Some very noobish questions.

[Gruntled]

Hi, I have looked through the forum and while the information here is very helpful, I have a few very basic questions that I can't get my head around.

 

The big one is if you are running an off grid system without batteries how do you manage the current. For example I can see how a controller/inverter can keep you at a fixed voltage but what does it do when current draw exceeds the power available? Does it just turn off if it can't maintain 220v? Also, how does the system manage excess production when you are producing more than the load? Does it just dump it to ground or something? Doesn't seem like a great idea to me.

 

Another lifetime ago running heavy DC mining trucks when you engaged the break retarders the system would a dump huge amount of energy into a bleed system that basically converted the energy to heat and blew it out of the truck with cooling motors. Do modern controller/inverters have the ability to do something like that? If so, how reliable/advisable is this kind of a set up. I hope I am not talking like too much of an idiot here but my biggest off grid concern at this point is not generating the power but rather how do I get rid of the excess when I have low or zero draw.

[Gruntled]

A further question I forgot to mention in the OP. I presume that once panels are installed they will immediately begin generating potential, if they are disconnected while exposed to sunlight does this damage them? Cause them to get hot or anything?

[Crossy]

If the load is less than the generated capacity the panels simply don't generate it, leaving them disconnected simply means that there is output voltage but no current, no damage will occur. Unlike wind there's no need for a power dump.

 

What is actually your aim? Off grid without batteries CAN be done but it's very limited as to application. It works for well pumps but it's of little use for your A/C or lighting.

 

EDIT If it is for pumping then there are specialist BLDC (Brush Less DC) motor pumps that can cope with the wide voltage range generated by the panels no need for an inverter.

[Gruntled]

Ok, I think I get it. The panels simply charge to their rated voltage and without any load it is like an open circuit so no current flows and no heat is generated. I was thinking of them as though they were being charged by the sun like you would charge a battery until it over charges and explodes or something.

 

To answer your question, the application I am planning is detailed in my other post.

 

Thank you for the prompt reply.

[Crossy]

Irrigation then, I'll respond over there.

 

[RocketDog]

14 hours ago, Crossy said:

If the load is less than the generated capacity the panels simply don't generate it, leaving them disconnected simply means that there is output voltage but no current, no damage will occur. Unlike wind there's no need for a power dump.

 

What is actually your aim? Off grid without batteries CAN be done but it's very limited as to application. It works for well pumps but it's of little use for your A/C or lighting.

 

EDIT If it is for pumping then there are specialist BLDC (Brush Less DC) motor pumps that can cope with the wide voltage range generated by the panels no need for an inverter.

Great answer Crossy. The only note I want to add is that the BLDC pumps probably have built-in 3 phase inverters that provide for variable input voltages down to a certain voltage. I just don't want people to think they can just get a pump and connect it to a BLDC motor like they can a brushed DC motor. A three phase inverter control is required to provide electronic commutation to the motor.

 

OP : your mining trucks probably used regenerative braking that converts the truck inertia into heat to dynamically brake it. That's where the heat comes from. Electronic exercise bikes use the same principle to provide pedal resistance. You drive a generator which dumps the power into resistors and dissipate it as heat.

BLDC motors don't work the same. As Crossy says, there is no equivalent with PV solar panels. Until the panel is connected to a load, no current flows except a very tiny leakage current internal to the panel diodes themselves.

Nonetheless, a panel in sunlight, if high voltage, represents a significant risk if you made your body a load! Anything less than about 50V is harmless.

Good luck.

[Crossy]

2 minutes ago, RocketDog said:

Nonetheless, a panel in sunlight, if high voltage, represents a significant risk if you made your body a load! Anything less than about 50V is harmless.

 

And it doesn't need much light, our solar strings are 350V ish and they will bite hard even as it's just getting light. The voltage goes to maximum almost immediately the sun stretches its arms, there's just very little current available.

 

[Elkski]

I replied on the other irrigation thread.   It would be an exciting project to do if I was retired in Thailand with good land and water supply and knew which crop won't be over planted.   I had a guy try to give me his rubber farm.   It's cool but it must be hard to find workers who will pull night shifts.   Now everyone is planting durian.   It seems like a free for all with no government help in planning for demand.  I'm betting in 6 years I can get some reasonable prices on durian here in the states.   Currently some Thai and Malaysia frozen is available for 450-600 baht pound or 1000-1500 baht/ kg.  It's not very good looking but tastes ok.  Have bought 2 in the last month. May wait for the fresh season.