Layout of a new 800w system

[UpNorthMan]

Hello Everyone,
I’m relatively new to solar power system and could use any wisdom you are willing to share. We are building a 800w 24v system for our off grid cabin in northern MN. I have purchased 8-100w Renogy mono panels and a Midnite Kid with temp sensor. As I understand, I should hook up in a 2, 4 (S, P) configuration to keep the temp down on the CC. I can also configure the panels in a 4, 2 configuration. I also am not opposed to buying one more panel to get a 3, 3 configurations if it makes more sense. The temp range is 105 F to -45 F or colder. It is not unheard of being colder than -50 F in the morning right after sunup. This is actual temp not wind-chill.  The panels will be mounted on a south facing wall with manual adjustments for angle. Shade is not a problem except for the end of the day or in the middle of the winter when the sun is low in the sky. My eventual plans are to use the load feature to heat the batteries in the winter months and a heat sink hot water heater system. If I use the 2,4 configuration, I understand my leads need to be the same length and will be 30-40 feet long. I want to keep my combiner box inside the cabin. My loads consist of 1 c-pap, .8amp 120v fridge, and the remainder is lights and radio. I’m planning on having 2- 210AHR 12V batteries. The Renogy panels have the following specs:  max power 100W  Voc 22.5V Vmp 18.9V Imp 5.29A Isc 5.75A
The Kid sizing chart is ok with both 2, 4 and 4, 2 configurations. I have the following questions:
1)   What panel configuration would be best?
2)   Is a SPD required? If so what size?
3)   Is this type of system required to be grounded?
4)   I’m looking at buying a pure sine inverter. Does a modern inverter use 100% input wattage under 10% load, or does a inverter only use required load watts plus conversion loss? I like to have a 2000W inverter for other misc loads that may happen.
5)   Is there anything that I missed that you would recommend?  I know fuses, correct wire size, and disconnect switches are required.
Thanks in advance,
Ed

[ClassicCrazy]

The general rule of thumb when planning out the series / parallel combination of panels is to try and keep the PV voltage closer to the battery voltage if possible because then the controller is more efficient and won't get as hot. But if you need the higher PV voltage because the  run from PV to cabin is long and you need higher voltage to reduce wire size , then that is a design consideration.

The other main consideration to how many panels you can put in series is the VOC during those very cold nights.
The Kid string sizing calculator should show you all the various options and what will work with the Kid.

Why do you want the combiner to be inside far away from the panels ? That kind of defeats some of the benefit of the combiner because you can run one pair of larger gauge wires for lower voltage drop back to the cabin instead of the 4 or 6 you seem to be wanting to do ?

Consider getting a Midnight Mini DC disconnect box . It will make wiring your Kid and inverter a lot easier and has breaker for inverter and spaces for the  PV , Bat , and DC load breakers as well. If you ever think of upgrading to something like the Classic for more PV ( and much better monitoring ) then it would be easy to swap out controllers. The Mini DC box also has spot to put the shunt with the Whizbang .  So if you get the MiniDC box you will have to know ahead of time which inverter you would get so you order the one with the right size breaker for the inverter.

yes you usually ground your system - all to the same ground rod so the potential is the same on all of them.  You will have to look up grounding . On my system I have ground wire coming from combiner box to ground rod and the controller , inverter cases and AC neutral are all grounded to it . It is a rather confusing topic in some ways so read up !

You can get the Midnite SPD  - one for the PV side and you can put one on the AC inverter side . You don't really need them but they are extra protection in case you ever get surges from nearby lighting strikes. If you look them up you will see the voltage ratings and you can use any one with the voltage rating for you system.

Consider getting an inverter with built in charger so you can hook a generator on it and charge your batteries. That is an essential part of the system in Minnesota and a lot of other places too. Otherwise you will have a short life time on your batteries. Ideally for long lifetime of batteries you only want to drain them 20 or 30% and they will last much longer. That doesn't sound like a lot of battery capacity for running a fridge in the winter if you are planning on doing that . So when your batteries get discharged by 30% if it isn't sunny out charge them with the generator. 

yes  your inverter will draw some power even when there is no load - depends on which inverter you get - specs should show no load current draw.  And then a modern inverter should be 80 to 90% efficient depending on the size of the inverter and the size of the load. Most inverter companies will show those efficiency graphs in their literature .

