I’m pleased to share relevant solar content as it applies to you, my passion and profession here at OneRoof Energy. This blog post will be the first in a series of three posts that will cover how different aspects of weather impact solar PV – or panel – performance, including: snow, cloud, rain, temperature, and wind. This week’s post will specifically discuss snow; which is fitting for our friends on the East coast who are getting a healthy dose of it this winter season. – Chris K.
Every year in the colder climates we expect that white solid precipitation – a.k.a. snow – to pay a visit and stick around for a while. The question we get from solar advocates is: how does snow impact the performance of my solar PV (photovoltaic or panel) system? Also, since system performance directly effects return on investment, how is my ROI impacted by snow accumulation? Well, the answers aren’t always straightforward, but we’ll try and explain them as simply as possible.
Snow accumulation has a specific consistency or density that is effected by local weather conditions such as: temperature, relative humidity, wind velocity, inclination or tilt angle of the area being effected, and other surface properties. Depending on these factors, snow can have a tendency to stick to the surface and accumulate or it can simply bounce and slide off a surface. For example, at ambient temperatures of -3°C or greater (that is, warmer), snow tends to adhere more to a surface and therefore sticks to a surface, which exhibits what we call adhesion. The amount of adhesion and subsequent accumulation is also affected by other environmental conditions in addition to temperature, including relative humidity and tilt angle of the surface relative to horizontal. On the other hand, at ambient temperatures of -3°C or less, snow particles tend to hit a surface and bounce off, which leads to the snow sliding off a surface, providing that it is inclined at an adequate tilt angle. For example a roof with a tilt of 30° from horizontal is an adequate angle.
Why do we care about this? Well, one reason would be that when it’s “warmer,” snow will have a greater impact on system performance since it’s more likely to build up. But at “colder temperatures” (<-3°C) snow is less likely to stick on your solar panels and more likely to slide off, clearing it from the solar panel array.
It is important for you to understand that snow accumulation on your solar panels does have an impact on the overall performance of your solar system and it increases as more snow accumulates. Some studies have shown that 20 percent of incident irradiance – the amount of relevant sunlight energy that can be converted to electricity – is still available for your system even when there is 2 cm of snow on it. At 10cm of snow, this value decreases to about 3-4 percent, which means your system isn’t producing very much during these times. Another interesting phenomenon occurs when snow is packed onto a solar PV array: it actually insulates modules! This means it retains heat that transfers energy from the solar panel array to the snow and helps in melting the snow so it will slide off of system. This would be dependent on other factors such as relative humidity and tilt angle, but we have found a pro to all this snow!
If you like the math behind all this snow talk – here it is:
Let’s get down to some numbers related to snow accumulation. A few studies gave us a little more insight into how average annual snowfall conditions might decrease your system’s performance:
- 40-60 percent loss in kilowatt (kWh) production occurs during winter months, depending on the tilt angle of the solar panel array (study from Northern California2)
- The following annual losses (averaged over year) are reported (study from Northern California2)12 percent reduction in production at a tilt angle of 39°
- 15 percent reduction in production at a tilt angle of 24°
- 18 percent reduction in production at a tilt angle of 0°
- Annual average snow losses ranged from 1-3.5 percent (study from Southern Ontario, Canada1)
- 1 percent performance loss per inch of snow accumulated, depending on tilt angle of the solar panel array, that is:
From this, we can figure that the more snow that accumulates, the less production will come from your solar panel system, especially at lower tilt angles. An important note: these percentages are based on local climates and environmental conditions and may not be accurate for all locations. But, the formula does provide you with a general idea of what your possible loss of production would be because of snow on your solar PV system.
Since ROI for solar panel systems depends on its performance, we can get a rough estimate of how much the snow’s impact on your production will have on your system paying for itself. Here is a formula for simple pay back that might help you calculate it:
Where % snow loss = snow loss as a percentage of annual energy yield. From this it devised that the simple pay back is proportional to:
For example, a system with simple pay back (excluding snow losses) of 8 years will have an adjusted simple pay back when including a projected annual snow loss of 8% of:
This means that an additional 36 weeks are added to the simple pay back if 8 percent of performance is lost due to snow accumulation.
That can be significant, especially if it adds years to the simple pay back of your solar PV system. These calculations are taken into consideration by companies like OneRoof Energy when we work on how to best finance and install solar PV in for customers in colder climates. For customers like you deciding whether it’s better to buy or finance solar PV systems, this timeline is also an important piece of the puzzle. Third-party owners (like us) are responsible for ensuring that systems under a Power Purchase Agreement – or PPA – are performing optimally. With a PPA, we take the risk on how a system will perform due to snow fall so you don’t have to assume it. On the flip side, customers who own their systems are responsible to manage snow accumulation on their systems and would be responsible for monitoring and clearing snow.
There is another positive impact that snow can have on solar PV performance albedo – or the way that light is reflected off the snow and onto the solar panel – gives us another way in addition to more direct beams of sunlight to produce energy from your solar panels. What’s important about albedo is if solar panel arrays are situated at certain angles where they are able to collect a portion of the reflected sunlight from the snow, then the solar panel system accumulates more sunlight than it would have if there wasn’t snow. In some cases, this may reduce or mask the potential performance loss due to snow. It is difficult to calculate the increase in system performance as it is on a case-by-case basis. One study showed an overall average 1 percent increase in system performance due to albedo irradiance at higher tilt angles.
To give you the short version, here is a list of pros and cons pertaining to snow fall on solar PV system performance:
PROS:
- increased albedo (reflected) irradiance at higher tilt angles can increase convertible sunlight into the solar panel array and *might* make up for long-term average snowfall production loss
- snow can melt and slide off solar PV array under specific environmental conditions
CONS:
- a drop in direct irradiance and overall system performance. This decrease can be higher at lower tilt angles and relatively warmer temperatures (>3°C) due to increased snow accumulation
- decreased simple pay back due to snow that is proportionate to percent loss in performance
To wrap it all up, we know we want to avoid snow accumulation on our solar panel arrays as much as possible to maximize production and ROI. In areas that regularly see seasonal snow accumulation, we have to find ways to adjust. For our customers, this means clearing snow frequently and factoring snow losses into your ROI calculations.
Check back next week for the next post in the weather series on how cloud cover and rain impacts solar production.
References:
- Andrews, R. W., Pollard, A. & Pearce, J. M. The effects of snowfall on solar photovoltaic performance. Solar Energy 92, 8497 (2013). DOI: http://dx.doi.org/10.1016/j.solener.2013.02.014
- Townsend, Tim; Powers, Loren; Solar winter output assessment: measuring snow-related losses; DNV KEMA Energy & Sustainability, 2012. http://www.dnvkema.com/Images/Solar%20Winter%20Output%20Assessment.pdf
