After over 30 years use of solar HWS, and now a heatpump, I find reliability and longevity equally as important as efficiency. If those parameters are also applied than the initial cost is not such a big issue.

Gandini said:
What is "efficiency" and does it matter?

In this context i see efficiency as getting the most out of any one system, not comparing one to another. It's not so simple though, as different systems would have different efficiencies in different locations at different angles-finding the right system, angle etc for your location is the real trick, but some simple things like say correct tilt angles or maybe double glazing a flat plate with a few sheets of lazerlight could make a huge difference.

Gandini said:
Most people are really only interested in the total cost of a system (of whatever type and area) that meets their needs.

That is largely true, but then there are nuts like me who care little about initial price, and are primarily interested in minimising the amount of (fossil-fuelled) boost required each year. Ignoring initial purchase cost, this also corresponds to the lowest running cost per year.

For a nut like me demanding near-zero boost even in the middle of a cold Canberra winter, the optimum system is probably a conservatively sized evacuated tube system, highly tilted to optimize winter performance, and with a large (eg 315 liter) storage tank.

The 'best' solar HWS depends on the climate, and the priorities of the individual, which both vary greatly.

What is "efficiency" and does it matter?

If we define "efficiency" as the amount of hot water produced by a given area of tubes or plates, and if the less efficient system is less expensive in $/sq m, then it means that we need a greater area of the less efficient system to produce the same amount of hot water.

Most people are really only interested in the total cost of a system (of whatever type and area) that meets their needs.

Does any one know if there has ever been commercially available double glazed flat plates? It would be good to see a comparison with all three. Maybe (oh if i had more time) i should set up 2 small scale flat plates and (another) arduino and log the results over winter. As you may know i have double glazed panels but have no comparison. Surely evac tubes where just the next logical step on from double glazing, because they are just double glazed "panels" with a vacuum between the glass. It just so happens they need to be round to be strong enough to resist the vacuum.....

peter69_56 and photonthief..

Interesting reading for sure ..it is a proven fact that flat plates reach higher temperatures within high ambient temperature parameters but there heat loss is far greater as opposed to Etubes
i think to widen the discussion to get a more full and broadened comparison you must also consider the efficiency performances overall of the flat plate v Etubes over a time frame of full 52 weeks within variable climatic conditions...it is a well known fact that Etubes perform so much better in cooler climates and that flat plate generally are the poorer performer in this instance where as Etubes perform 95-98% as good as a flat plate in summer...so whose the winner

I wish someone had set up similar flat plate and evac tube systems and done a comparison. I have been lead to believe someone did in Northern Terrritory somewhere but am yet to see the data. In any regard they both work so its like buying a Holden, Ford, Toyota etc. If you are happy with your system then thats all that matters. The rest is just smoke and mirrors.

I have evac tubes and split system and am happy. I know others who have flate plate and happy. I also know others have either flat plate or evac tube with tank on roof and happy. As long as they work and you are satisfied with what you paid for. Sadly most who push one type highlight the good of their system and gloss over the bad or less than ideal issues. Not a fully honest view. people should tell the good, bad and ugly to have an honest review.

Bad installations can make good systems seem bad. However, you can never make a bad system seem good.

I am happy I have solar HWS and really dont care about the politics of it. In fact it would seem to me most who are extremely passionate of their flat plate or evac tube usually have a vested interest (they sell them).

Hi all,

I have read through the postings with interest. I won't get into a discussion about the relative performance of plat panel and evacuated tubes as there are too many variables, but will instead comment on the interesting question in it's own right of whether the cylindrical evacuated tubes (E-tubes) 'track' the sun, and whether this provides an advantage (all else equal) compared to flate plate collectors.

Assume for simplicity that the flat plate and Etubes are both horizontal. Also for simplicity, consider an example where the area of the plate is 1.0 sqm, and assume an intensity of sunlight of 1000W/sqm.

The analysis of the flat plate is straightforward and well known. If the sun is directly overhead, so that the rays are at perpendicular to the plate, then in this example the power per unit area falling on the plate is 1000W/sqm - everyone will agree on that. If the rays are inclined at an angle theta with respect to vertical, then the power per unit area is given by PxCos(theta). For example, if in the early morning or late afternoon the rays are inclined 60 degrees to the vertical, then the plate will receive 1000 x COS(60) = 500 W/sqm. All very simple, and completely cut and dried.

