|regulator and fan||regulator||regulator schematic|
|Some notes on the regulator|
A buck converter can be extremely efficient, and as I paid good money for the solar cell, I consider every milliwatt precious! The input diodes are low offset Schottky diodes . Check them with a multimeter that has a diode scale, they should have about 400mV for forward voltage drop, unlike a bog standard silicon junction diode which has 600mV drop. The catch diode after the regulator MUST be a fast recovery schottky diode. A good source of these can be found in discarded PC power supplies. The inductance is not critical, the design calls for a 75uH inductance. This one came from the junk box originally from a dead switch mode power supply which measured about right. A usable choice would be to put 25 turns onto a yellow/white 1/2 inch toroid. These cores can be found in abundance in dead PC power supplies ( do you see a pattern here ?) The auxiliary diode switched input is provisioned so that I can run the fan from a mains powered plug pack during the evening. The regulator chip is not normally stocked by retail parts outlets but many now do stock equivalent 5 pin sm regulators that will perform the same function. I choose this chip because.....because....I found it in a dead PC power supply!
|But now, in my quest for power and global domination, I have another solar kindly given to me by a colleague who upgraded to an even bigger cell and I harvest even more power. Enough power to energize a little radio beacon experiment I have in mind.|
The lower cell is a BP Solar 10W array. It was quite expensive when purchased some 6 years ago. I am told it did a good job installed on a tiled roof facing north in Australia's withering summer heat. Here are some interesting observations on cell aging. The cells do age and degrade after prolonged exposure to the elements. This class of service is murderous on electronics for it gets all of the enemies of electrical systems. High temperature under load, rapid deep cycling of temperature, condensation at night and full rainfall exposure. After six years of this "normal" abuse the cell power delivery has fallen about 70% of what would have been its factory fresh output. Under the same conditions this larger cell array produces only a little more then the new 5W cell. A measurement of its open circuit voltage shows that a couple of cells in the array have short circuited internally. This does not hurt the array significantly. There was some evidence of water ingress, this is caused by moisture laden air being pumped into the array by diurnal pumping...normal changes in atmospheric pressure. I added yet more sillicone sealing to extend its life in my service.
The outputs of cell arrays with the equivalent open circuit voltage can be directly paralleled. The current sharing is done by the inherent high internal resistance of the cell arrays. The cells could also be connected in series for a higher terminal voltage, however, due to their different capacities would have resulted in an irregular volt-amp curve and significant I2R loss in the smaller cell array.
My advice then, for installers of larger cell arrays:
Mount the array offset from roof tiles. Tiles are good thermal insulators. There should be free air circulation behind the cells because they will heat up to punishing temperatures and this must decrease their service life. There should be no shadow at all on the cell array, except when the sun is low on the horizon and it does not matter then. Modern cells are surprisingly insensitive to the position of the sun, but try to position the array so that it directly faces the winter midday sun . Then the summer sun angle looks after itself ! Go mad with glazing grade silicon sealant, do not let water pool on the bottom glass-aluminum lip, diurnal pumping will force moisture ingress. Add extra sealant on the bottom lip. From what I have seen so far, I am somewhat skeptical that the typically constructed cell arrays can deliver their rated watt-hour output over their rated life. There is not enough rear thermal ventilation or moisture ingress mitigation to satisfy me. Moisture ingress resulting in inevitable electrolytic decay will significantly shorten the working life of the arrays.