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Data and monitoring: Reflective foil test

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  1.  
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      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011 edited
     
    I am always intrigued by the use of reflective foil as an insulator. It works well out in space, seemed to work well on my 'paint your tiles white' tests:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5643&page=1

    So to try and get to the bottom of it I tried a little experiment yesterday and am a bit perplexed by the results. So if anyone has any ideas about the results I could re-run the tests and see if it makes any difference (in the best scientific methods available to me).

    So what I did was get an 8W spiral CFL bulb, a glass container (Helman's jar, emptied), a power meter, couple of temperature data loggers and a plug in energy meter.

    First test was to just have the bulb on for 2 hours and then cool for 2 hours, noting the energy used and the temperature difference between the air in the container and the ambient temperature.
    Second test was the same but with the bulb covered in tin foil.
    Third test was like the first but the tin foil wrapped around the inside of the jar (this leaves some air gaps between the jar and the foil though).

    The Watts, Power Factor, kWh, AmP, V, VA, Hz were all near enough the same, except when the bulb was wrapped in foil. This showed half the energy use, though when I started the final test it had changed by 0.01kWh within 15 minutes, so this is probably down to the accuracy of the meter.

    So if anyone can think of an improvement to see what reflect coatings are actually doing I would be most grateful.

    Data and chart below:
    ,No Foil,Bulb Foil,Container Foil
    Watts,9.50,8.50,9.00
    VA,22.00,21.00,21.50
    V,242.01,243.42,241.85
    AmP,0.09,0.09,0.09
    Hz,49.99,50.07,50.05
    PF,0.43,0.41,0.40
    kWh,0.02,0.01,0.02
      Reflective Test.jpg
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 29th 2011 edited
     
    Where were the temperature loggers exactly? I'm guessing that one was some distance away, measuring the ambient temperature and the other was inside the bottle and receiving, radiated, conducted and convected from the bulb but not touching the bulb itself.

    The higher temperature with the bulb wrapped in foil indicates that the foil effective captured most of the radiated heat from the bulb and allowed it to heat up the air inside the container rather than pass through as light and IR, with a bit of UV as it's a CFL.

    The similar temperatures with the foil around the container and no foil around the container seem to illustrate that the foil did little to prevent heat loss from the container by convection/conduction. Presumably the foil reflected the radiated component back to the bulb and other surfaces, where it was absorbed and resulted in the container getting slightly warmer. Why the air didn't get warmer inside I don't know - maybe the foil was better at conducting/convecting heat from the container.

    If you could measure the surface temperature of the bulb and the outside of the container I suspect you'd be able to determine this for sure.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011 edited
     
    The ambient temp logger was on the bench, but shielded from any other heat source/light source. The one in the jar was towards the top (just as it starts to curve) and in when the jar was filled, it was between the glass and the foil. It was not touching the glass (blue tack was the thermal barrier).

    I am of the same mind as you but the power meter reading show a lower energy use (though I maybe should improve the meter reading intervals). Seems that less energy is used by the bulb but a much higher temperature is achieved. If that was so then wrapping everything in tin foil, shiny side in, would reduce energy use, and I can't see that being the case.
    Or am I missing something?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011 edited
     
    Posted By: JSHarrisIf you could measure the surface temperature of the bulb and the outside of the container I suspect you'd be able to determine this for sure.


    I should be able to do that, shall set up now.
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 29th 2011
     
    The energy difference may be an artefact of the CFL driver circuit sensing the higher temperature and reducing the drive current, or just the circuit unintentionally reacting to temperature in this way.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011
     
    I could test it in the hot box I have made, being of larger volume it may restrict the temperature rise. Would also be able to fit the foil to the sided of the box easier. Could try and get an incandescent low wattage bulb and see that that does, trip to MacS's I think.
  7.  
    Is it anything to do with the vacuum inside the bulb? Which is why reflecting foils work well in space but not inside the atmosphere where convection and conduction dominate heat transfer, and radiation becomes negligeable?

    It would be interesting if you could do the same experiment with a vacuum inside the jar.
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 29th 2011
     
    <blockquote><cite>Posted By: bot de paille</cite>Is it anything to do with the vacuum inside the bulb? Which is why reflecting foils work well in space but not inside the atmosphere where convection and conduction dominate heat transfer, and radiation becomes negligeable?

    It would be interesting if you could do the same experiment with a vacuum inside the jar.</blockquote>

    There isn't a vacuum inside a CFL bulb. They are gas filled, usually with a noble gas plus mercury vapour.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011
     
    Same as incandescent bulbs, they are usually filled with an inert noble (Could have been called an Archer a few years ago 2001-2003).

    Even if I had a decent vacuum pump I doubt if I could pull enough vacuum in a jam jar :wink:

    Unfortunately I had to go out half way though the latest test, so shall continue it later.
    • CommentAuthorEd Davies
    • CommentTimeDec 29th 2011 edited
     
    I think part of the puzzle here might be the relative emissivities of the aluminium foil at visible and thermal infrared wavelengths.

    http://en.wikipedia.org/wiki/Aluminium_foil#Properties

    Says “[t]he reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%” which imply an emissivity for visible light (and probably for short-wave IR and UV) of around 0.11 and .2 for the two sides. On the other hand

    http://www.engineeringtoolbox.com/radiation-heat-emissivity-aluminum-d_433.html

    gives emissivities well below 0.1 for thermal IR (long) wavelengths. Chromed materials which aren't otherwise heavily heat-sunk tend to get hot in bright sunshine because of these sort of differences and I think that aluminium foil will tend to do so, too. Though they're very poor at absorbing short-wave radiation they're even worse at emitting thermal IR.

