by **Chriswlan** » Wed Jul 26, 2017 1:38 pm

Here is my take on the VT and my "VT-60" and "YearRoundVT-60" - with the goal of LONGEST CONSTANT POWER....

At the Equator, we'll all want 2 (pairs) sub-arrays kinda hinged book-like at the binding, upside down. With a 60 degree included angle. Which means the outside "aiming angle" difference is 120. 120 being one third of the earth daily revolution, we, at the Equinox, want each module to be perfectly square to the sun at EXACTLY 8 hours apart. Could use a square and its shadow on the panels for example; does NOT matter (much) if we get the first array square at 7am or 8am... The other one must be done 8:00 hours later to get the included 60 deg. We'll get the magic 60. If book is a little leaning off to one side we can easily adjust for balance.

If we are rather close to the Equator, we are done -at least for the Equinox, and as well best fit for the yearly seasons overall.

Now imagine a schoolroom earth globe, on its side, axis parallel to a desk. Have a mini book, folded cardboard, open at a fixed 60 deg, straddling the equator, and let's say you want to slide it slowly down the side of the globe, to a bit higher latitude -and keeping its binding parallel to the earth axis. Then as you move away from the Equator, the "binding" will assume a slope-to-grouind equal to the latitude. As far as the equinox is concerned we are done. The binding "spine" MUST remain parallel to the globe axis as the whole idea is to compensate for the globe rotation.

Lets now move all the way to the pole: we now have the "book" sitting upright on end (on the ice...). Still being the equinox as we have 24 hrs sun, might as well add a third array, and we end up with a 3-sided prism, equilateral, "bolted" around to the globe/earth shaft. 24 hrs of steady power, and simple school cosine will show that power is constant +- 7.2%.

Now bring that prism slowly down latitude again, carefully keeping its axis parallel to the earth's, and the side facing the ground becomes useless, and we again end up with the "inverted open book". But still with its spine elevated according to the latitude. Remember all the above is true at the equinox.

NOW, in my case of interest (2 deg latitude), I may want to get fancy and, AFTER trading some midday energy off for more at the 2 ends of the day, in the name of near constant day-long power with a "VT-60", I may now ALSO wish to trade a bit of equinox power to increase the solstices that are lagging. Again it is easy, with a very few percent more PV area, in the name of constant power. Near the equator, split your 2 main arrays in halves, (ie FOUR subs) and have their facing azimuths skewed a bit away from aiming down the equator... (ie put a kink mid-length) I'm guessing one set about 10-12 degrees one way and the other set the same the other way. That gives us only a very slight seasonal variation in addition to the 7.2 % theorical within ea day. This I call my "YearRound VT-60"...

Of course for the poor souls stranded on more meaningful latitudes with increasingly gross/yukky season extremes, I'd think the extremes get harder to compensate for (think about the impossibility at the pole during the winter...) with an end result of renouncing more and more energy in the pursuit of "seasonal power consistency". I'll let ya'll take my YearRound VT-60, slide it parallel to the earth axis, to your latitude, and figure out how to contort it all to compensate for even more difficult conditions. Given it is impossible at the pole, I'd think part way up real seasonal symetry is already impossible. Perhaps start with much increasing the *area* of the 2 sub arrays that favor the equator (for winter solstice), while keeping the exact same "YR VT-60" geometries? Ultimately at higher lats one ends up paying for several hundred percent oversize PV area, to get meaningful winter power; at 50 deg North, I even see a few modules in the country side that are slightly looking into the ground to shed snow ASAP, to catch a wee bit of winter sun.

My 3 cents

Christian

NB: Many thanks to Gordon who introduced me to the VT concept of trading off a wee bit of PV area for constant power. Perfect for water pumping etc at low lat's.