MPPT charge controllers for PV panels

[sophiecentaur]

I know this is a 'how long is a piece of string' question but here goes, anyway.
I am planning to augment the wind turbine on my boat with a PV panel (both very low power - up to , say, 2A @ 12V).
Area is limited. MPPT controllers get more out of a panel but I wonder what the actual statistics are, for small panels.
The boat is on a swinging (drying) mooring, so direction is a factor, although the tide leaves her facing one particular way for almost half the time, usually, if the wind doesn't dominate.
You can read a lot about MPPT controllers on the web but it's mostly seller's blurb and aimed at higher power, domestic applications.
A 20W or 30W panel would be a much more convenient size than a 50W panel and I could adjust the elevation from time to time. I have read of 40% advantage with MPPTs, so I am on a bit of a cusp. Also, a small MPPT is cheaper than extra panel area.
Is there any practical experience out there in PFland? Cheers.

Omg. This should really be on the EE forum, I think. That's what you get on a smart phone. :)

 

[nsaspook]

If the IV power curve of the PV panels are matched to the battery or load profile and the power levels are low (<100W) then MPPT will provide little or no advantage under normal conditions.

A typical panel designed for a 12vdc system will have a power curve like this.

When using a typical solar charge controller the first stage normally pushes the max possible current up to a voltage limit. This is the stage where MPPT is useful as it takes power at the panel ideal IV curve point, uses that power to provide the most input current (as a constant current source) to the battery and it's where a battery normally has it's best coulombic efficiency. If the charge/load profile falls close to where the MPPT would be anyway then you would see little or no gain in normal light conditions. If the light conditions were poor for most of the time then MPPT would give gains in current but the added power would be much lower than in good light.

Once the current into the battery causes the battery voltage to be at the max voltage setpoint of the charger the MPPT function become much less useful as we are now limiting the power input to the battery, the coulombic efficiency is usually lower, so real efficiency gains are lost.

Panel is cheap now so simply buy more panel with a rock solid PWM controller instead of an expensive MPPT type controller (the cheap Chinese knockoffs are junk) for a simple battery system.

 

[sophiecentaur]

Thanks for that. Your last comment about cheapo Chinese versions is interesting. I shall have to measure up and decide just how big I can find room for. My needs are actually quite modest, with a chartplotter and tiny compressor fridge. LED lights throughout are a very small load.

 

[gnurf]

The length of your string will of course depend on your operating conditions, and a quick inspection of the panel's IV curve suggests that there's quite a bit to be gained (or lost rather) by controlling the operating point of a panel. If the panel's operating temperature--and hence the voltage at which it will produce maximum power (Vmp)--is fairly constant, a simple constant-voltage type of MPPT such as the LT3652 could be suitable if your willing to build your own controller.

http://www.linear.com/product/LT3652

 

[sophiecentaur]

Looks like a good idea to BMO. A bit of home construction is theraputic. I don't have access to so much equipment but this would only need my DMM.

 

[nsaspook]

Thanks for that. Your last comment about cheapo Chinese versions is interesting. I shall have to measure up and decide just how big I can find room for. My needs are actually quite modest, with a chartplotter and tiny compressor fridge. LED lights throughout are a very small load.

The realistic power in/out gain from just MPPT at same input/output voltage is usually about 5-15 percent. Spending more for extra panels for low power systems is usually the better choice.

 

[gnurf]

The realistic power in/out gain from just MPPT at same input/output voltage is usually about 5-15 percent. Spending more for extra panels for low power systems is usually the better choice.

If the 5-15% figure is relative to the PWM approach you mentioned in your first post, where I think you are suggesting that the panels are clamped to the battery voltage, the first statement seems to rest on an assumption that the battery depth of discharge is low. This might be a valid assumption depending on the OP's setup, but in general this configuration would move the operating point away from the MPP as the battery discharges and could ultimately produce the least amount of power when you need it the most, e.g. in the case of a drained battery.

Panel is cheap now so simply buy more panel with a rock solid PWM controller instead of an expensive MPPT type controller (the cheap Chinese knockoffs are junk) for a simple battery system.

The LT3652 is available at Digikey at $6.98.

 

[nsaspook]

If the 5-15% figure is relative to the PWM approach you mentioned in your first post, where I think you are suggesting that the panels are clamped to the battery voltage, the first statement seems to rest on an assumption that the battery depth of discharge is low. This might be a valid assumption depending on the OP's setup, but in general this configuration would move the operating point away from the MPP as the battery discharges and could ultimately produce the least amount of power when you need it the most, e.g. in the case of a drained battery.

Your points are all true but usually if you want to increase battery (a expensive consumable) life with lead-acid chemistry you balance the system by limiting the DoD to about 25% with a max charge rate of about 10% of the battery capacity. My general position about solar powered systems is if you're draining your batteries heavily daily then you need more panel or fewer loads because you're murdering them. If you can achieve the effect of more panel with cost effective MPPT then that's the way to go.

I think Morningstar has a fair discussion about PWM over MPPT for small systems here.
http://www.morningstarcorp.com/en/support/library/Trad-PWM-vs-TrakStar-MPPT-April-2013.pdf [Broken]