A query about the physics of greenhouses

[barx1963]

Here's a question for physics people out there. I have had a discussion at work today about whether the temperature in side a greenhouse is hotter than the outside temperature. My work colleague argued that the temp inside would never exceed the highest ambient outside temp on any one day, and that they only retain heat to keep the inside temp steadier.
I argued that they actually retain heat and so end up warmer than the outside temp and even exceed the peak temp on any particular day. I cannot find any documentation to support either point so I thought better refer it to the panel of experts that is IIS!!:D
I am certain I am right as the same effect should apply to cars and I know I have heard that cars can exceed 50deg C on some days, but again I have no documentation to prove it.
Plus my protagonist in all this is an annoying Kiwi and I need to prove him wrong!!:P

Malcolm

[alocky]

First bloke to describe this effect was Svante Arrhenius. Google him and your debate is over. First thing your Kiwi mate needs to understand is some basic principles of thermodynamics, and radiative vs conductive transfer of heat. The greenhouse will only violate the second law when it gets hotter than the Sun's photosphere. The number of climate change denialists who lack even this level of understanding of physics is remarkable. So I'm remarking on it.
cheers,
Andrew.

[GrahamL]

I think your Both a little right and wrong in that your arguments arn't on comon ground. :)

The greenhouse should be holding a steady temp night and day
through heating/cooling / night/ day .


5000 in today and knackered ,roof vents are open and purging
thankfully or we'd all be dead :thumbsup:

[Baddad]

Hi Malcolm,:)

I grew up in a market gardening area West of Adelaide.:)

There were glass houses everywhere. Its all houses now. The market gardens are further out.

I used to enter some of these glass houses and eat the odd tomato.
I can tell you these were built for a reason. It kept the frost off the plants. Tomatoes needed warmer temperatures than the cold winter days provided. It was always warm inside these "hothouses".:)

In summer glass panels were opened to allow cooling. In winter the glass panels were closed.

Glass panels in winter were covered with condensation. It is a fact that temperatures were warmer inside therefore. The glass surfaces were cold but the air inside the glass house was warm and humid.

Has your Kiwi friend studied any physics? Has he heard of the "Greenhouse affect?." What the H--l does he think what the greenhouse affect is?

Even in SE Qld we have a glass house. At the Botanic Gardens. Mt Cootha. The tropical plant display is housed in there. It is quite warm in winter upon entry. Without a thermometer I would say its 5 or more degrees warmer. It certainly is felt upon entry.

Ooh I hate arguing with >?^&%&%$. That's why I say never argue with a fool. He will pull you done to his level and beat you with experience.

Ignore him. If you can't, I hope this helps.

Cheers:)

[Barrykgerdes]

Simply without delving into the physics. It is about the relative efficiency of the transfer of energy by radiation, convection and conduction through the media of glass.

The sun shines through the glass which allows the heat rays to radiate through it virtually unrestricted. This heat is absorbed by the contents of the greenhouse which get hotter. This heat also gets re radiated but mostly it heats the air which circulates. However glass is a good insulator and it does not let the air convection give off its heat by conduction. As a result while the sun shines the heat accumulates in the greenhouse.

At night the greenhouse will lose most of its accumulated heat by radiation.

Barry

[Baddad]

Hi Barry (gerdes) and Malcolm,

What i know of greenhouses agrees with what Barry has said.
I do want to add that the cooling at night is slowed by the glass covering.
The internal temp stays higher than the external temp. Then is regenerated once the sun hits it.

The same principle applies to solar hot water systems. They work. If your friend is of the opinion that the internal temp can not exceed the ambient then SolarHart and Modern Solar and Edwards and others are in a lot of legal bother.;)

Again, Is this person worth the effort? I assume it is as you see him every work day.

I know people who believe for some fantastic stroke of logic that recycling water will drain the oceans. Don't Ask. I can not explain that kind of conclusion. That person really believes it. I can say this; he is not capable of dealing with mathematics, subtraction is beyond him. Oh! He does have a current driver's licence.

Cheers

[Kunama]

I was secretly hoping the Kiwi was right ..... then i would have an excuse not to build the glasshouse my wife has been asking for and I could use the money for the OBS instead. Oh bother !!!

[barx1963]

The discussion with said Kiwi was good natured, we have these sort of discussions all the time, he or I will make a statement, the other will contradict and then we have a discussion. All very lighthearted and fun!
Good way to while away a quiet day at work.
Thanks for all your input and PMs, I will attempt to win the argument on Monday.

Malcolm

[CJ]

Hmm. Not too sure about that part of your post Barry.

[GrahamL]

The automated systems in modern greenhouses do a fair job
the curved roof vents are always opening/closing and adjusting
air movement into the tunnels , horizontal air flow fans moveing the air around inside and pulsing water vapor into the air stream can keep temps a little under ambiant (if needed) on a good day.

let him Know how hot it can get there though with nothing running and all buttoned down , the management system pinged me at home recently with an alarm code for high temps , checking on monday it
got to 63 c. I'd shut it down as nothing was in there , thats pretty dam hot eh?

[Barrykgerdes]

That is the way we were taught in physics 60 years ago (before political correctness) about the methods of transfer of energy.


The point I am making is that glass is an insulator for conduction, and convection (it is solid) but it does not stop the radiation of infra red rays. That means simply that the high energy heat gets in by radiation but is absorbed by whatever is in the greenhouse so only a small portion is reflected or emitted to radiate out.

It is also why solar hot water heaters work.

Barry

[sheeny]

Malcolm,

Your friend is right... if the greenhouse wasn't a green house but an insulated masonry building.;)

Glass has the property of being transparent to visible wavelengths of light but not to infra-red wavelength (i.e. heat). The greenhouse affect works by allowing visible light through, which is absorbed by the matter inside the greenhouse. When light is absorbed it is converted to heat.

Heat from the matter inside the greenhouse is then re-radiated as infra-red, but the infra-red cannot escape directly. Some is reflected inside the greenhouse, and some is absorbed by the glass. Of the infra-red light (heat) absorbed by the glass, Some is re-emitted inside the greenhouse, and some is re-emitted outside the greenhouse. The amount emitted inside and outside depends on the difference in temperature on the inside and outside surfaces of the glass and the thermal conductivity of the glass, but it will always be more on the hot side.

Keep in mind also that the peak intensity of the solar spectrum is in the yellow/green part of the visible spectrum. Also the energy of each photon is proportional to its frequency, so a lot of energy falls in the small visible band of the spectrum, but the infra-red band is wider.

Al.

[Ric]

Interesting info guys, thanks for that. :thumbsup:

I'm just about to purchase a greenhouse for the farm. It's a 6mm thick polycarbonate setup rather than glass. Do you think the principles would be the same or would it lose its heat quicker in the evenings. :question:

[mithrandir]

Also the glass is transparent to visible light and much less so to infrared, so the heat re-radiated from the greenhouse contents can't escape as radiation. Conduction through the glass or hot air escaping through the vents does most of the cooling.

[CJ]

It was the "good" part that puzzled me.

In the architectural game glass is considered a poor conduction insulator. Granted, the fact that it's used in very thin layers allows conduction to occur more easily, hence the requirement for double and even triple glazing in certain countries, but it is the gas or vacuum in the gap between that does the conductive insulating.

It's a good insulator compared to metals, on a par with stone but poor compared to plastics, timber, paper, gases and pretty much anything with a cellular structure.

Cheers.

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html