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MPPT | 7 comments (7 topical, editorial)

Re: MPPT (3.00 / 0) (#1)
by jimovonz (username at matata dot co dot nz) on Fri Jun 8th, 2007 at 02:00:34 PM MST
(User Info)

The voltage that a panel puts out is mainly dependant on two things, the incoming solar radiation and panel temperature. Without going to extreme lengths, the best you can do to reduce the panel temp is to ensure that it is mounted so that there is free circulation of air around it (particularly underneath). A solar panel will put out somewhat higher voltage than its nominal rating (unloaded). This is necessary to maintain charging voltage over a variety of conditions from a hot overcast/cloudy day through to a cold bright sunny day. The watt rating of a panel is determined under standard test conditions that involve irradiating it with an artificial light source that approximates sunlight at an intensity of 1kW/m^2 at a temperature of 25degC into a matched load. The test is usually conducted over a very short time period (<1sec) so that the incoming radiation doesn't raise the panel temp over 25degC. In a real application maintaining 25degC on a panel with 1kW/m^2 falling on it would be rather difficult. In most situations (particularly in bright sunlight/high output) the unloaded panel voltage is significantly higher that the battery it is attached to. The ideal load for the panel that gives the most power out would bring the voltage down somewhat, but would still be significantly above battery voltage. For example your typical 12V nominal panel under ideal conditions is optimally loaded at around 16-17V with an unloaded voltage up around 20V. Attaching this panel to a 12V battery would see the system voltage brought down to somewhere in the 12-14V range. The difference in loaded voltage between optimal and the battery is essentially wasted. The loss can be over 30% and is worst when the panel output is highest. A MPPT controller will present the optimal load to the panel in order to get the maximum available power. It does this by acting as a DC/DC converter, maintaining a higher voltage on the panel side and a higher current into the battery. The controller will track the output of the panel and alter the load to maximise the output under all conditions.
The disadvantage of a MPPT controller is that it costs significantly more than your standard controller. The cost of adding MPPT to a system has to be weighed up against the cost of adding panels. In a small system, greater gains will be had by simply buying another panel rather than investing in MPPT. In addition to this, it is hard to quantify just how much additional power you will get from using MPPT but is usually considered to be in the 15-25% range. It basically comes down to a cost/benefit situation.

Re: MPPT (3.00 / 0) (#2)
by Flux on Fri Jun 8th, 2007 at 02:09:57 PM MST
(User Info)

The only thing that I can add is that the greatest gain from mppt is when the panel is cold. Clear winter days will produce a significantly higher voltage and the benefit from the improved matching is greatest.

In hot climates the gain is small and may be negative if you include the inverter losses unless the panels are chosen for a higher open circuit voltage than the usual.

Flux

[ Parent ]

Re: MPPT (3.00 / 0) (#3)
by jimovonz (username at matata dot co dot nz) on Fri Jun 8th, 2007 at 04:26:52 PM MST
(User Info)

Flux is that inverter or converter losses you refer to? I presume you mean the efficiency of the MPPT controller itself (converter).

[ Parent ]

Re: MPPT (3.00 / 0) (#4)
by boB on Fri Jun 8th, 2007 at 07:56:21 PM MST
(User Info) http://bob.gudgel.org

>>Flux is that inverter or converter losses you refer to? I presume you mean the >>efficiency of the MPPT controller itself (converter).

I think he did mean converter and/or inverter losses. Since MPPT can gain you from, like, 15% to sometimes over 30% relative to NOT using MPPT, even if the converter losses were say, 10%, you STILL win. (some)

boB

[ Parent ]

Re: MPPT (3.00 / 0) (#5)
by jimovonz (username at matata dot co dot nz) on Fri Jun 8th, 2007 at 08:16:14 PM MST
(User Info)

I don't think inverter losses are relevant. What Flux is saying is that in some situations you can get a net loss by having a MPPT controller connected. In a particularly hot environment the output voltage of the panel is low enough that the load presented by the battery is actually pretty close to the MPP. The less than 100% efficiency of the MPPT controller would make you worse off not better. The MPPT controller gives better results when the voltage differential between the panel and battery is greatest. As Flux points out, this is most likely to be on a cold clear winters day.

[ Parent ]

Re: MPPT (3.00 / 0) (#6)
by boB on Sat Jun 9th, 2007 at 12:16:59 AM MST
(User Info) http://bob.gudgel.org

Right. This is true because when the PV modules are hot, the MPP voltage may be
at or below the battery voltage. If below the battery voltage, a boost converter
rather than a buck converter might be in order. Right AT the battery voltage,
a relay would be best since there is ~some~ voltage drop from either the MPPT
converter or even a PWM controllers MOS-FETs. (less than a 1 volt drop if done right)

If the PV array is wired for say the next higher nominal voltage than the
battery, the MPP Voltage will always be above the battery voltage. However,
in this case, all else being equal, and not taking PV to controller wire
loss reduction into account, the controller will actually be less efficient
than if its input voltage (PV) was lower. (closer to the battery voltage but
higher than the battery voltage)

It's still typically better to wire the array for a higher voltage than
the battery voltage and use an MPPT controller. This can help when the array is partially shaded too.

boB

[ Parent ]

Re: MPPT (3.00 / 0) (#7)
by Flux on Sat Jun 9th, 2007 at 12:35:36 AM MST
(User Info)

Sorry about that, I did mean converter as you all correctly deduced.

Mppt does ease the restrictions on the choice of panels. Things like using 3 panels for 24v become practical as long as the panels are the same current rating.

Flux

[ Parent ]

MPPT | 7 comments (7 topical, 0 editorial)

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