Low Iron Glass - Double Glaze for Greenhouse

Wow, you're way into the research on the glass options available, but maybe you are now too close to the forrest to see the trees.

I've got no history of glass selection, but have built a couple of "heat retaining" structures for different purposes.

You are correct to be concerend about heat loss through the metal fittings. Double glazing is obviously the way to go, but why not use conventional glass? How much "sunlight" will your plants need to grow properly. That's the answer that you need. Some places here, I'm using 80% shade fabric to grow vegetables (like french beans) in summer and have used 25% in winter. (Summer protection from direct heat and winter as morning frost protection.) There's still enough sunlight for the plants to grow and produce effectively.

Also, have you investigated "dutch greenhouses" to get the thermal benefit by reducing the exposed (heat radiating) surface that is above ground. This also saves the absolute amount of glass needed since there are fewer wall sections.

The companies that make solar hot water systems have done significant research into the glass characteristics and some have spray on coatings for the inside surface to trap heat inside the collectors. You might like to review those options if heat retention is your aim.

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TedT,

A few observations to think about:

• There is a good site sponsored by the State of Florida, that rates solar collectors. You might be able to look there to see the relative savings for different construction details.
• I didn't see any statement about Low-E coating on the glass. This is more significant, in my opinion, than the low-iron glass. I would strongly suggest you include that in your specifications. Low-E coatings are optimized for the particular geographic area and application, so look carefully at that and don't be too generic.
• Balance the incremental costs for different options against the anticipated payback (in terms of energy savings, etc.). Also, look at the greenhouse as a total system, not just as individual components. A seemingly expensive option in one area could reduce the capital costs in another area and therefore pay for itself. Even more true if you look at a series of options that all add costs but have a very beneficial net benefit (in one house design, a number of modifications each added to the cost but the net effect of all together was to eliminate an entire cooling subsystem and drop the total cost to nearly the same as before but at a significantly better energy footprint).
• Don't forget to optimize the balance between storage and incident energy, to avoid or minimize overheating problems.

I hope this helps. Good to see your post and interest in the topic.

--JMM

The low iron glass will let in about 5 -10 % more solar energy

What about increasing the amount of light falling on the green house by including a large piece of Silver Reflective Mylar sheeting to the north (assuming you're in the northern hemisphere).

Yes Dear,

As part of Ph.D. in Solare Energy Appications, I have done some experiments for optimization of number of glazing and and transmission of solar radiation through them. Though experiments carried out with 3 glass covers attain marginally high temperature. Two layer of glazing were found to be optimum.

As far as the cost involved for low iron glass is concerned, kindly check with your pocket, In my openion there is no need to go for such a cost intensive option when you have some more options open to you which definitely will deliver the same results with much less cost.

The simplest that I can suggest you is, to go for a slightly higher input radiation area that will give all that you wish to get by deploying Low Iron glass. As the inner glass get much hotter than the outer one, in my openion use plain/normal toughned glass as inner glazing or both glazing toughned if you can. Two layer of glass with selective coatings on the absorber, can generate a stagnate temperatue of the order of 170-180 degree cels in Indian/south Asian terrain. As your range of temperature is much lower than the system will produce, I can assure you that heat losses goes down tremendously once you circulate more volume of working fluid be it air, water or any other liquid or gas. More you circulate more heat you gain.

Hope this answers partly your worries-

Dr. N.P.Singh, Chief Engineer, ONGC ( A Govt. of India Company)

I am available at npsdhillon@gmail.com

I have done my Ph.D. in glass and have worked in materials development for most of electronic and energy business which includes solar also.

The goal is to have glass which does not consume photon and reflectes solar energy back from surface reflection. If I am designing green house I will be wory about

1. Halogen ions like flourine and chlorine in the glass since most of photochromic glasses are made by dopping high concentration of silver flouride to make the glass colored and block sunlight. This is extreme case example if you have less you will not see but that will decrease heat in side green house.

2. Iron as long as in oxide and bonded and taken care of by doppant to make glass clear I will not be worry about this element.

3. During my days of glass manufacturing for solar what is impootant to look are

(a) Percent transmission through glass

(b) percent reflection back inside so all light photon is converted to heat. This is done using coating of the glass in inside side of green house

(c) double layer of the glass with optimum gap

4. Clearity of the glass

5. Glass free of pinholes and other

Masyood

I believe in recycling as much as possible. In your case, once you've done your research on glass types you want to use, you may want to see If you can get used double glazed glass for your purposes. I'm sure you will find plenty of companies that replace windows and are more than happy to give you the materials for free. You could have the glass coated to keep radiated heat on the inside of the greenhouse If it's still a big concern.

Good luck