System Design

quique

Ok here is a system design question. I have to supply 105kWh daily.

I have a system split up into 3 subsystems where 105/3 = three 35 kWh systems.

EACH 35kWh SUBSYSTEM

Each subsystem has 5 hrs to produce energy, thats 35kWh /5 hrs = 7kW.

If I need 7kW and Im using 290W panels, thats 24 panels of 290W, correct? This is without factoring in inefficiencies, right?

Photowhit

Re: System Design

Your getting way ahead of your self...

Is this a grid tied system or off grid?

Will all systems be independent?

Do you own a bank?

westbranch

Re: System Design

Photowhit, sounds like this is another of his "case studies" for school...

quique

Re: System Design

This is off grid.

I dont own a bank but my client pretty much does We know its about a $300,000 system.

westbranch

Re: System Design

to answer your first question , it is 24.1 panels so you can use 25 to cover off some derating, but not all.

Would be more like 34.5 panels at 70%

quique

Re: System Design

Yes I agree that for offgrid you want to add some extra umph in order to cover cloudy days, overuse and other such factors as inefficiencies. I understand its not so critical in gridtied systems, but the client does have a generator which he plans to use in cases of emergencies.

Ok I have a second question. Each 35kWh subsystem has been calculated at 36 panels actually, much like westbranch suggested, divided into 3 strings of 12.

If the panels operate at 44.7Voc and 8.51A(Isc). This means that 1 string of 12 would produce:

12 x 44.7V = 536V strings at 8.51A

So three of these in parallel produce:

536V at 25.5A, correct?

So at 5 hrs a day, that's:

5 X 536V at 25.5a that's 68,340Wh or 68kWh

This means that with 3 of these systems I could produce 68x3 = 204kwh daily. Of course thats using Voc. Using nominal its only:

12 x 24V = 288VDC @ 25.5A x 5hrs x 3 strings = 110.1 kWh daily.

Is this correct so far?

solar_dave

Re: System Design

Well for off grid you have to figure charging losses as well. and you want to keep it to about 20-25% discharge per day for a couple day in pretty strictly battery mode. I think the consensus is you need 4X panel rating to get a couple days autonomy.

quique

Re: System Design

Ok, but is my math correct for the:

USING VOC

1 Series String
44.7 Voc x 12 panels = 536V @ 8.51A

3 Strings in Parallel
8.51A x 3 = 25.5A & 536V

At 5hrs a day:
5 hrs/day x 25.5A x 536V = 68.3kWh per day

Which means 3 of these systems produce:
3 x 68.3kWh = 205kWh

USING VDC


1 Series String
24 Voc x 12 panels = 288V @ 8.51A

3 Strings in Parallel
8.51A x 3 = 25.5A & 288V

At 5hrs a day:
5 hrs/day x 25.5A x 288V = 36.7kWh per day

Which means 3 of these systems produce:
3 x 36.7kWh = 110kWh

westbranch

Re: System Design

You should only use Vmp, the Panels voltage at the MPP, for your calculations. Guessing that it is close to 34V for the 24v panels, 17 for the 12 v panels.

Where is the install going to be? Warm temperatures will derate the output, Cold Temps will increase the temps. Each panel manufacturer publishes Temp Coefficients for their panels... you have to use that to determine the CC to use with the arrray or reduce the size of the array to match the CC. It can be a trial and error process.

Photowhit

Re: System Design

VOC should only be used for peak charge controller ratings, as there is No potential for the panel to produce this much voltage with out extreme conditions.

For basic 'what it will produce' numbers at the panel, I would use NOCT values or PTC ratings.

quique

Re: System Design

The installation is in Honduras, very hot!

When estimating for how many charge controllers I was told I need to use Voc and Isc.

BB.

Re: System Design

You also have to know where that 35kWH per day is measured--Is that AC loads (after the inverter), or before the inverter (DC side). Are those loads daytime, night time, or split?

For example:

35,000 WH per day * 1/0.85 inverter efficiency * 1/0.80 battery efficiency * 1/0.77 panel+controller losses * 1/5 hours of sun per day = 13,369 Watt array minimum

If these loads are mostly during daytime, then the battery losses may be less, as an example (unless you have to account for a few days of bad weather if the loads must run during that time).

Also, would need to know surge load requirements... Lots of A/C and/or water pumps cycling, vs just office lights, fans, computers. Which could drie the battery bank larger or smaller. Which then hits with the 5% to 13% rate of charge (i.e., heavy cycling loads that run for short periods need a larger battery bank--which needs a larger array to hit minimum rates of charge).

Also, power during the day subtracts from power used to recharge the battery bank... So you may have your 5% to 13% battery charging current, plus any steady state daytime loads.

Off grid systems that charge during the day and use power at night (such as a typical off grid home) are a bit easier to design than system that have significant power usage during daylight hours (and high surge currents) vs industrial/business systems which have many other issues to worry about.

-Bill