Emissivity of glass

[Randall_James]

As much of the heat gain in tanks is directly related to the lighting, I know that low emissivity glass would be a big help. The issue is the amount of light lost (long wave length is the target of LE glass)

Gas exchange is the other issue, this is easy to take care of however. Mounting the glass midway in the hood and leaving the front section open (much like a bubble trap in a sump) and piping cooling air in at the bottom to travel forward/up and then out the back of the hood would take care of the gas exchange issue as well as remove convective heat.

The issue I am unsure of is the amount of light loss and how much additional wattage would need applied to make up for it. Does this additional wattage negate the effect of the LE glass?

http://www.mytankpics.com/tanks/albums/userpics/10002/hood.jpg

[stugray]

Randall_James,

I'm not sure what you are after... the emissivity of the glass is the characteristic of how well the glass radiates it's internal heat away. I'm not sure where this fits into your plan.

Stu

[Randall_James]

Radiant heat rejection = lower heat gain in tank

does this radiant heat rejection = light loss (likely as glass is filter)

If So
Light loss = need for more light

Is this loss significant enought to require addl lighting

IF SO
addl light = same heat gain as heat rejected at start
thus negating LE glass in first place

I do not have the ability to test light levels pre/post LE glass at floor of tank. The heat rejection is already documented I am sure.

[stugray]

Randall_James,

are you talking about using LE ( low-emissivity ) glass for the whole tank, or just the piece below the lights?

If it is for the piece of glass between the tank & the light, then the emissivity hardly matters. If the emissivity is low, then the glass will just get hotter before it radiates away the heat. If the emissivity is high, then the glass will reach a lower temp before it radiates efficiently. Either way, the overall heat into the tank wont change. ( there would just be a lag-time from when you turn off the lights and when all the heat goes from the glass to the tank )

Since this one piece is totally enclosed in the hood, then it's radiative capability is mostly moot. If one end stuck out ( of the hood ) a few feet, then I would use high-emissivity glass to help move the heat from the tank, and radiate outside the hood.

I imagine that the convective cooling ( fans ) will make much more of a difference than the E of the glass.

I would imagine that a parameter that is more of interest is the transmission of light ( of the frequencies we are interested in ) for our purposes.

Stu

[Randall_James]

Most of the heat is contained in the long wave length light. The glass can reject that radiant heat gain but at the expense of some light loss.

This reflected radiant energy is then removed from the canopy by the exhaust fans. The same effect so to speak as putting a chunk of glass between you and a campfire.

I guess I was hoping that someone had some experience with this glass and could say whether the heat rejection properties coincided with significant light loss.

I would have however thought that the setup would stop the heat gain by the water while at the same time that same energy was exported out of the canopy..

[stugray]

Randall_James

In fact, you may be right. If the glass separator was designed to absorb more of the infrared, and then have that heat removed via convection, it could benefit the tank's heat input.

IF - reef tanks dont need the infrared, AND the majority of the heat from the lights is transmitted to the water via Infrared, then glass that absorbs on those wavelengths may be a way to keep the heat from the water. ( as long as you actively cool the glass )

I'm not sue low E is the right glass for the task, but you may be on to something.
Stu

[45commando]

Doesn't any transparent surface increase the heat given off from a light source?I thought that is why we use greenhouses,whether made of glass or acrylic.It might be worth running a test with your idea,but you'll also going to have to allow for salt spray that is going to get blown onto the glass from the fans.Also will it interfere with regular maintainance etc.HTH.

[Randall_James]

Regular glass does in fact allow this transmission and capture of heat. (magnetic energy to be more precise if I recall my physics correctly)

We are not dealing with just plain old glass here. This glass is specially coated (vapor) with compounds similar to what is used in spacecraft and some oven doors I have seen. This compound has been extensively reworked to block as little visible light as possible and stop as much of the long wavelength magnetic energy as possible. (This is radiant heat and is the same type of heat you feel from a campfire, these coatings are metal based to reject the magnetic energy)

At any rate it rejects these "waves" in the first place. So the heat becomes trapped in the upper layer (this does require a similar heat shielding on the roof of the canopy also to presumably stop any conduction or convective heat transfer) waiting to be either reabsorbed or pushed on out into the room.

Salt spray is a non issue. There is none now so would be none by the simple addition of the glass panels.
I think if there was an issue with that, it would be on the MH, T5's or the PC lights that are there now.

Normal glass tops on tanks are victims of salt creep and evaporation normally. The fans already run and do not pickup any salt spray so I doubt that this is going to be of any concern.

