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DayCreek Journal

May 30 , 2004

A Kid in a Candy Store

Click on the above image for a better view and description of the solar heating and solar electric systems.

It's hard to describe the euphoric mood that instills you when you connect all the wires to a solar electric system and it actually works. Such was the mood at about 1PM on Monday when I completed the wiring of the first inverter and saw the inverter power up for the first time. Sky conditions were less than stellar with mostly cloudy skies, but nonetheless the panels were producing power. I was so excited I had to make a few phone calls to share the news with Jo and a few friends. It was the climax to lots of hard work and perseverance.

But the euphoria was short lived with plenty of work still ahead of me. I had two other racks to wire up and plenty of other testing to do, but the fact that the first array was doing its thing was quite encouraging. By the end of the day, all of the panels were wired to the inverters and I was amazed that there was 500 watts of combined power coming from the solar panels. This is just a fraction of what the system can produce, but considering there was no visible sun and darkening skies the panels were producing enough power to run a clothes washer. Although this may not seem very exciting, it most certainly was for me. It was nice to see the project finally coming together.

Changing gears just a bit, Tuesday was dedicated to hooking up the data feeds from the inverters. Xantrex never produced their own software to monitor the inverters from a PC, but thanks to Henry Cutler who worked extensively with Xantrex to redesign the Suntie inverter, I was able to download a copy of Henry's software.

Each inverter has an RS-232 port that can be used to retrieve data from the inverter, but the connection between the inverter must be optically isolated in order to protect the PC from electrical damage. In order to achieve this, there is a device called an opto-isolator that through a series of LED's and optical light sensors converts the electrical signal to light and then back to an electrical signal. There's no electrical connection, but yet the signal makes it from one end of the wire to the other.

Everything went smoothly except for one bad power adapter which was promptly shipped to me the next day. By Tuesday afternoon, my PC was recording data from the inverters. The statistics that I was gathering pretty much was what I was expecting—an efficiency of approximately 75%.

So why is the efficiency only about 75%?

Solar panels are rated under ideal test scenarios that are quite unrealistic. The manufacturers do at least test them under standardized test conditions, so you can compare panels "apples to apples." Unfortunately, we do not live in a test facility. Panels typically vary +/- 5% from their stated power, while panel temperature, dust, wire runs, and inverter efficiencies degrade the overall performance of the system.

Here's a summary of what to expect:

Environmental Condition Derating Factor
Module Production Tolerance
.95
Temperature
.89
Dirt and Dust
.93
Module Mismatch
.98
Wiring Loss
.97
Inverter Conversions (DC to AC)
.90
 
Total Derating Factor
.67

Considering I am seeing a derating factor of .75, I guess I should feel lucky! To put this in real numbers, our solar electric system is rated at 4.2 kW, but in reality the maximum AC output should be in the neighborhood of 3.15kW. This means that under sunny, clear skies during peak hours, I should be producing just over 3 kW per hour.

If you study the above chart a bit closer, the largest derating factor is temperature. Photovoltaic panels do not like heat. Unfortunately, there's lots of heat radiating off of the panels when in full sunlight, but with the panels ground mounted instead of roof mounted, they should stay a bit cooler. On the other side of the equation, when temperatures are below zero (F) during the winter months, the panels can actually produce more power than their stated values.

All and all, I hope to produce somewhere in the neighborhood of 3,500 to 4,000 kW's of clean energy per year. This means that 5,250 to 6,000 pounds of carbon dioxide will NOT be put into the atmosphere per year.

Back to our story...

Is it possible to Asian Beetle-proof the inverters? I doubt it, but I will try.

Wednesday I worked on building a bug-proof enclosure for the inverters and patiently waiting for the electrical inspector to show. He said that he would be there between 12:00 and 6:00 PM. He arrived at 5:58 PM—Cutting it rather close, but keeping his promise nonetheless.

I have had the pleasure of working with our local inspector a number of times in the past since he has had to inspect my house electrical work as I slooooowly build the house. He has always been fair and willing to give me a few tips here and there.

This for sure was a new area for him. He had never seen a solar electric system before, so I gave him the grand tour. All and all he seemed pleased with my work and there was only one (so far) problem that he said had to be corrected. It was running multiple ground wires to one ground rod. This is a no, no in the eyes of the NEC and he said that I needed to correct it. The proper way is to ground a single ground wire from the ground rod and attach the wires to a "pig tail" using a split bolt.

I told him that I would get that fixed ASAP and he said that he would return next week. He wanted to read up on chapter 690 of the NEC. Chapter 690 is devoted to solar electric systems and was one of the requirements for the Minnesota Solar Rebate program. I have that chapter and hopefully, I have read it correctly. If so, we should have our electrical inspection completed by Wednesday of next week. Once he approves the system, the next step will be the installation of the digital meter from our electric utility and signing of the state-wide contract. Until these approvals are completed, I am unable to sell power back to the utility and I cannot collect the Minnesota rebate. Let's hope the rest goes as smoothly as the installation.

This coming week is a short week for me, but I do plan to start work on writing the software necessary to put our solar electric system "live" on the internet. Don't expect anything for a while yet, but at some point during the summer I hope to be bringing to Daycreek hourly statistics.

Finally, don't forget The Midwest Renewable Energy Fair is June 18-20. I will be giving a workshop about our house on Friday, June 18th at 4:00PM. Rob Roy will also be giving a number of workshops on cordwood construction at the fair. Hope to see you there! If you would like to learn more, click here.

9 out of 10 Orioles prefer Welch's Grape Jelly. So the next time you're in a "jam", buy Welch's Grape Jelly— It's for the birds!