Southerly by David Haywood

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Southerly: A Tale of Two Iceblocks: Part 1 (Or How Analysis of Greenhouse Gas Emissions in New Zealand Can Cause Us To Do the Wrong Thing)

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  • Lilith __, in reply to Moz,

    Because almost all solar water heaters lose energy on cold cloudy days, and much of NZ’s land area has those for half the year.

    They shouldn't lose energy, any more than a wetback will when the fire's not on -- flow into the heater is controlled by valves.

    Consumer points to the high cost of the device as the main reason not to install one.

    Dunedin • Since Jul 2010 • 3887 posts Report Reply

  • David Haywood, in reply to Lilith __,

    Disappointingly, Consumer says they’re not great. Too costly for too little benefit.

    As Moz says, the issue with the consumer report (which cites other studies) is the “on average” bit.

    I know a bit about solar hot water systems: there is difficulty (and hence expense) in retrofitting them to existing houses as per Moz’s comment, but the problems with losing heat in cold weather (mostly due to the frost protection system during cold cloudless nights) aren’t really an issue with the new heat-pipe based designs. That said, it’s not really your latitude in NZ that’s important, but rather the number of sunshine hours at your location.

    Solar hot water heating can be economical (from an energy point of view) if fitted to a new house that’s designed with this in mind. As long as there's sufficient demand (e.g. from a young family), and depending whether or not night rates from the network (if any) can be exploited.

    The issues with appearance aren’t really relevant with a design such as the Thermocell unit, which is a flat-plate heat-pipe configuration that can be fitted invisibly into the roof cladding of a new house. Thermocell also had (when I last had anything to do with them) very skilled technicians at design and installation. (Disclaimer: the inventor (and former owner) of Thermocell was a member of the examination committee for my doctorate.)

    I’ve audited various households and recommended them not to go with solar hot water. On the other hand, I’ve also recommended solar hot water as viable to some people, including myself. The system we’ve got cost $4,999 to install and paid itself back (simple pay back period) in just over 4 years. We are now effectively getting free hot water to the value of over $1,000 per year (my kids use heaps).

    One of the problems with solar hot water is that the people for whom it would be cost effective (e.g. young families) can’t afford to have it installed. The people who can afford it (e.g. older couples whose children have left home) won’t find it economical because they don’t use much hot water. Furthermore the payback period (often around 7 years) is also the same as the period between house moves in NZ, i.e. your system just pays itself back and then you move and the next family get all the benefit.

    There’s a lot more to the subject of solar hot water, but I don’t have time to say it here , alas. It’s worth mentioning, however, that the “traditional” thermal approach is technology on the wane. A solar hot water system whereby PVs are used to store solar energy as hot water is ultimately the way things will go, as the installation (excluding the price of the PV cells) and maintenance are so much cheaper.

    Last time I did the numbers (a couple of years ago) this approach still wasn’t as good as traditional solar hot water heaters due to the PV costs. However I am aware of a power engineer who has built such a system for his own house using cheap Chinese PVs, no inverter, and his own system that “chops” DC direct into the element of his HWC (his “chopper” even incorporates MPPT). He assessed his payback period (excluding his own labour) at around two years.

    This PV-based solar hot water system would certainly get around the problems associated with grid-connected PV (in that supply and demand don’t match) from a lifetime energy perspective, but I don’t know what the story would be from a lifetime GHG point of view.

    EDIT: Obviously you can’t feed hot water into the electricity grid, rather you are storing solar energy as hot water to avoid using grid-sourced electricity for hot water heating (the electricity you avoid can be used to meet demand elsewhere in the grid).

    Dunsandel • Since Nov 2006 • 1156 posts Report Reply

  • David Haywood, in reply to David Haywood,

    This PV-based solar hot water system would certainly get around the problems associated with grid-connected PV (in that supply and demand don’t match)…

    To expand on that: the supply curve from a PV system is like the hump of a dromedary camel; the demand curve of the electricity network is like the humps on a bactrian camel. And these bactrian humps are ultimately the cause of most of the dirty energy generation. The reason grid-connected PV isn’t great (on average) in New Zealand is that you are trying to fit a dromedary camel into a bactrian-shaped hole.