The case of cylindrical E-tubes is more subtle. Consider an example where each tube is 1.0m long, and 0.05m (50mm) in diameter, with the tubes mounted at 0.1m (100mm) centres. For a meaningful comparison with the flat plate, assume that the total area occupied by the tubes is 1.0 sqm so, as the tubes are mounted at a 0.1m spacing, there are a total of 10 tubes. As seen from above, it would be fair to say that half of the total area is occupied by the tubes, and half by the emmpty spaces between the tubes.

Now let's do the comparison. When the sun is directly overhead, the EFFECTIVE area of each tube is 1.0 x 0.05 = 0.05 sqm. Note that the curved surface of the tube is irrelevant, all that matters is the effective flat area of the tube, perpendicular to the sun's rays. In other words, the area that matters is the area that you would calculate by taking a (2-dimensional) photograph of the tubes from directly overhead. So, with the sun directly overhead, the total effective area of the tubes is 10 x 0.05 = 0.5 sqm, and the incident power is 1000 x 0.5 = 500W. Of course, this is exactly as expected, as half of the total area is occupied by the tubes, and half of the total are is occupied by the 50mm wide spaces between the tubes. The bottom line is that with the sun overhead, the flat plate is receieving 1000W, while the tubes are receiving only 500W so, all else equal, the flat plate has the advantage at midday.

Now for the subtle part, to consider what happens to the E-tubes when the sun is NOT directly overhead. Unlike with the flat plate, the effective area of each cylinder is constant (0.05sqm per tube) as the angle of the sun's rays changes throughout the day. The result for this example is that the power falling on the tubes is a constant 500W, from midday until the sun's inclination is 60 degrees from vertical, at which time the tubes will begin to shadow each other. It would be fair to say that the tubes 'track' the sun, in the sense that the power received by each tube is constant as the sun moves across the sky. Any yet, hopefully you will by now smell the rat.

Hmmm. Let's summarize. The flat plate does not 'track', but receives 1000W at midday, falling off to 500W by the time the sun is inclined at 60 degrees. The tubes could be said to 'track', but receive only 500W of (constant) power over the same time period.

Clearly the flat plate will on average receive more power, given our intitial assumption that the total area occupied by each is the same, which is to be expected given that some of the sunlight falls in the wasted space between the tubes. How much more power, on average, is received by the flat plate? To answer that question requires a simple integration of the COS funtion, and for brevity I'll skip the math and give the result. For the example given, over the time period when the sun's rays are inclined at less than 60 degrees to the vertical, the flat plate will on average receieve 1.73 time more input power from the sun.

The advertising claim that the E-tubes 'track' the sun is arguably correct, but misses the much-more-important point that for the same total area, a flat plate collector will collect more energy.

Unfortunately, the flat plate collector will also lose more energy, so the argument is left to rage forever as to which is 'best'. Which is better, a 2m length of string or a 1.5m length of nylon <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_smile.gif" title=":)" class="bb_smilies" /> (well, it all depends ...)

It appears that the reflector is just pressed in behind the tubes in sections and each section clips together with a fold in the material. I saw from the stratco installation manual the following;

Wind Regions Covered
Flat-Mount Kit:
• Region A – N1, N2, N3
• Region B – N1, N2, N3, N4
(Checked to AS/NZS 1170.2:2002 and AS4055-2006)
Tilt Mount:
• Region A – N1, N2, N3 – Maximum tilt angle 38°
• Region B – N1, N2, N3, N4 – Maximum tilt angle 30°
(Checked to AS/NZS 1170.2:2002)

It appears both the solarplus and Stratco panels are made by;

Linuo Ritter International Co., Ltd
No. 30766 East Jingshi Road
Shandong Province, Jinan 250103
PR China

They are a susiduary of Ritter wich is the German parent I think (see the link);

http://www.linuo-ritter-international.com/about-us/organisation/

I suspect the same company that makes your panels as well, but not sure if they are all made at the same place. I think the mirror is not held by the plastic clips. I will have to get up and have another look at them. The edges seem held between aluminium and the sections engage each other with folds. I am assuming its wind loading on the roof etc and not the mirror itself although it may be adding to the problems thus the limit on tilt angle.

Am happy with the panels, just would have liked to tilt them to 50 degrees but now cant. Will cover one panel in Jan/Feb and have 4 panels exposed the rest of the year.

