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Author Topic: Comparative study - Flat Plat and Evacuated Tube collectors  (Read 13845 times)
billi
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« Reply #30 on: January 01, 2009, 05:20:44 PM »

But in the air collector section i have not found any available system with tubes , sure you get the collectors , but an approved system kit  Sad  and the collector only is not guaranteeing  a good system be it air based or water )

Billi

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« Reply #31 on: January 01, 2009, 06:27:09 PM »

Ivan,

Do you have a data about the temperature rise across the SFG panel? That would determine if a DIY flattie could feed into a SFG and produce air at 22C.
Likewise we need data about the temperature rise of the DIY panels on an overcast day.

Veco (his name yes Billi?) has possibly made a break through for building design. He adds insulation directly to the outer wall that improves heat loss 24/7. Then he boxes the insulation in the frames, adds building foil and the double glazed polycarbonate to trap the sun's heat.

Even at night he gets the benefit of 50mm of Rockwool and double glazing over the wall exterior, so all wall types including solid will benefit. In the winter sun he can get air temperatures up to 80C (presumably before the fans turn on - Ivan might confirm the SFG is similar). Just imagine if all walls from SE to SW are covered this way!

Thinking about Ken's cheap Arduino project we could all benefit from exporting data on the Internet on projects like these, to see how well they work.

-Paul
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30 tube thermal,
2.3kW PV see:
http://www.solarmanpv.com/portal/Terminal/TerminalMain.aspx?come=Public&pid=17067

LED lighting in every room
NO tumble dryer, +370 kWh per year
billi
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« Reply #32 on: January 01, 2009, 06:48:15 PM »

Paul credits to you that you studied that idea (Veco s air heater website)....   most houses ( in Germany no cavity) are insulated nowerdays with an external insulation  so there are ideas in this direction which covers solar gain and insulation

http://umwelt-wand.de/ti/index.html

Billi
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Ivan
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« Reply #33 on: January 02, 2009, 01:18:50 AM »

If sun is shining, with the fan running, it maintains an output of 55C until around 2pm (panel faces nearly East). Input air temperature will be around  16-18C. No idea of flow rate or how to measure it (hot filament like in a car airflow meter?)

On lightly overcast days, it still produces heat. On heavily overcast days, it produces nothing.

Whilst I was installing it, I was experimenting with various things, and a few times had no circulation during sunny (October) conditions. I could smell a slight 'cooked silicone' smell - so assume temperature was 150C+
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billi
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« Reply #34 on: January 02, 2009, 12:14:41 PM »

Ivan  how much are the air heating SFG panels  and how many tubes ? As well  how to install more then one panel ?

In parallel or in series ?  i expect the in and out pipe connections are not  big enough  to connect too many in series

Billi
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Ivan
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« Reply #35 on: January 02, 2009, 12:29:27 PM »

I think the price is the roughly the same as the SFB panel c.400 incl vat.

We've connected two in series for the Navitron showroom, but you're right - I wouldn't connect too may in series. I'd also recommend that all pipework between panel and heated space is kept as short as possible - air has a very low heat capacity, so it's easy to lose a large percentage of the heat, even in well-insulated pipes - hence through-wall pipework rather than down from roof.


* S6300296_small.JPG (54.27 KB, 359x269 - viewed 377 times.)
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Colin_A
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« Reply #36 on: January 02, 2009, 03:02:06 PM »

Hi Ivan
You can use a windspeed meter to measure the air velocity at the inlet and outlet.
From that you can work out the cfm flowrate and pressure loss in the panel.
You can add solar radiation data into the equation to calculate the efficiency.
I have a JDC Skywatch Xplorer 1, its a useful bit of kit for around 25.
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EccentricAnomaly
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« Reply #37 on: January 02, 2009, 10:33:24 PM »

I'm not sure that would be ideal. The vacuum tubes will produce heat on days when there's nothing available in the flatties, so you could actually pre-cool the air rather than pre-heat it on some days.

I think that depends on where the air is coming from.  In your SFG system you're recycling the air from the house, I think, so, yes, there would be a risk of pre-cooling the air if there was an inefficient collector before the ETs.  On the other hand, I was thinking of pre-warming incoming air for ventilation so, by its nature, it would initially be at the outside temperature.  If there's any warmth at all in the sunshine the polycarbonate flatties would provide at least a little warming.  Using an SFG panel to top this off is very attractive is it would rarely do much harm (other than introducing some flow restriction) but would often provide a big benefit.

Quote
The twin-wall plastic (you can also get triple wall) sounds like a good way to make a cheap large area panel. Cost per m2 would be very low - which means you could go for a large area (like Billi's German link).

For most British-type constructions a house needs some sort of ventilation gap (cavity) and rainscreen cladding.  My thinking is that the cavity should double as the solar collector and the rainscreen cladding should be polycarbonate.  This means that the incremental cost of a large solar collector over the same area of normal wall construction would be very low (a small number of pounds per square metre comparing the price a friend paid for western red cedar cladding and the cost of twinwall in Wickes).  In this case an important point is that there should be no condensation in the cavity - it should serve to remove moisture, not introduce it.  Hence it's probably not a good idea to feed in warm moist air from the house.

