Solar Winter Output Assessment: Measuring Snow-related Losses

Tim Townsend and Loren Powers, DNV Kema Energy and Sustainability
June 15, 2012 | 10 Comments

Historically, PV modules installed in snowy climates have been part of small, off-grid arrays mounted at very steep tilt angles. This is done both to shed snow quickly and to maximise winter output. Unfortunately, this concedes too much annual energy to be a good design strategy for larger contemporary systems.

Today's snowy climate PV systems tend to be installed at angles shallow enough to make them prone to snow loss, and as large-scale PV installations become more widespread in snowy locations analytical models are needed to estimate the impact of snow on energy production.

Both weather and array design factors influence the amount of snow loss. Weather factors include the quantity and quality (moisture content) of the snow, the recurrence pattern of storms, and the post-storm pattern of temperature, irradiation, wind speed, wind direction, and relative humidity. Array design factors essentially boil down to orientation (fixed or tracking, tilt, azimuth, and tracker rotation limits) and the surrounding geometry (open rack or building-integrated). Building features can also either help (e.g. melt) or hinder (e.g. dam up or drift) natural snow shedding.

Nonetheless, a generalised monthly snow loss model is introduced here which, despite some limitations, appears to deliver good- quality, unbiased monthly loss estimates which can now be used as inputs to the simulation programs PV investors rely on for decision-making.

Lake Tahoe Test Bed

BEW Engineering, Inc - now part of DNV Kema Energy and Sustainability - set up three pairs of 175 WP poly-silicon Mitsubishi model PV-UD175MF5 PV modules at fixed tilt angles of 0°, 24° and 39° on south-facing racks in Truckee, California, at the beginning of the 2009-2010 winter. The module pairs are spaced far enough apart to prevent row shading, even on the winter solstice.

Near Lake Tahoe, the station's latitude is 39° and its elevation is 5900 feet (1800 metres). The site receives an annual average of 200 inches (5 metres) of snow.

One module of each pair is manually cleaned and thermostatically heated. The three un-cleaned modules are allowed to shed or accumulate snow naturally and are bordered with two feet (0.6 metres) of similar material to minimise edge effects.

A datalogger saves hourly records of irradiance for the three tilt angles, short-circuit current and temperature for each module, along with air temperature and relative humidity. Meanwhile, an hourly webcam shot records snow depth and assists with quality checks. A second source of data is a 125 kWP Truckee Sanitary District (TSD) system located two miles (3.2 km) south of the BEW station and sitting at the same elevation.

For BEW's rig, snow losses are gauged as the difference in monthly amp-hours between the clean and uncleaned modules. For the TSD system, snow losses are gauged as the difference in measured energy and predicted energy for an always-cleaned array.

The TSD system faces south at a fixed 35° tilt, similar to one of the paired sets of BEW's test modules. The lowest edge of the 17 foot (5 metre) long rows are six feet (2 metres) above ground. While the District does not manually clean this array, they do regularly plough snow from between the rows to prevent snow from piling up. This maintenance practice proved to be especially valuable because snow is not removed from the array, yet ground interference does not occur. It is as if the array is very high above ground. Indeed, ground interference at the BEW site has resulted in twice the annual energy loss as the TSD site.

Calculate Winter Losses

Depending on tilt angle, wintertime energy losses of 40%-60% and annual energy losses from 12%-18% were noted in the first year of operation, though data from the TSD system were not included. The first winter was statistically very normal. The lost energy due to snow buildup in the seven-month winter season ranged from as little as 25% for the 39° tilt to as much as 42% for the flat orientation. The seasonal results project to losses in annual output of 12%, 15%, and 18% for the 39°, 24°, and 0° tilts, respectively.

While these results were hugely significant for this location, no attempt was made to project how the Truckee results would translate to other, less snowy locations based on the first year of measurements. The model development and fitting task was completed after the second year of measurements, after which BEW's generalised model was tuned enough to be provisionally applied to other locations. The current form of the model is:

Snow loss, % = C₁*Se'*cos²(T)*GIT*RH/T_AIR2/POA^0.67

Where:

C₁ is a fitted coefficient, 5.7x10⁴Se' is the 6-week rolling average effective snowfall in inches, with
Se = S (monthly snow, inches)*0.5*[1+1/N], where N is the number of snow events per month
GIT = ground interference term, defined in detail below
RH = average monthly relative humidity, %
TAIR = average monthly air temperature, C
POA = monthly plane of array insolation, kWh/m²

The GIT is further defined as:

