New System Design 7kw system

Ronin2004 · February 2013

I am currently looking into an off grid Solar panel set up right now I am looking at 24 panels from ET solar ET-P672290 looking at 4 panels in series and then tied the rest in parallel. The question I have is that from my research looking at outback power and I might need to get two charge controllers Let me run through the math

4 panels in series
V=145 I=8 P=1160
then 6 sets in parallel
V=145 I=49 P=6960

The outback Power charge controller
48 VDC systems 4000 Watts / 60 VDC Systems 5000 Watts

So I should need two charge controllers and break the 6 series into 3 right?
Now Do I need to do the same with inverters or can I buy a 8 kw inverter.

Cariboocoot · February 2013

Re: New System Design 7kw system

Welcome to the forum.

I hope you haven't spent any money yet, because you're going about it the wrong way. You need to define how much power you're trying to supply in terms of maximum Watts at any time and total Watt hours per day. Selecting a bunch of panels first will get you nothing but grief.

How did you arrive at a need for a 7 kW array? That much panel is very large for off-grid. It could charge 1120 Amp hours @ 48 Volts (needs two charge controllers as they max out at 80 Amps) or roughly 23 kW hours AC of stored power. Likewise the "harvest' from such an array would be around 15 kW hours AC daily (4 hours of good sun). That's a lot of power for off-grid.

BB. · February 2013

Re: New System Design 7kw system

Welcome to the forum... If I have the correct panels, this is their specifications:

System Rating: 290 Watts
Watts (PTC): 259.3 Watts
Max Power Voltage (Vmpp): 36.25 Volts
Max Power Current (Impp): 8 Amps
Open Circuit Voltage (Voc): 44.5 Volts
Short Circuit Current (Isc): 8.61 Amps
Max System Voltage: 1000 Volts
Cell Efficiency: 17.05%
Module Efficiency: 14.95%

4x Voc of 44.5 volts = 178 VDC at nominal temperature... Could be around 215 Volts, depending on how cold it gets in your area.

The standard Outback working maximum voltage is around 140-150 VDC maximum (I think--I do not follow the controller details as I am not in the business).

If you want to go with higher input voltage, you should be looking the Midnite Classic or Xantrex/Schneider MPPT charge controllers.

Will your home be "on grid" with emergency backup power, or will you need a pure 8kW off grid inverter? Will you have a backup genset?

We are kind of jumping in the middle here--Normally, we like to start with the loads (Watts peak, kWH loads, seasonal usage patterns), then battery bank, then solar array (based on location/hours of sun per day by season, etc.).

And the loads part--We generally like to talk conservation first and discuss needs (backup, full off grid, backup power sources, seasonal usage patterns, full time/weekend/seasonal, etc.)... It is almost alway cheaper and easier to do "extreme" conservation. After that cycle, then follow through the rest of the steps.

Once we have that all nailed down, then start selecting equipment to meet your needs.

-Bill

Ronin2004 · February 2013

Re: New System Design 7kw system

Its a university project for disaster relief the idea is to build a mobile power unit capable of providing power to key services after a natural disaster. We wanted it to be modular so its can be shipped in an container and then set up with minimal effort. Yeah the specs seem about right. The part of the project I am involved in is picking the components the 7kw system is set by our group lead. I am just trying to wrap my head around setting up the system so it can run up to 2-3 days with no sun. Just want to design a system that doesn't loss power due to equipment choices

Cariboocoot · February 2013

Re: New System Design 7kw system

Ronin2004 wrote: »

Its a university project for disaster relief the idea is to build a mobile power unit capable of providing power to key services after a natural disaster. We wanted it to be modular so its can be shipped in an container and then set up with minimal effort. Yeah the specs seem about right. The part of the project I am involved in is picking the components the 7kw system is set by our group lead. I am just trying to wrap my head around setting up the system so it can run up to 2-3 days with no sun. Just want to design a system that doesn't loss power due to equipment choices

Well, that's an impossible task you've been landed with. Without knowing the loads the panels are meaningless. Off-grid systems do not run without batteries, and the panels are there to recharge the batteries. The proof is in your own post: "so it can run up to 2-3 days with no sun". That's batteries, not panels.

