How to know 'good' Solar Panels ?

selfbuildache

Hi,
I posted a similar question a while back about HRV units and was sent on the SAP Appendix Q link (amongst other great advice) which I found invaluable for sorting through all the HRV units out there.
My question now is, is there a similar website or independent info out there for advising on what us innocents should be looking out for when researching Solar Panels?
(I'm trawling through the Boards' threads on this, copying and pasting all relevant info I can find but am just looking for some extra help.)

RVR

Most collectors on the market will now have been tested to the solar keymark standard by an accredited laboratory: http://www.estif.org/solarkeymark/index.php

If the collector doesn't have a solar keymark then it is probably one to avoid.

There are also different types of collectors, the main types being:

Flat Plate (Flat Panel) collectors,
Fluid type (u-tube type) vacuum collectors,
Heat Pipe type vacuum collectors

Some vacuum collectors come with a CPC reflector (Compound Parabolic Concentrator) behind the tubes. This is like a mirror with U shaped ridges and troughs which reflects energy passing between two tubes back into one of the tubes. Collectors with this can be smaller as they generally have a higher output and smaller area. Collectors without this generally make up for it by being larger.

Neither type is necessarily better - it is the Aperture area that counts when comparing two panels. This is the surface area of the collector that gathers energy. E.g. a large collector can have a small aperture (e.g. lots of gaps between tubes). On the other hand a smaller collector with CPC can have an aperture almost the same as the gross area of the panel.

Efficiency of the panels also vary. The quality of the glass, quality of insulation etc all affect how well the panel works. Google "solar optical efficiency a1 a2" if you want to learn more about comparing collectors.

You should also choose a supplier who publishes efficiency data. These figures are very hard to compare without inputting them into a BER assessment, but if a company doesn't publish them or make them freely available then it may be an indication of a panel with poor efficiency.

I hope this helps - google the terms to learn more!!! Of course, a good price and supplier with good backup if the final link in the chain...

ipod

i think the swiss panels are the best.we have lots of them fitted.realy good quality.

quentingargan

This is a good question. Very few panels actually give trouble for the first few years, so there aren't many failed units out there. Most people who own a solar water heater are happy with their results, so there isn't a database of gripes.

The EN12975 certificates give figures for zero loss efficiency and heat loss coefficients, but these figures work against most Chinese vacuum flasks because you also need to consider the Incidence Angle Modifier. Flatlates, tubes and flasks all behave quite differently. At the end of this post I will put an explanation of that as I see it.

It also depends on what you want your system to do. If you have a well insluated house and have the central heating off for a lot of time, you want to extend the operating season of your panels as much as possible. Flasks and tubes are better at this because of their better insulation, but flatplates are more durable.

I'm personally not a great fan of the CPC systems because they present a much higher wind load than either roof integrated flatplates, or surface mounted vacuum tubes without reflectors. I also wonder about the durability of the mirror surface and whether or not it will always be self-cleaning over their 20 year lifespan. They have their fan-base, but personally I would prefer to put on a few extra tubes and know that they will withstand storms.

Matching the panel to the cylinder, and to hot water use patterns in the house is also important. Other sloppy insulation practices will also reduce the systems efficiency by a lot more than a few percentage points in the zero loss efficiency of the panel.

The durability of even the best solar panels may be vastly increased by including a heat dump to prevent the system overheating (so called stagnation) during vacations or times when not enough hot water is being used. Again it ins't just the panel that is important - it is the whole system.

But back to the difference in how flatplates and flasks perform, in case you're interested in this angle;

_____________________________________________________________
Flatplates, flasks and Incidence Angle Modifier
Most flatplates and some vacuum tube systems show zero loss efficiencies close to 80%, but when you look at the EN certificates for most popular Chinese vacuum flask systems, they are often in the mid 50s. Many people assume that this reflects poor quality Chinese manufacture, but in fact it is a quirk in how we measure efficiency and ignores the issue of Incidence Angle Modifier, or IAM.
The efficiency of a panel is usually given for noon when the sun is directly facing the panel. With a flatplate panel, efficiency falls off either side of noon, but with Chinese vacuum flasks, the efficiency actually increases. There are two reasons for this;
1) Firstly, the curved surface of a tube passively tracks the sun.
2) At noon, the sun simply shines between tubes, but as the sun moves towards 40 degrees off due south, there is increased reflection from tube to tube, dramatically increasing absorption.
The effect of this change in incidence angle modifier is measured as part of the test procedure for EN12975 certification. To show efficiency at any particular time of the day, the zero loss efficiency should be multiplied by the IAM.