GIT = 1-C₂/exp(ϒ); C2 is fitted from data as 0.5; ϒ is the dimensionless ratio of snow received divided by snow dissipated, such that whenever the amount of snow received exceeds the ability of the array geometry to deposit it on the ground, shadow-like interference will quickly reduce array output by a factor of 2 to 1. BEW defines ϒ as:

R*cos(T)*Se'*2*tan(P)/(H²-Se'²)

Where:

R is the row plane of array dimension, inches
T is the tilt angle, degrees
P is the stabilised snow pile angle, nominally assumed to be 40 degrees
H is the drop height, inches

And Se', effective rolling-average snowfall, inches as defined above

For one of the US's snowiest urban areas, it was observed that annual losses of 12%-18% may be expected in a typical year for fixed tilt arrays mounted at tilt angles ranging from 39° to 0° (flat). However, monthly losses may be substantially higher; an entire month's output was lost for a shallow tilt angle unit when several feet of snow fell, for example.

On a rolling annual basis, the snow losses have averaged 6% for the TSD system, 13% for the 39° BEW module, 17% for the 24° BEW module, and 26% for the flat 0° BEW module. However, the principal use of this information is not necessarily to point out how much potential generation is sacrificed in a very snowy location, but to serve as a baseline for validating proposed snow loss models.

Developing a Losses Model

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10 Comments

Gary Best
April 7, 2013

Just wanted to follow-up after a winter with decent snowfall. As I just posted on my blog at http://masssolarinfo.com/wordpress/2013/04/07/monthly-solar-production-winter-2012-13-and-loss-due-to-snow/, I showed a 7% annual reduction in production due to snow loss this year just south of Boston.

Peter Nuspl
July 26, 2012

The problem with snow-related losses should be addressed in two parts: 1. at night (or any period of low sunlight), the accumulation of snow is the problem; one solution used by some is to cover the collectors and dump that snow before daylight. 2. in bright sunshine, there is not much accumulation of snow and what does actually will slide off quickly.
Also, flat-plate PV and heat collectors should be treated separately from evacuated tube collectors: different physics, geometry and purpose.
As to tracking collectors, they should usually be at high tilt angle evenings, nights and mornings; so the show is readily shed.
These need another model too.

Tim Townsend
July 19, 2012

The authors wish to thank REW's readers for their feedback. To gary-best, yes, our method does account for the lesser-weighted contribution to the annual energy total that is made during the winter months. Our model was fitted using 18 months of measurements, including two successive winter seasons, so each season's influence has been folded in. Your measured data covers three months with estimates for what your array might produce over the rest of its first year. Our model should apply well in Massachusetts since it is only 2-3 degrees north of our Truckee test site. The accuracy of the model has more to do with weather than latitude. To longwatcher, we agree that the mounting geometry of modules on a roof slope can have a bearing on snow-related energy loss. If modules are wired in series along the slope of the roof aligned in portrait, a small amount of snow near the bottom of the stack wil have a heavy penalty on energy, much like the severe mismatch caused by a hard shadow. Conversely, if parallel strings are being used and the modules are mounted in a landscape orientation and most importantly, if the strings are series-wired at the same physical height, then the energy loss from partially-cleared arrays will be somewhat less, perhaps half as severe as the portrait case. Lastly, we note the air temperature term in the first equation should denote Kelvin, not Celsius. Also, the results for modeled loss for sample arrays in Denver, Detroit, and Philadelphia were omitted in the article. These should be listed as 3%, 3%, and 2%, respectively.

Ruth Cooper
June 23, 2012

Thank you Tim & Loren for posting this detailed, timely and informative article. The ground-mount vs. roof-mount question has a myriad of variables, including the implications of snow coverage. For those installing systems of any kind, I propose that long term ROI & pay back period - based analysis should be taken with a large blue salt brick (vs. grain of salt), given that most jurisdictions can't predict electricity costs hour-per hour vs. over a 20 year period. I would like to propose that the decision process can be simplified by deciding whether or not an organisation or an individual is interested in worry about and/or dealing with the future uncertainty of energy supply and the associated pricing & delivery structure on a regularly basis. I am not. I advise others not to be regarding both electricity generation and the operation of their HVAC systems, which can both be more sustainably addressed using renewable energy sources.

Thomas M
June 23, 2012

All the more reason to consider ground mounted panels if possible. Not only can you clean off the snow easily, reorientate easily, maintain easily, you also get more reflective radiation off the white snowy ground cover. Other alternative is using a hose to wash off the snow if roof mounted. Small electrical resistance heaters such as those used to unfreeze piping can also be used to initiate snow melting. Turning on hot water circulator pumps for several minutes to heat up the panels is also another alternative method for SHW systems. As with most any technology, there are going to be drawbacks, so instead of focusing on them, figure out solutions to avoid them.

