Tilt Angle for Ground Mount Bifacials in New England?

[Turkey Hollow Solar]

We're installing a 13.8kW (DC) ground mount system with Bifacial panels here in Northern VT. This is for a net-metering system only at this time, so the objective is to maximize annual production.

The ground mount is the Nuance Osprey system, which allows for setting any desired tilt angle.

I did some quick analysis of various tilt angles using PVWatts. Here is a table of the estimated production vs. module tilt. It shows that 38deg is the optimal angle for our location.



A few observations on this data:

- tilt angle doesn't seem to matter that much, at least in the range shown here, which is 30deg to 45deg. The difference between the lowest (30deg) and highest (38deg) is only 139kWH, or about 0.8%.

- it predicts that 38deg is the optimal angle

- PVWatts doesn't yet have an option for bifacial panels, so I'm just using the "premium" option

If this was a rooftop, this would equate to the range between a 7/12 and 12/12 pitch. I'm surprised that there wouldn't be more response to tilt angle.

For people with experience in New England, does a 38deg tilt sound good to you? Does this low response to tilt surprise you?

We were originally planning to use 35deg, which is conventional practice around here. However, a little more tilt to 38deg seems like it would help snow shedding, and could help the bifacial panels pick up more ambient light from the back side.

We also considered that maybe a little lower tilt could be better to pickup more production in the summer months when the days are longer. But this data suggests that thinking isn't valid...at least based on the PVWatts models.

I'm interested in your thoughts!

 

[sunshine_eggo]

Your winter insolation is so poor that panel tilt makes little difference on an annual basis, but what you're doing is shifting "summer" production to "winter" production.

Is 35 conventional because that's the roof pitch? Since you're bifacial, I would go 45 - about your latitude, better shedding. Make sure the panels are elevated in accordance with guidance to ensure they actually get something off the back side.

 

[Maitake]

Optimum Tilt of Solar Panels

 

[sunshine_eggo]

Optimum Tilt of Solar Panels

I use that too.

 

[RCinFLA]

Attached is NREL data for Burlington VT. It provides the equivalent hours of full sun exposure per day.

 

[740GLE]

I’d be more concerned about splitting the array into east and west facing to maximize generation over the entire day.

Specifically weather wise sometime of the year the morning sun is bit more, and sometime of the year the afternoon sun produces a bit more.

 

[sunshine_eggo]

I’d be more concerned about splitting the array into east and west facing to maximize generation over the entire day.

Specifically weather wise sometime of the year the morning sun is bit more, and sometime of the year the afternoon sun produces a bit more.

Per OP's post, grid-tied, and goal is to maximize annual production.

 

[Turkey Hollow Solar]

Your winter insolation is so poor that panel tilt makes little difference on an annual basis, but what you're doing is shifting "summer" production to "winter" production.

Is 35 conventional because that's the roof pitch? Since you're bifacial, I would go 45 - about your latitude, better shedding. Make sure the panels are elevated in accordance with guidance to ensure they actually get something off the back side.

I don't know why 35 is conventional here....but I've heard that from a few suppliers and installers.

45deg is an interesting idea. Beneficial from both a snow shedding and backside uptake perspective. It could increase winter production from the bifacials (it really works...the modules go well above STC on sunny days with fresh snow), and not impact the summer yield much if at all.

The panels are about 3' off the ground. This should provide room for snow to pile up.

We went to great pains to keep the backsides clear of any obstructions. With microinverters mounted behind the panel like a roof mount would typically do, you can see the cells that are shaded because they are the last to thaw out and slide. So we've moved the microinverters to the side.

 

[Turkey Hollow Solar]

I’d be more concerned about splitting the array into east and west facing to maximize generation over the entire day.

Specifically weather wise sometime of the year the morning sun is bit more, and sometime of the year the afternoon sun produces a bit more.

Per OP's post, grid-tied, and goal is to maximize annual production.

Correct....we're not charging batteries, so the goal is the most kWH overall.

 

[Turkey Hollow Solar]

Attached is NREL data for Burlington VT. It provides the equivalent hours of full sun exposure per day.

Thanks for the helpful data. There seems like a ton more content on PVWatts that I haven't found time to explore!

 

[Turkey Hollow Solar]

Optimum Tilt of Solar Panels

Thanks for the link...very helpful!

 

[profileSOLAR]

Hi there, I'd have to say I agree with the angle of 38° South if you want maximum year round production.

I came to the same result a different way in case that helps you gain some confidence.

I pulled an entire year of solar & meteorological data from NASA POWER API for last year for you, for Burlington, Vermont, USA, at Lat/Long coordinates 44.4758825, -73.212072. I then separately calculated the ideal angle to tilt panels for those coordinates for every single day of the year and applied a weight to each individual ideal angle to tilt panels, dependent on the contribution of the solar PV output potential of that day to the overall solar PV output potential for those coordinates for the whole year.

My calculations also result in an ideal panel tilt angle of 38° South. No one has a crystal ball on exactly what weather will do in the future, but based on empirical data we can lay our hands on for last year, if you had of had that angle for your solar PV panel tilt last year, that would have netted you the highest overall kWh output for the year from a fixed angle solar PV system.

