Got bifacial panels? Please give me your opinion on a unique situation.

[IslandBill]

Given: two 20' shipping containers outside my warehouse, the tops of which will get a welded steel scaffolding for solar panels. That scaffolding will be a minimum of 7' tall to raise the panels above the container tops to avoid shading from the warehouse to their south. The tops of the containers tops will get a white elastomeric polymer coating to avoid rust when rain water collects atop them.

Intent: 6000 XP inverter, nominal 8kw bifacial panels (4kW + 4kW), 15'- 18' above the ground, DIY 15kWh battery.

The ground around the containers is tan dirt and tan dead grass in summer due to no rain at 16 latitude in the Caribbean. The panels will be mounted at a 1 degree angle to shed rain water since 0 degrees is recommended for my location by https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

Bifacial panels are touted to provide from 5% to 30% increase over the nominal rating. Where do you suspect my set up will fall; The low or high end? Don't want summer conditions to exceed the inverter's specs.

 

[Texas-Mark]

Don't want summer conditions to exceed the inverter's specs.

Why do you think this is an issue? The controller will only draw the current it is rated for. Cold conditions are what can cause the VOC to go up and exceed the controller’s voltage rating.

 

[IslandBill]

Why do you think this is an issue? The controller will only draw the current it is rated for. Cold conditions are what can cause the VOC to go up and exceed the controller’s voltage rating.

8000W + 30% of 8000W = 10,400W when the unit is rated for a max of 10,000 . If I derate panels by their temperature coefficient since they will live at 70-105F the entire year, I want to purchase panels that will deliver at or above 8000 Watts nominally to give me a reasonable amount of power during winter when the sun is north of my location. That means that during the summer I may exceed the specs for the inverter if nominally I purchase, say, 8500W, derated to 8000 due to temperature coefficient and get the upper end of the bifacial output.

 

[kromc5]

I would need to look up the specs, but getting cargo containers off the ground is very important. Once they start rusting from sitting on the ground, water will penetrate, and humidity in the container will skyrocket. Then, it will look like the "Last of US" was a documentary. I've helped two farmers near me with the same problems.

 

[hwy17]

I just don't count on the bifacial gain at all, and whatever it does generate is an unexpected bonus.

My 3.8kW array is about 3 feet off the ground at the front edge and I've only ever seen over 3.8kW out of it during cloud edging.

I would not worry about the output going over.

 

[IslandBill]

I would need to look up the specs, but getting cargo containers off the ground is very important. Once they start rusting from sitting on the ground, water will penetrate, and humidity in the container will skyrocket. Then, it will look like the "Last of US" was a documentary. I've helped two farmers near me with the same problems.

The containers are off the ground now about 6" but I'm raising them to about 14" since the ground keeps getting higher by wind blown dirt, plant life, wet season flooding, etc. I don't know what mechanism is at work, but the blocks I put under the corners are now fairly deeply embedded in the ground. It's one of those mysteries I'll have to investigate some day.

I just welded up some corner lift gadgets to use my hydraulic jacks so I don't have to dig a hole under the containers to position the jacks. The gadgets allow me to place the jacks next to the container corners and lift using the holes in those corners.

 

[Texas-Mark]

8000W + 30% of 8000W = 10,400W when the unit is rated for a max of 10,000

So what? People over-panel all the time. The unit will only draw 10K watts no matter how many panels you have. Of course too much over-paneling results in wasted power, but 400 extra watts will not affect anything. As I said above, the only thing to worry about is going too high on the string voltage.

 

[IslandBill]

So what? People over-panel all the time. The unit will only draw 10K watts no matter how many panels you have. Of course too much over-paneling results in wasted power, but 400 extra watts will not affect anything. As I said above, the only thing to worry about is going too high on the string voltage.

Why would the manufacturer put a limit on any spec if it's of no consequence?

 

[Texas-Mark]

Why would the manufacturer put a limit on any spec if it's of no consequence?

There are some issues with short circuit protection, etc.

Just search here for "over-panel" it's been covered over and over.

 

[Cronix]

Why would the manufacturer put a limit on any spec if it's of no consequence?

So you know how much is usable

 

[IslandBill]

There are some issues with short circuit protection, etc.

