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  • vacuum tube solar collector CPC absorber temperature

    Hi all,
    I am trying to demonstrate that a vacuum tube solar collector CPC have not caused the fire on the roof of wood structure. So I am trying evaluate the thermal condition of the plant, in particular the temperature of the absorber part of the collector. I found the dry bulb air temperature (it was 6°C) and the radiation on a tilt surface that is 120 W/m^2 (it was early morning).

    Since the trasmission coefficient multiplicated for the absorption coefficient is 0,7, the absorbed radiation energy is 0,7*120=84 W/m^2.

    Then I thought to use the Stefan-Boltzman low,

    Eass = (5,67*10^-8)*(Tabsorber^4-Tair^4)

    Where
    Eass is absorbed radiation energy
    Tabsorber is the temperature of the absorbe
    Tair is the already measured themeperature of the air

    So i found the Tabsorber since Eass and Tair were already known.

    Do you think that this method could be right? I personally have some doubt since I have not considered the temperature of the glass collector face, and I have not considered the convection.
    Do you have any advice? Have you ever heard of fire caused by thermal collectors?
    Thank you so much
    Hi all,
    I am trying to demonstrate that a vacuum tube solar collector CPC have not caused the fire on the roof of wood structure. So I am trying evaluate the thermal condition of the plant, in particular the temperature of the absorber part of the collector. I found the dry bulb air temperature (it was 6°C) and the radiation on a tilt surface that is 120 W/m^2 (it was early morning).

    Since the trasmission coefficient multiplicated for the absorption coefficient is 0,7, the absorbed radiation energy is 0,7*120=84 W/m^2.

    Then I thought to use the Stefan-Boltzman low,

    Eass = (5,67*10^-8)*(Tabsorber^4-Tair^4)

    Where
    Eass is absorbed radiation energy
    Tabsorber is the temperature of the absorbe
    Tair is the already measured themeperature of the air

    So i found the Tabsorber since Eass and Tair were already known.

    Do you think that this method could be right? I personally have some doubt since I have not considered the temperature of the glass collector face, and I have not considered the convection.
    Do you have any advice? Have you ever heard of fire caused by thermal collectors?
    Thank you so much

  • #2
    Originally posted by amantino88 View Post
    Hi all,
    I am trying to demonstrate that a vacuum tube solar collector CPC have not caused the fire on the roof of wood structure. So I am trying evaluate the thermal condition of the plant, in particular the temperature of the absorber part of the collector. I found the dry bulb air temperature (it was 6°C) and the radiation on a tilt surface that is 120 W/m^2 (it was early morning).

    Since the trasmission coefficient multiplicated for the absorption coefficient is 0,7, the absorbed radiation energy is 0,7*120=84 W/m^2.

    Then I thought to use the Stefan-Boltzman low,

    Eass = (5,67*10^-8)*(Tabsorber^4-Tair^4)

    Where
    Eass is absorbed radiation energy
    Tabsorber is the temperature of the absorbe
    Tair is the already measured themeperature of the air

    So i found the Tabsorber since Eass and Tair were already known.

    Do you think that this method could be right? I personally have some doubt since I have not considered the temperature of the glass collector face, and I have not considered the convection.
    Do you have any advice? Have you ever heard of fire caused by thermal collectors?
    Thank you so much
    Hi all,
    I am trying to demonstrate that a vacuum tube solar collector CPC have not caused the fire on the roof of wood structure. So I am trying evaluate the thermal condition of the plant, in particular the temperature of the absorber part of the collector. I found the dry bulb air temperature (it was 6°C) and the radiation on a tilt surface that is 120 W/m^2 (it was early morning).

    Since the trasmission coefficient multiplicated for the absorption coefficient is 0,7, the absorbed radiation energy is 0,7*120=84 W/m^2.

    Then I thought to use the Stefan-Boltzman low,

    Eass = (5,67*10^-8)*(Tabsorber^4-Tair^4)

    Where
    Eass is absorbed radiation energy
    Tabsorber is the temperature of the absorbe
    Tair is the already measured themeperature of the air

    So i found the Tabsorber since Eass and Tair were already known.

    Do you think that this method could be right? I personally have some doubt since I have not considered the temperature of the glass collector face, and I have not considered the convection.
    Do you have any advice? Have you ever heard of fire caused by thermal collectors?
    Thank you so much
    Since you ask:

    Do yourself a big favor, save a bunch of wheel spinning and see Duffie & Beckman, "Solar Engineering of Thermal Processes", particularly chap. 7, on concentrating collectors. You need some background on the basics of heat transfer, particularly as it applies to solar energy.

    Whatever your motives, I have a lot of doubts about your methodology.

    As for fires, back in the early days (40 odd yrs. ago or so) of my foray into solar energy when I thought I knew a few things from reading DIY articles/mags/blurbs stuff, I caused a home built solar collector to catch fire during an experiment with side booster mirrors in sort of a truncated CPC arrangement. Long story short, no real damage except to my then wife's sanity. I returned to school that fall. I've gotten dumber by the day since then.

    Comment


    • #3
      J.P.M. Thank you for your kind answer. I already read this book, and I agree with you about the fact that it is a good book. I tried to follow the basics of the heast transfer on this book. But I am not sure that I have enough informations to find the absorber temperature. In the book I did not find anything about similar problem where it was needed to find the absorber temperature, so I tried to simplify the problem.
      Do you have any idea about the correct formula that I should use?
      Thank in advance

      Comment


      • #4
        Originally posted by amantino88 View Post
        J.P.M. Thank you for your kind answer. I already read this book, and I agree with you about the fact that it is a good book. I tried to follow the basics of the heast transfer on this book. But I am not sure that I have enough informations to find the absorber temperature. In the book I did not find anything about similar problem where it was needed to find the absorber temperature, so I tried to simplify the problem.
        Do you have any idea about the correct formula that I should use?
        Thank in advance
        You're welcome.

        The possible solution(s) you may be looking for are not found in one formula. Rather, some analysis and hypotheses (guesses) as to what may be happening are one way to go.

        The basic scientific method.

        The technical side of what I'll call heat transfer technology can provide some tools and pointers, but not magic plug/chug solutions. I've found the process to be iterative with a lot of head scratching and observation, followed by more cut/fit/try this solutions based on educated observation and applying information/knowledge of the type found in textbooks or gained from experiment and observation. Repeat as necessary.

        When you get to a conclusion that seems to fit (explain) the observation(s) and makes sense with observations by others doing similar work, you're probably headed in the right direction.

        Good luck. Heat transfer tends to have a reputation as somewhat one of the engineering black arts.

        Comment

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