Off the Grid with Solar

[Perry]

LV (120VDC) has tougher rules applied and certainly at this level power is dangerous. The point to keep in mind is DC not only requires specific switching/protection (fusing or breakers) - AC ones won't do - it is more dangerous. One can get a shock at around 30V+ but this number is not exact.

But is any typical PV array-powered installation likely to have any such loads? I'm familiar with the switching issues, and well aware that AC ones won't do.

DC has mean arcing capabilities so therefore needs greater cable protection.

Nonetheless, it still seems like overkill, to me. (depending on the voltage) It's not as if you're arc welding! By accident or design.

[PTWhatAGreatForum]

But is any typical PV array-powered installation likely to have any such loads? I'm familiar with the switching issues, and well aware that AC ones won't do.

Here are the numbers in case you have under estimated them:

At array to controller - 150VDC (typical 130V) at upto 28.5A based on panel STC rating (Standard Test Conditions - could even go slightly higher in NZ)

Controller to inverter (wired in parallel with battery input) 47-62V DC at upto 60A

Battery bank to inverter - 47-62V typical and as this is the battery run we looking at 3 currents - normal peak, start up peak and max short circuit.

Normal peak - around 45A

Startup peak - not sure but estimated to be upto 80A

Max short circuit - 3,600A is the figure I have designed for based on using L16 style flooded (wet) lead acid batteries.

What does this mean?

Solar array to controller input - switchgear and protection (beakers / fuses) at minimum must be DC rated to handle peak Voltage and peak current + 25%.

Solar controller output - same story as solar input... switchgear/protection at peak Voltage and peak current + 25%

Batteries: Switchgear at peak Voltage and peak current + 25%. Additionally Cabling, connectors and protection (breakers/fuses and fuse holders) must be able to safely handle max short circuit current until protection is tripped.

This is why you must never use automotive or glass encased fuses in systems with deep cycle batteries even if the "normal" current rating is high enough because under max short circuit (Isc) conditions these deep cycle batteries have enough current available to blow those fuses apart, burn/melt things and potentially start a fire.

I hope this explains.

[Perry]

So the PV arrays are in series, then, to give that sort of voltage?

[PTWhatAGreatForum]

So the PV arrays are in series, then, to give that sort of voltage?

Yes. In groups of 3. I run a 48V bank so need to have minimum 2 in series. Each group has to be individually fused, which means an individual cable run to inside (used 180m of cable on this array) and there is only a limit to the number of fuses a combiner can take. Also, higher Voltage is more efficient.

Definitely go "150V" if an off grid system if possible.

Keep in mind a so-called "24V" panels is only called 24V in respect to the need to match Voltage (with the battery bank) on most lower end charge controllers. In reality it's about 38V..

So 3x 38V = 114V. Under load this goes down to around 100V.

There is a further Voltage called "Voc" (Volts open circuit). This is around 45V and needs to be considered in respect of the maximum input Voltage of the charge controller. This is what the panel will give in good light with no load but it's not usable Voltage.

All technical information I have offered here is a bit simplistic. If you are intending to make use of it please consult the manufacturers instructions or PM me to discuss these important specifics.

[Perry]

Kind of you to offer. I have an interest in sustainability, but not to the extent or fidelity that you have demonstrated. Some solar water pre-heaters, a small PV array for battery charging, a wood-fired boiler, heated using our own coppicing woodlots, for hot water and central heating. Plus fruit trees and raised bed gardens, seed-saving, etc. Yet I think we still spend too much at the supermarket! However, those things I've mentioned do seem to pale significantly, compared to your efforts.


I didn't think your locality had the sunshine hours you really needed, but it just goes to show.

[PC]

Have you considered adding a small wind turbine as well?

[PTWhatAGreatForum]

Have you considered adding a small wind turbine as well?

Yes and wind doesn't stack up price wise these days. For the price of a wind turbine I could add a bunch more panels which would give me some power on overcast days and a lot more generation over the year (by a factor of several times).

[Bobsyouruncle]

Are you running much on 12 volt and anything on 24 volt M?

[PTWhatAGreatForum]

Are you running much on 12 volt and anything on 24 volt M?

No and I advise against it for most fixed (ie: not mobile) applications.

230V AC mains is easy, cheap and efficient to distribute. All the switch gear, cabling etc is readily available and pretty cheap to buy on a trade account (assuming buying name brands - if not, even cheaper). When you buy a new gadget, just plug it in. No fluffing about.

Trying to run a bunch of stuff from DC is not only a pain in the arse, it's inefficient to distribute and requires expensive less common DC rated switchgear etc. Furthermore, and this is the part most people don't realise, is the DC side is not a clean 12V (ie: 13.5V), 24V (ie: 27V) or 48V (ie: 51.6V). It's subject to wide Voltage swings depending on whether it's charging, under load or somewhat discharged etc. It is possible to compensate for this using a buck/boost converter but it's just too much of a pain in the arse.

There is a few appliances designed for mobile (marine / auto) use and these will generally tolerate solar system DC conditions but these are expensive and risky in that you will be buying niche brands which use less common parts.

The best practice these days is to spend the extra money on more panels and a bigger battery bank and run everything off AC. You lose a little bit of efficiency in the conversion (typically 7-10% - the higher the DC Voltage used the lower this number is) but it's worth it.

[Anaru79]

This is awesome guys!