Hi Pete ,
I have been watching your postings with interest , obviously Sunplus pissed you off no end that it motivated you to search out the truth...as you know i own Skysolar that sells a similar system to your own but is Italian made instead of Chinese , you made a comment that your concern of the wind rating was the reason as to why you didnt pitch your collectors higher , i suspect part of your reason is that the CPC reflectors are held in place by the cup mechanism that holds the tubes in place and that your concerned they may also blow out...if that is the case screw them down with self tapper screws like the Skysolar collector...hope that helps...unfortunately my business is closed now so if you now of anyone who maybe interested in purchasing an ongoing solar heating business that is for sale i would appreciate there contact

On another note, if you have the "good" parabolic mirror evacuated tubes, the down side of them is that due to them having a mirror back, there is a limit to how much you can tilt them. If you tilt them too high and in cyclonic areas or high wind areas, the wind loading on them is much greater than the standard open built evacuated tubes. I have to find out what my wind region is as if its region A the max tilt is 38 degrees, if region B its 30 degrees. I am now thinking that if I am going from 25 degrees to 38 degrees its hardly worth it for the cost.

Made up a cover for the extra panel and it stays on over Jan/Feb this prevents overheating and then I remove it for the extra heat in autumn/winter/spring.I have had 3 36-38 degree days and the system stayed just below 80C, and if it risks getting above that then I can turn on holiday mode and it pumps water up the panel at night thus cooling it a bit. Drops the tank temp from 55C bottom/80C top to 76C top/42C bottom.

Just an update on my system (split, flat plate, 70deg tilt). I hit a milestone today in that my tank reached its cutout temp (which it does most days) at some time this arvo, and the panels reached 101.6deg (still no data logging so not sure when). It was hot today (37) but we did get some cloud later in the day. I'm hoping this is as hot as they will get, but i'll have to wait and see. Maybe i could have put them even steeper- but then i'd probably not gain the most in spring and autumn.

I agree with solarman. His analysis of the effects is accurate. When in stagnation the panel temp may rise to 120-150C so soon as you drop pressure in the system by opening a tap or the air vent opening, this hot water flashes to steam, if you then shut the tap and repressurise the system it then collapses and noises similar to water hammer are heard. We suffer this exact issue in our power station when starting the system up and it is so severe it shakes the whole building.

I have 24 parabolic tubes (equivelant to 30 Apricus), and have a vent, but understand the risks of having one. it was fitted when I had the system installed. The panels have stagnated once when I put on the extra panel and the pump didnt run after restarting (I had damaged the panel temperature sensor), and the expansion valve at the tank water inlet lifted and dumped water (as I have both steam vent and water expansion valve). My air vent valve has an isolating valve and I may chose to shut it off to prevent it from opening, and maybe only open it if I see the pump in cavitation (I have a flow meter on the pump line now). The cold water expansion valve leaks a little in normal use if the tank is heated during the day and water is not used from the system. It may lose about 1 litre per day due to this. If the system does stagnate, I can see the system will use a lot more water but at least its cool water and not 70-80C water from the tank relief valve. People should also realise if they heat their tank until the tank temperature/pressure relief valve lifts due to temperature it will dump all the hot water to the ground until the top of the tank cools below the valve set point (usually 92C), only then will it reseat. My solar system currently stops the pump at 80C so will stagnate then. After 3 hot days my tank is now at 77C.

