Transformation of solar energy into heat

[blaschle]

TL;DR

hi guys I need help in matter of transformation solar energy into heat

I have one project in my mind bu tI have no eductaion in this topic. please help.

question is:

if there is container of water. and we have certain number of solar rays getting into this container

how does process of water warm up look like comparing:

1. direct sun rays into water
vs
2. on top of this conatiner we have some optical lens ( parabolic solar concentrator) and so rays get concentrated.

( the total number of rays is the same- entry to conatiner stays the same)

First law of termodynamic says that number of energy will be the same..

but I wonder if in our case we have some effect of energy concentration

what happened if we leave container open and rays get it comparing to we leave container in shadow but we deliver hot water in number and temperature warmed up by tge same number of sun rays.

thank for help

 

[DrClaude]

if there is container of water. and we have certain number of solar rays getting into this container

how does process of water warm up look like comparing:

1. direct sun rays into water
vs
2. on top of this conatiner we have some optical lens ( parabolic solar concentrator) and so rays get concentrated.

( the total number of rays is the same- entry to conatiner stays the same)

Generally speaking, there is no difference. I would even say that focusing could be detrimental, because it could lead to an overall reduction in absorption.

In any case, direct absorption by water is not efficient at all. Water is after all transparent to a good part of the electromagnetic spectrum. You would want to put it inside a black container.

what happened if we leave container open and rays get it comparing to we leave container in shadow but we deliver hot water in number and temperature warmed up by tge same number of sun rays.

I don't understand what scenario your are thinking of here.

 

[some bloke]

There was a thread on this a while back, about swimming pools I believe, and whilst I don't remember the equations behind it, the evaporative cooling of the water exceeds the heat input of the sun. So generally, if the water isn't covered, it won't warm up all that much.

You need to reconsider the terms you're using. You need to think of it in terms of Surface Area and the Intensity of the sunlight.

If you increase the surface area you are heating, then you will absorb more warmth from the sun - an easy experiment is to get a large tray and a deep tank, of similar materials and colours, and put the same amount of water in them, and leave them in the sun. The large shallow tray will heat up more. The heat is actually going to be mostly from the bottom of the tray heating up, as the sun passes effectively through the water.

If you are looking for an efficient way to heat the water, see if you can get ahold of a black hose. lay the hose out in the sun, not overlapping, and use a slow pump to push the water around it. The water coming back out of the hose will be warmer than that going in. The black colour will absorb more energy from the sun than the water could have, and the surface area being heated compared to the amount of water being heated will mean the water will heat up appreciably.

et us know what application you have for this project and we can advise further!

 

[blaschle]

thank you "DrClaaude" and "some bloke".
you both agree that "direct absorption by water is not efficient at all. Water is after all transparent to a good part of the electromagnetic spectrum."

so..

if in the same conatiner with the same sun ray input we have some device that is designed to turn sun rays into energy./heat and can warm up water than this going to be more efficient comparing to direct water warm up by sun rays.

yes?

 

[russ_watters]

if in the same conatiner with the same sun ray input we have some device that is designed to turn sun rays into energy./heat and can warm up water than this going to be more efficient comparing to direct water warm up by sun rays.

It would probably help if you use proper terminology/definitions: solar energy *is* heat. So all you have to do is capture/absorb it.

 

[anorlunda]

Are you thinking of something like this?

 

[jrmichler]

I wonder if in our case we have some effect of energy concentration

No. Heat in is heat in, regardless of how it gets there. When heating a container of water, it does not matter if the heat is spread out over the container or concentrated in a small area, because the water circulates and distributes the heat.

Similarly, heat out is heat out. It does not matter how the heat gets out, just that it gets out. Heat can get out by convection to cool air flowing past, by radiation, and by evaporation.

When heat in is more than heat out, the water gets warmer. As the water gets warmer, the heat out increases. At some point, the heat out increases until it is equal to the heat in. At that point, the water temperature stays constant. Similarly, if the heat out is greater than the heat in, the water temperature will decrease until the heat out is equal to the heat in.

 

[anorlunda]

When heating a container of water, it does not matter if the heat is spread out over the container or concentrated in a small area, because the water circulates and distributes the heat.

Correct. But if the heat is collected over an area bigger than the area of the container, focusing does help.

Below are parabolic solar troughs.

Then there's this monstrosity in California.

 

[some bloke]

thank you "DrClaaude" and "some bloke".
you both agree that "direct absorption by water is not efficient at all. Water is after all transparent to a good part of the electromagnetic spectrum."

so..

if in the same conatiner with the same sun ray input we have some device that is designed to turn sun rays into energy./heat and can warm up water than this going to be more efficient comparing to direct water warm up by sun rays.

yes?

Are you thinking an electrical heater powered by solar cells?

I can only assume that turning solar into electrical into thermal energy is going to exhibit lower efficiencies than simply using the solar energy to directly heat the vessel.

The system used for Solar Panels (not solar cells) is to heat water with sunlight. It uses a zig-zag of copper pipe, painted black, behind a glass screen to keep the wind chill out. It is more efficient than using electricity generated from solar cells, but is also more weather dependent - electricity can be sourced from elsewhere (wind, or mains) but there's no replacement for the suns heat for a solar panel.

 

[Rive]

2. on top of this conatiner we have some optical lens ( parabolic solar concentrator) and so rays get concentrated.

The problem with concentrators is, that you have to aim them, so the system has to be adjustable in angle, and with flexible connections.

1. direct sun rays into water

Water is not a good absorber. You need an absorber what can be heaten up by the sun with high efficiency: you need water connected to the absorber, so it can pick up the heat: you need insulation, so the heat can be transferred only to the water.

... just google up 'pool solar heater' for examples