Some thoughts on MPPT & MPPC - Page 1
Forum section: electronics / projects

author: juanfermed
author_rank: Regular Contributor
timestamp: October 31, 2016, 03:22:02 am
content: Hello all,I wanted to share with you some thoughts, details and experiences related to "MPPT" devices. I hope someone find them as useful considerations or bring attention for comments from more experienced people. These are caveats that I have found while making a few projects around solar powered devices.As you might now MPPT refers to circuits designed totrackthe maximum power point of a solar array (panel) so that you always get the maximum power from it. Now, manufacturers tend to put under the same umbrella "MPPC" -Maximum power point Control- devices that DO NOT track maximum power point, but only let you define a setpoint to drive the voltage of the solar panel so that you approximately always get the maximum power, but it DOES NOT actively track the maximum power point.Now, these MPPC devices are a very good idea and work very well, since in reality the Vmp (voltage to get maximum powe) does not really vary much, ...right? Indeed, look at the following graph which is very typical.http://www.electro-tech-online.com/attachments/pro_bulb1-jpg.85436/In many applications, One could decide or compensate to live with that variation in Vmp. MPPC devices are cheaper than real MPPT devices, not only the IC or circuit itself but the total BOM, and design is simpler, also lab and field testing might be shorter. So a good trade off can be made. But some details might not raise up very intuitively, or only after you have made some deployment and want/need to make any changes:1) Solar panel cable length and its related voltage drop: The thing is that the MPPC setpoint voltage is set at the mere input of the IC. Inside the board the voltage drop might be negligible, but the drop in the cable is probably considerable. So you define your setpoint thinking only on the voltage in the panel should have right in its output connector, and the compensate for the drop in the cable. For example, you solar panel Vmp is 36V. The cable voltage drop is 2V (think of a roof top installation with the final storage device in another room), so you define the setpoint of the IC at 34V, because with the cable drop added, it will effectively regulate the voltage at the panel output terminal to 36V, and get out as much power as possible.Up to this point everything works fine....as long as the cable length does not vary. This is were you need to have a clear understanding of the application: will cable length always be the same? What if the device your power with the solar panel is farther away and need a longer cable? What if you needed it to be shorter? If the MPPC voltage setpoint is set with a resistor divider (which is very usual), definitely thinking about desoldering and resoldering a new resistor is not a solution when the number of devices deployed is very large or of difficult access. A potentiometer? yes, well, maybe because though it makes changing the setpoint possible with no complex tools (a screwdriver), how are you going to measure that setpoint? a predefined resistance? now need a multimeter and definitely cannot put the end costumer to make the change, measure output power and display it as you vary the potentiometer? well that defeats your first purpose of implementing MPPC and saving $1-3 or so in the added BOM, now you need to measure power with enough precision to set the setpoint correctly. You would not have this issue with real MPPT.2) Solar Panel Power and Vmp: Very similar to the previous but not directly intuitive. Lets say now you need to change the solar panel to a higher/lower power, or one from a different manufacturer. The Vmp will change. Maybe or maybe not dramatically. Again you need to be able to set the voltage setpoint of the MPPC correctly. If you have devices already in customer facilities, it will not be as easy as simply changing the solar panel. Now you need to also somehow be able to define the new setpoin. Again, you would not think of this with real MPPT.3) When there is very little sunlight, performance can be equal or worse than direct energy transfer - DET-. Put simply, if the input voltage regulation network the MPPC circuit cannot regulate, you wont be able to get any energy from the solar panel. This happens, for example when there is very little sunlight hitting the panel. In the Direct Energy Transfer -DET- method, were you simply connect the panel directly to its load, you are not getting maximum power, but if there is any power it will be delivered to the load, as small as it is (not efficiently of course). But with MPPC and depending on which IC or circuit you use, you might not be able to get those small amounts of power that even DET would get...which might not be ideal. You would probably suffer this with MPPT too though.With the previous I dont want to make you think it is necesary to avoid MPPC devices. They can be a very good option in many applications. But details like the previous are not mentioned anywhere that I know, and might imply a considerable aftersale work/cost that you might want to take into account from the beginning.All comments are very welcome!!

