Domestic solar energy comes in two forms: electricity from collected light and heat from collected light. One excites electrons until they warm up, die other pops them down a wire. You use those electrons arriving at the other end of your wire to excite others until they warm up. Bodi methods will heat a hot water cylinder.
Whichever way you collect solar energy, you need to store it until it’s wanted. In a passive-solar house the store is the thermal-mass which holds the excited electrons in an insulated floor or interior walls. With solar-electric panels — which I’ll hereafter refer to as PV (photo-voltaic)
— it has traditionally been about storing electrons in a battery bank.
But nobody has had much luck turning battery-stored energy back into heat. You either have to have a lot of batteries which cost you, or you have to dip deep into their charge which shortens their life and also costs you. With solar-heated water, it has
You’ve got a normal house, connected to the grid and with a conventional hot-water cylinder. You want to be less vulnerable to rising power prices and have been told that hot water is the best place to start. There is the evacuated-tube system that will heat the water in your cylinder, but you’ve heard the price of PV panels has plummeted. Can you use them to directly heat your water? Have they become cheaper than solar/ liquid water-heating systems?
The short answer is yes, the times they are a’changin. Michael Lawley has lived off-grid for more than 15 years and runs Ecolnnovation which specialises in renewable energy products and installations.
“SI0,000 spent 011 solar PV on your roof will give you more usable energy than SHW can for the same cost. It will not freeze or boil and will be more reliable than SHW as there are no moving parts (other than
The price of PVhas dropped below what was once seen as the grid-parity cross-over figure of $3 a Watt. I’ve seen it as low as SI a Watt, and heard it said that if you imported a container-load you’d be down to 65 cents a Watt. China is being accused of‘dumping’ panels in Europe (they have retaliated by complaining about French wine being ‘dumped’ in China!) but any reduction in European consumption must be further good news for NZ buyers.
Other advantages of PV panels in terms of hot water are that they don’t leak, don’t need pumps, don’t freeze, and can be placed anywhere.
Alternatively, the advantages of the various systems which use the sun to heat the water itself (either directly or by heat-transfer as the evacuated tubes do) include more tolerance to being not perfectly angled to the sun and that they take up less area and therefore less space on your roof. This is because PV panels have an efficiency of about 15%, whereas tubes claim «nearly 80%».
For the record, experts claim evacuated tubes work better in our colder southern climes than flat-plate water systems, but I’ve heard conflicting views as to whether evacuated tubes or PV works best on cloudy days. Let’s say that both claim to work in overcast conditions, but neither work nearly as well as on a sunny day.
SHOULD YOU BE GRID-TIED…
A grid-tied system means you don’t need to use on-site batteries. Any power you generate is fed into the grid and your ‘storage’ system becomes a bucketful of water at height, somewhere in Lake Benmore or Lake Karapiro. That comes back down the line when your hot water cylinder thermostat requests it. There are some inefficiencies in a system like that, but water-at-height is one of the simplest, most renewable, least polluting options.
If you are grid-connected, you can simply buy some PV panels, feed the power they generate back into the grid (remembering that this requires an installation and a deal with a generating company) and keep your existing grid-to-element connection. That may pad the pockets of those who make and sell the grid-tied set-up, but more importantly it leaves you at the mercy of the power suppliers. They essentially control the price they will ‘buy’ your power at and the price they will ‘sell’ it back to you. It’s not unlike a bank, making its profit from the margin between the borrowing and lending rates.
There was a period where there were so few folk involved that the quantities didn’t register and the power companies offered 1:1 rates, the exercise being pretty much a goodwill one. Don’t expect that to continue. Dare I say, don’t bank on it.
…OR HAVE AN UNTIED SYSTEM?
The good news is that you can now have PV-to-hot-water-storage economically and there’s no need to connect to the grid. There are dual elements you can buy — or you can have them made — which fit into a standard hot water cylinder fitting.
Some cylinders come with two elements, but typically one is at the bottom and the other is a booster at the top, but you really want two at the bottom (or more depending on how much PV you opt for). One of them continues doing your grid-connected business-as-usual, the other(s) is/are your solar input. At a basic level, you can just switch your ‘normal’ hot water off when the sun shines as you’ve simply chosen to provide your own power rather than buy it from a power company, and the systems are entirely separate. Further up the cost, convenience and complexity scale, you can also add in a controller to do the sunny-day switching for you.
Beyond that cost again is the grid-tied option.
Some of us off-grid enthusiasts get by, switching tilings in and out, watching our voltmeters, ammeters and thermometers, the weather and the season, but most folk will want more fit-and-forget levels of automation than that. Whether you go for electric or direct, control equipment will include thermostats, comparators, and charge controllers (or pump controllers for water-to-water heating). One of my suggested links — Navitron — serves as an example, but it’s hard to be objective in comparing which is better in this respect.