At its most extreme point, with the sun at an angle of 60 degrees, the output of a flatplate would be reduced by 10%, whereas some flask systems' efficiency is increased by 44%.
Chinese vacuum flasks are like a thermos flask with two layers of glass and a vacuum between these layers. There are many advantages to this;
· The vacuum element is a single piece of glass, whereas many other vacuum systems rely on a join between the copper heat pipe and the glass forming a seal to maintain the vacuum. With extreme temperatures, especially in stagnation, this seal can fail.


· Flasks are modular. When they lose their vacuum (typically expected after 20 years) only the flask itself need be replaced.


· Replacement flasks are in common sizes of 47mm and 58mm diameter, produced in numerous plants in China, so you are not reliant on one supplier for spare parts.


· Replacement flaks are typically about €5 each, whereas replacement tubes are much more expensive.
For this reason, Chinese flasks have been widely adopted, and many (but not all) are of excellent quality and durability.
Modern houses with their improved energy efficiency could benefit more from vacuum systems because the central heating is operating for a shorter season, and vacuum systems have a longer operating season which complements this nicely. In the case of a system I have worked with, the ratio of the average IAMs would be about 33% (though this is a very simplistic approach). This would bring the zero loss efficiency from a stated 55% to 73%.

When added to the improved heat loss coefficient of vacuum systems, it becomes apparent that vacuum flaks systems not only extend the operating efficiency through their improved insulation, but are extremely efficient at all times, often at a lower capital cost.
Lastly, it should be stated that this situation does not apply to vacuum tube systems which have a small flatplate inside a single glass tube. Their IAM more closely resembles that of a flatplate panel.

Merch

Interesting stuff Quentin, I think evacuated tubes are the way to go, I'm keen to maybe do an install myself. The part I probably would get someone to do would be the roofing aspects, mostly so I'd be covered from an insurance point of view.It's all a while away yet anyway.
Until I read your post I assumed I would consider a flush type mounting but that I would have to modify the angle to compensate for the angle of the sun throughout the year as diagrams I saw showed the vacuum tubes laid out top to bottom, but if I understand you, the tubes would be horizontal and automatically compensate for this. I still wonder would there be a significant advantage for being able to modify tracking the sun east to west or would the extra cost outweigh the benefits?

If these work off solar radiation/light would there be much less/any benefit if I wanted to stick some on the north side of the apex roof also? or would the cost outweigh the benefits?

I'm interested in calculating the efficiencies but I'm not sure what you mean by zero loss Quentin, or how that differs from how the Chinese manufacturers would calculate it? surely the manufacturers would be trying to use the figures that expressed the max efficiency anyway?

Replacement flaks are typically about €5 each, whereas replacement tubes are much more expensive.

Also I assumed flasks were the evacuated glass part/ (transparent/semi opaque coated glass) component? you say these are cheap? I thought they would have been expensive?? what do you mean by tubes?

Is there any indicator component in the evacuated section that acts as a tell tale in case the vacuum has been lost due to damage (something like a a chemical bag/blob that changes colour (perhaps with exposure to moisture if vacuum is lost (does this even exist?) ) and if it doesnt how can a person tell if the vacuum is as it was when it left the factory?

Any opinions, ideas or links quentin or anyone?
thanks

quentingargan

Hi Merch,

That article may have been slightly misleading. I've re-written with pretty pictures it and posted it here. For most flaks systems, the tubes do need to be laid out top to bottom, because the heat pipe in the centre needs to be at an angle of between 15 and 80 degrees.

I have worked with vacuum flask systems that have a U tube inside the flask and these can be mounted any way you like, and on commercial buildings are often mounted horizontally, but there are complications in getting these to empty efficiently in the event of a power cut or going into stagnation (they can occasionally "thermo-drive" pushing steam around the system).

Anyhow, if the tube is mounted horizontally, while it will track the north-south angle of the sun, it will no longer track the sun as it goes from east to west.

It isn't generally worth tweaking your angle throughout the summer - it may be cost effective with solar PVs (which are expensive, and commonly use dual-axis trackers). How well your system will work through the seasons depends on your roof pitch. At 30 degrees you will be optimised for mid-summer. 45 degrees is close to being optimised for spring and autumn, which is usually when you are trying to squeeze the most out of your system.

There is simulation software that can estimate the output of a system, given the characteristics of the panel, roof pitch, location, angle of roof towards south etc. There is a free software package from Retscreen, but we usually use T-Sol which has more variables.

As for self-installing, if you want to avail of the grants from SEAI (TAFKA SEI) you will have to use a registered installer. Some installers are happy for you to do the roof work, and they plumb in the system and sign off on it. I haven't heard of installers doing this the other way around.

"Zero Loss" efficiency is the efficiency of the panel if the fluid was the same as the air temperature with no heat loss. It is calculated as part of the tests done on panels by independent laboratories such as www.solarenergy.ch There are then other variable such as heat loss coefficient and IAM which are combined in software simulations to estimate the output of a panel.

