In our experimental site, we accommodate a CPC collector with the circulation of heat oil. During the process as the oil gains heat from the absorber and exchanges in evaporator, it returns to oil tank.
As it was seen in the experiments when the solar radiation decreased, the inlet oil temperature increased very rapidly.
Is there any one who can explain the physics behind this matter??
Dear Hussein,
The explanation lies in the characteristics of the heat exchanger.From your description it is so that the heat exchanger becomes more effective as the temperature of the oil increases. Which means as the temperature of the oil is relatively low the heat transfer from the oil to the exchanger is relatively low leading to more heat accumulation at low er radiation intensity. Please review the characteristics of the heat exchenger to see how it transfer efficiency changes with temperature.
wish you success
Dear Dr. Zekry,
Thanks for your outstanding comment, as I must add about the heat exchanger, it's such a simple tube and plate type where oil is cooled inside the tube by floating water on outer. Thus It would be acceptable that the amount of heat which is exchanged, differs by the outlet temperature. However, consequently oil returns to tank which smooths the inlet temperature from the changes in heat exchanger. Meanwhile, we observed such a similar relation in an open circuit current of air through the collector. How do you see this situation?
I would be so glad to hear your profound comments.
Dear Hossein,
what do you mean with open circuit current of air through the collector? would you please describe the system in more details such that my contribution be more useful..
Best wishes
Dear Dr. Zekry,
Of course, in this matter, I mean just like other conventional solar air heaters where there is always a fresh air at inlet and the heated air at exit.
Interestingly, when we tested the collector with air the same behaviour for decrease in radiation and increase in fresh air temperature was observed, would you see any relation with sensor error detection.
regards.
Dear Hossein,
The collector is in form of black metal sheet with metal tubes contacted intimately to the collector plate. Then you push air in the metal tubes to heat it and then take it away as a hot fluid. You observe that the outlet temperature of the gas is higher for lower radiation intensity. Is that okay?. iT is expected as the radiation increases the the temperature of the air streaming in the tubes increases. This is intuitive.
You found the opposite of what is expected. Is that right?
In order to scientifically analyse this problem and explain the results correctly we have to measure the temperature of the collector. The temperature of the tubes and the plate.We have to verify that it is homogeneous through out the plate.
I expect that as the radiation increases, the thermal contact resistance between the absorbing plate and the fluid tubes increases. E,G, their separation may increase.
Therefore the evidence is in the temperature of the tube and the temperature of the platen.This is my explanation to this effect.
Sorry for the late reply
Best wishes
Dear Dr. Zekry,
Thanks for your interesting reply. This information is actually perfect in case of there is a plate and tubes. However in our situation, there is not any plate in the collector. To clarify the configuration of this CPC I must add that like other concentrating collectors, this one has just tubular receiver (pipe) for the absorber element.
Meanwhile, there is another important note, as I said before, while the radiation decreases the INLET temperature increases. which is considerably detectable by our inlet sensor.
Actually the problem is here, Why the inlet temperature increases ?
Any way I really appreciate your time and very outstanding information.
Regards
Dear Hossein,
Thank you for describing your system in more details. Let me tell you what is my understanding about your system. It is so that you draw fresh air from the environment and push it in the concentrated radiation heated pipes. The problem is not with the outlet temperature, the problem is with the fresh air that is directly drawn from the environment. Is that right. Still it is the matter of the temperature of the virtual container representing the environment. You can preheat this fresh air by solar radiation if you passed it a relatively long tube and so control its inlet temperature.
best wishes
Dear Dr. Zekry,
Thanks for your profound statements. It would be a good idea in this case to preheat the air before collectors entrance. By the way, I really enjoyed your comments and ideas.
Regards
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