Mistake in Startup Construction Project

georgemiller

Hello everyone, 

I am working on a startup project to give the operator of construction equipment a ‘rear-view’ camera. The unit uses a solar system to power one IP CCTV camera and one radio transmitter in Washington, DC. It is mounted at the top of a large construction crane, in a place where it is impossible to run power.

We built a prototype but and the customer is reporting that the system is ‘browning’ out. I did not design the system to provide enough power. I was hoping an expert could have a look over my specs, and see if they can find where I’ve made the error.

I. The System

1. Panels: 2 x flexible 100W hung back-to-back and mounted vertically (i.e. 90 degrees) to a railing. They are attached back-to-back as the equipment they are attached to moves around, and this ensures one panel is always facing the sun. Here is an image roughly conceptualizing the layout. https://www.amazon.com/gp/product/B071JQQL84/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
2. The panels' MC4 connectors are connected to barrel connectors https://www.amazon.com/gp/product/B072MQZV6R/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
3. Connected those barrel wires to a Tycon SCPOE charge controller. 15V - 36V @ 8A max. 0.5W self-consumption. http://tyconsystems.com/documentation/Spec%20Sheets/TP-SCPOE%20Charge%20Controller%20Spec%20Sheet.pdf
4. Which powers a Tycon POE converter: 12V in, dual 802.3at out. 2W self-consumption. http://tyconsystems.com/documentation/Spec%20Sheets/TP-xx-48Dx2_Dual%20Ouput%20PoE_Spec_Sheet.pdf
5. 40Ah LiFePO4 battery. https://www.lithiumion-batteries.com/products/12v-50ah-lithium-ion-battery/
6. Ubiquiti Radio. 8W draw. https://www.ubnt.com/airmax/nanostationm/
7. Axis IP CCTV Camera. 8W draw. https://www.axis.com/gb/en/products/axis-q1765-le/support-and-documentation

II. Calculations

I predicted that the solar output would be 60W (2x100W panels, hung vertically but only one is facing the sun at any point). In Washington, DC during the winter 3.5 hours of sunlight per day. So 60W x 3.5hrs = 210 W/day output.

The total Watt draw of the system is 0.5 (charge controller) + 2 (converter) + 8 (radio) + 8 (camera) = 19W/hr. The system runs for 10hrs/day. So total consumption is 19W x 10hr = 190W/day consumption.

Storage capacity (for cloudy days etc.) is from a 12V 40Ah LiFePO4. Which is 12V x 40Ah =  480W/hrs storage. The unit runs 5 days a week, allowing the battery recharge on the weekends.

III. Conclusion

The customer is reporting that the unit turns off frequently, and won’t turn on until later morning/early afternoon. Somewhere, I have made a mistake.  

Some possible errors include:

• Should I use a charge controller with a higher Amp rating (mine is 8A)?
• My estimate of 60W output from my panels is too high?
• Maybe I need 4 x panels each responsible for 90 degrees of sky instead of 2 x panels which are each responsible for 180 degrees of sky
• The panels will frequently be in-and-out of shade as shadows from the crane’s frame passes over them. Will this effect output?
• Installation error….???

Even if you think I need more panels, I’d also love some advice on how to optimize the system further. I've also cross-posted this in the 'Technical' forum.

Many thanks!

George

Estragon

A quick look at the mc4 barrel connector link appears to show a connector with one female mc4 end and one male mc4 end going to a barrel connector. To me, this suggests there would be a male and female connector remaining, which would be connected together to complete the circuit. If so, it would make a single series string, with one half mostly shaded, which likely wouldn't produce anything near what you expect. The panels should be in parallel (positives combined, and negatives combined) to (ideally) produce ~10a at ~18v. In series, they would (ideally) produce 5a at 36v, which as well as producing much worse with one panel in shade, has the potential to overvoltage the controller on a cold morning.

More generally, it seems to me that in refining the concept it would be helpful to incorporate some sort of logging to better assess actual loads, pv production, battery performance, etc under actual use conditions.

BB.

Welcome to the forum George.

And one thread is  best. I have closed the other.

Have fun,
Bill

westbranch

I see one major calculation error, you did not use  the standard ,  50%,   derating for  the amount of power you can draw from the battery. Going below is the fastest way of killing any battery.  Most batteries used for solar are killed first time around...
You also have probably assumed the panels will be producing their FULL rated wattage, but this only occurs when the panel is aligned exactly will the suns position, so they are not able to deliver what you have estimated... maybe use 40% of output? PS standard is ~70% for PV's

Estragon

Any shade will lower output - even a nearby wire or rigging can reduce it considerably.

