I would like to share a small project that I completed few months back. And hopefully it can help others also, who are looking for a similar solution.
In short it is a switching-system that turns ON a relay depending on the strength of sunlight.
Reason that I needed something like this was that I am using a UPS with Solar panels(through batteries) and wanted to power the load via solar-panels during strong-sunlight hours.
As per my understanding the Day-Night switching options found in Charge-Controllers are voltage dependent and nonadjustable(mostly). By the time when they switch to night/evening mode, it is quite late in evening and there is very little power coming in from panels, causing the batteries to be discharged to some extent.
I wanted something which I could adjust/set and would help me in keeping the battery-discharge to minimum when the sun was not strong enough to keep-up with the load(in evening, rain and fog).
A separate 50 Watt Solar-Panel sits next to the main array. This panel is shorted via a resistor and a shunt, the voltage across the 50-watt panel and the current flowing through the resistor(and the shunt) is being measured by the DC Volt and DC Amp meter seen in the above image. On top of the box you can also see the resistor.
Value of resistor was chosen based on the panel's Max-Power Volt(Vmp) and Max-Power Current(Imp) and putting the values to the formula V = I . R
(I do have doubts if this is the correct way or not for getting the resistor value. But for now it is working)
As solar-panels are current source and their output is linked to insolation levels, I selected this method.
The volt and amp meters have Relay output. One can adjust the set-points to activate/deactivate the relays inside the meter.
The relay inside the ampere meter activates the timer seen above, this helps in avoiding short cycling when the current value is moving close to the set-point in the meter.
When time completes the external relay is turned On by the timer. It is wired such that the utility/wapda gets disconnected to UPS and the loads shift to battery power which is being topped via solar-panels.
Benefit is that the utility is disconnected when sun is strong. In evening, rain or fog when there is not enough power being produced by solar-panels, the UPS stays on utility and batteries are not drained. And whatever little power is being supplied by main array is helping to keep the batteries topped-up.
There will be better and cheaper options available but for me this has worked out as per my requirement.
I will try to update this post with more information. If anyone has any questions, I will be happy to help.
Current setup prioritize the use of main pv array, first. Utility, second. And the batteries, third.
Looking at the current values of Volts and Amperes of 50 watts panel, provides a rough estimate of expected output from the main array.
For example as per the picture above at 13.21 V and 2.3 A, it is 30.38 watts. This 30.38 watts is 60.7% of the installed 50 watts panel.
((30.38 / 50) x 100)
And if the main array is of 1000 watts, then as per above the main array should provide with atleast 600 watts.
By adjusting the set-point on ampere meter, I can set the system to shift to utility when the main array's output is less than the average load on UPS.
For example if average load on UPS is 300 watts, I can set the ampere meter to 1.1
(600/300 = 2---> 2.3 / 2 = 1.15)
Surely, comparing a fixed resistor to an MPPT will not be accurate. But still it does give some figures to work with.