Flat solar thermal panels or evacuated tube?

[russ]

Like Roscho04 says - depends on your location and requirements.

Both types of collectors can collect similar amounts of heat - flat plates are better for low temperature applications (such as household use hot water) in temperate climates while evacuated tube types do better when higher temperature water is required, in cloudy situations and during the winter in cold areas.

I have the flat plate type - mild climate here. A neighbor has a big bank of evacuated tube types and keeps a good portion covered all summer.

Roscho04 is just the reverse of my situation - for his location the evacuated tube type is better.

[roscho04]

Hello Stone hope all is well:
I really don

[Stone Liu]

Just because 80% of flat panel are used around the world is not a good reason to use flat panel.
That would be like you telling to vote for someone because 80% of the people are going to vote for some one.I am not saying that flat panel are no good. sorry had to jump in.

Hi, glad to talk to you, how about your suggestion of using flat panel collector or using evacuated tube?
I promote flat panel collector mainly reason is besause of there is no risk of vacuum tubes fried and easily breakable.
I am looking forward to hearing from you.
regards.
Stone

[roscho04]

Just because 80% of flat panel are used around the world is not a good reason to use flat panel.
That would be like you telling to vote for someone because 80% of the people are going to vote for some one.I am not saying that flat panel are no good. sorry had to jump in.

[Stone Liu]

There's a lot of good information contained in this thread. I work for a manufacture where we distribute both flat plates and evacuated tubes.

It all comes down to value - which system produces the most in your area for what you spend. However, there are a few general pros and cons for each technology to consider.

Evacuated tubes produce more on cloudy / overcast days than flat plates. They also produce higher internal temperatures (our manifolds generally stay around 160-170F during the day), and are easier to maintain.

Hope this information helps!

Thanks for your detailed information. I agree with your opinion.
Flat panel collectors are widely used in project than evacuated tubes.

[Stone Liu]

We suggest flat panel thermal collectors, because 80% of this style are widely used around the world, except China, in China, 80% use evacuated tube.
We are Solar products manufacturer, Product Range:

Solar Photo-thermal: Solar Flat Panel Thermal Collectors, Air-source Heat Pumps, Solar Water Heating Systems.

Solar Photovoltaic: Solar Panels (Mono/Poly/Thin film), PV Roof Systems, Solar Street Light, Smart Grid (On/Off) Power Generation Plant, Building Integrated/Attached Photovoltaic (BIPV/BAPV), Solar House.

Regards.
Stone Liu

[russ]

Hi John, I was born in Seattle and grew up in Central OR - some beautiful country in the NW!

The cheap hydro power does tend to make solar less attractive. And, as you pointed out, the abundant cloudy days on the coastal side of the mountains.

I remember that when I lived in Portland we were rather surprised to see the sun between mid December and mid June - a bit of a stretch but not much.

If you ski though - wonderful - you are about the first in the country to hit the slopes.

Russ

[Thermal John]

Thanks Russ, good to be here!

I must admit I'm pretty jealous of the sun hours over there. Surprisingly we don't see all that many installers on that side of the mountains, cheap electricity seems to limit interest all over the state no matter how much sun.

John

[russ]

Hi John - Welcome to Solar Panel Talk!

Move east of the Cascades - then no problem with sun. Can be a bit cooler though.

Russ

[Thermal John]

The choice really depends on location. Flat Plates will generally perform better during optimal sunny conditions. The vaccuum that helps an evacuated tubes performance in cold conditions hampers its performance a bit in warm conditions as it cant take adavantage of ambient air temperatures. if you live in a cloudy or windy climate the tubes will be a good choice as long as you dont have extended snowy conditions (no snowmelt ability) otherwise you need a 60 degree tilt to shed snow effectively.

There are a lot of evacuated tubes out there that are closing in on flat plate prices so cost isn't really as much of a factor these days. Stagnation can be an issue, if you are installing a closed loop antifreeze system, but there are glycerin based fluids that are both non toxic and extremely durable with boiling points of 460F or so. This at least helps prevent vapor lock if you lose power to your pump. They also wont turn acidic like propylene glycol can when stressed over and over again (around 260F).

I'm based in the Pacific NW and we can see decent performance for DHW but space heating just doesn't pencil out for decent financial payback, because of our 226 fully cloudy days a year where I'm at. For space heating contribution Evacuated tubes probably make more sense but you still need good winter sunshine to make an actual contribution without oversizing your system to greatly.

I discourage people to go solely by efficiency curves on collectors which can be misleading, as they dont acuratelty represent other factors such as the incidence angle modifier. An identically sized collector with a higher efficiency rating can actually be producing less heat than a lower efficiency collector with a differently shaped absorber surface. You should really check SRCC charts to see what the collectors produce BTU wise rather than their efficiency %.

Keep researching as this isn't a decision you want to make hastily.

John

[russ]

Those are the common claims made by manufacturers but - observations I have made from my system and my thoughts have trouble with their statement. I collect power readings from four meters every morning and hot water system data about every hour. Solar insolation is collected from my Davis weather station both Global solar radiation and UV are recorded.

1) I consider that my system is functional for about 85% of available light. The very early morning and late evening light no system will pickup. If the insolation for a day is 2000 watts/m2 then you really have 1700 watts to work with. Even if the vacuum tube collectors extend the day you might say 50% of nothin is still nothin.

2) My hot water system electrical backup is set for 50 degrees. I don't need or want water hotter - now for a radiator heating system OK hotter is needed. For in floor radiant heating I have the temperature set at 35

[JeremyMC]

Generally speaking,

1) Tubes enjoy a significantly longer and more productive solar day. Evacuated tubes begin BTU production earlier in the day, and continue to produce BTU later in the day than flat plate collectors. The round shape of a tube allows solar gain no matter what the angle of the sun. In contrast, a flat plate collector has a low solar angle in the morning and afternoon, with direct angle occurring only at the middle of the day.

2) Tubes operate at higher temperatures than flat plate collectors. The old rule of thumb for flat plate collectors is hot water at approximately 50 degrees above ambient temperature. This rule of thumb appears accurate. Evacuated tubes greatly exceed this threshold.

3) For a high temperature tank or for winter radiant demand, evacuated tubes perform better.

Check out: http://simpledrainback.com/ for more information on this subject.

[Jbergess]

Article

Here's a link for what Russ is referring too:

http://www.solarpanelsplus.com/solar-water-heater/Evacuated-tube-vs-Flat-Plate.pdf

It's actually hosted on Jbergess's website...not sure if he just put it on there, or if it's a coincidence.

Just a coincidence - we've had the article on our site for a while now since it came out. We try to bridge the gap between flat plates and evacuated tubes, showing the advantages and disadvantages of both vs. pushing just a single technology.

It's a very well written article, and goes into some technical details regarding design, tilt, and differences in efficiency based on what Delta-T you're working with. Definitely a good read.

[Jason]

Here's a link for what Russ is referring too:

http://www.solarpanelsplus.com/solar-water-heater/Evacuated-tube-vs-Flat-Plate.pdf

It's actually hosted on Jbergess's website...not sure if he just put it on there, or if it's a coincidence.

[russ]

Thanks Joe,

Excellent post from someone who is in a good position to know.

Take a look at a pdf from Home Power - issue 132 date August September 2009 with the title Flat-Plate & Evacuated-Tube Solar Thermal Collectors.

It is a free download - would be interesting to hear your take on the article.



Russ