Given that back to back panels can only produce ~5a ideally, an 8a controller should be ok.

Using pvwatts.nrel.gov
... for Washington DC with 100w thin panel at 90° tilt, I get ~3hrs equivalent Dec sun with azimuth south. Changing the azimuth to 270°, assuming the panel averages out to 1/2 way between north and south as the crane moves around, cuts that to 1.23hrs.

In spring/summer, the tilt will be more of a problem, offset to some extent by longer daylight hours.

Changing the tilt to 0° (horizontal) increases sun to 1.74hrs in Dec, and production would double if both panels were in sun. If practical, it would appear that orienting both panels horizonally should produce more than vertically back to back, even in Dec. You may want to try pvwatts youself with various angles and assumptions.

Estragon

> @westbranch said:
> I see one major calculation error, you did not use  the standard ,  50%,   derating for  the amount of power you can draw from the battery. Going below is the fastest way of killing any battery. 
...

Being a LifePO4 battery, the charge/discharge profile is a bit different, generally kept between ~10% and ~90% is recommended, vs ~50 to 100% for lead acid. Lithiums are apparently happiest sitting in a 50% or so SOC, whereas a LA battery won't be happy at all sitting at 50% for long periods.

One possible issue with using lithium in this application though is they don't like being charged in freezing temps. Discharging is okay, but they should be warmed to 0°C to charge. The battery may have self-protection to prevent low temp charging, or possibly even some sort of internal preheat, which may be a factor in current performance issues?

georgemiller

Thank you, everyone, for your excellent replies. @Estragon, I would like to pick up on two issues you identified. 

1) Shade, even a small amount, will significantly reduce output. Unfortunately, the location of the panel makes frequent shade inevitable. As the crane moves around, its own superstructure will shade parts of the panels. This is potentially disastrous, as the location of the panels is somewhat fixed. Do you think there is any way to mitigate?

2) Charging LiFePO4 in freezing temperatures. Unfortunately, this is also inevitable. And as running a heater would draw even more power, I'm not sure how I could solve this. I do not believe the battery, nor the charge controller, has a temperature cut off. 

georgemiller

BB. said:

Welcome to the forum George.

And one thread is  best. I have closed the other.

Have fun,
Bill

Thank you, Bill. It is good to be here and get such great feedback! Thank you for closing the other post, and please excuse me if double posting was rude. I was not sure which was the more appropriate forum for my question.

Best,
George

georgemiller

Estragon said:

A quick look at the mc4 barrel connector link appears to show a connector with one female mc4 end and one male mc4 end going to a barrel connector. To me, this suggests there would be a male and female connector remaining, which would be connected together to complete the circuit. If so, it would make a single series string, with one half mostly shaded, which likely wouldn't produce anything near what you expect. The panels should be in parallel (positives combined, and negatives combined) to (ideally) produce ~10a at ~18v. In series, they would (ideally) produce 5a at 36v, which as well as producing much worse with one panel in shade, has the potential to overvoltage the controller on a cold morning.

More generally, it seems to me that in refining the concept it would be helpful to incorporate some sort of logging to better assess actual loads, pv production, battery performance, etc under actual use conditions.

The panels are wired in parallel. Apologies for not specifying that in the original post. 

BB.

George,

No problem. You are among friends here. And there are no "mind readers" here--Mistakes and misunderstandings happen. And "editorial"/moderation is pretty person specific (and I am not a mind reader either ).

In general, just keep it friendly and "business/family" verbiage. We are a open forum and pretty much allow any reasonably worded questions and answers (including links to off site/host sites).

I try to avoid the--Link to the cheapest solar panels or charge controllers type Q&As. Our host is a solar wholesale/retail operation and they are paying (and software support when needed) to run this site. They have good prices and very good service and we try to respect them here (but do not hide behind censorship if, for example, there are problems/questions with a product that they sell).

Otherwise, we are all volunteers here (including me).

-Bill

BB.

Regarding your system... a couple of "prediction" links for solar panes:

http://pvwatts.nrel.gov/
http://www.solarelectricityhandbook.com/solar-irradiance.html

You can play with the numbers and see what happens (flat, vertical, etc.). Note with panels at less than (roughly) 5 degree tillt from horizontal, They tend to collect dirt. Having some tilt gives the chance for rain and stuff to wash off a bit. For panels with frames, they create a "dam" when flat, making dust collection/failure to wash, even worse.

