Renewable Energy: Outstanding solar thermal collector performance too good to be true?

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- CommentAuthorJeremy S
- CommentTimeJul 21st 2012
We are in the middle (or, hopefully, near the end!) of specifying a solar thermal installation for a new build - we're pretty much decided on 5-15sq_m of collectors (scaling over time), with PV powered 12V DC pump and controller as indirect supply to 800L store (with future contingency for wood/pellet stove). DHW will be mains via coil in store, with top up from in-line electric heating (Zip CEX).

I've been researching collectors - both flat plate and evacuated tubes - and using efficiency as a function of mean panel temperature [Tm: 10-90degC] and global radiation [G: 300-1000W/sq_m] as a means of comparison (calculated from eta0, a1,& a2 per method in EN12975/Solar Keymark). As might be expected, the resulting plots exhibit two main trends corresponding to tubes and flat plates and, generally speaking, there's not a great deal to choose between the particular models in each group, although some (older?) models are definitely perform more poorly...

EXCEPT: there is one product that appears to perform as well or significantly better than it's competitors, being a clear outlier on the graphs; comparing efficiency, across the ranges I've looked at, against the mean of the other 27 units, one product exhibits between 0 & 60% better performance.

So, why isn't everyone raving about Baxi Solarflo tubes?

Since this is a clear outlier, I was initially sceptical; the data I've used are drawn from product brochures; in some cases manufacturers publish the actual EN12975 reports. I obtained the EN12975 report from Baxi and the test results are indeed as presented in the brochure. When I've figured out how to do it, I'll put graphs and data up; in the meantime, anyone who cares to check my calculations (please), get in touch and I'll email the spreadsheet (.ODS format, but can do .XLS). Currently I've gathered data for: Navitron, SusTech, AES, BAXI, Worcester-Bosch, Genersys, Clearline, Dimplex, Consolar, Thermomax, SonnenKraft, & Gasokol. If anyone cares to suggest others - or better still: post eta0, a1, a2 - I'll add them to the mix; for completeness, the parameters I've collected are: type (plate/tube), aperture area, eta0, a1, a2, IAM50_long, IAM50_lat, H, W, D, kg.

Finally, I'd welcome advice on other important performance measures, e.g. when does stagnation temperature become critical? should we be lobbying for all manufacturers to present the off-axis correction factor (IAM50)?

Best wishes - Jeremy
- CommentAuthorfostertom
- CommentTimeJul 21st 2012
This sounds seriously useful!
- CommentAuthorowlman
- CommentTimeJul 21st 2012 edited
As far as evacuated tubes are concerned a quick look on alibaba gives dozens of Chinese manufactures with many others wolrlwide. I suspect that most branded ST kits use third party tubes, some possibly of dubious origin, and which can be changed at will, sizes appear pretty standardised. So, should your research centre on the tube and not the whole branded kit, admittedly more difficult. The manifold looks pretty simple to me. and the rest is usually just third party pumpstation and pipes.
- CommentAuthorJSHarris
- CommentTimeJul 21st 2012 edited
<blockquote><cite>Posted By: owlman</cite>As far as evacuated tubes are concerned a quick look on alibaba gives dozens of Chinese manufactures with many others wolrlwide. I suspect that most branded ST kits use third party tubes, some possibly of dubious origin, and which can be changed ad will, sizes appear pretty standardised. So, should your research centre on the tube and not the whole branded kit, admittedly more difficult. The manifold looks pretty simple to me. and the rest is usually just third party pumpstation and pipes.</blockquote>

I'd agree. I've looked at a fair few branded offerings and found that you can often find the exact same tubes from one or more Chinese vendors.

There seems to be a large sector of the ST (and wind) market here that is importing no-name Chinese units, branding them with a UK name and selling them at a premium. Nothing at all wrong with that, as these companies are providing a warranty and some sort of quality control, but often the UK companies have little control over the products they sell.
- CommentAuthorowlman
- CommentTimeJul 21st 2012
These seem to be one of the big boys JS, I'd never heard of them before.
http://www.linuo-ritter-usa.com/about-us/organisation/our-partners/
- CommentAuthorChris P Bacon
- CommentTimeJul 22nd 2012
Jeremy have you come across the DRC-10 manufactured by AMK Solac in Switzerland?

http://tinyurl.com/c877gpm

I have come across various resellers touting them as having very high performance, would be interesting to see how they compare to the Baxi tubes.
- CommentAuthorMarkBennett
- CommentTimeJul 23rd 2012
Unless you have restricted roof space (which many will have) is efficiency really very important?

I can see that a more efficient panel might achieve a slightly higher temperature during winter but anything else can be made up by having more panels, albeit at the cost of more embodied energy.

