Water layer heat transfer

[dianad]

Hi,

I have a complicated question to make.
I have a solar system that stops when the solar collector reaches 100 ºC. At this moment, there is 40m of copper pipe filled with water that isn't circulating.
My problem is that from this moment ahead, the solar pannels continue to transfer energy to the water until it evaporates (if that's the case).
How can i calculate the temperature the water temperature along the pipe, so that i can predit it's value 5 m under the collectors position?

I think this is a conduction problem, of heat transfering within the water.

Does anyone know how can i solve this?

 

[IRstuff]

Why wouldn't simply be the solar heat input dissipated by conduction and convection along the pipe?

TTFN

FAQ731-376

 

[kevindurette]

A temperature measuring device may be your best bet. If you want to go closed-loop, overshooting by a little won't hurt anything because it takes a LOT more energy to boil water than it does to change it from, say, 95C to 99C. Once you hit 100C, extra heat will just change the mixture quality.

This is largely a radiation problem, actually. When you concentrate the sunlight, you're EFFECTIVELY moving the object closer to the sun (by increasing the radiation density). If you end up with the power of 2 suns, it's as though you're 0.71 au away.

I made up a little something at home with these calculations because I've recently been interested in solar energy. I derived the solar energy density from it and came out to within a very high degree of precision with respect to measured values. I'm at work right now, however, so I don't have it front of me.

If your collector stays at sane temperatures (below 2000K) I would go ahead and assume 1300 W/m^2 of radiant energy. As the collector gets hotter, it actually absorbs less energy from the sun because it starts to radiate it back out. If I remember correctly, a collector at the melting point of tungsten winds up being only something like 80% as efficient.

I hate not having this in front of me because I already did all the work. Bug me so I'll remember to come back on here and expand on this: kevin.durette@yahoo.com

 

[dianad]

Thank you all for the replies.

I still think this is mainly a conduction problem through the water layer, but i admite that can also occur a little natural convection if the water starts to vaporize.

My intention is to use heat transfer formulas to make his calculation. I've tried with some formulas but the results were not good.

My question remains in the air, for someone that can give me more help

Thank you all once more!

 

[guilloec]

I assume that you have a Flat-Plate Collector, probably Kevin was refering to a concentrating collector.
In your case you have to model the collector (heat from sun - Losses (radiation+cond+natural conv)-heat to flow of water), when you shut down the flow losses will go up, and some of this will conduct through the piping.
Once you estimated the losses through the piping, you need to model the pipes (heat conducted through the pipe and water - losses to air by natural convection- radiation would be negligible), that would give you a rough estimate.

It would be easier to a surface thermocouple at your position instead of performing the calculations.