    By storing the energy from your PVs in batteries or as hot water you are able to make your dromedary supply fit the bactrian demand. (See also: the edit on my previous comment).

    As with most things in life, it all comes down to humps!

    (Addendum: of course, there are also exergy-efficiency* issues with using electricity to produce hot water, but I don’t want to go there…

    * ‘Exergy’ isn’t a misspelling of ‘energy’, by the way.)

    Dunsandel • Since Nov 2006 • 1156 posts Report Reply

  • steven crawford,

    cheap Chinese PVs

    dunt, dant, da…

    I just arrived back home with a six year old vacuum tube solar hot water system. So I am very happy. In my case, it’s only the price of a return ticket for the Toyota hylux to sail across the cook strait. And I’ve noticed these setups are starting to show up on TradeMe for around a thousand dollars. It’s probably a good idea to check out the local water before buying a second hand one, just to make sure it’s not been collecting metals. I assume the calcium from concrete tanks and pipes isn’t as tricky to remedy.

    On the PV water heating, I’ve seen a “heat pump” hot water tank which looked stylish…

    Atlantis • Since Nov 2006 • 4311 posts Report Reply

  • David Haywood, in reply to steven crawford,

    I just arrived back home with a six year old vacuum tube solar hot water system.

    This will almost certainly be an example of the heat-pipe solar hot water that I mentioned earlier, Steven. I’ve heard good reports on some of the Chinese systems of this type.

    On the PV water heating, I’ve seen a “heat pump” hot water tank which looked stylish…

    I’ve designed and built one of these from scratch, but there can be issues with longevity. You really have to do your numbers, look at the temperature profile of your location, etc. One of the problems is that manufacturers quote performance based on dry air at the outdoor heat-exchanger (which it won’t be in New Zealand) and so their data doesn’t take account of the effects of frosting, etc.

    EDIT: I’m sure this is quite unnecessary to mention, but do use a header pipe on the hot water cylinder of your solar hot water system so that it’s vented to atmosphere. Be a pity to build a steam bomb rather than a water heater.

    Dunsandel • Since Nov 2006 • 1156 posts Report Reply

  • steven crawford, in reply to David Haywood,

    I’m sure this is quite unnecessary to mention, but do use a header pipe on the hot water cylinder of your solar hot water system so that it’s vented to atmosphere. Be a pity to build a steam bomb rather than a water heater.

    That’s a good thing to mention?

    Another one is to shade the heat pipes when there isn’t any water in the system, the seals get cooked.

    Atlantis • Since Nov 2006 • 4311 posts Report Reply

  • Alfie,

    When we were planning our PV install we looked at adding solar hot water. The wisdom these days is that you’re better off putting in a slightly larger inverter and adding a few more panels at around $220-250 each. Obviously that only applies when you’re already installing a PV system.

    Picking up on Moz’s comment about the ideal angle for panels, most installers take the easy route and go with the existing roof angle. If I ever get around to building my ideal little zero-energy, off-grid house, I intend to ground mount the solar panels, hinged at the base, with some small motors to lift the angle in winter. And I’d like to add a small windmill to the system, but that’s another story altogether.

    Dunedin • Since May 2014 • 1386 posts Report Reply

  • Moz, in reply to David Haywood,

    We are now effectively getting free hot water to the value of over $1,000 per year

    We use a total of about $200 of hot water a year, which drastically affects the value of any of these systems. At 8c/kWh off peak and 4-8kWh a day of consumption, it was hard to justify $50 worth of extra insulation for the tank, let alone $5000 for a heat pump one.

    We also have problems with the interaction between PV and hot water, in that you either run it off peak at 8c, or on peak at 30-80c with PV input, you're not allowed to have both off peak and local PV heating the water.