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 02, 2016, 02:16:45 am
content: Thanks for the info ;^)One thing that always confused me with MPPT is how exactly it manages to get peak power from the solar cell.For example say I have a DC-DC converter that initially sets the input voltage to Vmp and is happily supplying a load at Imp (max solar cell efficiency). But what if now the load needs less current, does this mean that the DC-DC converter input voltage needs to increase (since we want less current), if so wouldn't this mean that you are operating away from Vmp and hence at lower efficiency?

author: jt
author_rank: Regular Contributor
timestamp: November 02, 2016, 02:43:55 am
content: QuoteTo add to blueskull's reply, often the load is a battery. If the application requires maximize energy throughput, the battery would be sized to always present enough of a load such that the MPPT can throttle the charging current to the solar array's max power point.juanfermed seems to have a good grasp of the trades. Perhaps one omission is solar cell temperature which will move Vmp around. I believe some of the MPPC devices (Linear Tech I think?) include a thermistor input as a calibration input. You'll notice the linked IV curve plot specifies a 25C temperature - in reality temperature will vary over time of day / time of year.Solar array age can also shift Vmp. Solar cells are gradually degraded by UV (the cover glass helps slow this way down though).Solar cleanliness probably also perturbs Vmp (I think).Also to note - in some cases DET can be more efficient then MPPT or MPPC. If the battery (load) voltage is well mached to Vmp, and the array trackes the sun. This might be more of a pathological case though.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 02, 2016, 06:37:10 am
content: If you have a DC/DC converter capable of controlling the input voltage, then the addition of actual max power point tracking is a total no-brainer, with absolutely zero additional BOM cost, and very little development cost (compared to the development of the whole product, anyway).The only reason it costs more is marketing, and this is totally non-technical. This is not unusual; when you have a feature that adds quite some value to the product, even if the feature comes "for free" for the manufacturer, it can be sold for a relatively high price. This is a funny situation because it can suddenly change due to competition; if a product that sells the "premium" feature for the "base" price gets some serious traction, then suddenly everyone else enables the MPPT code, which probably already is there, just disabled, and people stop discussing anding about the "magical" MPPT. I'm surprised this hasn't happened yet, given the number of solar companies out there (and the utter triviality of MPPT).Every idea of "improving" the fixed setpoint strategy is probably going to be more complex than just implementing the tracking, which is the easiest solution to the easy problem - and ages old.

author: jt
author_rank: Regular Contributor
timestamp: November 02, 2016, 03:07:00 pm
content: QuoteIn addition to blueskull's comment of needing additional components (BOM cost) to measure power, MPPT hardware needs to also incorporate additional components (BOM cost) to run the MPPT algorithm. MPPC can be implemented by choosing a regulator that controls input voltage (as you point out), but MPPT requires running an algorithm (such as perturb & observe) to track the peak power point. So this means either adding an MCU or a specialised ASIC with baked in firmware that you wouldn't necessarily need for MPPC.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 02, 2016, 03:23:06 pm
content: Today, most people want user interface and/or some monitoring. If there is a product at all, i.e., it has some sort of casing, user manual, marketing, etc., it is most likely going to cost at least in the range of $50 and up. In this case, MCU (20-30 cents top, more likely 10 cents) is a total no brainer to have there anyway. If the DC/DC is the 90's, not 80's technology (i.e., current mode), current monitoring is there anyway. We are talking about a few tens of cents of BOM cost, which is a miniscule percentage of the total BOM+manufacturing.I understand that some $2 Ebay module might go this "MPPC" route, but then again, they are not integrated products, but low-cost building blocks.Heck, even a trimmer resistor to adjust the "MPPC power point" costs almost the same as an MCU.Really trivial stuff with lots ofaround it.Any real designer can easily achieve this without significant extra cost, but marketing can still sell it at a much higher price because it gives so much added value to the customer; as a side effect, this gives the (false) impression that MPPT is technically complex and/or expensive to implement, fueling the talk about it.

author: Seekonk
author_rank: Super Contributor
timestamp: November 02, 2016, 03:35:57 pm
content: The graph misses the point. 25C does not exist. Power point varies with temperature. There are some controller ICs that have a temperature sensor and that is all the tracking they do. In reality, MPPT does not matter for most battery systems because by design they have to have excess panel capacity and that is just wasted most of the time. I run my system at approximate power point because it is used only in one season and temperatures do not vary that much. It also allows me to set priority on each load by selecting a different power point. Lowest power point wins and that only has to be a couple hundredths of a volt. These loads are resistive heating. If I was to track I would do it by temperature. No one in their right mind does large arrays into a single MPPT controller.

author: Kilrah
author_rank: Unknown
timestamp: November 02, 2016, 05:28:53 pm
content: Quote from: Siwastaja on November 02, 2016, 03:23:06 pmThe cost of the MCU is irrelevant, the cost is of course in the development, testing and maintenance of the code that runs on it and the longer time to market it causes.