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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Rich of Observationz, in reply to
I think the problem with islanding is not so much worker safety (as you state, electricity network techs are trained to treat all circuits as live at all times) as problems of restart and load.
If your local substation trips, then a small number of inverters will try and drive all the local loads – which they obviously can’t do, so they’ll go into a trip/restart cycle, which might break stuff. Then, if an island does get established (middle of the night, most loads switched off) it’ll be out of sync with the grid, so on restart, everything will trip (possibly accompanied by blown pole fuses) again.
I guess there’d be ways to deal with this, like locking inverter phase to a GPS time reference, but it isn’t implemented now (and of course, since we import all such things, NZ can’t take a lead here).
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linger, in reply to
zero-energy iceblocks in winter
Pretty sure that method won’t lead to any major energy saving: either you’re pumping liquid water, in which case the energy needed to then actually freeze it is of the same order as that needed to cool it from 25°C to 0°C, or else you’re trying to pump semi-frozen slush, which takes more energy.
If you have subzero nocturnal temperatures, surely you’d be somewhat better off just leaving a tray of water out overnight to freeze, then bringing it in next morning? -
True as, forgot latent heat. But yes, making the ice blocks in the day and leaving them out at night would work in many climates.
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David Haywood, in reply to
True as, forgot latent heat. But yes, making the ice blocks in the day and leaving them out at night would work in many climates.
This was a genuine trick employed for icecube production in colonial India (apparently it was one of the big spreaders of typhoid amongst the elite).
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Moz, in reply to
If your local substation trips, then a small number of inverters will try and drive all the local loads
No, no, no, that would be insane and dangerous. I can't imagine anyone certifying an inverter that does what you're suggesting. The idea is that you mechanically switch so you're isolated from the grid and the "island" is your house. Modern inverters all electronically switch, but to get an install signed off you need a separate mechanical switch.
The mechanical switch is so that electricity and emergency workers can see easily that this is the case, and smite you with fire if you bypass that and back-feed a dead grid (which probably wouldn't work, but you still don't want to be the one holding the wire when it happens).
My somewhat simpler plan is to use a UPS-style system: grid feeds house, UPS plugs into wall and charges battery, solar also feeds UPS/batteries, I plug stuff into the UPS. Most small off grid systems have a generator input what can also run off a single phase socket, to look at it another way. But there's a couple of companies in Oz doing UPS-sytyle systems for on-grid urbanists like me. Largely as a regulatory hack - in OZ it's almost impossible to get any feed-in payment at all if you have batteries, and almost as impossible to be allowed to buy off-peak power if you have batteries (yes, they are that dumb). But a plug-in system doesn't count (otherwise even a battery backed alarm clock would trigger their "no grid connected battery systems" clause)
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Paul Campbell, in reply to
This was a genuine trick employed for icecube production in colonial India (apparently it was one of the big spreaders of typhoid amongst the elite).
I believe the Egyptian Pharaohs got their ice the same way (maybe even their typhoid ...)
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Alfie, in reply to
If your local substation trips, then a small number of inverters will try and drive all the local loads – which they obviously can’t do, so they’ll go into a trip/restart cycle, which might break stuff.
As Moz says, that ain't gonna happen. Our solar system is grid-tied and while we have manual isolation switches with bloody great warning signs, an essential part of the sign-off is automatic shutdown whenever the grid fails. We had a six hour (planned) outage last week and our solar simply shuts down until it detects live power from the mains. Then it boots up and resyncs again.
What Moz is discussing is actually quite cute -- a dual system with mains power wired seperately from an off-grid solar setup charging a UPS. That would also provide a way of getting around the lines companies stupid "solar tax" which the Commerce Commission seems set to approve.
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Rich of Observationz, in reply to
There has been interest in the electricity profession in enabling islanding (which normally refers to multiple connection points rather than a single house) - e.g. this paper: http://vbn.aau.dk/files/19638822/Publication in order to improve network resilience. I thought that was what you were about.
However, your idea, which is basically engineering your system to detach it from the grid when the grid fails is perfectly sound. For NZ, it's questionable if the cost of breakers and/or UPSs is justified by the frequency of power failures.
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Substituting battery storage with a large UPS and keeping it seperate from the mains supply was actually Moz's idea, and it's a damn good one. ;-)
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Ice has a rich literary heritage as well
who can forget the slush the Trask's trainload of lettuces became when their Ice-packed carriages got held up by snow then sidetracked in a Chicago heatwave ( East of Eden, John Steinbeck) ]]
or Fat Boy the ice machine in Paul Theroux's The Mosquito Coast
filmed by Peter Weir an acknowledged Ice Wrangler -
So, I was just talking about why does everybody not have hot-water-heating solar panels and my sister pointed out that our neighbour does. This is apparently what they look like!
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Disappointingly, Consumer says they're not great. Too costly for too little benefit.
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Moz, in reply to
One point to keep in mind is that most of NZ's population lives in Auckland, so there is a big difference between "most people" and "most land/most towns/most houses". If you restrict discussion of solar to "Auckland and points north" the numbers look much better. Not good, but better.
why does everybody not have hot-water-heating solar panels
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.
Now, some pointed remarks. The ideal angle is somewhere steeper than your latitude, so for NZ that means 45 degrees. Which doesn't really go with most rooflines. The one pictured will largely stop working at the start of winter, even on sunny days - it's just not going to collect enough sunlight to be useful. A more sensible setup won't really peak at all - it will heat all the water as hot as it's allowed to, for more than 70% of the year. But for most of NZ that means 2-5 square metres per hundred litres of tank which is ridiculously expensive.
You want to over-steepen, so the orientation is best in winter and in summer when you more collector than you need the orientation isn't ideal... but that doesn't matter, because 3x the area you need working at 80% of potential... is still 2.4x as much collector as you need.
There's a circular process at work too - they don't make sense, so there aren't many of them, so there are no skilled, experienced solar hot water system designers or installers, so most installs are sub-optimal, which means the economics are even worse than expected, so people don't install them,
In Sydney it's the other way round - there aren't a lot of heat pump hot water systems, the ones we have are mostly imported, few plumbers know much about them so it's hard to get one fitted, so few people install them. It's getting better, but it still is not great. But then, we also have the opposite "problem" to NZ - solar hot water systems work well, are cheaper than electric, are not powered by coal, and there are a lot of options from a lot of installers.
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Lilith __, in reply to
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.
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David Haywood, in reply to
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).
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David Haywood, in reply to
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.)
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David Haywood, in reply to
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.
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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.
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Moz, in reply to
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.
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Rob Stowell, in reply to
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.)
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Lilith __, in reply to
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. -
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.
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Rob Stowell, in reply to
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.)
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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.
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David Haywood, in reply to
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...
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