    For our granny flat I spent a while playing with numbers and talking to installers, and in Sydney it's much, much cheaper and easier to go solar thermal hot water with grid-tied PV to whatever level you can afford. The feed-in tariff in NSW is whatever the power company wants, which seems to be about 5c. So the margin you're making when comparing PV+heat pump to off peak resistive is only 3c/kWh... you have to be using a lot of hot water to pay for $5000 of capital at that rate. Or about $2000-$3000 for solar thermal. I decided that I'd rather pay for solar hot water mainly for green hardline and resilience reasons rather than the largely imaginary cost saving, and ditto the extra $2000-odd to get an inverter that will work without a grid present.

    An important caveat is that all those costs are valid at time or research only - they change, sometimes significantly, from day to day. Currency changes, new products or new versions of old ones, government whims, they all make life exciting.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • Rob Stowell, in reply to David Haywood,

    Thermocell also had (when I last had anything to do with them) very skilled technicians at design and installation.

    Nick Williamson (Arthur's son) is still involved - a great guy - but more in the manufacture. Solar City, who have taken over the sales are, are ... hopeless (in my experience.)

    Whakaraupo • Since Nov 2006 • 2091 posts Report Reply

  • Lilith __, in reply to David Haywood,

    EDIT: I’m sure this is quite unnecessary to mention, but do use a header pipe on the hot water cylinder of your solar hot water system so that it’s vented to atmosphere. Be a pity to build a steam bomb rather than a water heater.

    Gah!
    Another is not to do what a friend of mine did when the HWC started boiling – turn on all the hot taps to cool down cylinder. Waste pipes aren’t built to take boiling water. He had a long and smelly job under the house replacing the melted waste pipes.

    Dunedin • Since Jul 2010 • 3887 posts Report Reply

  • steven crawford, in reply to Lilith __,

    Another is not to do what a friend of mine did when the HWC started boiling – turn on all the hot taps to cool down cylinder.

    I hate when that happens. I would like a radiator system to use all that extra hot water on. It’s just the mater of getting pumps to circulat it, I'm told.

    Atlantis • Since Nov 2006 • 4311 posts Report Reply

  • Moz,

    Gizmodo Oz has a review of the Tesla Powerwall with a headline David might like "can save you money, but only in very specific circumstances". They also go into opportunity cost/time value of money at the end, which is nice.

    Some Oz solar thermal hot water systems used to dump the whole tank if the water got too hot. Usually late in the day, when there wasn't time to reheat. I believe due to the overheat valve being purely mechanical, and the reset time was long enough for empty all or most of the tank. These days that doesn't happen, but the heat pipe ones do need to be Oz-rated as some problems have been found with systems that fail if the collector gets much over 100 degrees... so they need active cooling on hot days. Not sure how they get round that.

    Using the excess heat inside the house is difficult, as you're most likely to have it on hot days, when you probably don't want extra heat in the house. But solar-hydronic systems are used here, where a solar-thermal HWS heats pipes in the floor or radiators.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • Rob Stowell, in reply to Moz,

    I looked at the specs on the Tesla powerwall. It’s a very expensive battery bank. Sadly lead acid still stores more watts for the dollar – although the specs weren’t that clear on how many cycles (they give a big range – to be fair, lead acid are like that too) and how low you could drain the powerwall. 8 kw hours isn't that much (we use about 7 most days.)

    Whakaraupo • Since Nov 2006 • 2091 posts Report Reply

  • Moz,

    Batteries are still not ready for mass market, that's for sure. But realistically, because they need care and feeding bigger ones with full-time staff are likely to be better. Same was as I have a driver take me to work when I don't ride - the days when even the biggest egos have their own train and drive it themselves are long gone.

    There's a subdivision in Perth with their own communal battery, and a local cohousing group have one budgeted but they're still selling off round three of the empty plots and getting ready to build the first new houses, so it'll be a while before that goes in.

    Meanwhile, I'm in inner-city Sydney wondering why my internet connection is so awful today. Ditto the different provider cellphone internet. First world problems... but it does remind me why I want to be grid-agnostic for electricity.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • David Haywood, in reply to Moz,

    with a headline David might like “can save you money, but only in very specific circumstances”.

