Capacitance for solar system

I have a friend that wanted to know what capacitance he would need to smooth out the load curve/harmonics on a 12kw solar system, but for the 12v (max 15v) dc side of the system. It is 4800amp hrs.

One more question; what size wire for the lead of the cap?

(I dont know about his system, but told him someone here probably knows the answer)

Thanks,
James

jminer99er said:

I have a friend that wanted to know what capacitance he would need to smooth out the load curve/harmonics on a 12kw solar system, but for the 12v (max 15v) dc side of the system. It is 4800amp hrs.

One more question; what size wire for the lead of the cap?

(I dont know about his system, but told him someone here probably knows the answer)

Thanks,
James

Click to expand...

Something is not quite straight here..... WHAT harmonics? What is their source? Why is a 12 KW solar system operating at 12V, instead of a higher voltage where the current is more reasonable and the losses much less?

There are a lot of factors totally not mentioned.

I will assume that there is some sort of step-down battery charger type device between the (presumed) photovoltaic panels and teh battery or 12V distribution.......

But......

1) teh maker may already spec the capacitor

2) the battery is a fine capacitor for lower frequencies.....

3) what is the system limit for ripple (the harmonics you mention possibly are ripple)?

yeah Im thinking ripple, so not harmonics sorry, but he said " a cap acorss the pos and neg on the 12v dc side for the load curve". Where he has this there is no power, so he is using solar pannel to charge somehting like 130 batteries that go though an 8000w inverter for basic home power.

jminer99er said:

he said " a cap acorss the pos and neg on the 12v dc side for the load curve".

Click to expand...

I sure don't understand what he's trying to do.

A fully-charged automobile battery exhibits capacitance of about 1 Farad. So 130 batteries would surely exceed 100 Farads, which is a couple of orders of magnitude more capacitance than he's likely to buy and connect to the system.

And what load curve is he concerned with? It sounds as though he expects the added capacitor to function as a voltage regulator, which it definitely will not.

Depending on the type of inverter used to generate house power, it might put some noise out on the DC input line. But that's of absolutely no consequence unless it's of sufficient amplitude to be picked up on an AM radio. If so, adding caps won't do anything to attenuate it.

Another remote possibility is that there's some kind of oscillation in the system that's causing the DC voltage to fluctuate. If so, a detailed system analysis would be required to determine the cause and the proper corrective action.

- Leigh

Noise on the inverter output is best addressed with a filter on the 120V output.

4800 A-H is a huge capacitor.

It is true that the capacitance is "on the other side of" a considerable inductance and possible AC resistance, and so less effective.

I am starting to get the idea that he has a voltage drop at high power outputs...... and that this question is really about that, maybe he has heard about the "power boosting" capacitors on car stereo setups.

In that case, he should consider the system design...... At 12V, 8 KW is a lot of amps........ and that means big drops. he'd be better off at 48V, or higher. I realize he HAS this stuff now.......


If the equipment is a given, we need to see the wiring, and equipment list... I will bet money it can be improved a LOT with a better wiring setup, assuming changing the voltage is not happening.

I printed out the replys, and he said this pretty much answered his question.

A fully-charged automobile battery exhibits capacitance of about 1 Farad. So 130 batteries would surely exceed 100 Farads, which is a couple of orders of magnitude more capacitance than he's likely to buy and connect to the system.

Click to expand...

Thanks guys!!! From my understanding, when he was sweaty, he layed his arm across the 12v buss with the inverter on and said " he could feel the frequency" (got me?)

Since I visit this place often to browse posts, I figured I would ask here! This forum ROCKS!

jminer99er said:

I printed out the replys, and he said this pretty much answered his question.



Thanks guys!!! From my understanding, when he was sweaty, he layed his arm across the 12v buss with the inverter on and said " he could feel the frequency" (got me?)

Since I visit this place often to browse posts, I figured I would ask here! This forum ROCKS!

Click to expand...