Hi Sunshine ...you said "No Hills systems that I am aware of, suspect due to Apricus not having a relief valve on the roof", I sell a U pipe system with CPC...peter69-56 has something similar from a different supplier...I have never ever installed an air vent valve on any of our systems and do not recommend them either , i believe they cause more problems than they help.
So.... what is the job of an air vent valve? Australia and New Zealand have successfully bastardized the true setup for solar water heating (apart from passive solar)which is through a heat exchanger(indirect heating), we in our wisdom looked at ways of selling solar heating systems in NZ and OZ as "cheap as possible" because indirect is expensive and so we come up with (direct split system solar water heating)...contact any European and they will tell you the same...I am digressing I know but its to help prove my point.
The problem with an air vent valve(AVV) on a direct system is yes it expels air from the manifold but it also ends up being defectively used as a "steam valve"..not its intended purpose , that is to say that when air is in the process of being emitted from the AVV it will create a slight pressure drop therefore lowering the boiling point... which if the timing is right can turn the water in the manifold from water to steam , the steam will look for the least point of resistence and rush in behind the air being emitted from the AVV and turn it into a steam valve...
So.... because the valve is now steaming it continuously will vent because it is constantly aiding in keeping the pressure lower which constantly lowers the boiling point that creates steam ...additionaly that steam is also boiling the water in the return pipe to the tank and when all that water in the pipe is now steam and entering the inlet to the tank the geothermal explosion noise is horrendous creating also the problem of overheating the tank...always have a stainless steel tank for this very reason....If yourb system is sized correctly and the collector pitch and orientation is right then the chances are you will not have these problems which are mainly caused by systems going into stagnation because they have overheated
Originally it has always been a requirement to have to relieve trapped air from a "closed circuit system"(indirect heating) because air gets trapped in a closed piped circuit and it does need to be exhausted from the system because as it circulates through it gets trapped in the pump and causes cavitation.
It does not get trapped in an "open circuit system" because eventually the air is pumped to the tank where it is absorb in the tank water or expelled through the household taps.
Installations of "direct systems" with ET's or flat plate are still in my mind an unproven science which we are still working out the best ways because we have created an unknown system...the knowledge of "direct system" installs are based on "indirect systems installations which is a nonsense because they are very different animals
Noise is created in all "direct" solar water heating systems whether it be flat plat or ET's...the reason is varied..here is a couple of them to help you understand...
1) When you turn on the hot tap and simultaneously the collector is either at stagnation or it is extremely hot then result is this......you get a pressure drop in the solar system which lowers the boiling point in the manifold turning the water to steam therefore the excess pressure and extreme temp of the water/steam in the manifold of the collector is sucked into the tank as your drawing hot water off from your tap because the cold water entering the tank to replace the hot water causes a venturee effect...the crashing and banging that is heard is the superheated water/steam mixing with lower tank temp water ..it creates a mini thermal explosion in the tank..that's which reverberates its way back up to the collector through the pipework....My understanding is Apricus have only recently adopted a similar attitude to my own company recently of not using AVV and good on them... I recommend to use a Cold water expansion valve(CWEV) so any excess pressure build up is relived through the valve rather than pressurizing into the tank restricting overheat issues...Expansion vessels are helpful for sure but a CWEV is best in my opinion <img src="http://www.ata.org.au/forums/my-plugins/bb-smilies/default/icon_smile.gif" title=":-)" class="bb_smilies" />

I am about to find out. I have added 6 more parabolic tubes to my 18 existing ones. Was going to tilt it to 50 degrees but am trialling it just laying on the roof at 25 degrees. Has a roof steam vent and a tank expansion valve. It is apparently the equivelant of 30 Apricus or standard evacuated tubes. If I have to shade it or modify it I will advise you. Circ pump was adequate for the job and flows at 2.5 litres/min per my installed flow gauge.

May need operation of the vslves on the side of the system to bleed any air pockets in the pipes..

My dads hills (30 tube split) makes noises....

The flat plate splits have alot more water in them than the manifold of an ET system, which only holds 2 litres of water.
I've never heard of ET glass overheating.
They are made of borosilicate glass, so only prone to heat damage when heated rapidly or unevenly, so not an issue on a roof.
http://en.wikipedia.org/wiki/Borosilicate_glass

Haven't heard the noise either, seems to be only some Apricus systems with that complaint as far as I can see. No Hills systems that I am aware of, suspect due to Apricus not having a relief valve on the roof, where the Hills has. Other than the valve the two systems are just about the same.

Has anybody ever really experienced a damage caused by overheating glass tubes?

And the dreadful noise is not so bad. The dragons in Skyrim are much worse. (;-)

I am referring to a split system, the controller shutting the pump off to stop the tank overheating and the water in the panels boiling to the point where they can damage themselves, if not just cause some awful noises.

There's alot of talk about overheating of solar hot water systems.
I believe this could the case with flat plate systems with the tank on the roof,as there is no control when system gets hot, whilst on split systems the circulating pump simply stops, and the heating of the tank stops.
Has any member here experienced it with split systems?
I haven't seen any evidence of this on mine and system is over 3 years old.

Add a roller blind on top of part of the array and couple it's controller to a temp switch, so that it rolls down as the temp in the tank gets to serious temp, and reverse in winter.

Those security roller blinds are designed for long term outdoor use.