    Oh dear, you're quite right. This should probably be written on my tombstone.

    Very impressed by your energy efficiency Moz and Rob. Post solar water heating we're down to about 14 kWh per day. A not insignificant chunk of this is due to the stupid wastewater system that our dictatorship (ECan) insisted upon. I've quite seriously considered going back to the composting lavatory system that we used for six months after the earthquakes -- hugely cheaper and you don't get shit all over the lawn several times per year.

    I won't seriously attempt to justify our hot water usage (by the sounds of it we use more in a month than you do in a year, Moz) but my kids just get so dirty. I've often had to strip them at the door, wipe off the worst of the mud, and then carry them into the shower. Both of them are so full of energy that a nighttime bath has always been helpful in calming them down. I fear that I do much of my parenting via hot water.

    My (small & homebuilt) PV system as well as my wind turbine have been sitting in the loft of my workshop since the earthquakes. Quite unforgivable, I'm afraid. My plan is to have the wastewater system switch between solar + wind and the grid depending on availability. Unfortunately I couldn't make this work without a small battery, and the cost of that sunk the whole thing. I've now figured out how to do it with cheap supercapacitors (I think) and my son Bob is keen to do the job for educational purposes. We'll see how he does...

    Dunsandel • Since Nov 2006 • 1156 posts Report Reply

  • Ian Dalziel, in reply to Moz,

    the body electrick…

    I’m in inner-city Sydney wondering why my internet connection is so awful today. Ditto the different provider cellphone internet. First world problems… but it does remind me why I want to be grid-agnostic for electricity.

    We (the planet) may still be in the tail end of a Geomagnetic Storm…
    see:
    http://spaceweather.com/

    Christchurch • Since Dec 2006 • 7889 posts Report Reply

  • Ian Dalziel, in reply to David Haywood,

    My (small & homebuilt) PV system as well as my wind turbine have been sitting in the loft of my workshop since the earthquakes. Quite unforgivable, I’m afraid.

    Totally forgivable - it's good to have 'lofty aspirations'...
    ;- )

    Christchurch • Since Dec 2006 • 7889 posts Report Reply

  • Alfie, in reply to Rob Stowell,

    Solar City, who have taken over the sales are, are … hopeless (in my experience.)

    I was speaking with a local Solar City installer last week who told me they've been instructed to flat mount all panels to save money. So instead of lifting the rear of panels by mounting them on a framework to achieve the optimum angle, they now have to go with the existing roof pitch. That's an extremely poor decision from one of our biggest solar players.

    Dunedin • Since May 2014 • 1386 posts Report Reply

  • Moz, in reply to Alfie,

    Solar City

    Over here that name is one used by a succession of phoenixing solar scammers, IIRC. www.whirlpool.net.au is a geek forum with a useful green sub-board for that stuff.

    Over here they just charge you if you want tilt rails, and most companies will do it if you ask. With the granny flat I was torn between an off-centre roof peak to get the north face to 50 degrees, and a single-piece roof with more panels at about 10 or 15 degrees. The cost difference meant it was cheaper to go one-piece, but the winter performance was noticeably worse. Still arguing with the architect (my partner) while the granny flat is on hold - we're probably moving instead.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • Moz, in reply to David Haywood,

    composting lavatory

    We have one in the shed, packed away, because I never really solved the vinegar fly problem to my partner's satisfaction, and with six people we really do need the second toilet working all the time. The "toilet room" is big enough to fit the composting toilet in front of the water-waster, but having it there makes the water one unusable. I really want to put squat rails on it, too, but have not yet got round to that. Maybe I should do that today rather than sitting on the interwebs.

    I've often had to strip them at the door, wipe off the worst of the mud, and then carry them into the shower.

    You're too kind. My mum used to hose us off on the lawn, then make us strip and go through the bootwash barefoot before we were allowed in to shower. We quickly learned that cold, windy days were a time for caution around mud.