Personally, I am not a fan of flexible solar panels. If they are plastic, I think that 2-7 years it about it. Between the plastic top sheet failing (UV exposure?) and flexing causing cell bonds to fail, they are not that rugged for long term use.

As Estragon says--ANY shading on solar electric panels--There is a good chance of at least 50% loss of output. Remember, you have a stack of 0.5 volt cells in series--That go "high resistance" if not illuminated by sunlight (basically, they are current sources that output current in proportion to the amount of sunlight on the cell).

Most panels over ~12 volts have (usually) 1-3 "bypass" diodes that will (for example on a 24 VDC panel), will bypass the current around the dark cells--Cutting output voltage to ~12 volts (instead of 24 volts).

And, roughly, the amount of current generated is equal to the Cosine of the angle to the sun:

• 8 amps full sun * Cos 0 degrees = 8 amps
• 8 amps full sun * Cos 60 degrees = 4 amps

So, the more often/more time a panel is pointed not at the sun, the more energy harvest you lose.

Using the second link above for Washington DC:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Washington
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.08	2.80	3.85	4.80	5.43	5.76
Jul	Aug	Sep	Oct	Nov	Dec
5.61	5.02	4.33	3.48	2.35	1.90

Measured in kWh/m2/day onto a solar panel set at a 36° angle from vertical:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
3.51	3.91	4.43	4.52	4.45	4.44
Jul	Aug	Sep	Oct	Nov	Dec
4.44	4.44	4.65	4.77	3.82	3.44

continued next post

BB.

You are in a tough situation... you might be better with two panels flat (or 5 degree tilt for cleaning)--That would give you nearly 3.8 hours of sun in December (with two horizontal panels).

So, two panels flat, December average sun (this is long term average, if you have one or more days in a row of really dark storm clouds, you could get at little as 1% to 5% of "predicted output" those days:

• 200 Watts * 0.77 panel+controller deratings * 0.90 battery eff (assuming LI ion or AGM battery) * 1.90 hours of sun per day (Dec) =  263 Watt*Hours per average December day

Assuming 2watt+8watt+8watt running 24 hours per day:

• 18 watts * 24 hours per day = 432 Watt*Hours per day

During winter, I do not see how two panel back to back (with random sun orientation) or even two panels flat without shading can keep up with December sunlight (let alone during bad weather).

I suggest 10% to 13% rate of charge for a "full time off grid power system" as a starting point... If you have 18 watts of load and limited solar panels, you may need to up your panel size/number to get enough power to both fully charge the battery bank and run your daytime loads... Also, I think your battery bank AH (Watt*Hour rating) is a bit on the small side (about 1 day storage--suggest at least 2 days of storage and 50% maximum discharge--Most rechargable chemistry batteries will die if taken to zero or negative state of charge)

Something like this would be a suggestion (two days of "no sun" operation and not too hard on battery banks--Note that some battery banks using various LI Ion chemistries really need a Battery Monitor System to keep the batteries in an acceptable operating voltage range for safety and good battery life--Another post):

• 432 Watt*Hours per day * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank = 144 AH minimum battery bank @ 12 volts

And to charge such a battery bank from solar:

• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.05 rate of charge = 136 Watt array minimum (weekend/non-winter operation)
  
• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.10 rate of charge = 271 Watt array nominal
  
• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.13 rate of charge = 353 Watt typical cost effective array maximum

Because you are running loads during the daytime/during charging, I would suggest taking the above results and adding:

• 18 Watts * 1/0.77 panel+controller derating = 23 Watts additional panel (charging while running system--Not a big deal in this case).

And then there is sizing the array based on your loads... Note that most off grid systems have variable loads (don't do wash/pump well water during bad weather or run genset if needed). The typical maximum load from "predicted numbers", I would suggest ~65% to 75% of predicted numbers... For example:

• 432 Watt*Hours per day * 1/0.65 "continuous load fudge factor" * 1/0.61 typical solar derating for off grid DC system * 1/1.90 hours of sun per day (December) = 573 Watt array minimum (December flat array)

You may have some charging current issue (depends on battery type, charge controller type/model, etc.)--But I would be suggesting a minimum of 573 Watt minimum "flat mounted" array for the minimum amount of field maintenance/irritated customer calls.

Anyway, the above is just a starting point based on a lot of assumptions on my end... Your thoughts George?