I am considering tilting my evac array. Its at 25 degrees now with 18 tubes parabolic mirror. I am adding another 6 tubes (equivelant to about 30 standard tubes). Its not in an ideal spot as the wind roars down that side so the structure will have to be good and solid. With 30 tubes I am thinking I may not have an overheating problem. I guess I can try it and see, if it overheats I can always redo the job and have a frame put up. This is why I was curious as to how much gain/loss it will really have. Is it really worth it?

But to get rebates people should keep the following in mind as this is the response I got from Sustainability Victoria!

"Sustainability Victoria do not have any issue with modifications to your installation provided that the collector tubes are oriented between 10 degrees and 50 degrees to the horizontal and face within 60 degrees east or 60 degrees west of true north. Repayment of the Victorian Government Rebate is not required"

From my experience tilt is everything. My 4m2 of flat panels (double glazed) are at 70deg tilt, facing nnw (Canberra). I have not had a winter with them yet, but they have not yet gotten hotter than 85degC this summer, and are still providing plenty of hot water. They are wall mounted (gable end of house) so wind is not an issue.

My plan is to put more up for heating, but i was worried about stagnation in summer, but it seems it won't be an issue at this high angle.

I just need to wait 6 months to see how they go in winter.

dymonite69 said:
The amount on energy collected is based on Lambert's cosine rule. Ideally the collector is perpendicular to the direction of sunlight. You would need to have a tracking system which is complicated, expensive and needs maintenance. 25 degrees off angle loses 10%. 45% off angle loses 20%.

Just to make it clear to me, if it is say mid year so sun inclination is per my latitude of 38, if I am at 50 degrees then I will lose close to 5% radiation compared to being at the correct tilt of 38? therefore similarly, if its mid summer and the required angle for the sun is at 15 degrees and I am at 50 degrees I will lose a bit more than 15% radiation so that will be the cooling factor in summer

mick_queensland said:

Second, there is a simple, practical and cheap way to load balance between summer and winter, and that is to tilt the array for winter, and I mean not a half hearted attempt at tilting, but a correctly calculated tilt angle. Getting the tilt right will flatten the insolation curve out quite a lot and you will end up with slightly more output in spring and autumn.

Except that tilt frames are not cheap and would require additional engineering /council approval in cyclone prone areas.

Mick, Dynomite is talking about Adelaide. In the tropics, there is no reason why solar would not work effectively year round.

If you size a system in Adelaide to be adequate for winter, you will have a problem in Summer with overproduction and also a problem with your wallet. That is the underlying reason why most Solar systems down south land up running boost during winter.

Hi all,

Dynomite said "Simple maths shows that insolation varies 2-4 fold (depending on latitude) between winter and summer. Here in adelaide it varies 3.7x.

Even with a 100% efficient system (with no losses), the potential amount of hot water that can be generated is at least half that compared to summer. There is no real practical cheap way of load balancing solar hot water generation. "

First issue is with your ratio of winter to summer, not so here in Townsville, its more like 1:1.5 Winter to Summer insolation.
Second, there is a simple, practical and cheap way to load balance between summer and winter, and that is to tilt the array for winter, and I mean not a half hearted attempt at tilting, but a correctly calculated tilt angle. Getting the tilt right will flatten the insolation curve out quite a lot and you will end up with slightly more output in spring and autumn. Further, you should consider your prevailing weather patterns and allow for rainy or cloudy seasons too. So, then calculate your summer, spring and autumn average requirement and size for that, you will find your winter output will probably never need boosting or if it does it wont be much.

One more thing, Apricus USA sell a heat sink to deal with over production high output days, I havent seen it on the Aussie Apricus website but it is on the US site. It's purpose is to let you produce what you need for winter and dump excess for summer, now that a good idea, hey. (downside is it costs money up front)

Cheers, Mick

Interesting survey on showering habits in the UK.

A survey, using innovative technology, has offered an insight into people's showering habits.

The average shower lasted eight minutes - much longer than previous studies suggested, using almost as much water and energy as the average bath.

http://www.bbc.co.uk/news/science-environment-15836433

During winter, the performance of the system is compromised by several factors: Reduced sunlight intensity and duration; Lower entry water temperature; Higher domestic demand for hot water. So, not only can the system make less hot water, it has to work harder to do so at a time when demand is highest.

The amount of boost is dependant on demand. In my circle of friends my non-scientific research tells me that boost is on for a significant part of winter and for some, virtually all of winter. The perils of dealing with sharp salespeople I guess. Hardest hit are those in the hills with no access to gas and systems on normal tariff. Have managed to help convince a couple of those to convert to offpeak.