    But the difference between six adults who all understand that we have 300 litres of hot water each day, the end, and two children who like to play with dirty things is huge. We also live in Sydney, where heating is basically optional and cooling is becoming less so. So a lot of our "efficiency" comes from living in a mild climate.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • Brent Jackson,

    When we had our solar hot water installed (an Edwards LX305 in 2004), I asked about increasing the angle of the panels, and was shown a convincing graph of the efficiency based on angle, and basically the efficiency does not fall off all that rapidly. Basically, if your panel is facing within 30degrees of north, and is tilted between 15 and 50 degrees you'll be getting over 95% of the optimal annual solar input for a fixed panel.

    Our return on investment calculation at the time, was that the system would never pay for itself (we'd make more money by investing the funds, and paying more for our electricity). We installed it anyway because we felt it was a good thing to do. As it turns out, investment returns have dropped, and electricity prices have increased, so it was probably a good investment. We turn the mains feed to the cylinder off at the start of November, and turn it back on at the end of April, so basically have free hot water all summer. Of course it is still helping the rest of the year, so our power bill is less than similar families (2 teenage children). We use 14kw/day around this time of year (electric blankets and heaters), and about 8kw/day in summer (really should get a more efficient fridge).

    When our dishwasher was installed, I asked for the plumber to connect the hot pipe to it, but he refused, saying it would invalidate the warranty. Of course the dish-washer is out of warranty now, but I don't think the $200 (or more) it'd cost to get it changed over now would be recouped, but it does annoy me that we have to unnecessarily heat water to run it in summer.

    Auckland • Since Nov 2006 • 614 posts Report Reply

  • Moz, in reply to Brent Jackson,

    Lost my reply, FFS.

    was shown a convincing graph of the efficiency based on angle

    Was that the misleading raw output one or the seasonal output graph that actually matters? Until you can save the excess hot water from summer to use in the winter, it doesn't matter how much excess you have in summer.

    This graph is probably what you were shown, and it's a class case of "technically correct but irrelevant and misleading". For PV it works because most people have a year-round surplus from their PV during weekdays, and don't store their PV output. So all they care about is maximising their annual output.

    With hot water that is very rarely true. Most systems have storage, and the cost of extra collectors is low compared to the overall system cost so it makes sense for most people to over-collect. That means the desirable system characteristic is to maximise output in the winter trough in order to reduce the need for boost heating.

    Especially for people who are switching from off peak resistive systems to on-peak boost (as in Australia and very probably NZ) because the boost cuts in in the evening when it's clear the solar has not got the water hot enough. A poorly set up system can end up increasing energy costs because while you're buying less total electricity, the electricity you do buy costs a lot more.

    This blog post amusingly manages to both quote the correct text, then immediately summarise it but get it backwards. The bold text is wrong :) Full pdf (2 pages) and http://solarpaneltilt.com/ has better graphs of how tilt affects seasonal output.

    Sydney, West Island • Since Nov 2006 • 1198 posts Report Reply

  • Brent Jackson, in reply to Moz,

    Thanks for that Moz. According to that we should have the panel at 33 degrees for maximum annual solar input with a fixed panel. But as you say that is not what we should be trying to maximise. What we want is sufficient hot water all summer, extended as far as we can for spring and autumn. Based on when we currently turn the power supply on (May to October) it appears that we need about 5kwh/m2/day, so setting to the winter tilt angle won't give us sufficient heating all summer. So, eye-balling the Adjusting the tilt four times a year table, probably 45 degrees would maximise electricity savings.

    However, this may be overly simplistic, as the turning on of the power booster is usually the result of 4 cloudy days in a row, which will occur much more often in the seasons other than summer.

    Especially for people who are switching from off peak resistive systems to on-peak boost (as in Australia and very probably NZ) because the boost cuts in in the evening when it’s clear the solar has not got the water hot enough.

    We don't have peak and off-peak power charges, and the booster is not timed, so as we mainly shower in the morning, the booster will heat the cold water being added to the tank, which would be unnecessary on a sunny day, as the sun itself can achieve it by the end of the day.

    But better to spend a little bit more than to suffer luke-warm showers in the colder months.

    Auckland • Since Nov 2006 • 614 posts Report Reply

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