etc etc) all ready to go.If you plan to use an MPPT then 36V all series would be ok. Of course since you mention tracking then it would appear you plan to use it outdoors, so making it waterproof/outdoor-proof is likely your biggest challengeI wouldn't bother with tracking (especially such a small output array) - just set the angle to your latitude and that will be 'good enough'.Are the cells you have mono-crystalline or? The mono cells are currently the best structure cells and provide good performance off-axis and with cloud cover etc.12V mppt means 12V OUTPUT. Check what their input capability is.Also, you haven't even mentioned you planned use for the array....cheers,george.

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:35:50 pm
content: Ah okay, outputI did mention the use in the beginning. It's to charge a bank of batteries, that will power a heat pump in our new shed, which will keep it heated to 5~10°C, and when we go to work there, keep it at ~20°C Now I know a 130w arrays is far from enough for that, but I'm hoping a battery bank will keep me good for the 5-10°C, and then have it use x amount of juice of the batteries, until it jumps of the solar system, and jumps on the grid instead.The panels are sadly not mono-crystaline, but poly-crystaline. Partly why I got them at that, as for weather proofing, that really isn't that difficult. It just has to be waterproof, and I have a lot of plexi and silicone laying around. Here in Denmark we don't really get it that ruff, and for the rare storm we do get, we'd either bring it in, or just bolt if firmly down. I'll see about that tho.Now I probably want to throw more panels at it later, but for now this will do as a sorta break in try

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:38:42 pm
content: Also, what do you think about this one?http://www.ebay.com/itm/Made-In-USA-MPPT-8A-Solar-Boost-Charge-Controller-12-24-36-42-48V-LA-Battery-/260939301031?pt=LH_DefaultDomain_0&var=&hash=item3cc131c0a7Seems rather expensive tho. Also do you have a particular MPPT that you recommend?

author: georges80
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:45:09 pm
content: What voltage is your battery bank going to be configured for to run the pump?The ebay link is to boost to a 48V bank (presumably 4 12v series batteries).cheers,george.

author: mtdoc
author_rank: Super Contributor
timestamp: March 19, 2014, 07:28:04 pm
content: A few points:1) making your own solar panel is rarely worth the effort. It is difficult to build one out of tabbed solar cells that comes anywhere close to matching a commercial solar panel in quality or perfomance. Now with high quality panels easily available for less than $1 per watt it rarely makes financial sense. But if you're doing it for fun/education -2) regardless of home made or commercial panels - the question of whether to use a MPPT charge controller or less expensive PWM controller depends on a few factors- String voltage versus battery bank voltage.- Overall size of your solar array.- MPPT versus PWM for equivalent solar arrays/battery banks will give allow you to harvest 10-30% more power from your array.How much and whether it is worth spending more money for a MPPT controller depends on a combination of these factorsFor example - to reliably charge a 12 volt nominal battery bank you need a string Vmp of at least 17 V or so. If your array (or single panel) puts out about that then you will get very little advantage from MPPT. You'd get a much better cost/benefit by using a PWM controller and buying more PV.MPPT really shows it's advantage when your string Vmp is far above your battery bank voltage. MPPT are basically buck converters that allow you to bring PV string voltage down to charging voltages without losing power. They allow someone to run a PV array with string Vmp of say 100+V and charge a 12 V battery bank. Higher string voltage allows longer wire runs with less copper and less voltage drop.Bottom line for small PV systems with PV Vmps close to battery bank Voltage - MPPT cost is rarely justiifed.A few more important points:Solar panels rarely put out their STC rated output. On average plan on about 75-80% of STC power outputin full sunIt's critical to size a battery bank to match your PV output. Goal is to be able to charge at least 10% rate. i.e. if you have 12V, 100AH (20 hour rate) battery bank - plan on having enough PV to put out 10A at 15-16 Volts.Power drawn should never bring yourDeep Cycle Batterybelow 50% state of charge (assuming lead acid batteries)Failure to pay attention to these rules will lead to an early, expensive death of your batteries.To size your overall systems, you need to know your loads. In generally anything making heat (evan heat pumps) draw a lot of power.Sorry to say but you have no chance of powering even a small heat pump with your 72, 1.5w cells. More battery won't help (see above)FWIW - I have a large 4500 watt solar system composed of 2 large arrays with string Vmps close to 100V feeding a 436AH 48V battery bank via 2 Midnite solar classic charge controllers (MPPT).I also have a small shed system (for lights only) that uses a single 12V nominal, 145W solar panel (Vmp of 18V) to charge a single 12V (85AH) battery via a PWM controller.BTW - for informed discussion and advice on solar systems - I highly recommend theNAWS forum