Safe solar collector?

[ScienceNostalgia101]

So I've recently been getting back into my fascination with concave mirrors, and was considering using them and/or some other means of a solar collector for boiling water as we get closer and closer to those winter months.

 

However, before I float this with my landlord and/or boss for my home and/or workplace, I want to make sure I can keep potential fire hazards to a minimum.

 

A. If I make a concave mirror whose focal point is inside the mirror (ie. half-sphere concave mirror) does that mean that there is no risk of it causing a fire if it falls off the balcony or comes loose?

 

B. If I were to instead of using a concave mirror, use a marginally less efficient arrangement of several flat mirrors pointed at the same water-boiler, would that be free of potential for fire hazard? Would it depend on whether I used an actual mirror or just covered several flat surfaces in aluminum?

[swansont]

That’s not enough detail. The temperature limit for sunlight is the sun’s surface temperature, about 6000 K. You potentially have several hundred watts per square meter of light, so overlapping mirrors, or light outside the focus of mirrors/lenses can still get quite warm. Flat mirrors arranged analogous to a Fresnel lens could easily start a fire.

[ScienceNostalgia101]

Well, for starters I live in a town where the solar noon is presently 24 degrees and decreasing. So... sin(24)=0.4, about. Multiply that by 1368 watts per square metre and you get 556 watts per square metre.

 

I most definitely wouldn't go for the Fresnel-lens approach, more like, approximating the shape of a concave mirror through several flat mirrors. (Ie. Half an octagonal prism instead of a half-cylinder... or something like that.)

 

So obviously all 8 sides of an octagonal prism wouldn't equally reflect light onto the water boiler, as they wouldn't have the same angle with the sun's rays. So it'd be not less than or equal to 556, but strictly less. Where would I find information on the rate at which wood absorbs energy from sunlight?

 

Of course, thus far this is presuming I bring the concave mirror/solar collector back inside by February, which I doubt. Odds are I'd need to use a solar angle corresponding to sometime between February and May, as I don't want to have to put away a valuable source of heat energy. This is just more so to figure out whether or not I'm on the right track before I take this idea any further.

[Sensei]

14 minutes ago, ScienceNostalgia101 said:

Multiply that by 1368 watts per square metre and you get 556 watts per square metre.

~1370 W/m^2 receives the Earth.  ~1050 W/m^2 reaches the surface. The rest heats up the atmosphere.

Edited November 14, 2020 by Sensei

[swansont]

46 minutes ago, Sensei said:

~1370 W/m^2 receives the Earth.  ~1050 W/m^2 reaches the surface. The rest heats up the atmosphere.

If the sun is directly overhead

[HallsofIvy]

You appear to be using "concave" where you mean "convex".   A "concave"mirror cannot  collect light, it disperses it.

[studiot]

1 hour ago, HallsofIvy said:

You appear to be using "concave" where you mean "convex".   A "concave"mirror cannot  collect light, it disperses it.

 

Respectfully, I think you have misremembered this one.

 

Coancave mirrors have the capability of focusing incident light.

 

https://en.wikipedia.org/wiki/Curved_mirror

[ScienceNostalgia101]

Okay, another question now. What would be a good foundation for such a solar collector? I'm hearing that a halfpipe costs a minimum of $800 to build. Is there any cheaper alternative onto which I can place the tinfoil and still concentrate sunlight comparably well?

[Externet]

Solar heat collector:  I do not know what a 'halfpipe' is, but a whole pipe for sure works for well under $800 unless you go lenghty big diametres.  Irrigation pipe is not expensive.

Place under a clear plastic sheet/box/film so wind does not takes away the heat collected; or/and recirculate to a storage tank in basement. Maintenance is near null.