Sounds like money being wasted to me. Generators are far more efficient emergency back-up power sources than solar panels + batteries + inverters. Yes, I know; burning fossil fuels isn't the "in" thing to do. It's not PC. It's not environmentally friendly. But it makes a lot more sense for emergencies than solar, which makes more sense in non-emergencies because the more you use it the better the value.

Ronin2004 · February 2013

Re: New System Design 7kw system

Yeah It seems that way. They want solar and a battery backup system. Lets just ball park the numbers here they want 4 kw of use with the rest of the power being sent to battery backups to provide that 4 kw for three days if their is no sun. So we have 3kw to charge batteries that will need around 288 Kw of total power or 4kwh or around 88 amp hours based on 48 volt battery 3 days no sun 4kwh if we have sun the whole time and the batteries are fully charged they want the 7kw to go a grid to expand services. So 4 homes using 1kw a piece that's no peak loads or anything then after the batteries are fully charged they want to be able to power an additional 3 homes. Does this make sense i mean from an example point not should it be funded part.

Cariboocoot · February 2013

Re: New System Design 7kw system

What makes sense to this single-minded old fool:

You need XkW hours of power per day. That is provided by a battery bank of Y Amp hours @ Z Volts.

Think of ALL the power coming from the batteries, not some from solar with some of the solar available to recharge the batteries. That's not off-grid thinking. That's confusing grid-tie, where panels feed power needs directly because there's this massive "AC battery" called the power grid, with off-grid. That's probably how they got the specs.

So, when we talk about battery capacity we use the "20 hour rate". Low and behold we talk about recharging them in 4 hours of good sun per day. 20 + 4 = one whole day's power. So if the battery's 20 hour rate includes supplying the power that will be used while recharging then all you have to worry about from the panels is getting enough to recharge with.

Talking in terms of wanting "4 kW" doesn't help either, as that is a rate of power not a quantity. 1 Watt for 1 hour is 1 Watt hour. So is 2 Watts for half an hour or half a Watt for 2 hours. Differing rates: 1, 2, 0.5. But the same quantity once the time is factored in.

So you could have 4kW hours per day for three days: 12kW hours total stored capacity. Or you could have 4kW hours over three days, which is 1.3kW hours per day.

"88 Amp hours based on a 48 Volt battery". Okay, 88 Amp hours @ 48 Volts is roughly 4.2 kW hours, not including losses from conversion (inverters are only about 90% efficient). You would need at least double that in battery capacity to avoid going below 50% state of charge (which is the 'tipping point' for recovery for most batteries) so you have 176 Amp hours of 48 Volt battery that needs to be recharged.

Normally we'd go the shortcut with 10% peak charge current and get: 17.6 Amps * 48 Volts = 845 Watts, less typical panel & controller derating: 1097 Watt array. If the DOD was only 25% this would work fine on a day-to-day basis with good sun. Deeper discharge requires either more time or more current to recover from, and there are upper limits as well as lower ones.

Are you seeing the pattern here?

Ronin2004 · February 2013

Re: New System Design 7kw system

Ok so does that mean that with 4 hours of good sun my solar panel array could provide 583 amp hours @ 48 Volts. [7 kw*4hrs /48 Volts] or for one hour we get 145.8 Ah. Ok and saying that our power output is 4kwh for 24 hours how do we that means we need 176 amp hours. what does that mean for how much battery we can charge. Is it amp output/2.

Whats the shortcut with 10% peak charge current?

westbranch · February 2013

Re: New System Design 7kw system

you are overestimating the 'usable' watts you will get from your system, the rule of thumb (50%) would be 145A * .5 = 72Amps per hour (always round down)

add: that is what you will get out of the system based on what went in.

hth

Cariboocoot · February 2013

Re: New System Design 7kw system

The "end to end" efficiency of an off-grid system is about 52%: the "nameplate" Watt rating of the panels * hours of good sun * 0.52 = AC Watt hours available "at the socket". Some things can make this better, or worse.

So 7kW of array for 4 hours ends up being 14,560 Watt hours per day "typically".
More hours of good sun = more power, providing you can make use of it: once the batteries are full it is necessary to make use of "opportunity loads" to take advantage of it. This is not something you want to plan on being able to do. Sure as you do, it won't happen.

Something else you have working against you is not having a fixed location. Since this could be put in place anywhere, you can't predict what conditions will be prevalent and how it will affect the output.