Largely it depends on how long your unit works before it fritzes itself. My only advice is that the simpler it is, the less ‘black box’ technology and the more serviceable your system is, the better.
At some point, depending on how much sunlight there has been, how much collector you have, how big your cylinder is and how hot it was when you started, you may reach that ‘hot-enough’ point, the same as when a battery is fully charged. One person I know just runs off a little hot water at that seldom-reached juncture, but other less down-home options include shading your PV or water panels (either manually or automatically), or the other option is to have an alternative receptacle for your excess hot water. Possibilities could include an under-floor tank, a second hot-water cylinder, a radiator system, jacuzzi…
We went for the second hot-water cylinder, with a manual tap which we open the moment the cylinder starts bubbling. If your system is grid-tied, you could sell it but don’t expect to retire 011 the proceeds.
You will need a back-up system, as in practical terms nobody can pre-store a dull fortnight’s worth of energy. A wetback is the perfect complement to solar; you’ll be more than likely using the fire on cold, gloomy or short days. Grid-tied folk (presuming there’s no power outages) can just fall back on Plan A, using it to make up any shortfall.
These can be a good option says Ben Stanton of S4 Solar.
“Using a grid tie inverter or battery system and a heat pump for water heating is a very efficient way to do it for high water use households.»
Fcolnnovation’s Michael Lawley concurs.
«I am off-grid and have two heat pumps — the low cost of PV means you can do things today that would have been crazy just a few years ago.»
The key is probably the ‘high water use’ comment as you are the only one who can choose your outlay, use and pay-back time.
The times are indeed a’ changing. Those ideas we’d all held about the price of PV have gone out the window, and it’s likely the process will continue. They have definitely become an economic option for water-heating.
PV may well turn out to be one of the better investments too. If the banks default or the markets crash, you’ll still be singing in the shower, knowing that a big part of your power bill has been paid-up years in advance.
Musings on the Good Life
You may remember I wrote an article (NZ Lifestyle Block, January’ 2010) about three young ladies who were planning on sailing around the world, one of the ultimate self-sufficiency adventures.
I’ve just been to a talk by one of them, Laura Dekker, and I wish you’d been there. You would have met an unpretentious, unselfconscious, completely honest 17-year-old who has piled more into her life thus far than many of us will do full-span.
She was born on her parent’s yacht while in New Zealand. Five years later, that voyage ended back in Holland and her parents subsequently separated.
Laura opted to live with her father, mostly on a larger vessel he is still restoring.
Early on she started to sail. At 10 she asked for — and was given — the use of a small keel-boat which she and her dog adventured in.
At 11, she bought a similar vessel using money earned from busking and newspaper rounds. Aged 12 she sailed it across the English Channel.
The story goes that the English police flew her father to France (she had apparently overlooked telling him where she was going) and he asked what she intended to do. «Sail back,» was the reply. «Well,” he said. «If you got here, presumably you can get back” and he flew home.
The English Channel between Holland and Lowestoft is all tides, sandbanks and traffic — no walk in the park, even for an old salt. She got back.
I looked around the audience at about that point, a packed house of competitive southern sailors, partners and offspring, with a century-long history. Lots of testosterone there and would-be winners every one, yet all I could see was humility and respect etched on every face.
By the time she’d matter-of-factly recounted her voyage, listeners proud of their achievements — and I freely admit to being one of them — had been simply, quietly outclassed. For the record, she sailed from Holland across the Atlantic to St Martinique in the Caribbean, through the Panama Canal, across the Pacific to the top of Australia, on to Cape Town and back up the Atlantic to St Martinique.
That ‘crossed her lines’, as she put it, but she then carried on through the canal and across the Pacific to New Zealand. One-and-a-half circumnavigations, most of the maintenance self-done and on a shoestring (she chose to shun major sponsorship).
Inevitably, someone asked her if she had made contact with Australian circumnavigator Jessica Watson. Dekker answered no, as she feels their adventures were «different”.
She’s right. Both are incomparable efforts compared with what most of us will ever do and that’s as far as we need delve.
I don’t think I’ll ever forget that talk. If I’m ever looking wearily at some project which looks dauntingly big (thinning half our trees being a looming example) I’ll conjure up the image of that slightly-built young lady who placed an Everest in front of herself and climbed it before she turned 17, and I’ll start sawing with a smile.
Back before she departed, I wrote my thoughts on her intentions.
«Be warned, a youngster who does something like this will never re-join the crowd, and you wouldn’t want them to.»
I’m not so sure that’s right in hindsight. It’s probably best to say «thanks for the inspiration», set our own goals, and wish her well whatever she does.