It's worth experimenting of course, but I suspect that once you start going to triple-wall the losses from the sunlight absorption would exceed the gains by extra insulation.  Some in the US (even in cold and dull parts) prefer single wall because it lets more light in even though it loses more heat.
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Brandon
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« Reply #38 on: January 02, 2009, 10:49:27 PM »

hmmm, this gets more interesting as it progresses

I am embarked on a large external insulating mission on our house, the front of which is south facing, I am going to lime render the sides and back of the house, but for the front I am now considering something along these lines.  I have previously thought about a 3' "conservatory" up the front of the house, the height of the eaves, the height of the eaves, and the width of the building, to act as both a heat gain and to insulate the house, this however would side step the summer over heating issue.

Now do we let it act purely under gravity, as we would a side arm PHE on a cylinder, or do we control and pump it?

Answers below please.

PS what would be the most affordable selective coating. or is black boat paint the best option, answers as before.
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changing the world, one roof at a time.

Quality is never an accident; It is always the result of
high intention, sincere effort, intelligent direction, and
skilful execution; It represents the wise choice of many
alternatives.
Ivan
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« Reply #39 on: January 02, 2009, 11:07:33 PM »

A south-facing conservatory to the eaves is a very good idea (looks nice too, and also gives you somewhere warm and sunny to sit during the winter - which can be very pleasant). Maybe use a pair of double-glazed doors to open up into the conservatory so it becomes part of the house when the sun is shining - allows the heat to enter easier. Whether to pump the air or not is a matter of personal choice, I think. It does allow for automation, though.

For DIY panels etc - I'd stick to black paint. It's not as good as a selective coating, but doesn't deteriorate in damp conditions like selective coatings will.
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Brandon
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« Reply #40 on: January 02, 2009, 11:19:07 PM »

thanks ivan

I was meaning that I am now thinking of the panel idea, the conservatory idea faded somewhat when the large pile of windows from the armoury that I was welcome to got skipped by an over eager labourer fume
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changing the world, one roof at a time.

Quality is never an accident; It is always the result of
high intention, sincere effort, intelligent direction, and
skilful execution; It represents the wise choice of many
alternatives.
rt29781
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« Reply #41 on: January 03, 2009, 05:43:31 PM »

Hi, I have just removed a very large conservatory from the south side of our house on the grounds that the heat supplied is indiscriminant and difficult to control.  As a replacement I have 5 x 30 tube Navitron Evac tube panels on a sw facing roof with underfloor heating.  That is controllable and works a treat.  In the summer the conservatory cooked the house.  So we needed air con...
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Nowt currently, Aberdeen.....well actually very well insulated extension with passive solar that seems to heat the house....
EccentricAnomaly
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« Reply #42 on: January 03, 2009, 05:49:46 PM »

PS what would be the most affordable selective coating. or is black boat paint the best option, answers as before.

I've not heard of a selective coating which is affordable over these large areas.  I think that with the relatively low temperature differences involved selective coatings wouldn't be so important, anyway.  (The temperature difference being limited by the relatively poor insulation of any practical covering, particularly one which will admit a large percentage of the arriving sunlight).

By the way, I've done a little calculator web page to show roughly how much sunlight will get through a panel containing one or more layers of various materials, rather than be reflected off, at different angles of incidence.  It doesn't take into account the effects of absorption in the materials, though: http://www.edavies.nildram.co.uk/2008/10/fresnel/

One rule of thumb I have read somewhere is that the thermal collector (black bit) should present three times the area to the air being warmed than it does to the sunlight.  A simple double sided sheet would present twice if the air could flow on both sides.  Some sort of louvre like structure, like the radiator on the back of a fridge, seems ideal.

Another trick is to try to keep the warm air behind the collector (away from the glass or polycarbonate or whatever cover) to reduce heat loss.  This is done by angling the collector outwards from the bottom of the panel to the top so the cool air flows up between the collector and the cover, through the collector at some height where it is warmed then up behind the collector to the top.

The articles referenced from this page may be of interest:

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

The rest of that site is worth a look, too (i.e., lots of good stuff).
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djh
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« Reply #43 on: January 03, 2009, 06:15:39 PM »

Aesthetics are probably the major reason for choosing flatties.

On a new build, there's also the advantage that you can use them as the external cladding, usually on the roof, thus saving the cost of tiles or whatever.
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Cheers, Dave
Brandon
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« Reply #44 on: January 03, 2009, 06:33:43 PM »

the selective coating comment was rather tongue in cheek.

the other information you gave is very interesting, thanks for that.

the conservatory idea is not happening for exactly the reasons that you state, the uncontrollable heat in the summer.

Unlike you, I have no easy way to dispose of the excess heat in the summer, and therefore the 5x30 tube option is not suitable in my situation alas.

I am looking for a 600l store connected to 60tubes, the WBS and pellet boiler, all in good time.

I have approximately 25m2 of south facing wall that needs cladding, and if it can somehow benefit the house by providing some usable heat, then all the better.

Trying  to give the air 3x the collector area is going to complicate the construction rather, but as you say, a louvered effort would help , and is not impossible.

The beauty of the setup that billi linked to is the simplicity of it, and the ease of build.  The cost needs to be considered, as if it spirals up, then the whole project can be thrown off course, as some element of payback has to be considered, (not that I want to open that payback pus infested wound).
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changing the world, one roof at a time.

Quality is never an accident; It is always the result of
high intention, sincere effort, intelligent direction, and
skilful execution; It represents the wise choice of many
alternatives.
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