The other important point is the cost. How much are these highly efficient panels compared to the cheap Chinese tubes? Can you get enough extra Chinese tubes to make up the power at a lower cost?

My point being that I'm not sure efficiency is *always* the best metric to use when selecting ST.
- CommentAuthorJSHarris
- CommentTimeJul 23rd 2012
I'm inclined to agree, efficiency isn't, perhaps, the best indicator of overall suitability for any requirement. Personally, I'd want an array that is optimised for performance in winter, with an ability to start producing hot water even when the air temperature is very low.

ETs seem to do far better than flat panels on very cold, but clear and sunny, days, AFAICS. There doesn't seem much between the various types, though, and even the cheap Chinese heat pipe ones seem to perform remarkably well for their price.
- CommentAuthorMartinH
- CommentTimeJul 23rd 2012
I can recommend compound parabolic collector (CPC) panels from Solarfocus in Austria. These have a series of compound parabolic "troughs" with a collector tube in each trough, all in a glazed box. There is no insulation because it isn't needed. They can generate high grade heat because of the concentration of the solar radiation by the parabolic reflector - probably far hotter than a flat panel. They work well in bright overcast conditions too.

The thermal store (850 l) was up at 77C today (it would have got hotter, but I use the cooling function on the controller to dump excess heat into the central heating system, particular into the coolest rooms in the house - constructed of cob). The panels were running at 87C. Based on today's performance, I suspect that the panels could take the store temperature above 80C
- CommentAuthorChris P Bacon
- CommentTimeJul 24th 2012 edited
Posted By: MarkBennettis efficiency really very important?

I tend to agree that the best way to compare panels is on a "bang for buck" basis.

But it is a minefield out there and I wonder about the long term quality of some of the products available.

It may be scaremongering by some sellers but some have questioned whether the panels you purchase really perform as well as the test cert would have you believe and some claim that there are unscrupulous retailers selling panels which are completely different than those to which the cert relates.
- CommentAuthorSteamyTea
- CommentTimeJul 24th 2012
One way you could assess the performance of ST panels/tubes would be to compare the rate at which they can heat up a fixed amount of fluid from a fixed temperature for the same solar input. If the actual collector area is the same (this is really just the black bit, not any frame or glass).
I suspect that the the EVs would perform better at higher fluid temperatures than the FPs.
Another method to assess performance would be to compare price per kWh generated, but that would favour the FPs on two grounds, one on the greater area per money unit and the other on the better low temperature performance (or useless energy).

Really what needs to be done is fix a minimum water store temperature, say 35Â°C, fix a target temperature of say 65Â°C and either say you have up to a fixed area of roof to play with or you have a fixed price.
Think of it as the 40 mph to 60 mph acceleration figures for a car rather than either the 0 to 60 mph or the top speed.
- CommentAuthorfclauson
- CommentTimeJul 24th 2012
Have you done you return on investment for ST - its typically 15 to 20 years - its not actually the best bang for your buck when it comes to saving energy
- CommentAuthorSteamyTea
- CommentTimeJul 24th 2012
It may not be under out current subsidy system.
Take away incentives and false pricings and I suspect that things start to look different.
And there is nothing to stop you selling some excess heat to a neighbour, infact it is probably a sensible thing to do, not that sense ever comes into it.
- CommentAuthorskyewright
- CommentTimeJul 24th 2012
Posted By: SteamyTeaIt may not be under out current subsidy system.
And there is nothing to stop you selling some excess heat to a neighbour

Wouldn't transmission losses become significant quite quickly except for adjoined or very close neighbours?
- CommentAuthorJeremy S
- CommentTimeJul 24th 2012 edited
I agree that the next step is to weigh up the economics, but it seemed to me that getting a handle on the performance is the most important first step.
Interestingly (and as you'll see if I manage to post the graphs - they need some reformatting to make them clearer), the Baxi units appear to perform comparably with the rest of the field at low temperatures, and way outperform them all at higher temperatures.
In Scotland optimising for winter is critical, hence the largish store (to make heat while the sun shines) to take advantage of the cold, bright, clear days that JSH refers to. So here efficiency would seem more critical than in, say, Spain (hence, http://bubble-sun.com/new/" >these).

(why can't I get an html link to work?)
- CommentAuthorEd Davies
- CommentTimeJul 24th 2012
Some of my thoughts in this general area (which have been mentioned on this forum before):

http://edavies.me.uk/2011/12/pv-heating/
http://edavies.me.uk/2012/01/pv-et-flat/
http://edavies.me.uk/2012/01/solar-per-area/
http://edavies.me.uk/2012/03/et/
- CommentAuthordjh
- CommentTimeJul 24th 2012
Posted By: Jeremy Swhy can't I get an html link to work?