If he's got a long run from the batteries to the inverter and the wire is a bit thin for the current drawn, he would get some ripple, otherwise there's no problem. He may be better off fitting a second cable run to the inverter, or alternatively moving them next to the batteries.

As mentioned difficult to tell without more info, scope traces etc. At the sort of current he has there is likely to be a considerable AC component on the line due to internal resistance of the batteries, resistance of the battery connections, battery hookups and DC cables. The whole setup sounds like trouble waiting to happen.

Few areas to look at:

Minimise the AC component with capacitance, the exact details depend on the situation, frequencies involved etc but the 1:100 rule of thump may be appropriate ie 1 farad, 10mF, 100uf, 1uf. The I farad obviously a car one the rest low ESR types. If the DC wireing is any length have this at each end. A few 10mf, 100uf, 1uf spread around the batteries would help too

Edited to add the reason for the range of capacitors is to deal with limits in performance due to induction and resistance of big capacitors. Careful engineering can minimize the quantity and range required but better too many than not enough.

Mechanical design of DC wiring. The mutual inductance of these wires is going to cause forces in the wire and vibration due to the AC component which leads to harmonics generated at higher frequencies.

This AC component may feedback (magnetically if nothing else) to the solar panels which is not going to help reliability of connections to and connections within the individual panels. Probably need a scope to look at this but the answer is more 10mf, 100uf, 1uf spread around. Consider how many caps can be bought for the cost of a single panel

As previously mentioned 48V is used for a reason, the reduction of current by 16 will same him a lot of expensive grief.

How is it even possible to have a 12v 12Kw system? Large panels, anything over 75 or 100 watts, put out somewhere around 32Vdc under full load. Your buddy didn't do a 12Kw system with 50 watt panels did he? This would be a less than ideal design, to be very nice about it.

If he is actually running his DC side at 12V into an inverter he is seriously screwing up. He should run the DC side at the highest voltage that he is capable of, legally, and that his inverter will handle. Just laid out a 30KW system for a new customer last week last week, 144 panels of 200 (or so) watts (somewhere between 200 and 225). 12 banks of 12 panels, each bank running at close to 400Vdc with full load. This will be going into a 28.8 Kw three phase inverter, a very capable system for sure.

Just consider wire sizes! A 12 panel bank makes 2400 watts in full sun, at 33 volts, that 72 amps, at 400 volts its only 6!

Lets get one thing real clear, 400 volts DC WILL KILL YOU DEAD AS A HAMMER! Unless he does this kind of thing all the time your buddy should hire a professional. There is a reason we do the final wiring on these bigger systems at NIGHT!

The whole thing seems screwy, naturally starting with the voltage vs power as several of us have stated.

I'd like to know what the equipment list is. # batteries of what voltage, etc, etc, method of charging, etc.....

8kW at 12V sounds more like a sears air compressor rating.... I suspect it isn't delivered power. If you could get it, you'd have something in the area of 700A.

the stated 4800 AH divided by 220 AH for traction batteries, gives 22 12V strings of T105 6V batteries, for example. The wiring to connect that number of strings is physically large enough to require careful layout to equalize the draw on each string.

Sounds a bit like a setup that might possibly not have been planned that well............ particularly given the "feel the frequency" comment, which suggests testing might not have been that exhaustive as well...............

Thanks for all the replys, and everything everyone has mentioned is SPOT on! He wanted to do 24v, but everything was free except the 8kw inverter.

"The system grew from 6v (12v) golf cart batteries and a coleman gen. Originally It was all 12v due to existing lighting and equiptment in the cabin.

There are 12 banks of 10 cell NiCad wet batteries which feed through large diode main buss. The panels are 75w at 24v (and he said, as you know, there isnt enough, but he comes across them from time to time so he is adding) They are fed through a 80a charging brain with diodes to each bank of batteries regulating it to 14.3v float voltage.

The ac load is delivered through two inverters a 2500w and 8kw depending on what circuit is plugged in. There is a Onan 10kw generator connected to the house panel through a transfer switch, disconnecting the inverters and chargers when 240v is needed."