Heating

Peter
You should add some aluminium foil where you can (ie non glazed areas).
Shiny side inwards facing the airspace.
About 80% of your heatloss will be from radiation.
The polystyrene and bubblewrap insulation is effectively only going to be 20% efficient at best. It is transparent to radiant loss.

A chest freezer lined with 12" of bubblewrap instead of aluminium sheet wouldn`t stay cold for long for the same reason :D

 

Hex - that's interesting - its not something that I have considered. I had assumed that at the low temperatures involved radiation was pretty low. Have you got any sources on this? Its difficult to Google as most articles on radiation are to do with high temperatures.

 

Hi Peter,

Anything warmer than absolute zero (-470F approx) will emit radiation.. people,plants and just about everything else.
It is invisible and has no temperature until it hits something.. distance is no object, it travels at 186,000 miles/sec :D

The amount of radiation emitted from a surface depends it`s "emissivity" factor and the amount of radiation absorbed depends on it`s "absorptivity" factor, both are related.

In simple terms, every surface will reflect and absorb radiation to some extent.
The colour of the surface makes little difference.

Radiation is energy and is not necessarily the same as sunlight.. a mirror is great for reflecting light but very poor for reflecting radiation.

A perfect (but impossible) situation is to have all the radiation travelling between objects inside the greenhouse and not to anything outside.

At night especially, it`s likely to be one of the main modes of heat movement as it redistributes the energy gained in the day from one object to another.

Regarding information, this section on Radiation isn`t too technical (scroll down), and there`s info on the other types of heat movement etc too.
http://www.unu.edu/Unupress/unupbooks/80a01e/80A01E04.htm

 

Ver interesting Hex I think it's the right way to go using the heat sink theory in the greenhouse if only you could control the rate ofemmision at night time

 

Hi Walnut
Greenhouses are designed mainly for light
which makes them terrible for slowing down heat.

These day`s, energy efficiency seems to be a paramount concern and people are more aware of how to save energy (not leaving the tv on standby overnight, for instance)

How many people would open all the house windows at night and then put the heating on?
That`s pretty much how energy efficient a greenhouse is

 

Hex that reference is very interesting. I have had to have a think about this one.

If I may, I will disagree about the chest freezer. If it has a lid it is a closed box, so there will be no radiation involved other than between the inside walls and the contents, which will all be at the same temperature. Heat can only get in by conduction, so if the walls are insulated with enough foam etc it will work well. Actually my summerhouse is mostly made of wood, with just a front of glass. Consequent it works largely like a chest freezer, which is why I have never bothered to think about the problem. But I do agree that there will be radiation loss through the glass.

In the case of a proper greenhouse, you are absolutely right. Its a totally transparent box. So nearly all the heat comes in as radiation (sun light - short wave radiation) and nearly all will be lost at night through radiation (long wave radiation). And as you say, if you could drop reflective blinds at night, you would keep a lot of heat in.

Most people think of radiation as being high temperature (short wave) radiation, and its easy to forget low temperature (long wave) radiation. A good example of the importance of low temperature radiation is the fact that in winter clear nights are much colder than cloudy nights. On a clear night radiation is lost into deep space, which is so cold that it sends virtually nothing back, as the heat radiated is proportional to the 4th power of the absolute temperature. But on a cloudy night the earth will radiate as much energy outwards, but the cloud layer (which will be much warmer than deep space) will be radiating a lot of heat back. Acting like a big blind.

 

Hi Peter
I think the perception of heat is relative to us humans.
We might think -400F is pretty cold, but it`s still very capable of giving up some heat.

Better to think of radiation as radio waves, which it exactly what it is (electromagnetic radiation)

If you sat in a polysterene box with your tranny radio would you still be able to pick up radio 2?

Absolutely..because the foam insulation wouldn`t act as any form of barrier. A lead lined box would be more effective as the radio waves would be reflected

A mylar survival blanket (a sheet of silver foil in effect) has no value against conductive heatloss as it`s only a few microns thick.

It reflects the body`s radiant energy back, which passes straight through layers of clothing..just like radio 2 :D

Ideally you need to use different types of barrier to address each type of heatloss.

I think the reason the clouds keeping the heat in is mainly to do with the water vapour content,it absorbs longwave radiation quite well.

The air temperature at 40000ft is approximately -50 deg Celsius.. pretty chilly

[ 14. February 2007, 06:18 PM: Message edited by: Hex ]

 

Hex, you have exercised my brain cells more than they are used to! Its an interesting area that I have never really thought about. In Googling I came across this http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/eartherm.html#c1 Its a bit off topic but I would never really have thought that you could measure body temperature with any accuracy by infra red. You do learn by chatting to people and getting stimulated into a bit of googling.

I take you point about low temperature radiation just being an electromagnetic wave. My intuition keeps telling me that radiation can't play a part in the heat loss of a chest freezer, but I am searching for a reason. I was trying to find the wave lengths involved. Radio waves are measured in many meters and can pass through non metalic walls which are many orders of magnitude thinner. So you could hear your radio inside a polystyrene box. But I think that the waves could not pass through a wall that was thicker than the wavelength, they would be scattered and would not get through. This link says that body temperature radiation wavelength is about 9300 nanometers, which I think is 0.009 millimeters. I don't think that could penetrate the wall of a chest freezer.

However, I have been reading about this and it is clear that modern insulation uses a combination of ordinary insulation, and air gap insulation (which is the best conductive insulator) in combined with reflective sheets to prevent radiation across the air gap. Come to think of it nothing is new. Do you remember the old Thermos Flasks. The guts consisted of a glass vacuum flask, which was silvered.

 

Hi Peter
It`s good to question things, it`s the way things improve.

The old vacuum flask is a good example, the vaccum is one of the best insulators for convective and conductive heatloss and the silvering reduces the radiant losses.

As effective as the combined insulation is, the contents will still warm up or cool down eventually.
All the suns energy travels through a vacuum to get here, so a vacuum won`t stop radiation

Electromagnetic energy is quite potent stuff and will usually find a way to do it`s thing regardless.

Any temperature difference above absolute zero, results in energy being transferred by one or more methods, the best you can do is attempt to slow it down

 

Dragging the topic back to its original subject and away from gcse Physics, I am looking to put in a propane powered greenhouse heater and wondered if anyone had experience of the running costs of such a beast.

Electricity would be a lot of work to get in and paraffin isnt really convenient either. We had it in our last greenhouse but I would rather have something that doesnt need topping up quite so often.


I await the collective wisdom of the board...

 

Hello Dendys work collegue.

[ 08. March 2007, 05:26 PM: Message edited by: Scotkat ]