Hard News by Russell Brown

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Hard News: Going solar?

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  • Aidan,

    Bit late, but we've got a 30 tube/250L electrically boosted system. Here in Canberra (more like Christchurch weather, hotter in summer though) we don't need to boost Sept->Apr. We've got a timer switch in the fusebox for the electric boost, and it comes on from 5.00-6.20pm, and it will top it up if we've had a poor solar day.

    We have hard frosts, so the tubes make much more sense here. People on various enviro forums reckon you get a much bigger surface area for panel systems for the same cost, so if frost isn't an issue this is a better way to go. Also the integrated thermosiphon jobs are very well understood, very low tech (I've been quoted $1000 to replace the controller on our system, which is outrageous. I wouldn't pay that, but it gives you an idea of what some are paying). Don't write off flat panel systems, but I don't know enough to recommend any.

    Your water use patterns matter a lot. If you all tend to shower at the same time of day it helps. We scrub the kids in the evening, but the adults are morning showerers. This means the tank is half full of cold water overnight. It is supposed to stratify and not mix too much, but it still has an effect.

    We tilted our tubes up to 50deg, as we have an over-production in summer, but our shoulder and winter production was disappointing. There was a substantial improvement after tilting, with the added bonus of less over-heating and venting in summer. A wetback + solar system would be a great combination, as a solar system sized for summer will struggle in winter. Sized for winter it is likely to be expensive and still struggle on very cold dull days.

    We average about 5kWh/day electricity usage in Summer, 8 kWh/day in winter.

    We also have a 1.5kW PV system. This produces about 2/3 of our power, but as we got in when there was a generous FiT, we don't pay power bills, they pay us.

    CAUTION: PV does not like shade! If there is ANY shade on the panels your output will be substantially reduced. The cells turn into resistors when shaded, so well designed panels have by-pass diodes that, as you may have guessed from the name, bypass that section of the panel. All it needs is a small part of a panel to be shaded to bypass a much larger area of panel: there is a limit to the number of bypass diodes. Upshot of this is you can have a quarter of your panels shaded, but get a 75% reduction in output. It depends very strongly on the geometry of the panels and how the shade interacts with that geometry.

    If you have significant shading from 10am-3pm I think PV would be a mistake.

    I understand thin-film PV suffers less from shading (and possible also better in overcast conditions), but I haven't done any significant research into that.

    Canberra, Australia • Since Feb 2007 • 154 posts Report Reply

  • Lucy Telfar Barnard,

    So, I had a reply from the local distributor I was hoping might pick up importing Dovre. They said "The testing process for the last fire we tested cost closer to $50,000 and the manufacturer will not test any more unless we order a minimum of100 fires . The NZ standards are much more stringent than the European and US testing.
    Sorry but it is not worth it." I don't blame them in the slightest. The retail price overseas for the Dovre Vintage 30 seems to be about NZ$3,000 at discount, but I don't know what the markup would be. Still, 100 fires would be $300K worth of stock to shift.

    So looks like I might be in the market for a locally produced funky model too. In some places, where open fires are still allowed, you could avoid the whole testing regime by doing open fires (like some of those uber-cool focus ones).

    Otherwise, yeah, it'd be the testing that would be a cow.

    Incidentally, some years back, I asked Pyroclassic if they had any plans to make woodburners that were a bit more ... aesthetically pleasing. At the time they responded saying no, and that they thought it was a "classic design" or something equally blind ( :-P ), though I see that since then they have introduced colour options for the surround.

    Wellington • Since Nov 2006 • 585 posts Report Reply

  • James Green,

    So I've come up with my design for a one household experiment on whether a drain water heat recovery system is worth it. Will update once I have some results
    http://distractedscientist.wordpress.com/2014/07/25/drain-water-heat-recovery-weve-ordered-one/

    Dunedin • Since Nov 2006 • 703 posts Report Reply

  • Aidan,

    Another thing to carefully consider is the size of the water tank you attach to your panels. Often people are advised to get a larger tank as it can store hot water for longer, potentially covering for a cloudy day or two. The danger of too large a tank is twofold: not being able to heat the large mass to a high enough temperature, and having to boost a large volume of water on cold winter days.

    A properly designed tank can have the booster heating element placed higher up, so when it does boost it is only heating a smaller volume of water. Again, it relies on stratification.

    I believe that newly installed electric boost systems here in Canberra are no longer allowed to turn off their booster, as they are paranoid about legionella. This is unfortunate, as it will be a great deal less efficient, harvesting much less solar energy if the tank is always maintained at 60 degC. Not sure what the regs are in NZ.

    It might also be worth it to lag all your hot water piping while you’re redoing things. Especially if the new system is located further away with longer runs of piping. Make sure they use the plastic coated pipe lagging for exterior work exposed to UV. The other stuff breaks down very quickly in sunlight.

    We also had to have a tempering valve in our setup, which we had never had before. Again, new regulations. This drops the outlet temperature to 50 degC to prevent burns, but it also meant we had lukewarm water in the kitchen in winter, until we lagged the pipes.

    Canberra, Australia • Since Feb 2007 • 154 posts Report Reply

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