You asked a lot of questions - this is my summary answer !
Larry

[UpNorthMan]

Thanks Larry! I appreciate the input! I'll try to explain some more of my thoughts.
In the winter, I plan on using mother nature for a fridge. A cooler in the porch has worked well in the past. It's been more of a concern of stuff freezing than anything else.
As far as the configuration, the 2,4 with all the extra accessories to do it right would cost an extra 500 $. I'm not trying to be cheap, but I could use that money elsewhere. I believe that the 4,2 configuration could be done with a inline fuse and a standard connector. Please correct me if I'm wrong.
If the CC runs warm, but not hot, I wouldn't be afraid to run a pancake fan on it. If it is likely going to run hot, then the extra money will be worth it.
I want to have the combiner box inside so I don't have to worry about anyone messing with the system. The cabin is located in the woods and vandalism or mischief has been a problem for a couple of the distant neighbors.
For the near future, this will be a weekend cabin, and the occasional week long stay.
I hope this helps with the discription of the direction I'm going with the system.
Ed

[Vic]

Hi  U N M,

The PV String configuration can also be dictated by the exact battery that will be used on the system.

With your mention of a 210 Ah 12 V battery,  perhaps you might be thinking of using a 4D AGM.   AGM batteries do not require high Absorb or "EQ" voltages.   This can mean that the KID (or other MPPT Charge Controllers (CCs),   will be able to find a generally effective Max Power Point voltage, which would maximize power production from the PVs under most any set of conditions.  Some AGM batteries  need only a fairly narrow range of charge voltages verses changes in battery temperature.

If you are planning on using Flooded Lead Acid (FLA) batteries,  it would be quite possible that strings of 2 PVs in series would not provide the CC sufficient headroom to produce optimum power output.

Exactly what brand and model number of battery are you planning to use?

Thanks,    Vic

[UpNorthMan]

Hello Vic,
The actual battery I was looking at was Trojan J185H-AC  12V 225 AH Battery
This would be 225 Ah not 210 as my original post.
The specs per cell are
absorption 2.35-2.45v
float 2.2V
equalize 2.58v

The specs for series (24v)
absorption 28.2-29.4v
float 26.4V
equalize 30.96v

I haven't looked to hard to find a local distributor, but one 10 hrs away has some at a reasonable price. I live 45 miles NW of Brainerd MN.
I haven't made up my mind if these are the ones I am getting. Price and quality seem to be what I'm looking for. I am open to suggestions, but these are what I'm leaning towards.

Ed

[Vic]

Hi Ed,   Thanks for the added info on the batteries you are considering for the system.

In looking at this Data Sheet:
http://www.trojanbattery.com/pdf/datasheets/J185HAC_Trojan_Data_Sheets.pdf

The  Absorb voltage is very close to what you mentioned,   but the EQ voltage is 32.4 V.   The EQ voltage (Veq)  is high,  which can mean several things  ...   It might not be possible to EQ your batteries with common,  inexpensive Inverters,  perhaps ever,   and,   with cool/cold batteries  EQing batteries,   even with the "best" inverters might not be possible.

Also,  you will need to look at the  MNKID String Sizer results,  particularly for warm/hot weather.   In warmer weather,  it might not be possible to reach even Absorb voltage (Vabs) with strings of two PVs,   let alone,   when trying to EQ.

It is customary to design string configurations with a minimum  Vmp string voltage of about 30% greater than highest battery charge voltage that the system will ever see.   Generally for FLA batteries,   this would be EQing cool/cold batteries.

For cool batteries,   charge voltages need to increase,   and for warm ones,  the opposite is true.   Your proposed Trojan batteries need this Temperature Compensation of Charge voltages needs to be compensated (by that KID Battery Temp Sensor  --  BTS) at  -5mV per battery cell,  per degree C for temperatures below about 27 C,   and above 27C,  by reducing charge voltages by that factor  --  this is built-into the KID Firmware,   as you probably know.

DO look at the ambient temperature where the KID reaches HyperVoc with strings of three PVs.  This should be at a very,   very low temperature,   but just check it,  please.

More later,  Vic

[UpNorthMan]

Vic,
Thanks for sharing your knowledge!
I'm wondering if I referenced a older spec sheet.The info I posted was correct per the data sheet from the suppliers website. I'll use the values directly from the manufactures. Good catch on values
Bulk 29.64 v
float 27.0 v
equalize 32.4 v
As far as the battery temp in the winter, I was going to try and stay around 55F. I was going to have removable insulating panels on my battery box. Use extra venting in the summer and much smaller in the winter. 24v cartridge heater 100w?? in size with a thermostat. I've been a machinist for many years an have access to the equipment for personal work.
I've run the numbers for all 3 setups. 2,4 and 4,2 come out OK. 3,3 is where the Kid doesn't like it. 841 watts/31.3 amps, Kid requirement of 1.1 With the cabin located at 48.25 degs north, I most likely won't get full production. From what I've read, it sounds like the Kid can handle extra input watts. It just won't  output any more then the 30a @ 24v With 8 100w panels and 24v batteries, the output power is 27.8A per the sizing chart.