 

[Turkey Hollow Solar]

Hi there, I'd have to say I agree with the angle of 38° South if you want maximum year round production.

I came to the same result a different way in case that helps you gain some confidence.

I pulled an entire year of solar & meteorological data from NASA POWER API for last year for you, for Burlington, Vermont, USA, at Lat/Long coordinates 44.4758825, -73.212072. I then separately calculated the ideal angle to tilt panels for those coordinates for every single day of the year and applied a weight to each individual ideal angle to tilt panels, dependent on the contribution of the solar PV output potential of that day to the overall solar PV output potential for those coordinates for the whole year.

My calculations also result in an ideal panel tilt angle of 38° South. No one has a crystal ball on exactly what weather will do in the future, but based on empirical data we can lay our hands on for last year, if you had of had that angle for your solar PV panel tilt last year, that would have netted you the highest overall kWh output for the year from a fixed angle solar PV system.

Wow...thanks for checking that! I appreciate the help....it does help gain confidence!

 

[Maitake]

None of these answers (including mine) address the bifacial aspect.
Bifacials benefit from at least one meter gap between the array and ground. This may require taller posts on your ground mount than you were planning.
March is a big month for solar and very big with bifacials in the northeast, if your inverter is sized to catch it. Bright sun, cold weather, high albedo, can easily take your MPPT to 100% early in the day and keep it pegged until afternoon.
This effect led me to choose 1:1 DC/AC ratio to catch more of this peak.

 

[Hedges]

We're installing a 13.8kW (DC) ground mount system with Bifacial panels here in Northern VT. This is for a net-metering system only at this time, so the objective is to maximize annual production.

Annual production of watts, or dollar credits?
Do you have a flat rate per kWh regardless of season, time of day, usage per month?

I originally had a rate schedule which charged more per kWh for higher kWh usage. Same applied for net-metering credits.
I opted for time of use, because rates were higher Noon to 6:00 PM, lower at night. Later time of use because mandatory.

You may want to orient and tilt to maximize production when higher credit will be earned. If not implemented yet, rules could change in the future.
Think also about how to set up the system in case rules change in the future so you get zero credit for export (use it or lose it), or to charge batteries. For batteries you would want to match production to time of day you consume (reduce capacity of battery required) and align production to consumption for winter and summer.

Is 35 conventional because that's the roof pitch? Since you're bifacial, I would go 45 - about your latitude, better shedding. Make sure the panels are elevated in accordance with guidance to ensure they actually get something off the back side.

We were originally planning to use 35deg, which is conventional practice around here. However, a little more tilt to 38deg seems like it would help snow shedding, and could help the bifacial panels pick up more ambient light from the back side.

We also considered that maybe a little lower tilt could be better to pickup more production in the summer months when the days are longer. But this data suggests that thinking isn't valid...at least based on the PVWatts models.

Production difference due to 3 degrees difference probably not noticeable, except maybe when sun is low in winter.

I've read that facing straight up produces best on heavy overcast days, because clouds are the light source. That isn't considering bifacial and wouldn't be factored into sun-angle and sun intensity based calculators. May matter more for off-grid where that little bit of power helps, compare to net-metering where only annual total is important. My system output drops to 10% on a lightly overcast day, 2% with heavy clouds.

We're installing a 13.8kW (DC) ground mount system with Bifacial panels here in Northern VT.

Has system been designed including breaker size allowed in your panel based on 120% rule?
13.8kW if AC would be just over twice what is allowed for some 200A breaker panels, 32A continuous through 40A PV breaker. Many models of 7.6kW inverter for that reason, and 3.8kW to fit 100A panels.
Other connection schemes (after meter and main breaker, wires split to main panel and separate PV aggregator panel) it could be larger.

If AC production is limited by such rule, spreading power out over the day and aiming for more winter production would allow more per year.

This is for a net-metering system only at this time, so the objective is to maximize annual production.

If you may want battery backup in the future, speak now or forever pay the extra cost.

 

[Turkey Hollow Solar]

Annual production of watts, or dollar credits?

Goal is to accumulate maximum credits...local power allows transfer of credits to other customers, but not cash payment, for excess credits.

Do you have a flat rate per kWh regardless of season, time of day, usage per month?

Flat rate for Net Metering

I originally had a rate schedule which charged more per kWh for higher kWh usage. Same applied for net-metering credits.
I opted for time of use, because rates were higher Noon to 6:00 PM, lower at night. Later time of use because mandatory.

No option for that with our power company

You may want to orient and tilt to maximize production when higher credit will be earned. If not implemented yet, rules could change in the future.

This was my thought also, which is why I did the analysis on PVWatts.

Think also about how to set up the system in case rules change in the future so you get zero credit for export (use it or lose it), or to charge batteries. For batteries you would want to match production to time of day you consume (reduce capacity of battery required) and align production to consumption for winter and summer.

No plans for batteries at this time. Might add later to work on off-hours consumption.