What works against:
Short daylight, cloudy conditions, "marine layer", high temperatures, low elevation.

What works in favour:
Long daylight, clear conditions, cold temperatures, high elevation.

The difference can be drastic.

Now to go sideways, it's not difficult to determine how to wire up that much panel (two controllers minimum because 7kW can produce more than 80 Amps @ 48 Volts) and how to (Voltage allowances for strings). In all likelihood you'd be better off with the MidNite Classic 200 controllers as Bill mentioned.

Even so, the array needs to be matched to the battery bank. This can mean running a smaller battery bank than the array can support under good conditions so that it will be able to support it under not-so-good conditions. This would mean programming a lower limit to the output current so that it would not be exceeded when conditions are good.

BB. · February 2013

Re: New System Design 7kw system

I will do an example here... Say you want honest to God 4kW x 24 hours per day. On a genset, just for comparison, that is around (very roughly) 1 gallon per hour of fuel.

4kW genset ~200 lbs
4days * 24 hours * 1 gph * 6 lbs per gallon = ~576 lbs of fuel

~$2,500 for genset + $384 for 96 gallons of gasoline ($4 per gallon) and less than 800 lbs (plus tank+trailer)

For a solar power system running 4kw*24hours*with 2 day battery backup:

4,000 Watts * 1/48 volt battery bank * 24 hours per day * 1/0.85 inverter efficiency * 2 days battery backup * 1/0.50 max batt discharge = 9,412 AH @ 48 volt battery bank

Crown Industrial Battery - 24 Volts, 1875 Amp-hours Price: $7,180.66

9,412 AH / 1875 AH per 24 volt battery pair = 5 pairs of batteries (24 volt * 2 in 5 parallel strings or 10 batteries total)
10*$7,180.66 = $71,806.6 for battery bank (20 year working life--if treated well)
10*2,844 lbs = 28,440 lbs for battery bank

Now, we recommend two methods of calculating solar array... First is based on the size of the battery bank--More batteries, more solar panels needed for charging. The second is based on hours of sun per day.

A 9,412 AH @ 48 volt battery bank with 5-13% rate of charge:

9,412 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 36,059 watt solar array minimum
9,412 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 72,118 watt solar array nominal
9,412 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 93,253 watt solar array "max cost effective"

It is not my money, but I would suggest that 10% rate of charge is the minimum recommended for industrial batteries (and your unknown/variable loads-weather-usage).

A 72,118 watt array using SolarWorld SW-250 Monocrystalline panels would need:

72,118 Watt array / 250 watts per panel = ~288 panels
228 panels * 250 watts * 1/59 volts charging * 0.77 panel+controller derating = 966 amps battery "nominal" charge rate
288 panels * $290 per panel = $83,520 for solar array (plus ~$30-$70,000 for mounting hardware???).
288 panels * 46.7 lbs per panel = 13,449.6 lbs of panels (plus unknown weight for mounting structure)

The typical solar charge controllers we use in RE are around 80-96 amps maximum to the battery bank--So, you are looking at >100 charge controllers at $610 each:

100 * MPPT charge controllers * $610 each = $61,000 in charge controllers
100 * 10 lbs per controller = 1,000 lbs of charge controllers.

Now, assuming we have 288 * 250 watt panels and you want 4kW*24 hours per day, how much "minimum sun" would be needed to "break even":

4,000 WH * 24 hours * 1/0.52 system efficiency * 1/(288*250 watt panels) = 2.56 hours of sun per day "break even" point

Anyway--That is a rough back of the envelope calculation.

You are looking at ~$250,000 for the solar RE system (really rough numbers) that will (pretty reliably) supply power for years on end with only minor maintenance (filling battery bank with distilled water once a month, checking connections, etc.). And it will weigh around 50,000+ lbs.

The generator will cost you $3,000 for 4 days of power, and every four days you will have to bring in $400/600lbs of gasoline.

Assume that gasoline costs $10 per gallon to bring in and the hardware cost nothing (obviously, oil changes and other maintenance is needed).

$250,000 for solar / $10 per gallon of gasoline = 25,000 gallons of gasoline
25,000 gallons / 24 gallons per day = 1,042 day supply of gasoline

So the "break even point" for generator vs solar is probably around 2-3 years.