-Bill

oil pan 4

If it's been real cold the LiFePO4 battery could be suffering from capacity damage and numbing from the cold.
Probably should have used lead acid.
Pretty much if the application is going to be used much below freezing, don't use LiFePO4.
Do they remember to turn it off at the end of the day?

Estragon

There's probably not much you can do about shading other than plan for some loss of output. I have a treed ridge to the south that means basically no sun from early Dec to early Jan., so had to install a couple of winter float arrays to maintain batteries when I'm away, and have to run a generator if I'm there.

Likewise, you can't change the weather. We've had weeks of temps <0°F, which is pretty normal for this time of year here. DC shouldn't be nearly as extreme, so you may be able to get away with charging cold. The problem is apparently lithium plating of anodes charging cold, which is permanent damage. The battery should generate a bit of heat when charging, which may mitigate the issue to some degree. Locating the battery out of the wind, or even insulating it may help.

Lead acid can be charged at much colder temps, and won't freeze until very cold (-70ish) fully charged. In this application though, I'd be concerned that a partly discharged battery might sit overnight in temps cold enough to freeze it in the discharged state. Charging a frozen lead acid battery is potentially dangerous.

georgemiller

BB. said:

You are in a tough situation... you might be better with two panels flat (or 5 degree tilt for cleaning)--That would give you nearly 3.8 hours of sun in December (with two horizontal panels).

So, two panels flat, December average sun (this is long term average, if you have one or more days in a row of really dark storm clouds, you could get at little as 1% to 5% of "predicted output" those days:

• 200 Watts * 0.77 panel+controller deratings * 0.90 battery eff (assuming LI ion or AGM battery) * 1.90 hours of sun per day (Dec) =  263 Watt*Hours per average December day

Assuming 2watt+8watt+8watt running 24 hours per day:

• 18 watts * 24 hours per day = 432 Watt*Hours per day

During winter, I do not see how two panel back to back (with random sun orientation) or even two panels flat without shading can keep up with December sunlight (let alone during bad weather).

I suggest 10% to 13% rate of charge for a "full time off grid power system" as a starting point... If you have 18 watts of load and limited solar panels, you may need to up your panel size/number to get enough power to both fully charge the battery bank and run your daytime loads... Also, I think your battery bank AH (Watt*Hour rating) is a bit on the small side (about 1 day storage--suggest at least 2 days of storage and 50% maximum discharge--Most rechargable chemistry batteries will die if taken to zero or negative state of charge)

Something like this would be a suggestion (two days of "no sun" operation and not too hard on battery banks--Note that some battery banks using various LI Ion chemistries really need a Battery Monitor System to keep the batteries in an acceptable operating voltage range for safety and good battery life--Another post):

• 432 Watt*Hours per day * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank = 144 AH minimum battery bank @ 12 volts

And to charge such a battery bank from solar:

• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.05 rate of charge = 136 Watt array minimum (weekend/non-winter operation)
  
• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.10 rate of charge = 271 Watt array nominal
  
• 144 AH * 14.5 volts charging * 1/0.77 panel and controller derating * 0.13 rate of charge = 353 Watt typical cost effective array maximum

Because you are running loads during the daytime/during charging, I would suggest taking the above results and adding:

• 18 Watts * 1/0.77 panel+controller derating = 23 Watts additional panel (charging while running system--Not a big deal in this case).

And then there is sizing the array based on your loads... Note that most off grid systems have variable loads (don't do wash/pump well water during bad weather or run genset if needed). The typical maximum load from "predicted numbers", I would suggest ~65% to 75% of predicted numbers... For example:

• 432 Watt*Hours per day * 1/0.65 "continuous load fudge factor" * 1/0.61 typical solar derating for off grid DC system * 1/1.90 hours of sun per day (December) = 573 Watt array minimum (December flat array)

You may have some charging current issue (depends on battery type, charge controller type/model, etc.)--But I would be suggesting a minimum of 573 Watt minimum "flat mounted" array for the minimum amount of field maintenance/irritated customer calls.

Anyway, the above is just a starting point based on a lot of assumptions on my end... Your thoughts George?

-Bill

Hi Bill- thank you for such an in-depth reply! Below I have adjusted your calculation, and in a second post I will reference your point on shaded panels. 

Two quick notes on your calculations above:
a) In Line 7 you put the system was running for 24hrs/day. This system will be running for 10hrs a day, meaning 10hrs*18W = 180W/hrs per day
b) In Line 19 (3rd calculation on this post) you used a 50% max discharge (1/0.5). I'm hoping to get 90% max discharge from the Lithium batteries, meaning I can do 1/0.9, as opposed to 1/0.5. The Lithium battery I am using does come with a BMS. 