Tim Dolan
June 20, 2012

@phil-manke-79191, Although I wanted them roof-mounted anyway, the gap between the panel and the roof is fairly good for airflow so it is not adding that much heat back up to the panels in summer (and I am not sure it is really adding any given the A/C cooling leaks from the attic may cool the panels a touch in summer. However, all that said, I live in a historic district and there is NO WAY they would have allowed me to put in pole mounted panels that were high-enough to clear the neighbor's trees. Having a two-story house, the roof is high-enough to clear most trees fairly early in the morning and throughout most of the day. And since my panels are producing MORE then the models say they should by a small bit, not too worried. I am currently getting 115% of my needs when the models said I would get 90% of my annual needs (a change of A/C unit and about 3 or 4 light bulbs to CFLs between estimate and final results looks to have made part of that difference, but based on A/C power sucking not the full difference. I am guessing somewhat that I got luckily on the production of MY particular individual panels that or the models expect more of something to be reducing the output then is the case with my array. trivia, my A/C unit went from old I think late 70's 10,000 BTU to a 2010 era 12,000 BTU A/C so I upped the A/C power, but the net was a lessened electrical use. My panels were not on long-enough before I had to replace the old A/C to confirm how much it was using versus the new, but comparing meter readings from base load to current usage, my use has gone down only a little bit with the new A/C, as mentioned not enough to off-set the amount extra I am getting.

Jack Farrell
June 20, 2012

Sounds about right. Significant benefit to homeowner when two way meters are used. Size of batteries smaller and payback around noon on air-conditioning days is large. Community benefits by local energy sharing midday from spring thru fall; self sufficient mountain communities reduce demand on electric grid during highest demand days. Largest benefit is increased comfort and safety to all, but retired people needing assured access to electricity benefit the most.

Phil Manke
June 20, 2012

Longwatcher; Your shading of the roof in summer is costing you production by heating the panels. 15% or more is lost at high PV insolation temperatures. This is, little doubt, why your production was high in Dec also. It may have been even higher if your attic insulation was better. I have panels pole rack mounted for this reason, with seasonal tilt for shedding and summer flattening. Doing it this way also leaves architecture or roof lines unaffected. Roofs get hot, PV, best not.

Tim Dolan
June 19, 2012

Gravity may cause multi-panel arrays to get rid of snow faster (presuming they are angled). Adding in the data from my roof top array (30 panels in 3 rows of ten) in SE Virginia. I have noted that during the few times it has snowed in the past couple of years, the snow from the top row will go first and actually push the snow off the next row leaving some accumulation on the bottom of the third row that has to actually melt off. Also since it is a house the loss of heat through the attic causes the snow to slide off faster then it might with a panel by itself in the open. I have noted that the snow seems to come off faster with the panels then it did with a slate roof that used to be present, but the data set is of course limited. having spent many winters in Tahoe region (my father had a house there and I liked to ski) I am familiar with the conditions there. The largest benefit will be the cold clear air for most of the year. Also for my own system, the highest one day power production was in December near solstice, it was a case of heavy rain at night, followed by cold and clear the next day. Even though the not the longest days of the year, the cleaned panels with the cold air produced enough to out produce the best I have gotten from any other time. For trivia my panels are fixed at nearly perfect for maximum peak at equinox at my latitude. Just happened my roof was that way. And the home is in a historic district(built in 1915), which is why it leaks like a sieve; because I am some-what restricted on energy efficiencies to the home by its design and historic district status. but it helps clear the panels faster. I also note the panels do seem to keep the house cooler in the summer, but have no clue what the savings from reduced A/C is; minor in any case. Just a data point.

Gary Best
June 19, 2012

I question whether your methodology takes into account the relatively little amount of production yielded by solar panels during the winter compared to the rest of the year in more northern locations. In New England, for instance, we had a very mild winter with only one day of production lost to snow cover. Yet my panels only produced about 16% of their annual production during mid-December through mid-March according to my May 2012 blog post on output located at http://masssolarinfo.com/wordpress/2012/06/02/monthly-solar-production-may-2012/ and projected throughout the rest of the year. At 40-60% loss, I would show roughly an 8% loss in annual production, not 12-18%. So while I think your formula may be applicable to locations with latitudes similar with Tahoe, it would need to include a reduction factor for more northern locations. Thoughts?

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