Use text mode, instead of Html, and just post a raw link.
- CommentAuthorSteamyTea
- CommentTimeJul 24th 2012
Posted By: skyewrightWouldn't transmission losses become significant quite quickly except for adjoined or very close neighbours?

Yes, that is what I meant by neighbour, hot water is not really a global tradeable commodity. Still won't stop some people trying
- CommentAuthorskyewright
- CommentTimeJul 24th 2012 edited
Posted By: SteamyTeaYes, that is what I meant by neighbour

Ah. Context counts.
Around here a neighbour less than 100m away is unusually close, & the term could easily be used for someone several miles along the road.
- CommentAuthorborpin
- CommentTimeJul 24th 2012
Posted By: fclausonHave you done you return on investment for ST - its typically 15 to 20 years - its not actually the best bang for your buck when it comes to saving energy
True, but in a new house I want it to use as little energy as possible from the start rather than try to reduce what I use. With no VAT and a low cost of borrowing, the economics do look better.
- CommentAuthorrenewablejohn
- CommentTimeJul 25th 2012
<blockquote><cite>Posted By: owlman</cite>These seem to be one of the big boys JS, I'd never heard of them before.
<a href="http://www.linuo-ritter-usa.com/about-us/organisation/our-partners/" rel="nofollow">http://www.linuo-ritter-usa.com/about-us/organisation/our-partners/</a></blockquote>

Ritter panels are useful if you want to heat thermal oil for higher temperature applications
- CommentAuthorcrusoe
- CommentTimeJul 29th 2012
I think there are some typos in Baxi's literature. If Ritter and Co don't know about it - sold in this country as Vaillant, Worcester-Bosch and Solfex, it is highly unlikely IMHO that Baxi - using no doubt someone else's tubes - have sneaked a quantum leap in here somehow.

What is more important, again IMHO, is that you take an overall look at the system as it is applied and designed rather than focus on a spot efficiency, nice as it is to quote that. For this reason, I reject two of the dearly-held views above, to wit:

1: That the best way to compare panels is on a bang-for-buck basis (Crispy Bacon). My old Citroen BX16 well out-performed my later Mazda 626, and was relatively cheap, but the Mazda was around to tell the tale long after the Citroen had become a solar panel, or some other recycled commodity. Ergo performance without longevity means nothing, as the SMART Energy example I quoted on another thread conclusively shows (fully half of the 17 000 installations going faulty within a few short years, I attended one recently , hence recent thread on Replacement ETs). And those longevity assessments are useless if done by the manufacturer themselves. And are affected by the installation methodology. Which leads me to the 2nd point...

2. Solar thermal taking 15-20 years to repay investment (ROI) (FCLauson). A projection of performance hence payback based on historical sunshine-radiation figures in a climate change scenario is a nonsense. Likewise in a Peak Oil situation when fuel figures may well spike way beyond projection, meaning that payback may be achieved ten years earlier than if you hadn't installed the technology. And vice-versa with global recession when Brent crude prices fall.

But we have to start somewhere, so putting the two things above to one side, the 15 to 20 years payback is based on an average family for DHW only, which is actually pretty generous in my view, with Which magazine coming out in 2010 and saying that the average family would save only Â£75 PA.

It is this installation methodology - large collector, relatively small tank, which I have been castigating for many years now. Why? Because you can only store so much sunshine in a small tank. And if the sun's not out next day, instead of using the stored sun from a larger tank, you have to revert to fossil fuel. Which means that when the sun DOES come out to play, the system stagnates, overheats, cooks itself into an early grave. Because a sealed system is easier to fit than a drainback/flowback system, which would overcome that problem.

A usefully-designed solar thermal system is a bit like a farmer;s harvest. He can't generate instant crops, so he keep some in reserve, in storage. At times he may seem to have far too much, but he makes hay while the sun shines, and stores it til the sun shines again to grow his crop. I'm mixing metaphors, but the drift is clear - if you have oversize storage you can harvest sun for as long as there IS sun, rather than saying 'Ooops, tank's full, better stop getting free sunshine now...'

That alone will bring your payback down by several years. But if you add oversize arrays into the equation, and start using that heat in the space heating too (usually the much greater part of the energy consumed in existing housing stock) then you are harvesting much more sunshine, which translates into much less fossil fuel and much smaller payback ROI times.
- CommentAuthorGaryB
- CommentTimeJul 29th 2012
I have flat panel solar thermal and very happy with it, especially when I received 50% funding for its installation 4 years ago.