Obviously, I have made lots of assumptions. And the trade off between not having to bring in a $250,000 power system and 50,000 lbs (two semi trailers of weight) vs a 1,000 lbs for 4 day supply + another 600-1,000 lbs every four days for fuel/oil is something to think about.

And there is the central power vs distributed power issue... What is it you are trying to power? A few lights/emergency HAM radios/cell phone chargers--Run run "homes" with central heat (maybe even electric heat).

Do you bring in a bunch of 900 to 1,600 watt generators and a couple extension cords for each and setup fuel distribution. Or a larger 4kW genset with some way of distributing power around the neighborhood?

What is the right answer--it probably is different for every situation. But--You can see what happens if you assume a constant 4kW load--It becomes insanely difficult to support with "portable" solar power.

Oops--forgot the 4kW DC to AC inverter and backup battery charger (way too small of AC charger to be of use on this large of battery bank):

Magnum Energy MS4448PAE 4400 Watt Sine Wave inverter 120/240 Volt Price: $2,159.20 and 55 lbs weight

A 4kW inverter is almost "nothing" in the grand scheme of this system design.

Please note that none of us here work for NAWS or (for the most part) in the solar business. The above are just starting points for your research. Your thoughts?

There are lots of tradeoffs that can be made--But, as you can see, supplying 4kW*24hours per day = 96kWH per day is not easy. The average home probably uses 6-34 kWH per day (more if A/C and electric heat/cooking).

-Bill

Ronin2004 · February 2013

Re: New System Design 7kw system

Thanks everybody I new the calculation part is what really was getting me. I know the economics don't make sense for this project but hey no one wants to work with old dirty diesel or even natural gas generators for urban locations. But thanks for the help it was so appreciated.

Ronin2004 · February 2013

Re: New System Design 7kw system

Ok so i am working through the numbers and here is what i get

Panel to battery Bank Calculation
# Panels 24
Panel Power(W) 290
System Power(W) 6960
System efficiency 77%
Power to Batteries(W) 5359.2
Battery Size(V) 24
Available amps(A) 111.65
Total Power output 949.025 w continuous

Sun Hours 5
Total Power(Wh) 34800
Overall System efficiency 52%
Power Available for charging(Ah) 754
Ratio of Power available 0.675324675

Do these calculations seem about right. I was thinking that it would be around total power*((sunlight hours)/24) because for every amp i pull out of my battery i have to replace it. I know it not a real 1 to 1 ratio but for rough calculations.

westbranch · February 2013

Re: New System Design 7kw system

replacement amps is in the range of 110% to 120% of amps used, 115 would do for estimating

BB. · February 2013

Re: New System Design 7kw system

Also watch power (Watts) vs Amps...

Lead acid batteries are close to 100% efficient for Amps out = Amps in... Where much of the losses come into play is you are discharging the battery bank at (for example) 12 volts but recharging at 14.5 volts so:

12v/14.5v = 82.8% efficiency

PWM charge controllers only pass "amps" from the array.

MPPT charge controllers are power conversion devices and will pass Watts in * 0.95 (+/-) = Watts out

Since Watts Out = Volts * Amps, a lower battery voltage (in the 0-80% state of charge), the current will be higher... But as the battery approaches 100% full, the charging voltage will rise to ~14.5 to 15+ volts--So, Amps out falls.

-Bill

Ronin2004 · February 2013

Re: New System Design 7kw system

what is the ratio from power output in amp hours to charge input from solar panels. Should the forumla read (Solar panel amp input for daylight hours)/(Amp output 24 hours) should that be around 20% to make sure that the solar panels are keeping the battery charged

Cariboocoot · February 2013

Re: New System Design 7kw system

Ronin2004 wrote: »

what is the ratio from power output in amp hours to charge input from solar panels. Should the forumla read (Solar panel amp input for daylight hours)/(Amp output 24 hours) should that be around 20% to make sure that the solar panels are keeping the battery charged

Not quite so simple, because the output from the panels will not be constant. The charge cycle will start when it can, with perhaps as little as 1 Amp of current being fed to the batteries. As the sunlight intensifies the panels' current output will increase up to their peak Imp rating. But in the meantime the battery Voltage is going up because they are charging. At some point it will cross a threshold where the batteries do not need as much current as the panels are capable of, and the controller will reduce output accordingly. Then it must spend time in the Absorb cycle where Voltage is fixed and current continuous to drop off until either the End Amps or max Absorb time is reached. And all that is without accommodating loads which will vary the current demand at any given time depending on if they are on or off.

