I described and showed the changeover switch for switching circuits in the house from grid to off-grid on a timer so as to take advantage of cheap night time electricity, and to adjust the off-grid use in winter when the solar energy in the UK simply isn't enough.
I've had problems with my battery bank tending to take too much stored energy out of it in the autumn leaving it too depleted in the winter with danger of sulphation of the batteries.
Accordingly I've been wanting to model the amount of solar power being generated so as then to control switching back to the grid so as not to take out more energy than is put in.
There are now available 500uF 2.7V supercapacitors on ebay and I think that people are having fun putting them into banks to start car engines instead of batteries. I have made up a 24V bank of 12 of these to put on the terminals of my inverter to reduce ripple but have found that there was no reduction at all and perhaps its not impossible that these small size capacitors have too much inductance to smooth ripple but are actually better suited simply to storing energy.
My off grid system has about 8kW of solar panels fitted to a barn roof, some facing east and others facing south with a fence made of 8 broken panels (cheap) facing southwest at a near vertical angle to catch late afternoon sun. This feeds a battery bank and an inverter, and then is connected to the house through a 300m or so cable. With the cable we laid a couple of CAT5 cables to provide signalling functions.
I seem to have troubles with the CAT5 cables at points from time to time nibbled by rodents or suffering for other reasons so that of the 16 available wires only 6 or so :-( are working.
Through the available signal cores, in the house, I'm able to monitor battery bank voltage, one voltage of one set of solar panels before entering the controller, and the voltage of another set also. I put LED lamps between the negative of the monitored voltage of the panels and the negative of the battery signal - the solar controllers are common positive. This means that the lamps flash according to the off periods of the PWM controllers and stay on when the batteries are at full voltage so that the controllers have cut off, letting the solar panel voltages raise to their open circuit value.
Luckily there's one more pair of wires in my CAT5 cables that are working that are available.
As a start of a control circuit, at the barn on the roof I mounted a small 12V 5Watt panel. I was experimenting using that to work two electromagnetic relays in the barn, intended for control use. From that solar panel feeding the relays, providing a load, the voltage is fed into the CAT5 pair of wires. In the house, this voltage has been feeding a voltmeter and a pair of 12V led lamps in series which light when solar energy is available in goodly excess.
In veiled sun the resulting voltage at the house from this monitoring panel has been around 9V and in full sun the voltage has gone up to 18. This accords with my real system where in veiled sun around 80amps is going into the battery bank and around 180amps in full sun - so between 2 and around 4 1/2 kW input.
But much of the time in England this winter has been dreadfully cloudy and at times I've been lucky to have 10-15amps going into the batteries.
Having the monitoring voltage coming into the house I had the idea to feed it to a supercapacitor as an analog to the battery bank.
Whilst much can be done with electronics or arduino control, to be frank, in 25 years time people won't understand component level electronics nor arduino code. So I have wanted to do things in a really dumb basic way that is so simple that anyone in the future will be able to understand it.
Protecting the supercapacitor from exceeding 2.7V - OK zeners are fine but LEDs are pretty :-) A red LED fires at around 1.5V and a green starts to fire at around 2.2V and blue at around 2.5V. So that's a good start for protection. With the quantity of current available from my 5W solar panel, the voltage on the green LED was approaching 2.7 volts and even 2.75 (ouch - but I assume a small margin might be tolerated). So I'm going to be using 3 or 4 green and blue LEDs in parallel as well as a red LED with a small resistor, ideally keeping the supercapacitor below 2.5V.
On my first test, it took a day of 50% veiled sunshine to charge the supercapacitor, the 5W panel powering the two relays in the barn and feeding the capacitor through the resistance of the 300m cable. As a guide, from memory the pair resistance of that cable is around 180ohms.
I was worried that the relays on the barn end of the cable would discharge the supercapacitor over night, but on account of the low voltage, low current resulting meant that the voltage on the capacitor went down only to 1.5V and this is a promising start.
On the next day, I've found that the 2-2.5V on the capacitor is enough to fire a 6V relay, and that on the low voltage, the current is small and the discharge therefore is still preserved in terms of being a long time. It will be interesting in due course to find out at what voltage the relay drops out.
But certainly here I have the workings of a system that integrates the solar power input into my battery bank and can fire a relay all the time that the solar power is a healthy amount. My next task is to model the analog of the power consumption.
I'm proposing to use a current transformer on my live wire from the off grid supply, to put it through a full wave diode bridge to generate a DC voltage, and to apply that reversed through a resistor across the capacitor so as to discharge it proportionately to the power use.
If it's possible to model the balance of power in and power out reasonably well then this system should be invaluable in preventing progressively taking out more power than is being supplied from the solar panels during Autumn as we head into Winter and I then need to shut it down and take next to nothing for at least a couple of months.