Even coupled to a reasonably generous 210 litre cylinder, it regularly reached stagnation temperatures in summer because the controller was set to 60 deg C at the factory. I eventually found my way into the engineering set-up menu on the controller (good old trial and error) and reset the limit up to 80 deg C which is the maximum recommended for my cylinder. This dramatically increases the effective DHW storage capacity of the cylinder (for water used at 40 deg C the delta T is doubled from 20 K @ 60 deg C storage to 40 K @ 80 deg C storage). Even now it will occasionally reach stagnation.

Note - always fit a blending valve if the control setting is raised above the 60 deg C factory setting.
- CommentAuthorcrusoe
- CommentTimeJul 29th 2012 edited
That is high temps for a domestic hot water tank GaryB. Vented I expect? Thermal store with DHW recovery, these temps are quite normal, but with TMV as you say.
- CommentAuthorGaryB
- CommentTimeJul 29th 2012
No Crusoe, it's unvented with a 90 deg C/7 bar P/T valve. Only very occasionally will it reach 80 deg C, especially this 'summer'.
- CommentAuthorChris P Bacon
- CommentTimeJul 29th 2012
Posted By: crusoeI reject two of the dearly-held views above, to wit:

1: That the best way to compare panels is on a bang-for-buck basis (Crispy Bacon)

Ahh but I did include the rider that I wonder about the long term quality of some of these products!

It's my belief that some Chinese tubes are of dreadful quality but likewise some are of exceptional quality the question is how is one to find out which are which?
- CommentAuthorJeremy S
- CommentTimeFeb 10th 2013 edited
Updated graph attached - this is a reduced selection of the models I've collated, with examples across the range of technologies.

So, when the collectors are operating at really useful (not low-grade) temperatures, i.e. above 40degC, it really pays to go for evacuated tubes (heat pipe or direct flow), whereas flat panels are only really better for low-grade heat, i.e. generally <20degC.

Whilst many of the ETs perform roughly the same, I've included one that's significantly less good (navitron's offering) - I'd say it pays to investigate whether the much-touted unbadged Chinese 'budget' alternatives would appear in a similar portion of the performance spectrum, although I take the point about whether installation area is a limiting factor. In our case it's more the pressure drop due to multiple manifolds that is making us steer away from the 'more of less' tactic.

Jeremy
- 130210-STcollector_cf.png
- CommentAuthorbillt
- CommentTimeFeb 10th 2013
Interesting. You've done the trick of removing the bottom half of the graph, which has the effect of making the differences seem more significant than they are.

I'm surprised that the Navitron collector is noticeably worse than the other samples that you've chosen. In the comparisons that I did, with similarly priced collectors, they seemed to be no worse than the competition and better than some.

In view of the small differences, it would be interesting to have a price comparison; it looks as if there may well be a 2 to 1 or more price range but only a small difference in performance. Of course there's no way to know of any potential longevity issues.

It looks as if the legends for heat pipe and direct flow are the wrong way round.
- CommentAuthorJeremy S
- CommentTimeFeb 10th 2013
@billt: it's less of a 'trick', and more designed to show what distinction exists between models - I'm hoping everyone sees through it as easily as you did!
- CommentAuthorEd Davies
- CommentTimeFeb 11th 2013
Posted By: billt: â€œI'm surprised that the Navitron collector is noticeably worse than the other samples that you've chosen. In the comparisons that I did, with similarly priced collectors, they seemed to be no worse than the competition and better than some.â€

I'm surprised, too. Is there any possibility of confusion as to which areas are quoted? ETs don't collect from the whole area of the roof they cover (especially when the sun is straight on, better when it's oblique) so various areas are usually quoted (gross, aperture, etc)?

Still, efficiency in terms of percentage of sunlight hitting the panels turned into useful heat is not very interesting unless you are limited on roof area. What's more useful to know is power output you get per pound/euro/dollar of panels.

Also, all of these panels will work well enough with 1000 W/mÂ² arriving square on. What's more interesting is how well they'll work when the sunshine is a lot less bright or arriving at a funny angle. It's not just a matter of proportional efficiency as the losses are fixed for a given temperature difference (except using PV). If you've got a panel with 70% direct conversion efficiency but losses of 140 W at the current temperature difference but only 200 W of sunlight arriving then it's a net zero efficiency. Another panel with only 70 W losses would be 50% efficient under the same conditions.

As I said on another thread, I've got quite a few especially cheap Navitron (actually without branding) ETs which I'm happy with (in storage at the moment - only played with a couple of tubes briefly). However, I'm now thinking that with the low cost of PV it's better to use solar thermal, if any, at low temperatures (e.g., UFH levels) then use the PV to boost the heat to higher DHW levels with via simple resistance heating or a small heat pump.

Calculator for exploring these ideas here:

http://edavies.me.uk/2012/11/pv-dhw/

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Renewable Energy: Outstanding solar thermal collector performance too good to be true?