Just search here for "over-panel" it's been covered over and over.

Thanks for the lead.

 

[IslandBill]

So you know how much is usable

On the 6000 XP, 8000 is usable and 10,000 is stated as max. This still doesn't explain what that limit actually means and why it exists.

 

[Texas-Mark]

This still doesn't explain what that limit actually means and why it exists.

It means that is the max input it can take for it's max rated output. Adding more input will not increase the output. However, adding too much input can be wasteful as clipping will occur. As I noted earlier, people over-panel all the time to compensate for cloudy days, poor panel angle, etc.

In your case, i doubt you will ever even come up to the panel ratings of 8000 watts, let alone the +30% you factored in.

 

[IslandBill]

In your case, i doubt you will ever even come up to the panel ratings of 8000 watts, let alone the +30% you factored in.

That 30% is just a number that some claim some panels are capable of under perfect conditions. My original post was to try to gauge what I might expect to see given the surroundings I mentioned - height above the ground, ground color, bright white container top coating. So far no one has piped in to give me an estimate of what they think the bifacial component as a percentage might be.

I've used primarily two sites to attempt to figure out what my location is going to provide for solar energy. The first:

SunCalc sun position- und sun phases calculator

Application for determining the course of the sun at a desired time and place with interactive map.

www.suncalc.org

Shows in the drop down for "More Solar Data & Photovoltaics" that in the majority of the year their model claims I should see above the 1000w/m^2, admittedly at above 25C since it rarely gets that 'cold' here.
The second site:

JRC Photovoltaic Geographical Information System (PVGIS) - European Commission

Photovoltaic Geographical Information System (PVGIS)

re.jrc.ec.europa.eu

after I key in the specs for 8000 watts of panels produces the following chart:

These are models but are all I have to work with for at least some information. At elevated temperatures for some of the panels I've looked at, factoring in the Pmax -.36 to -.39 factor, I get above 4000 watts assuming the 1000W/m^2 . It's the unknown of what a bifacial will add to the output that had me place the original post. I understand adding extra nominal capacity for December, January, in my case for example, but I don't want that capacity to cause an issue in the high output months.

For now, I'm looking at non bifacial panels due to the unknown of what bifacial might produce.

 

[Citlalatonac]

PVwatts will give a decent estimate with most of those variables included: temp, sun angle, sun time, average weather. It generates an hour by hour estimate based that location. It doesn't let you do customization down to your specific panel, but most solar panels are close enough for a decent estimate and mounting fall roughly into some basic groups, yours being a bit of an outlier.

In the advanced section it will let you choose bificials for a ground mount. Reflected light on the back can vary widely by deployment so it might not be the best indication of what actual results will be, but better than a wild-ass guess. You can look at how the estimate comes by checking doing the hourly download on the results page.

 

[Texas-Mark]

My original post was to try to gauge what I might expect to see given the surroundings I mentioned - height above the ground, ground color, bright white container top coating. So far no one has piped in to give me an estimate of what they think the bifacial component as a percentage might be.

I think there are just too many variables to give anything except a WAG. But since you said the panels will be nearly flat, I'm not sure how much reflected light will come from the white container. My guess is you will be at the lower end of the spectrum. But regardless of any of that, you are not going to harm the charge controller as long as your controller string voltage is not exceeded.

 

[IslandBill]

Thanks for getting me to try PVWATTS again. I tried it a month ago and it kept giving me an error to come back later. I tried all day then and it never worked. This morning I tried it again and got the same come back later message but kept trying and it finally did work. I got detailed results on an hourly basis.

I thought it was failing due to being outside the US, but since it worked once, that can't be the reason for the intermittent service.

 

[IslandBill]

I'm not sure how much reflected light will come from the white container.

Because the bottom of the panels will be 15'-18' off the ground a large ground area has access to the back of the panels and that ground area is pale dirt and dead grass in summer. I'm thinking the back should get plenty of reflected light. I know that when I walk outside the light is so strong I shield my eyes with one hand and close one eye to walk to my vehicle to get to my very dark sunglasses. I'm also at 16 latitude.

As you say, it's all a WAG until it's actually tried. Thanks for your input.