Posted by: nik | January 28, 2013

house cleaning

I’m not going to lie to you, our house is pretty cluttered. Before we have company over we always have to spend the better part of day putting things away, dusting, scrubbing, mopping, etc., etc. It’s not that I don’t like a clean house. I always feel much more comfortable when things are tidy, but I guess it’s just a matter of priorities. But there is one type of house cleaning that I get really excited about. Cleaning off the top of our house! Why? Because that’s where we keep our power plant!

It’s snowing today but this past week saw a string of five brilliantly sunny days in a row. If you don’t live in the northeast, you might not be able to appreciate the significance here, but take my word for it, that’s a big deal. Sun in January in Vermont goes a long way, especially if you get your energy from the sun. And I get both my psychic energy and my electric energy from the sun.

The forecast for Saturday called for cold temperatures and lots of sun–ideal conditions for solar power production! It might be counter-intuitive but solar electric panels actually operate more efficiently when it is very cold out. The output voltage increases as the cell temperature decreases.  This is a result of the semiconductor effect and the same reason we have cooling fans blowing across the processor chips in our desktop and laptop computers. The days are much shorter this time of year and sunny days are few and far between, but we will often see the highest power readings during the cold months of January and February. This is primarily a function of low moisture content in the air, but also the temperature coefficient of the solar cell can have a significant effect.

So I woke up Saturday morning to a bright sunny day looking forward to spinning our electric meter backwards, a rare occasion this time of year. I looked out the window and was discouraged to see that we had received a light dusting of snow Friday night, only about an inch or so but that’s plenty to totally shut down the power plant on the roof. Not to worry though, Li Ling the Fearless, was on the job!

House cleaning!

Li Ling clearing the snow off our solar panels with a plastic roof rake. 8:40 AM

She got out the extension ladder and the telescoping roof rake before I even had my long johns on. The plastic roof rakes does a decent job but, as you can see in the photos, it’s not 100%. There are little bolts that stick up in between each solar module. The roof rake catches on these and you have to maneuver around them leaving some snow behind. When it’s warm outside (above freezing) the snow will just melt off, no problem. But remember, the forecast was for cold, damn cold!

Here’s the photo our daughter posted on facebook earlier in the week:

cold, cold, cold, cold, cold

Screen capture from the national weather service website. All temperatures are in Fahrenheit.

So I wasn’t sure if the modules would warm up enough throughout the day to melt the small amount of snow that was left behind. This was a bit of an experiment so I decided to take photos every half hour until the panels were totally clear.

Here they are at 9:00 AM (immediately after raking is complete.)

The roof at 9:00 AM

The roof at 9:00 AM, 0 degrees F, 3000 Watts

According to our online data monitor, at this point our solar system was producing 3,000 watts (30% of the peak capacity.) Keep in mind that this is still very early in the morning (for a Saturday at least) and the sun is only glancing across the front of the modules from low on the horizon.

So the question remained, could we recover in these frigid temperatures?

I took some garbage to the dump. Got another coffee and warmed up by the woodstove. I went back out at 9:30 and little had changed. Here’s the photo.

By 10AM there was a noticeable amount of melting happening and by 10:49AM the snow had totally melted. Here’s the photo:

Clear roof! 10:49AM, -2 degrees, 7kW

Clear roof! 10:49AM, 8 Degrees Above Zero, 7,000 Watts!

The high temperature of the day was only 14 degrees Fahrenheit (-10 C for those of you in the developed world.) But even though the air temperature never got above freezing, the solar panels warmed up enough in the January sun to melt the snow off by 11am. We generated over 31 kilowatt-hours for the day! That’s about 200% of what we normally use. The meter was definitely spinning backwards. I would call it a wildly successful experiment and a very rewarding one too. This is the type of housecleaning that really pays off!

Solar power output on left scale, temperatures on right

January 26, 2013; Solar power output in Watts on left scale, temperatures on right


Responses

  1. Cool, I mean HOT, experiment! Not only did you get warmed up raking (or rather Li Ling did) but you also raked in the energy savings. Su-WEET deal. 🙂 -Amanda

  2. This is both interesting and enlightening (so to speak): “It might be counter-intuitive but solar electric panels actually operate more efficiently when it is very cold out. The output voltage increases as the cell temperature decreases. This is a result of the semiconductor effect and the same reason we have cooling fans blowing across the processor chips in our desktop and laptop computers. The days are much shorter this time of year and sunny days are few and far between, but we will often see the highest power readings during the cold months of January and February. This is primarily a function of low moisture content in the air, but also the temperature coefficient of the solar cell can have a significant effect.”

    By the way, at an Ace Hardware store I found a professional grade telescoping pole (6′ – 12′) for our 24″ soft rubber squeegee. Do you worry that your plastic roof rake will scratch your panels?

  3. […] to understand. The solar photovoltaic process has a significant temperature component. As I have described before, The typical solar cell is a silicon semiconductor, just like the CPU chips in our computers, and […]


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