04-19-2018, 02:13 PM
My understanding of solar PV theory is admittedly crude and it goes something like this. Picture photons of light coming in like hailstones and hitting the PV cells like hailstones hitting water. Each photon "splashes" an electron up through an electric field like the hailstone splashes drops of water up into the air against gravity. If a collection device (cup, bucket, gutter) is placed where the droplets of water can be collected before they fall back down to the water's surface from which they started out, then you could run a water wheel and get useful work. Similarly if the electrons are captured somehow they can be used as "electricity" as they attempt to return to the voltage they were originally dislodged from. The million dollar question in each case is how high to you hang the bucket? The higher you collect the droplets/electrons, the more energy to be had per droplet/electron as it rides the waterwheel/motor back down but the fewer make it that high. It's a "how sharp is the bell curve" thing.
In the case of the hailstones hitting the water I can see less intense hail splashing the rain less high meaning a wider bell curve. If there is a similar relationship between less intense light and less energetic electrons then as the light weakens due to clouds the panels will put out both fewer electrons and less energetic electrons so you should hang the bucket lower.
TL/DR: An MPPT charge controller will allow you to add an extra panel to each string since it automatically adjusts the height of the bucket. The benefit on cloudy days when the panels might otherwise not produce at all more than makes up for the modest penalty for doing so extracted when the light is high. Truest if VOC varies a lot with light intensity. Less true if all electrons knocked loose by each photon are equally energetic (always splash equally high) and vary only in number of electrons produced. The salient point is that there is a rather precipitous cut-off in production as the light dims and the native voltage produced by the panel string is no longer enough to push electrons into the battery bank. MPPT controller lets you design for that worst case.
In the case of the hailstones hitting the water I can see less intense hail splashing the rain less high meaning a wider bell curve. If there is a similar relationship between less intense light and less energetic electrons then as the light weakens due to clouds the panels will put out both fewer electrons and less energetic electrons so you should hang the bucket lower.
TL/DR: An MPPT charge controller will allow you to add an extra panel to each string since it automatically adjusts the height of the bucket. The benefit on cloudy days when the panels might otherwise not produce at all more than makes up for the modest penalty for doing so extracted when the light is high. Truest if VOC varies a lot with light intensity. Less true if all electrons knocked loose by each photon are equally energetic (always splash equally high) and vary only in number of electrons produced. The salient point is that there is a rather precipitous cut-off in production as the light dims and the native voltage produced by the panel string is no longer enough to push electrons into the battery bank. MPPT controller lets you design for that worst case.