    The main mechanism for cooling the foil inside the glass jar will be by emission of thermal IR. The foil outside the jar will also be cooled much more effectively by convection so will be a lot cooler.

    Posted By: SteamyTea: “,No Foil,Bulb Foil,Container Foil
    [...]
    kWh,0.02,0.01,0.2”

    Is that last number missing a zero immediately after the decimal point?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011 edited
     
    Posted By: Ed DaviesIs that last number missing a zero immediately after the decimal point?


    Yes it is, shall go back at edit it at the top.

    So a test with shine side in and shiny side out as comparison may be helpful? As well as loggers on the glass.

    I just love tabletop physics, who needs CERN, knowing why my house weights several tonnes is not going to pay my electric bill :wink:
    • CommentAuthorEd Davies
    • CommentTimeDec 29th 2011
     
    Directly measure the temperature of the foil?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011
     
    Shall try that, I like wrapping things up in tin foil :bigsmile:
  14.  
    Was it a covered CFL?
  15.  
    maybe the foil wrapped on the CFL is helping the elements inside heat up a bit quicker to operating temp, reducing overall energy consumption.

    The CFL without the foil heats up the gas inside the CFL plus the air inside the jar, but with the foil wrapped around the CFL, the gas inside the CFL warms slightly quicker?
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 29th 2011 edited
     
    <blockquote><cite>Posted By: bot de paille</cite>maybe the foil wrapped on the CFL is helping the elements inside heat up a bit quicker to operating temp, reducing overall energy consumption.

    The CFL without the foil heats up the gas inside the CFL plus the air inside the jar, but with the foil wrapped around the CFL, the gas inside the CFL warms slightly quicker?</blockquote>

    There aren't continuously on elements inside a CFL either.............. (sorry.....)

    They light comes from current passing through the ionised gas, which creates radiation that then excites the phosphor coating on the inside of the tube. They work best at low temperatures, typically around 40 deg C.
    • CommentAuthorEd Davies
    • CommentTimeDec 29th 2011
     
    Have you considered a test with the foil wrapped round the outside of the jar?
    • CommentAuthorCWatters
    • CommentTimeDec 29th 2011
     
    It's quite possible the operation of the CFL is being affected by the foil. Because it's an electrical conductor.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 29th 2011 edited
     
    Just got back in

    Posted By: bot de pailleWas it a covered CFL?

    No, just a cheap stumpy spiral one.

    Posted By: Ed DaviesHave you considered a test with the foil wrapped round the outside of the jar?

    Yes, was on my list of test to do, shall be starting again tomorrow with a bit more care and take readings from the plug in meter at timed intervals.
  20.  
    "here aren't continuously on elements inside a CFL either.............. (sorry.....)""

    How long do the elements stay on for in a CFL?

    If I understand correctly the heating elements are there to create a mercury vapour, after which the mercury acts as the conducting element?
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 29th 2011
     
    The elements fire initially for a second or two to vaporize the mercury to vapour, as you say, after which it's maintained as vapour by ionisation and the elements effectively turn off. The temperature at which this happens is pretty low, around 40 deg C or thereabouts, much lower than an incandescent filament that typically runs at around 2,800 to 3,000 deg C.
    • CommentAuthorCWatters
    • CommentTimeDec 30th 2011 edited
     
    Here is my take on the experiment..

    You might think that the foil reflects the light but actually it absorbs most of the light. What happens is the foil reflects the light many times but each time it absorbs a little until eventually it's all absorbed and converted to heat. Obviously some is also absorbed by the bulb but lets ignore that for the moment.

    Where the foil is located matters...

    In the case of the foil covered bulb all the light is converted to heat near the middle of the jar. It's conducted to the outside world through the air and glass of the jar.

    In the case of the foil lined jar the light is converted to heat near the inside surface of the jar. It's conducted to the outside world via the glass jar. Some of the insulating properties of the air in the jar have therefore been bypassed because the energy travels through that as light not heat.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 2nd 2012
     
    Bit of a disaster. The iButton that was on the bulb 'popped its case', so all is lost.
    Running out of iButtons now so this may well be on hold. I have some data and shall see if I can make any sense of it late.
    Think we can assume though that the bulb went above 85C.
    • CommentAuthorPeter_S
    • CommentTimeJan 2nd 2012
     
    Posted By: CWattersHere is my take on the experiment..

    You might think that the foil reflects the light but actually it absorbs most of the light. What happens is the foil reflects the light many times but each time it absorbs a little until eventually it's all absorbed and converted to heat. Obviously some is also absorbed by the bulb but lets ignore that for the moment.

    Where the foil is located matters...

    In the case of the foil covered bulb all the light is converted to heat near the middle of the jar. It's conducted to the outside world through the air and glass of the jar.

    In the case of the foil lined jar the light is converted to heat near the inside surface of the jar. It's conducted to the outside world via the glass jar. Some of the insulating properties of the air in the jar have therefore been bypassed because the energy travels through that as light not heat.


    I was thinking the same thing, so would it be possible to carry out this experiment with a foil 'balloon' neither touching the bulb or the jar to minimise material conduction? This is pretty much what the foil manufacturers are trying to get us to do in the installation guides.

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