Substituting those two, the calculation becomes:

• 180 Watt*Hours per day * 2 days storage * 1/0.90 max discharge * 1/12 volt battery bank = 33.4 AH minimum battery bank @ 12 volts

Therefore this is in line with the size of my current battery which is 40 AH. Do those adjustments to your calculation make sense to you?

georgemiller

Two general notes on panel orientation and battery type:

1) I am alas stuck with vertical mounted panels. This is because it is hard to place a mount in the location where this will be installed. Also, vertical panels can simply be ziptied on, leading to a minimal installation time.

2) I also prefer using Lithium batteries as they weigh less. As you'll be able to see from the below images, there is not much space in the trolley, and getting the unit there is a bit of a pain. Small, light batteries are much better.

Below are images of where the panels will be installed (that box to the left of the shot).

georgemiller

@BB. and @Estragon

My conclusion from your above recommendations is that as I cannot control the weather nor shade, I just need more panels. I can put in two more 100W panels, increasing the size to total four 100W panels all mounted in a similar vertical fashion if necessary.

Do you think it would be better to use 24V panels as opposed to 12V panels to decrease the effect from shade? (BB. you mentioned them having output diodes).

If you have any recommendations on a type of suitable flexible panel I would welcome them (BB., I know you don't like these types of panels but my application only has a 2-3 year shelf life so replacing the panels isn't much of a problem). These are the current ones I am using. 

BB.

I would probably suggest a maximum of 70-80% "useful" battery capacity. But, yes, you can adjust to your needs. Nothing "magic" about my assumptions.

More panels will help. If the batteries are exposed to sub freezing weather--You may have to (long term) look into insulating the battery box and/or preventing charging until the Li Ion batteries are above freezing.

For the panels, make sure you strain relief the cables to the panels, and put a drip loop where the wires leave the panels and enter your electronics "box". If the cables are not in conduit, look for UV outdoor rated wiring.

Using 24 volt panels will probably not help much--And you would need to use a more expensive MPPT type (maximum power point tracking) charge controller. The higher end MPPT controllers ($400-$600+ charge controllers 45 amp to ~90 amp output) are nice, and some even support Ethernet connectivity for logging and programming. May or may not be useful for your needs.

The bypass diodes to not "fix" shading issues... There are there to prevent damage to solar cells when multiple panels are used in series (or you use "Grid Tied" type solar panels with Vmp>>12 volts and Pmax ratings >~140 Watts) when subjected to "any" shading. None of this will offer use much improvement over your present 100 watt panels (for large solar installations, ~200-300+ watt panels are used because they are much less expensive $$$/Watt and less wiring... 1x 300 Watt panel vs 3x 100 Watt panels worth of mounting and connections). Larger panels (>~175 Watts) usually need two people to carry/install each panel (size and weight).

I don't have any specific panel experience (I am not in the solar power industry--Just a retired systems engineer)... Talk with your supplier about there recommendations and experiences.

-Bill

georgemiller

Hi Bill- could you clarify the phrase "strain relief the cables to the panels"? Is this to ensure the cables aren't taught or tear the connectors?

Thank you for you comment!
Dalton

BB.

Dalton,

Yep, that is correct. When you have panels on a moving structure (mechanical, wind, ice, etc.)--You want to make sure the copper wire does not work harden and break (finely stranded cables are used in areas where lots of flexing occurs--Such as line cords, welding cables, locomotive wiring, etc.). Solid and coarse stranded cables where they are not subject to moving/bending--House wiring, fixed wiring in cabinets, etc.).

You don't (for example) want a run of cable that can sway in the breeze--The constant motion will work harden the copper and possibly crack the insulation at fixed points (random wire tie, gland nut entry, etc.). And plastic wire ties generally do not do well in full sun (there are UV/Outdoor versions, but I would still suggest metal bands/ties). Your electrician may have some alternatives/advice.

Entrance of flexible/moving cables into "the box" or panel j-boxes will focus strain and flexing right at the gland nut (or whatever you are using).

-Bill

mike95490

Below freezing, you should not charge Li batteries.  They are efficient and do not generate self heat, their temp is what it is.

Estragon

I mostly agree with @mike95490 - round trip efficiency is supposedly ~92%, so it will heat some, but not much, and a lot of whatever heat is generated will be near the end of the charge cycle. That said, I understand why you'd prefer lithium, so it might be worth finding a way to deal with the freezing charge problem.