The only thing I ever enter the hood for is to feed fish. I always remove the canopy for light service (learned that the hard way)

Also it would be easiest to put the plates in 3 segments so that they could be slid over each other should it be necessary to get them out of the way.

[42]

Originally posted by 45commando
Doesn't any transparent surface increase the heat given off from a light source?I thought that is why we use greenhouses,whether made of glass or acrylic.It might be worth running a test with your idea,but you'll also going to have to allow for salt spray that is going to get blown onto the glass from the fans.Also will it interfere with regular maintainance etc.HTH.

The reason glass houses get hot is that the glass or plastic will let high wave length light pass through it but reflect light with longer wave lengths.
Thus the light from the sun which is a relatively high wavelength passes into the glass hous, light then hits the soil/ plants etc and is reflected and radiated at a much lower wave length. This lower wave length light cannot pass through the glass and is effectively trapped in the glass house.

[stugray]

We must be careful what we are talking about here.

Low E-Glass is Low-Emissivity glass. that means that if it gets hot it will resist radiating the heat away. The reason low-E glass is used in windows, is so that it will not radiate the heat from the inside of the house outside. ( or at least it must reach a higher temp before it radiates away the same heat as a high-E glass ).

I think the kind of glass that Randall_James is talking about would be essentially a filter. It would reject the low frequency light ( Infrared - which is where the majority of the heat is transmitted ), while allowing the higher freq. ( Ultra Violet ) light to pass through.

If you were to use this type of glass ( High-Pass filter ), then the heat could be stopped ( and taken away via convection ) before it reaches the water.


42 - your wording was a bit misleading ( "high wave length light " vs " longer wave lengths" ), but it is essentially correct. Most of the sunlight gets in, but the IR doesnt get out. For our tanks, we want the opposite: Filter what light gets in before it has a chance to raise temps.

Stu

[42]

You're right, I didn't mean to mix and match like that. High frequency short wave length and low frequency longh wave lenght.

[Puffers]

I thought it was the UV light that also transfers heat, isn't that why I put on UV sunblock so I don't burn my skin in the summer?

[Randall_James]

Originally posted by Puffers
I thought it was the UV light that also transfers heat, isn't that why I put on UV sunblock so I don't burn my skin in the summer? Infrared transfers the "heat" (is above the range of the human eye)

Your sunblock knocks down UV "Ultra Violet" (what would be below the range of the human eye)

Sunblocks work on type A and B (280-400nm range)

UV comes in 3 types
UVA (partially blocked by ozone and what causes tanning)
UVB (barely affected by ozone, what burns the skin)
UVC (completely blocked by ozone, but bad bad stuff, used by sterilzers and are totally dangerous rays)

[Puffers]

So is it possible to block IR without blocking UV and allowing the visible light through? Would something like a hot mirror work in this situation?

[Randall_James]

Supposed to be able to do this with the glass in question. I do not have a lux/par meter to do any testing however.
Keeping in mind that all these waves are in fact magnetic, the metallic coating used in this special glass "Should" be able to selectively filter out the unwanted IR wavelength. I have a feeling the only way I am going to get good info is to purchase a lux or par meter.

Recommendations would be appreciated by anyone that has used/purchased or owns one.

[Puffers]

I'm assuming we need to test the par/lux to see or measure how much visible light we are or would be getting through?

[david8956]

May or may not be interesting to you, but I thought it was a good article about UV

http://www.advancedaquarist.com/issues/aug2004/feature.htm

[Randall_James]

Originally posted by Puffers
I'm assuming we need to test the par/lux to see or measure how much visible light we are or would be getting through? Precisely. The heat gain is easy enough to calculate thru chiller run times or simple logging of cause/effect over a few days.

However if the light loss is such that either the lighting period or intensity needs expanded, then perhaps the gains are not worth the efforts.

[FunkieReefJunkie]

There you go. Cost of materials vs. actual long-term savings. But it's very interesting and gives us all something to wrap our brains around. Just how much are we talking per pane of glass?

[Puffers]

Maybe I'm missing something, but if the par is too low with the new glass it won't matter how long we run the light cycle. The intensity of the light will remain the same.

Being that the higher K MH bulbs have lower pars than 10 k's I think we would be ok. We might be able to use different k bulbs to help accommodate the intensity with the new glass. Though I'm not sure the effect of IR heat of a 10k bulb vs a 20k bulb. Or we could lower the light by a 1/2" to make up the difference.