Is my builder being honest with me

[Agile]

The efficiency of solar power depends on collecting more heat than is lost. The lower the collection power the less net gain.

For example, suppose the panel loses 500 w by conduction and radiation from the back of the panel. If it receives 900w then there is a net gain of 400w. However if it only receives 550 w then it only gives a net 50 w.

Further north the sun's infra red radiation is traversing the atmosphere obliquely and less heat arrives at the earth's surface.

Tony

 

[ChrisRoberts]

I've heard different things re evac tubes. some say best thing ever. some say the coating which is exposed to the atmoshepe degrades in about 5-8 years. Plus it's always pishing with rain in scotland.

 

[Nigelo]

The efficiency of solar power depends on collecting more heat than is lost. The lower the collection power the less net gain.

For example, suppose the panel loses 500 w by conduction and radiation from the back of the panel. If it receives 900w then there is a net gain of 400w. However if it only receives 550 w then it only gives a net 50 w.

Further north the sun's infra red radiation is traversing the atmosphere obliquely and less heat arrives at the earth's surface.

Except that the collector losses are not static eg When collector temp = Ambient temp there is Zero temp loss. That's presumably why the thermal losses are usually expressed as a % in the 5% to 10% bracket.

Agree that solar power is reduced the further north we go but there is only 5.75 degrees difference in lattitude between my neck of the woods (Maldon in Essex) and Aberdeen. This approximates to half of a season so that Aberdeen's sun inclination is 6 weeks behind mine at this time of year(or 5 deg less if you prefer). There is still no reason I can see why the Jocks cannot use Solar power provided correctly specified.

Chris, you're the expert but I believe that Schott found the answer to deteriorating evac tubes - They place the mirror within the outer Tube under vacuum. That said I don't know who uses Schott tubes or equipment in the UK - Do you?

 

[Agile]

I expect that the reflective coating is aluminium.

Presumable this is deposited in the same way as a "getter" in a CRT. The tube is evacuated and the getter element is "fired" by an inductive loop and that heats and vapourises the aluminium which then condenses on the outer part of the tube ( outer as seen by the getter, inner as seen by you. ).

In the same way that a CRT leaks at the metal/glass seals I presume that the solar tubes also leak and the aluminium reflector oxidises and becomes dark and cloudy and less effective as a reflector.

Does this problem affect most of the solar tubes after 3-5 years or only the odd ones?

Tony Glazier

 

[Nigelo]

Agile,

Schott claim that the reflective coating is pure silver and that "The mirror cannot become soiled and does not age, thus maintaining a permanently high angle of reflection."

Regarding the manufacturing process, I think you may be right as I recall reading that they vaporise some compound within the vacuum which then deposits silver on the inner surface of the cool outer glass tube to create the "mirror" - Damned if I can find it now but further info is on link
http://www.schott.com/solarthermal/english/download/icr_eng_end_secure.pdf

Looks very good kit to me but I have no idea how cost compares to eg Sonnenkraft

 

[ChrisRoberts]

Sonnenkraft is a fair bit more expensive. But thier ceramic linned cylinders are the nuts.