Posts by David Haywood
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Southerly: A Tale of Two Iceblocks: Part…, 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...
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Southerly: A Tale of Two Iceblocks: Part…, 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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Southerly: A Tale of Two Iceblocks: Part…, 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.)
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Southerly: A Tale of Two Iceblocks: Part…, 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).
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Very interesting indeed, David! Thank you for all your hard work in investigating this for us.
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Southerly: A Tale of Two Iceblocks: Part…, in reply to Rich of Observationz,
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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Southerly: A Tale of Two Iceblocks: Part…, in reply to Lilith __,
Could it ever be feasible for each house to have a thermal-solar system on the roof to heat the hot water?
Sorry Lilith – hasn’t realized you were asking about CSP (my earlier answer referred to solar thermal for hot water production)…
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Southerly: A Tale of Two Iceblocks: Part…, in reply to Shaun Barker,
In response to the question asked by Steven Crawford, above – the analysis of the net benefit (or otherwise) of solar power (photovoltaics) in NZ has been done – in it’s current form solar has little or negative GHG benefit in NZ if a lifecycle analysis is done (published by Luke Schwartfeger and Allan Miller, EEA Conference & Exhibition 2015, 24 – 26 June, Wellington, available here.
Thanks for posting that link, Shaun! Photovoltaics (grid-connected) don’t make much sense from an energy engineering perspective in New Zealand either (from the viewpoint of the whole country). I hadn’t seen that paper and must say I’m surprised that the difference is quite that much in terms of GHGs. I shall read it with interest.
EDIT: Steven Crawford’s quite right then – the electricity companies should be making a huge deal of this…
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Southerly: A Tale of Two Iceblocks: Part…, in reply to Lilith __,
How does that compare with solar thermal? I’m guessing the storage problem is the same, but are the manufacturing issues any better?
With solar thermal the energy is stored as hot water in your tank -- it's a whole different kettle of fish (a much better kettle) from an energy engineering perspective, and I would have thought (though I may be proven wrong) from a GHG perspective.
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Southerly: A Tale of Two Iceblocks: Part…, in reply to Mike O'Connell,
For the technically minded, the 3rd Edition of the New Zealand Energy Handbook is a good resource describing our country’s energy resources
Cheers, Mike -- I should have thought to mention that myself...