The HyperVOC values for 24V are as follows
2,4 -1323.4 F
3,3 -655.6 F
4,2 -322.6 F

It appears that 2,4 are out of the equation, the panel output voltage is to low under certain conditions.

I choose the current panels because of there physical size/shape as they relate to the planned mounting on the cabin.
Another thing I am considering is that I am maxing out the CC in 24V mode. There won't be any room to expand except by purchasing another. It might make sense to go to 36V or 48V system. I could buy smaller A-hr batteries. 48V seems to be the next step. From what I've seen, 36V isn't the most common.  If I went to a 48V system, it looks like two more panels. If my math is correct, 84V min for batteries figured 77F with a1.3 factor, add up to 8V more for cold battery extreme. With a 4,2 configuration, only 75.6 V VMP.  I don't require 24V for any certain appliance. I don't recall the name of the device, but a voltage reducer from 24/48V to 12V would work for me. 12V led lighting, radio, pumps, ect are very common.  A 12V 25 amp load would be a very heavy load. The biggest load would be the addition of a compressor fridge, and that would be on a inverter.

Below is the calculated values for a system in my area.



I currently share a cabin with two brothers, we have a tiny 100W system that we have been using for a couple years. It has given me some limited incite on usage, capacity, and conservation.

Many things to consider!! The good thing is that I don't need to have my mind made up until April/May at the earliest.

Ed

[mike90045]

....I want to have the combiner box inside so I don't have to worry about anyone messing with the system. The cabin is located in the woods and vandalism or mischief has been a problem for a couple of the distant neighbors.....

Those big shiny PV panels are vandal bait.  Expect losses.  Copper wire is easily re-sellable, boxes, not so much.

[CDN-VT]

VOC is system voltage plus rated unit !!!

So a classic 150 is adding 48 Vdc so VOC is 198Vdc MAX from panels

150 + 24Vdc system is 174Vdc

JFI

VT

[UpNorthMan]

Thanks for the info!

[CDN-VT]

The biggest load would be the addition of a compressor fridge, and that would be on a inverter.

Danfoss is one good compressor system & they are in many units in chest freezer to coolers .

With the use of a Kill-a-watt & measure the freezers & I use a freezer as a fridge with a different Johnson thermostat system .
Cuts the KW down to 1/3 less from freezer & frost free fridges draw the most .

JFI
VT

[UpNorthMan]

I've read that somewhere else, that sounds like a interesting idea that I'll have to put on my list to research.

Right now I'm trying to find some acceptable DC light switches, 12v or 24v system. From the my research, it sounds like the older snap light switches are compatible. The other option I've read about is via circuit breakers. I am planning on using 10amp breakers in a MNPV12 and a extreme switch load would be 36w. Most would be around 15w. I like the traditional setup used in AC wiring, a switch by point of use. And I would rather not wire everything as AC. I don't want the inverter running all the time.

I did come across a manual motor on/off switch from Square D model# 2510KO1
I don't know if they are legal. Anyone have any thoughts?
See link below for specs.
https://www.schneider-electric.us/en/product/2510KO1/manual-switch---open---2p---toggle-operated---no-indicator---600vac-/?range=7476-type-k-manual-switches&node=166379550-2510k-manual-switches


There are a lot of auto/marine products out there, but I am trying to keep the system as close to code as possible. So I'm assuming those products are out due to wiring code.


Does anyone have any suggestions on any DC products?

Ed

[CDN-VT]

12 Vdc or 24Vdc , controlling WHAT ?
Low light amps  ? 
you need to remember  , cars use relays  for 15-25 amps and are just contacts .
It's not till you reach voltage that will draw an ARC !! .

Then again many Square D breakers are DC rated in the Pro series (Silver contacts  )

If you can rethink the lighting grid & use a SS300 @ 120Vac  , then ?
Use lowes stuff !

VT

[mike90045]

I think after all the mess trying to wire for 12V lighting, just get a morningstar sure sine 300w inverter and hard wire it, and use conventional.  300w will run a lot of lights and cell chargers

[CDN-VT]

I agree on the SS300 , with that system hard wired & just use for lighting with regular switches (none back lit ones, causing a draw) So when all lights are off the ss300 should go to sleep . Using some led types the ss300 in search mode might make the led wink , so what I did was use SPDT type switches . As for LED tubes 22watt 120Vac ones , I haven't seen a wink.

Best is to test first & then see how to wire them in using standard 14/2  lumax/romax

VT