Flat solar thermal panels or evacuated tube?

[MikeSolar]

I was the importer to Canada for the Schott vacuum collector. They would not import to the the US due to the belief that they would get sued for just about anything if there was an issue and made me promise that I would not sell to the the US. The PV end is a totally different animal, as we know.

These were the most elegant and efficient tube on the market at the time and had a stagnation temp of 300C. This brought on long term problems controlling the temps and pressures and Tyfocor had to come out with a special glycol mix to handle the temps and the as it is a liquid to glass contact, it had to keep the glass clean.

In Europe, Viessmann sold them for a number of years as did a few other companies.

The problem with tubes, and all tubes suffer from this to some extent is when there is no load, the degradation of glycol or glass stress becomes quite a PITA. We all know that the only way SDHW has a payback is when it performs well for a couple of decades and flat panels will do this. Only the rare tube system will do it.

[Naptown]

I agree with you Mike. Flat plates are the best overall value for hot water and perhaps heating. Yes they are a larger footprint for the same BTU output but the overheating issues etc with evac tubes far outweigh the benefits.
I personally only use them in specific instances. Generally on flat roof urban installs where wind loading is an issue, and commercial installs where there is a constant high load on the array.

[MikeSolar]

Howdy, I've been in this business for almost 30 years so i have seen a few things. First, even though Germany has less sun than Canada or the US, 80% of all panels there are flat plate (that ratio is not really changing either) and the reason is simple. When you are heating DHW, 90% of the time the panels will be at 70C or less, 80% of the time 60C or less. At these temps, a flat plate will put out as much heat as a vacuum tube for the same absorber area, on an annual basis.
Second, longevity.....flat panels have been around for decades, in their modern form, can take quite a beating and still work but a vacuum tube, any vacuum tube, has a 5-6% chance of losing it vacuum within 5 years. Simply put, it is under a lot more thermal stress than a flat collector and as they say "somethings gotta give".

I bought and installed 2 container loads of Chinese (Sydney tube) panels before I decided that I would stick to flat panels. I did a survey of my systems about 3 years ago and found that, even in really good running systems, there was an average of 2 tubes in a 60 tube system that had lost vacuum for some reason (systems had been installed from 2000 to 2008). I had researched the best manufacturers in China for a full year before buying the panels. These failure numbers are well known in Germany and in china and it is why there are stalls on many streets selling replacement tubes for $1 or so.

This is not to say that tubes don't have their place and the best place for them is doing high temp work such as a chiller. The secret in this situation is to have a small deltaT across the tube to reduce stress. Be prepared to change tubes out after a few years.

In the real world a flat plate will give you more heat for a longer period than a tube will and with less maintenance. An average flat system for DHW will give perhaps 500kwh of heat/m2 of collector annually. A system sized for heating will be less, perhaps 350-400kwh/m2 for collector area and if you size for winter time, you will overheat in the summer... big time.

My panels are on the wall of my house (90deg) and feed my DHW and floors directly. When they are lower in temp than my storage tank, they feed my ASHP to increase the COP. I have 10m2 (4 Viessmann flat panels).

It is my first post so i won't blabber on any more. Cheers

I tried Solar Air warming for about 4 decades. The section was effective in producing very hot air. The section would heat-up by 9am, fan convert on (@ 100 degrees) & gradually get warmer. It was not unusual to heat-up beyond the thermometer's restrict of 160 levels, with the fan blowing
(300cfm) regularly.

[russ]

evacuated tubes for sure!!

Well - so much for salesmen's blather - this guy has no idea but is busy trying to get suckers lined up. Both types of systems have their disadvantages and disadvantages.

[Gabriel.H]

evacuated tubes for sure!! u can read this about solar thermal energy :

and there is also a comparasion betewen vaccum tube and flat panels :

Mod note advertising links are not allowed If you want to advertise contact username Jason.

[russ]

I think evacuated tube (vacuum collectors ) are better but depends on lot of factors. Some will say flat panels has longer and proven history and last longer but vacuum collectors has increased efficiency, so better do some research over net and you can find solution.

Depends - in a cold climate the evacuated tubes can perform better - in a warm sunshiny climate a flat plate

For data http://www.solar-rating.org/default.htm

[billmckinstry]

I lived in Papua New Guinea for 30 years and we only used standard Solar Hot Water systems ie no heat exchanger as it never freezes.
The Govt passed a Law in the early 1980's banning electric domestic water heaters in coastal regions which means a Solar unit can not have an electric backup element.
This was revised soon after to allow a One Shot Switch which when activated will allow an element to heat the water storage until the thermostat cuts out and then no more power until the switch is re pressed. This means in days of big usage and or a lot of cloud, you can boost the water temperature, once.

The end result is most units had either no electric boost or if installed, it was never used.

We got used to luke warm showers when we had rain for more then one day (seldom happened).

Legionares desease was never an issue and in most cases the water would not reach 70deg c.

We never had any health problems.

The water in the storage tank (180ltr 1 panel or 360 ltr 2 panel) would have changed a few times a day and it was Town supply cold water mains pressure feed.

[Yianie123.]

I own a 90 tube system

If you live in cool climates go with evacuated tubes I have them on a stand in the yard. *All piping is buried. *I used copper pipe inside of corrugated drain pipe The copper pipe was centered with spacers and filled with spay foam insulation.

[russ]

Legionella would be more of a concern on open type systems, IE if you have an open to the atmosphere water storage tank or pumping from a lake or pond. If pumping from an open water source you will also have other organisms to worry about. Legionella is transmitted by air, it does not come from water unless it is a contaminated source, chlorination or better yet treating with ozone will kill it. Ozone is the best disinfectant available it kills all viruses, bacteria, and protozoa. Ozone generators are readily available and simple to install and maintain.

Ozone generators are OK providing they are working though there are different schools of thought about how effective they are.

Like I said, I am not concerned as there is first chlorination and then UV in my system.

[LucMan]

Legionella would be more of a concern on open type systems, IE if you have an open to the atmosphere water storage tank or pumping from a lake or pond. If pumping from an open water source you will also have other organisms to worry about. Legionella is transmitted by air, it does not come from water unless it is a contaminated source, chlorination or better yet treating with ozone will kill it. Ozone is the best disinfectant available it kills all viruses, bacteria, and protozoa. Ozone generators are readily available and simple to install and maintain.

[russ]

I have a bit of trouble understanding the current concern and government recommendations concerning hot water temperature. There are areas in any system both up stream and downstream of the water heater that the proper conditions are available for such growth.

I pump from a well to a large sump (35 m3) where chlorine is metered in while pumping. From there water goes to a carbon filter, to a softener and then to distribution between 5 homes and common users. In each home there is a UV lamp where the cold water enters the home. I am presently buying a reverse osmosis unit for several reasons - one our water is very hard 500 mg/l hardness approximately and I want RO water for drinking/cooking - I have no idea of how good the well water is and testing facilities such as are easily available in the states are more costly and not so common here.

To me, a lot of the concern over legionella is like mandatory safety warnings on many things - iron ore powder standard in a bottle has a warning that makes it sound lethal - I lived in and worked in the stuff for a lifetime.

Keeping a healthy system is important - how to do it has many ideas and solutions - many of which work equally well.

[mtmtntop]

ok. let me re phrase it. i would not use this water for drinking or washing dishes at my house.

[emarq]

About the hot water consumption i think it depends on how hot the water is delivered by the solar heater. The highest the water temperature the lowest will be the consumption of the hot water from the reservoir to reach water at 40-42

[emarq]

Directly using the heated water.
Not a pumped system.

About the bacteria. That