Production difference due to 3 degrees difference probably not noticeable, except maybe when sun is low in winter.

Agreed...the data showed little effect, even over a range of 10+ deg.

I've read that facing straight up produces best on heavy overcast days, because clouds are the light source. That isn't considering bifacial and wouldn't be factored into sun-angle and sun intensity based calculators. May matter more for off-grid where that little bit of power helps, compare to net-metering where only annual total is important. My system output drops to 10% on a lightly overcast day, 2% with heavy clouds.

Makes sense...but can't do zero tilt with the snow we get. Might experiment with vertical bifacials someday.

Has system been designed including breaker size allowed in your panel based on 120% rule?
13.8kW if AC would be just over twice what is allowed for some 200A breaker panels, 32A continuous through 40A PV breaker. Many models of 7.6kW inverter for that reason, and 3.8kW to fit 100A panels.

System ties in on the line side of the main panel and has a dedicated breaker (its not a service entry, otherwise it would need a fused disconnect), so it skips the main panel entirely. System is sized for 25kW capacity, which is just over 100A continuous so it uses a 125A breaker.

Other connection schemes (after meter and main breaker, wires split to main panel and separate PV aggregator panel) it could be larger.

If AC production is limited by such rule, spreading power out over the day and aiming for more winter production would allow more per year.



If you may want battery backup in the future, speak now or forever pay the extra cost.

I'm sizing to add a small hybrid inverter some day...maybe a SolArk 5K...so the microinverters can be AC Coupled in case of an outage. However, our grid has been very reliable...we have a backup generator we never use....so I'm not in a hurry on that and I can wait for battery prices to keep coming down. In the mean time, Net Metering is excellent for our needs, and I appreciate our power company for providing it.

 

[Turkey Hollow Solar]

None of these answers (including mine) address the bifacial aspect.

I totally agree on this...PVWatts does not (yet) consider bifacial panels in their main tool. Maybe included in other more complex tools that have?

Before building our system this spring, we setup 4 bifacial panels for testing over the winter. We had 2 on an APSystems YC-600 (2 panels, 300W each), and 2 on a Victron DC charge controller. The results all winter were quite impressive, especially after fresh show. We have 365W panels, and regularly saw 400W+ on the DC system, with some high points over 450W. Even on the winter solstice, the APSystems unit clipped for most of the day.

Bifacials benefit from at least one meter gap between the array and ground. This may require taller posts on your ground mount than you were planning.

Yes, we set them up to be 1M off the ground. We also went to great pains to not block the backside of the panels. When testing the bifacials, we noticed that the location where the microinverter shaded the panel backside would be the last area to thaw. So when it came time to build the full array we located all the microinverters away from the panels. We still have the strut channels behind the panels for mounting...couldn't find a practical way to avoid that without significant modification and cost.

March is a big month for solar and very big with bifacials in the northeast, if your inverter is sized to catch it. Bright sun, cold weather, high albedo, can easily take your MPPT to 100% early in the day and keep it pegged until afternoon.
This effect led me to choose 1:1 DC/AC ratio to catch more of this peak.

Same with us....based on the data we collected over the winter, we ended up using the new APSystems DS3-L, which are a 768VA unit for 2 modules, so 384W per panel. This is 20W over the panel rating, but even in the warmer months we see near peak production. This ends up being a DC:AC ratio of 0.95.

This is unheard of using conventional panels, but for bifacials it makes sense based on the data we collected. Conventional wisdom would have applied say a 1.25:1 ratio, which would have specified a 300W microinverter like the APSystems YC-600 or Enphase IQ7+. But our data showed that this would lead to extensive clipping and a significant loss in power production.

We also quickly noticed the bifacial panels tendency to warm up and shed snow. Most days our panels were clear the morning after a snow, and we noticed neighbors with conventional panels that took days to clear.

Overall we're completely sold on the bifacial panel concept, and I'm currently working on a design to use them as awnings for our south facing windows....they really look nice from underneath and I think would have impressive performance if elevated up the side of the house.

 

[peakbagger]

PV Watts does not take into account reflection from snow in front of the panels in winter. I have an adjustable vertical tilt and I get more output in winter with snow on the ground. PV Watts also does not take into account snow buildup on panels. Unless you clean off the panels after every storm, a steeper winter panel angle will melt off quicker assuming that the bottom edge of the panel is high enough off the ground that snow sliding off and piling up does not block the bottom edge of the panels. My pole mount was modified to hold more panels so I lost some clearance. I have to run my snowblower in front of the array every few storms to keep the "lump" down.

A lot fo the big solar farms in Mass are built too close to the ground and I have seen some arrays with the lower edge buried for weeks after a big snowfall. Some tried to clean out the lumps but its lot of work as they have to move it with bobcats.

 

[Turkey Hollow Solar]

Good point about snow losses in PVWatts. The default settings is zero (under systems losses), but if the panels get covered with snow for days or weeks, it must become significant.

We have seen impressive spikes in bifacial output after a fresh snow in winter. So it makes sense that conventional modules would see some benefit also.