The rule-of-thumb is that batteries want to see a peak net charge current of between 5 and 13 percent of their Amp hour capacity. Lower rates tend not to charge but only maintain and higher rates become expensive and then hazardous. If you shoot for 10% it usually works out okay: enough power to replenish batteries completely on a good day providing SOC hasn't been allowed to go too low - works very well around 25% DOD. Greater DOD or not as much sun and you have to come up with more charging power from somewhere - either larger array or a generator.

BB. · February 2013

Re: New System Design 7kw system

Take a look at the formulas I used... They usually are pretty self explanatory. The "killer" in solar (and any engineering project) are the "derating" factors. In solar, about 1/2 of the system is "losses" (and marketing exaggerations). But that is not too bad--For construction, the usual factor of safety is 10x or greater (allows for defects in materials and construction).

So, the deratings:

81% for solar panel marketing numbers (cold panels produce more power, hot panels 20% less).
95% for solar charge controller efficiency
85% for "typical" AC Inverter efficiency
80% for flooded cell lead acid battery charging (90% for AGM; depending on how system is operated, can be >95%).
System Eff = 0.81 * 0.95 * 0.85 * 0.80 = 0.52 end to end efficiency

And there are hours of sun per day... In North America, a typical "sunny" region will get around 4 hours of sun, or more, per day for 9+ months of the year. On the coasts and Northern US, you can see 2 hours or less in winter/fog season. In the South West, you may see 7+ hours in summer (monsoon season can cut back to 5-6 hours). So, the formula would look like (using 3.3 kWH per day or 100 kWH per month as a "nice off home" system--with lots of conservation):

AC Watt*Hour Load * 1/(system efficiency) * 1/(Hours of Sun per day) = Solar array Size
3,300 Watt*Hours * 1/0.52 * 1/4 hours of sun = 1,587 Watt array (minimum)

You should only plan on using around 66-75% of predicted output of a solar power system every day... Unless you have a backup genset to help. And, that is why I use 4 hours to plan a 9+ month system. And any system will need a generator during winter/bad weather when you may get 10% or 1% of predicted power under heavy clouds.

Sizing a battery bank--We use 1-3 days of "no sun" and 50% maximum discharge (for longer battery life). Using 2 days as "nominal", then the calculation would be:

3,300 WH Load * 1/48 volt battery bank * 2 days no sun * 1/0.50 max discharge = 275 AH @ 48 volt battery bank

Then you also need to meet the charging current needs of the battery bank. We use the 5-13% rule of thumb here. 5% rate of charge (C/20) is the minimum and would only be recommended for a smaller system that charged during the day and supplied power at night. Otherwise, 10% is a good nominal rate of charge and 13% tends toward the maximum cost effective array size (need thermal management, and other load requirements can justify a larger array--just using rules of thumb for a conservative system design):

275 AH * 58 volt battery charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,036 Watt Array Minimum
275 AH * 58 volt battery charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,071 Watt Array Nominal
275 AH * 58 volt battery charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,693 Watt Array "cost effective maximum"

So, using the above imaginary system, the solar array should be around 1,587 to 2,693 Watt array to supply 3.3 kWH (or more) per day for ~9 months of the year.

There are other rules of thumb too... For Flooded cell batteries, the maximum continuous power is around C/8 (fork lift duty--discharge battery bank in 8 hour shift. So the maximum inverter you should support would be:

275 AH * 24 volts * 0.85 inverter eff * 1/8 hour discharge = 701 watts AC continuous power

And the maximum starting surge is around C/2.5 rate of discharge:

275 AH * 24 volts * 0.85 inverter eff * 1/2.5 hour discharge = 2,244 watts AC max surge power

Note--If you get into the details--You find that a lead acid battery appears to have different capacities with different rates of discharge. We normally use C/20 rates to design off grid systems... But at C/8, a battery will have less usable Amp*Hours--So plan accordingly.

-Bill

New System Design 7kw system

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