A practical  use for a solar cell

abstract:  ventilation fan suitable for a small shed is driven continuously  by a solar cell and simple switch mode power converter

This project was no so much inspired but compelled by the very hot summers here in Oz.  Even though Melbourne is traditionally known for its excessive rainfall  (not so true in the last few years)  there are a large number of summer days that can drive the interior temperature of an unventilated tin shed to over 50C degrees. This is not compatible with life !  It certainly is not possible to do any actual work in a shed like this.  I would also like to have the temperature below this to preserve  my stock of goodies when I am not in attendance.  I could put in a powered ventilation fan, however, for safety reasons I will not tolerate the mains electricity permanently powered on in the shack without my direct supervision.  This left the only possibility of a dedicated outlet running a little plug pack power supply or something more noble.

Thus I have done the moral equivalent of re floating a whale, hugging a tree and lying in front of a bulldozer! and purchased a solar cell.

A supplier of surplus electronics in Melbourne , see www.rockby.com.au  ,  had a stock of attractively priced solar cell arrays of assorted capacities.  There was a nominally 5W rated cell priced at only AU$30 , actually an extremely attractive price, they are usually double that. I bought one, not expecting much really, but the expenditure of $30 would not consign me to a paupers' grave.  

I was very agreeably surprised when in the full Melbourne sun it actually produced 5Watts into a dummy load. I was further surprised when I discovered that the cell was not significantly affected by orientation with respect to the sun.  The power output was sensibly constant as the sun covered its sky course during the day.  The cell with a notional output voltage of 12V would drive  a 12V DC muffin fan to full power.

I cast about in my junk box looking for a suitable fan. Some are very sensitive to supply voltage, some are very reluctant to start from cell power. The reason is that the solar cell has a very high (compared with battery or mains PSU)  internal impedance.   My cells optimum operation point was when it was putting 16 volts into a load.  The startup current drawn by some muffin fan was greater than the short current of the cell, however at short circuit current there is zero voltage !  Some fans were reluctant to start at partial cell illumination. I wanted my ventilation fan to run from sunrise to sunset and only one Papst fan coped  well with a wildly varying cell output.

The solution was a little switch mode buck regulator. The solar cells open circuit voltage would rise to over 20 volts, the average fan motors load would drop it to about 10 volts.  With this buck sm regulator, the cell voltage hovers at around 16 volts for most of the day and drives the  fan properly with a rock solid 12Volts.

The regulator is based on an LM2596S  surface mount SM regulator, that is normally used for embedded computers and supplies a fixed 5 volt output, but on dividing down the sampled output voltage it regulates to 12 volts very nicely.  This regulator I obtained from a discarded PC motherboard by sweating it off with a heat gun.  The inductor and fast diode diode were recovered from a discarded PC power supply. The  chips data sheet design spec called for a 75uH inductor  and this one had about the right number of turns on it.  Its precise value does not matter, however, too much  is as bad as too little inductance.  The little bit of heat sinking provided by the PCB keeps the LM2596 regulator nicely cool.  They are very efficient, if it is getting hot then you have done something wrong.  The output capacitor is the only critical component, it must have a low ESR ( equivalent series resistance)  and the only way of ensuring this is to use a high voltage rated capacitor,  mine uses a 100V rated capacitor from a dead CRT monitor and has proven to be very satisfactory in this class of service.

The design effort was saved by using a trimpot to set the output voltage. You could just calculate a pair of resistors , but GEE ,thinking takes effort!  The only modification I made to the cell array was to put silicone sealant  along the lip of the aluminum frame to positively eliminate the possibility of water ingress.

Does it work ?  Yes it does!   Also keeps the shack interior at external air temperature instead of 50C degrees !  At the time of writing this article ,Feb-2010,  the fan runs continuously from 08:00  to 19:45  AEST, still spins when heavily overcast but with less enthusiasm.

The cell is mounted high to be clear of all shadow. Be warned, do not run a cell in partial shadow it will not work. The reason ? the cells are wired in series, if only one segment is not illuminated it will go into a high resistance state.  Do try this trick at home. Just cover one little segment and watch the entire cell output drop in half.

regulator and fanregulatorregulator 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.


page created Tue Feb  9 18:13:50 EST 2010