Less demand but more production from Solar panels

dhaval12345

There are two solar panels(SP) each with a blocking diode connected in parallel such that cathode of both the panels are connected and anode is connected to respective + terminal of solar panel. Both -ve of solar panels are common.If boost converter(without MPPT) is connected to anode and then to a battery charger which charges battery woth a constant current. If solar panels are unable to provide the current battery charging stops.

If different irradiance falls on both such that SP1 can produce 2Watt and SP2 can produce 1W.But the load for solar panels is only 1.2W. So which panel will provide how much power?
Will SP1 provide all the 1.2W power? Will SP1 and SP2 provide 0.6W each?? Will SP1 give 0.8W and SP2 0.4W??

dhaval12345

Less demand but more production from Solar panels

I have a circuit which operates only above 3V and requires 1W to operate. If it gets less power it turns off.
I have connected Solar Panel to this circuit. Characteristics of solar panel : 2W at Maximum power point , Voc = 5V.

Now power vs voltage graph has two points where 1W can be produces corresponding to Solar panel voltages(2V and 4.8V). So which operating point will be selected???
If 2V 1W point is selected my circuit will not start and if 4.8V 1W is selected, my circuit will start.

On what factors will this depend??Will my circuit start??

CALLD

Re: Less demand but more production from Solar panels

I'm not sure how to understand your problem. Imo it's not much use looking at points on the power graph to work anything out. The exact shape of the power graph depends on so many things for example, light intensity on the panel and the temperature of the panel. Resistance in conductors will also play a part. In practice the best thing to do is make sure you have the minimum voltage and current available under worst case scenario at the load, then install a voltage regulator at that point to keep the voltage within acceptable range. Add a battery who's nominal voltage is the same as what your load requires to keep it working when a cloud passes over for instance.

BB.

Re: Two Solar Panels in parallel with blocking diodes

It sounds like you have a very specialized application.

First--You really do not need a blocking diode for applications below 12 volts (Voc less than ~21 volts) when connected to a battery bank (solar panels are already diodes and the leakage current from a 12 volt battery bank at night through the panel is very small). And if you have a charge controller, then no blocking diode is needed (controller should block reverse current flow).

Are your power needs really this low (1-2 watts)? If so, is there some reason you cannot use small cells in series to get your working voltage--Using some sort of Boost Converter (inductive or capacitive) seems like a complication (unless you are trying for 100's-1,000's of volts).

Matched solar panels/cells have roughly the same Vmp (relatively stable) over operating light/current ranges. Vmp does fall with rising temperatures--So if the panels are mounted at different angles, the "hot panel" under full sun will have lower Vmp voltage.

Solar panels are, very roughly, current sources. They will output current in proportion to the amount of sun hitting the panel. If a panel is receiving 50% of rated sun, it will output ~50% of rated current.

Solar cells/panels (at least for standard crystalline silicon cells) are not affected by load--If the current draw is less than rated/available current the operating voltage will increase a little bit (lower internal resistance losses mostly).

In the end, you can put multiple strings of cells in parallel, all pointing the same or different directions (some facing south east, some facing south west--so you get more hours of sun per day--i.e., "virtual" tracking as an example) without the need for blocking diode per string (plus you save the diode forward voltage drop of ~0.2 to 1.0+ volts).

Once your circuit (if MPPT/boost converter) attempts to draw more than available current (not enough sun), the output voltage will collapse (again, solar cells are "current sources" with relatively flat current (i.e., ~1 amp from 0 to Vmp, and if you draw less than "1 amp", then Vcell will rise towards Voc--Voltage open circuit Voc which is ~20% or so above Vmp rating).

Is this helping? I am guessing at your needs.

-Bill

BB.

Re: Less demand but more production from Solar panels

I moved your first post/thread into this one--They related:

dhaval12345 wrote: »

I have a circuit which operates only above 3V and requires 1W to operate. If it gets less power it turns off.
I have connected Solar Panel to this circuit. Characteristics of solar panel : 2W at Maximum power point , Voc = 5V.

Now power vs voltage graph has two points where 1W can be produces corresponding to Solar panel voltages(2V and 4.8V). So which operating point will be selected???
If 2V 1W point is selected my circuit will not start and if 4.8V 1W is selected, my circuit will start.

The operating point will depend on your circuit. Cells are a current source--So if you circuit starts loading (without surge) it should hit the 4.8 volts. If your circuit has surge current and can operate at 2 volts--Then, it is possible for the circuit to operate at the 2 volt point (bi-stable). But it really depends on your circuit design (voltage mode, current most, constant power mode, etc.).

On what factors will this depend??Will my circuit start??

What is your circuit? DC converters are, many times, constant power devices:

Power = Voltage * Current

So... For example, if your circuit needs 2 watts from a 5 watt panel (and enough sun), as voltage rises, current falls, so it would be expected to operate at the higher voltage point of the power curve.

If, however, you have a lot of surge current, or your DC converter attempts to draw too much current (during starting conditions, low sun in morning, etc.), it can become "stable" at the low voltage intercept of the IV Power curve... I.e., as you get a little more current available, the converter tries to increase output voltage, which increases output power requirements--So, your circuit could become "trapped" at the low voltage point on the curve (drawing too much current for the array to supply).

One way around this can be to "burp" your converter. Put a small buffer capacitor on the solar input, and stop the converter for a second, let the capacitor charge, then start the converter again--The capacitor will supply starting surge current and get you over the initial current surge to start your circuit/converter. Then the solar panel will continue to supply power at the higher voltage (P=V*I).

Obviously, you will want to "over size" the solar array so you can supply sufficient energy to run your circuit most of the day--Not just at high noon on very sunny days.

Note that any shading/clouds can easily cut cell output power (current) by 1/2, and on very dark/cloudy days (stormy weather), it is very possible for the array to output only 5% of rated current/power.

-Bill