While you might expect the vacuum flask to be the expensive part of the system, in fact the manifolds and particularly the heat pipes make up the bulk of the cost. The flasks are basically long glass bottles and quite cheap to produce. That is the benefit to the flask system - when it is broken or has lost its vacuum, you only change the flask, not all the gubbins inside it.

All vacuum systems will lose their vacuum. There is a barium getter which collects any air that enters the flask. When this is exhausted, there will usually be white streaking from the barium along the glass part and this is usually visible from the ground. Older posters may remember valves in radios and tellies that had a silvery bit at the top. That was barium, and when the valve lost its vacuum, it went all white. Same thing.

championc

In many cases, the only sure way of knowing is to see a setup in action. Maybe you might know of a neighbour who has had a system installed. Personally, I recently had a Kingspan 30 tube system installed and I'm delighted with it. I'm in Sth Dublin and today, we didn't have an ounce of sunshine and yet, at the temp sensor at the tubes on the roof, it was producing 30 deg C and the pump was running fine.

Last week, my 180L store got up to over 65 deg C thanks to the bouts of sunshine. I have a combi boiler so I have no way (or need) of supplementing the heating of my DHW. This was achieved solely by the sun. So in my opinion, you need to get data from a system and then get confirmation as to how my deg C a store rose in temp of say a 4 hour period. An approach like this can be fairly useful but I have noticed that once you push above 50 deg C, it gets harder and harder to raise the temp every deg C.

The more water in the store you have to heat, the harder it obviously will be to heat up. And with this in mind, I went for a smaller store to try and maximize my temperature gain during Spring and Autumn since I believe the worst of systems will supply you with hot water through the summer months (not saying that any systems are bad). And so this is why I went for a 180L rather than the recommended 300L store. Time will tell but I'm delighted with the results now, even on cloudy days.

I got a controller that records all data onto an SD Card so I can review it at any stage.

Hope this helps someone


C

quentingargan

championc wrote: »

In many cases, the only sure way of knowing is to see a setup in action. Maybe you might know of a neighbour who has had a system installed. I'm in Sth Dublin and today, we didn't have an ounce of sunshine and yet, at the temp sensor at the tubes on the roof, it was producing 30 deg C and the pump was running fine.

Last week, my 180L store got up to over 65 deg C thanks to the bouts of sunshine.

The more water in the store you have to heat, the harder it obviously will be to heat up. And with this in mind, I went for a smaller store to try and maximize my temperature gain during Spring and Autumn since I believe the worst of systems will supply you with hot water through the summer months (not saying that any systems are bad). And so this is why I went for a 180L rather than the recommended 300L store.

I'm afraid it isn't that simplistic in relation to the hardware. For example, in your own system, obviously a 30 tube vacuum system of any type will bring 180L water to 65 degrees in the weather we've been having - that is a very small cylinder.

The whole system is what works - the level of care and attention given to detail to prevent heat loss. I've seen very good panels fitted too far from the cylinder, or with the boiler pipes from the cylinder losing heat because of other apsects of the configuration.

The longevity of the system will depend on whether there is a heat dump installed (particularly if you've a set up like yours where the cylinder is very small for the panel, or if you have a family that regularly goes on holiday). There are also systems with reflectors which will work very well when new, but I would need reassurance that the reflectors will perform so well in ten years time.

I've also seen the good panels in a budget system with heat exchangers outside the cylinder, with very mixed results.

Yes, you may like to have a referral or a recommendation, but

1) Very few people are honest enough with themselves, much less their friends to enable them to bad mouth a system that they just shelled out four grand for and
2) You need to look at how the whole system hangs together, how it is designed for the needs of your family, time of day that the house is occupied etc.
3) Installation is every bit as important as the brand of panel used.

paddy147

What about these Irish made solar panel kits that you can buy from*******************?

The state that they are made soley for the Irish climate.

Anyone know anything about how good or bad they are??


edit: do not name particular companies.

syd

quentingargan

Without reference to any company in particular, there is, for sale, a panel which is claimed to be Irish designed, or designed specifically for the Irish market, and has exceptional performance.

It is, in fact, a bog standard vacuum flask panel manufactured in China. Nothing wrong with that - there are a lot of very good Chinese panels out there and their fundamental design (the flask system) is very good.

I looked over the certs for this panel, and it would be at the upper end of the range in terms of "zero loss" effciency, but below average on its level of insulation. This would suggest that it is less suitable for the Irish climate than many other similar panels.

More worrying is that I tried to verify the TUV certification for this panel, and failed to find it on the TUV site. In email correspondence, a TUV representative said;

The written information are not right. The so called certificate is a fake. We'd never tested a collector for <company>.

We are trying to get in contact with <company> since February this year, [2009] but without success.

Please don't trust this company and their statements

All that glitters is not gold.