Solar Electricity

In early February, my long awaited grid connect PV solar panels were installed by the Solar Shop. Installation took only about half a day. The frames were cut on site, and an electrician wired up the inverter and by 2pm I was getting free electricity from the sun!

Here are the guys at work:

Here is a picture of the installed PV system. In the foreground is my ancient (but now effective) evaporative air conditioner. In the middle left you can see two paler panels – they are the solar hot water system, which is also working great. Free hot water, and my gas bill has been decimated.

In South Australia we have a big rebate for Solar PV installation (about $8k), so the system cost me $20k – $8k = $12k installed. The $8k rebate holds for smaller systems, for example a 1kW system is $13k – $8k = around $5k installed. However I have recently heard of some damn good deals – like 1kW systems for $2-3k (after the rebate) when people get together and group buy.

Our PV system has now been running for 3 months. My wife and I love it. Our state (South Australia) is about to introduce a new feed in tariff, where they pay us twice the going rate for energy we feed into the grid. With this new tarrif I estimate our Electricity bills will soon be about $0. They were approximately $2,000/year 12 months ago, as explained in my post on halving my electricity and gas bills.

However it’s not just an issue of economics. It’s just plain smarter to be using renewable technology rather than relying on dirty, non renewable fossil fuels. Feels great too, making my own electricity. I mean, when you think about it, non-renewable means it won’t last. It will stop one day. Maybe soon in some parts of the world. Gas, oil, uranium, coal – all transient technology that will be history in my children’s life time.

Grid Connect Solar – How does it Work?

For a while I got to see my old school electricity meter spin backwards.

Then they replaced it with a new electronic export/import meter. During summer I was actually using about the same as I was making over the course of a day, as the we are now drifting towards winter we are generating about half of our daily power.

The system is rated at 2kW but the peak I see on the inverter panel is 1700W. Not sure why. The Solar Shop guys estimated about 9kWh/day average over the year, which seems about right. It was peaking at 13 kWh/day in February and is now down to about 8-9 on a sunny day, maybe 5kWh if it’s cloudy or raining.

The blue box is the inverter. It converts the roughly 300V DC from the PV panels to 240VAC that is synced up to the grid current. Now here is the clever bit. So clever that no one can tell me how it’s done. First it supplies power to our house. Any excess is exported to the grid. If the PV system is not making enough electricity, it imports the balance from the grid. So the grid is like a big battery.

The bit I don’t understand is how it actually makes this export/import happen. Let me explain with a simple electrical model. Imagine the grid as a big battery with a voltage Vg, and an impedance Zg. My PV system has a voltage Vs and impedance Zs. My house is the load impedance Zl, which is time varying.

Assume its a bright sunny day and my wife and kids aren’t home. The PV system is generating more electricity than the house is using. Lets say we are generating 1500W and the house is using 500W. Some how the PV system is supplying exactly 500W to the house and the balance (1000W) to the grid. Somehow the inverter must fiddle it’s voltage and impedance dynamically such that this happens. Thats the bit I don’t understand. For example if Vs and Vg are fixed at 240V, and Zs=Zg then the current supplied to the load will be shared by the solar system and the grid. But somehow the system fiddles it so that the house first uses 500W of the solar (zero W from the grid), then exports the balance back to the grid.

For current to flow back into the grid I guess Vs must be slightly greater than Vg. The inverter must fix it’s output phase to exactly match the grid. Perhaps it makes it’s output voltage a little higher than the grid to facilitate flow of current back into the grid. I dunno – can some one explain this to me please? I have asked all the solar and electricity company people but they can’t explain it to me.
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6 thoughts on “Solar Electricity”

  1. Hi David,
    congrats for you choice: I’m planning PV array as well.
    PV inverter is very tricky: I’m not a power conversion expert, so will not try to explain it: just google for ‘synchronous inverter’ and you’ll find out all the gory details. It’s quite a complicated piece of electronic, and is usually DSP driven: basically the DC from PV array is converted to AC in a DSP-controlled way, driving voltage in a way to keep PV array always on an optimal load (that one that maximizes PV power production), while keeping in sync with grid voltage in order to make the exchange possible. AFAIK PV array is connected straight to the grid and the house is connected to the grid as well: balancing is, so, automatic.

  2. Hi David
    I read your ATA mag article with interest. Am curious about the Pilkington Comfort Glass detail. P are now in Oz as http://www.viridianglass.com and show a huge array of building glass options. I’m sifting through their tech stuff to find the PCG equivalent. As a former (retired) renovations builder but doing some consulting for friends (building technology) the matter of alts to high cost double glazing keep coming up so a mid way is interesting. Any leads?
    Lucky you in SA with feed in. Our stupid Vic Gov has gone for nett not gross, and worse yet, capped an array at 2000W. Our 2006 installed system at 2004W puts us at 5W above the cap. Madness. And the Feds $100K income cap is equally brainless.
    I have been a cycling advocate for more than 40 years, involved with rail trails (cf http://www.railtrails.org.au) and lately on the East Gippsland council’s Environmental Sustainability Advisory Board – working to get more PVC and other (e.g. transport, buildings) improvements. Member of ATA. Regards
    Michael

  3. Hi Michael,

    Thanks for your comments – thats a shame about the Vic Government and I agree completely on the 100k means test for PV. The government just doesn’t get it. Yet.

    I have some other friends in Vic in the same situation. It’s recently got even better here in SA – the export rate is now 66 cents/kWH. Quite remarkable – it means that a PV array actually earns around 8% on the capital cost.

    The quote I have for the glass says it’s “6.38 Neutral Comfort Plus”. The Neutral refers to the tinting – you can get darker tints for windows that receive direct light, we opted for Neutral to increase the light into the house in Winter.

    Cheers,

    David

  4. Interesting but I calculate payback to be 6 years after the rebate. And that’s in sunny Australia. Outside of the American Southwest, solar panels are still not cost-effective in North America.
    However, solar thermal collectors ARE cost-effective in many parts of the US. My neighbor across the street has one installed.
    Solar thermal collectors are usually cheaper than solar panels because of lower manufacturing cost and cost of materials. Again, at least in North America, don’t know about Australia.
    Cheers. Love reading the blog by the way

  5. David,
    Fascinating blog! I am just getting into embedded systems and currently work for a company that makes battery-based inverters. Your analysis of a gyrator was what got me here in the first place and I decided to poke around.

    As far as the inverter is concerned, it behaves like a current source – the MPPT (maximum power point tracking) algorithm finds the sweet spot on the solar panel V-I curve for the given operating conditions, the inverter converts just that much power and sources it to the grid. This is accomplished by controlling the (AC) current, not the voltage, as voltage THD can vary with loads, distribution transformer size, grid impedance etc.

    Grid tie inverters such as yours attempt to have a 1.0 pf relative to the grid waveform – i.e. Iout = k*Vout. This is mostly because the inverter cannot process reactive power efficiently – that is the grid’s job.

    The easiest way to think of it is that whatever power is not used in your home, ends up on the grid.

    Hope this answers your question,
    John

  6. Hi John,

    Thank you – that is an excellent explanation. Thinking of the inverter as a current source is the paradigm shift I needed!

    Cheers,

    David

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