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Issue #211 February 2008
Intelligent Energy Solutions
Solar-Powering the Circuit Cellar
Part 3: Wiring & Electronics
by Steve Ciarcia
Start | Back to the Solar Panels | Wiring and Inverters | It's All About Architecture | Maximum Power Point Tracking | So, Does It Work? | Conservation | Soaking Up Some Photons | Sources & PDF
SO, DOES IT WORK?
As I write this, the PV system has been operational for three weeks. I haven’t seen any 10,760-W output, but that’s primarily because the roof array pitch is maximized for the middle of the summer and not mid-fall. Ignoring edge-of-cloud effects, I did see 10,000 W being produced one afternoon, but it has typically been about 8,000 to 9,000 W and I have been racking up a bunch of kWh credits on the meter. (I was intrigued to see the system even generating 1,500 W during a moderate rainstorm last week. I guess a lot more light filters through the clouds than I realized.) Because I haven’t been using a lot of air conditioning this late in the fall, I expect that my net-metered electric bill for October could be $0, with perhaps even some additional solar credit toward next month. Of course, next July might be a whole different story. ;-)
Unfortunately, because this has been a work in progress, I haven’t had the usual six months to a year to design and implement elaborate solar monitoring software that seems part of everyone else’s solar system article these days. The Xantrex inverters have an RS-232 data port that enables connection to a computer for analysis and a CAN bus that connects to an LCD for local monitoring. The only display software I’ve seen thus far is user-supplied and not very user-friendly. While I was able to configure two GridConnect NETDIO serial-to-Ethernet converters into a “tunneling RS-232” arrangement that at least allowed this software to run on a computer located farther away than a 50¢ RS-232 cable, I discarded it for a more innovative kludge. I simply mounted a web cam in front of the Xantrex LCD monitor and called the power display up as a web page. I had to resort to these methods because I wanted immediate display solutions, but fortunately, Xantrex has a better answer for the long run. In the first quarter of 2008, Xantrex will have a TCP/IP Ethernet/wireless “Gateway” that connects to the CAN bus and displays both real-time and archive recordings directly. The next time I present an article about my system, I should be showing this new device installed.
Still, seeing what the solar system is producing in real time is only half the story. How do I know whether I am producing more power than I am using or just how bad is it when I turn on the 5-ton air conditioner in the solarium? Obviously, I can wait and rely on the power company to tell me there’s a kilowatt-hour surplus or deficit on my monthly electric bill (nah!), or I can close the loop myself. The only way to know whether I have a surplus is to monitor the power consumed by the entire house in real time and compare it to the amount being generated at the same time by the solar system.
The classical method for monitoring AC power is to put current transformers around the AC wires in question, connect the core secondary to a true RMS converter, and then write a bunch of software to display and plot the results. Great idea, but I was a bit limited on time for building all my own circuitry. Instead I found an almost perfect off-the-shelf solution from PICO Technology. Using an EL040 current monitor with three split-core current clamps (rated at 0.1 to 300 A AC) and an EL005 RS-232 output data logger, I am able to record the current on each side of the 240-VAC line feeding the main AC distribution panel plus the 240 VAC output from the inverters. The PICO data logger comes with PC software called EnviroMon that graphs and displays the three readings. The data logger can hold about 60 days of recorded readings when taken once per minute. Photo 6 illustrates a recent daily recording of two data points. The blue trace shows the AC current feeding the main distribution panel (the total house load) and the red trace is the solar system output (the current contribution from the solar system). The power difference in magnitude between the blue and red traces represents the power being redirected to the grid (red higher than blue) or consumed from the grid (red less than blue).
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| Photo 6—This screenshot illustrates a recent daily recording of two data points. The power difference in magnitude between the blue and red traces represents the power being redirected to the grid (red higher than blue) or consumed from the grid (red less than blue). |
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