I am trying to perform an energy balance on surface 1 as shown in the attached figure. The main difference is that I am also trying to account for an airflow through the duct. Could someone give me some guidance as to how to account for all the radiation, convection and conduction terms?
First, I don't know how detailed calculation you want to do.
Second, do you mean the airflow trough duct is between black plate and glass cover?
If it is so, I think the simplest one is below (CMIIW):
1. Since there is air flow you can ignore conductive heat transfer at the air between black plate and glass cover. It is because the convection might be much higher than conduction.
2. Heat in: radiation absorbed by plate--> increase the plate temperature
3. Heat out: heat absorbed by air flow + heat loss trough the glass cover.
4. Heat loss through glass cover can be calculated by assuming temperature of glass is the same with air flow (after heated)
Of course above is the simplest estimation. If you want more detailed calculation you can use finite element method but it is very complex calculation.
Tri Ayodha Ajiwiguna, thank you for your response.
The airflow is between the black plate and glass cover.
I have the following equation for the energy balance at surface 1:
(Q Solar Radiation) = (Q Solar Absorbed) + (Q conduction (2-->1) + (Q convection 1--> Air)
I can help you Kasim Mohammed , but I need to understand what do you mean with: "the duct" ? Are you calling "the duct" to the interior of the solar collector ?
By the wat, I have in my Research Items (here in RG) my masters dissertation, is written in Spanish, but maybe you can get to understand the part where I also did an Energy Balance onto a Flat Plate Sollar Collector. You can try to see that too, if are pleased. I think is chapter 4.
The name of the document is: FLAT PLATE SOLAR COLLECTORS: ANALYSIS AND TEST METHODS. is inside the section 'Thesis'
Regards !
Kasim Mohammed : I apoligize, I just read your last reply, I wrote my answer first, and then read everything else.
Now my question is:
Why are you considering an airflow inside the collector ? I mean, this would be the case if the system were not in the steady state, would n't be ? But, I guess you want to work the problem considering the system (air inside the collector + absorption flat plate + bottom insulation, etc., ) in steady state.
If so, the velocity of the air above the pipes will very small, which means that you can neglect the Convection Heat Transfer between the pipes and the inner surface of the glass. The only mechanisms you will have between these two points will be Conduction and Radiation.. I don't know If you are following my argument ?
If you want to compute any of the temperatures, either the temperature of the absorber plate or/the pipes welded over the plate ... and/or the temperature of the glass, the only mechanisms you have to consider is Conduction through the almost motionless air between the plate and the inner glass and Radiation mechanism entering (or exiting) the collector
What do you think ?
But then, in your next reply, when you say: " determine the heat gain of the air as it leaves the collector " there I don't understand the configuration anymore. I think I need you to send me a more detailed diagram. Where is this air flow in your diagram ?
Regards !
Franklin Uriel Parás Hernández I am building a solar collector similar to the one in the example, I want to be able to determine the heat gain of the air as it leaves the collector and also the temperature of the surface T1.
I am assuming steady state but I feel like I am missing some terms as the outlet temperature is coming out greater than T1, which shouldn't be the case.
Kasim Mohammed :
Why are you considering an airflow inside the collector ? I mean, this would be the case if the system were not in the steady state, would n't be ? But, I guess you want to work the problem considering the system (air inside the collector + absorption flat plate + bottom insulation, etc., ) in steady state.
If so, the velocity of the air above the pipes will very small, which means that you can neglect the Convection Heat Transfer between the pipes and the inner surface of the glass. The only mechanisms you will have between these two points will be Conduction and Radiation.. I don't know If you are following my argument ?
If you want to compute any of the temperatures, either the temperature of the absorber plate or/the pipes welded over the plate ... and/or the temperature of the glass, the only mechanisms you have to consider is Conduction through the almost motionless air between the plate and the inner glass and Radiation mechanism entering (or exiting) the collector
What do you think ?
But then, in your next reply, when you say: " determine the heat gain of the air as it leaves the collector " there I don't understand the configuration anymore. I think I need you to send me a more detailed diagram. Where is this air flow in your diagram ?
Kasim Mohammed :
This will be the Energy Balance on the Absorption Plate. The final expression didn't end too explicitly to find the value of Tplate. But maybe you can solve it with an iterative method, if you can know the Temperature of the Air inside the collector and all the other properties and geometrical parameters ... The Tsky can be computed with some adjusted-to-experimental-data model, found in some Solar Thermal Energy Engineering.
another likely useful thing to notice is: as you show it on the forum of your Question; The Heat Flow transferred from the air flowing inside the collector, to the absorption plate is Qconv=mcp*DELTA(T), I'm sure this expression could be helpful too.
And finally, If you are also looking for the Heat Gain on the air mass flowing over the plate, I think this could be directly the Qsun
Regards , and Good Luck !
What is energy balance for? Is it for a certain time? aka what does the answer mean Kasim Mohammed Franklin Uriel Parás Hernández Tri Ayodha Ajiwiguna
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