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a firmament greenhouse proposal

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Posted by Toby Anderson on July 20, 2004, 5:02 pm
 
Greetings solar heads,

Greenhouses suffer from the fact that the thin layer of
glass/plastic/polycarbonate etc., not only lets light through,
however, it also lets heat through as well ---

IN the summer, Greenhouse temps are too hot and often require fans or
evaporative cooling. In the winter, they are too cold.

The reason for this is that the glass/plastic doesn't offer much
thermal resistance --- heat can flow through it rather easily.

Some techniques help…

I personally built a greenhouse with 2 solar chimneys and a cooling
tower with misters. ---- or planting ‘cold tolerant' plants in the
winter.. or using floating row covers in the winter, or using ‘angel
hair (translucent) insulation in the winter. Having a large thermal
mass (like a pool) inside your greenhouse also helps. Nick Pine
suggests stuff like ‘bubble walls'.

Many of these ‘techniques' are helpful because they improve the
‘insulation' or ‘thermal resistance', but I think there may be another
way which focuses, instead, on using the phase change properties of
water.

Suppose your greenhouse's roof was made of water --- perhaps distilled
water. Sunlight passes through the water, evaporating some of it and
heating up some of it as well. Perhaps, when it gets too hot, it could
be used to water plants or used with a heat exchanger to help heat
your pool… or it could be pumped into a holding tank and then pumped,
at nite, into a large surface where it is cooled at nite by ‘nite sky
(black body) radiation', and then re-used the next day.

In the summer, when heat is a problem, my calculations show that 45%
of the sun's heat would be used in evaporating the water in your roof.
This evaporated water vapor (really distilled water) could be
condensed in a liquid bath (perhaps the holding tank mentioned above).
Some of the remaining heat is absorbed by the water. The result is
that much less of the sun's heat enters the greenhouse.

The book of Genesis speaks of a Pre-Flood 'firmament' around the earth
and 'the waters' above. Many bible scholars speak of this acting as a
way to moderate the earths temperature and humidity; further, they
say, it block the harmful part of the sun's ray, and perhaps kept the
oxigen lower in 'atmosphere.. hence, Pre-flood, people lived up to
1000 years old.

a lively stone,
  Toby

Posted by Toby Anderson on July 22, 2004, 12:46 am
 
Dear Solar Heads,

I brought this same topic (a water-roofed greenhouse) up about 6
months ago, and didn't get much of a response. Let me present a bit
more evidence. Any fisherman knows, that when you go fishing,
1. he's likely to get sunburnt --- the water reflects alot of the
light
2. the fish look bigger in the water than out of it -- sunlight gets
refracted (bent) by the water. --- resulting in the upper layer of
water absorbing a large percentage of the heat.

We can conclude, that if you had a 3inch deep water-filled roof made
of glass or twin-walled 'plastic'; then alot of heat from the hot
summer sun is not going to enter the greenhouse because some of it is
reflected back into the atmosphere, and some of it is absorbed into
the water. As the water temperature rises, some of it will evaporate
which takes heat out of the water, not due to convection or radiation
or anything like that, rather it is due to the phase change of water
going from liquid to vapor. The Potential Energy in the form of heated
water, is used in performing work in the form of Kenetic Energy.

The following article 'LIGHT PENETRATION IN WATER'
(http://www.utoronto.ca/env/jah/lim/lim02f99.htm ) says this:

"The absorption of water (in %) is very high in the infrared portion
(long wavelengths) and results in rapid heating of water by incident
light. Approximately 53% of total light energy is transformed into
heat in the first meter of water."

That is, in the first 36 inches of water.

The rest of the article is below my signature, and below that, I mess
around with some equations.

 a lively stone,
   Toby

•    Light Irradiance - measure of the number of photons passing through
a unit area (uE/s/m2)
•    Light Attenuation - in water there is a rapid reduction in light
irradiance with depth due to scattering and absorption of solar
energy.
•    The most common way of expressing the transmission or absorption of
light in water was developed by Birge:
% Transmission - 100(Iz/I0) = 100e ^(-n)
% Absorption - 100*(Io-Iz)/Io = 100(1-e ^(-n))
where Io is irradiance at the surface of the lake or some discrete
layer within the lake, and Iz is the irradiance at depth z, usually
taken at 1-m intervals below Io and n is the extinction coefficient.
•    The absorption of water (in %) is very high in the infrared portion
(long wavelengths) and results in rapid heating of water by incident
light. Approximately 53% of total light energy is transformed into
heat in the first meter of water.
•    Absorption by pure water decreases markedly in the shorter
wavelengths to a minimum absorption in the blue range and increases
again in the violet and ultraviolet (300 nm) wavelengths. Thus, below
depths of about 60 m, the only visible light left is usually blue.
Dissolved organic carbon (DOC), even low concentrations, greatly
increases the absorption of short wavelengths, resulting in more rapid
attenuation.
•    Light attenuation with depth increases exponentially according to
Lambert's Law:
Iz = Ioe ^(-nz)
ln Io - ln Iz = nz
where n is the extinction coefficient and Io is the irradiance at the
surface and Iz is irradiance at a particular depth z.
•    The extinction coefficient, n, is constant for each wavelength and
all wavelengths obey Lambert's Law.
•    Total extinction coefficient (nt) is influenced by water (nw),
absorption of suspended particles in water (np), and absorption of
dissolved colored substances (nc):
nt = nw + np + nc
nt values vary from 0.2 m-1 (about 80% transmission) in very clear
lakes to 4-10 m-1 in highly colored lakes or lakes with high
turbidity.

----------------
Let's find out what Iz/Io is for %Transmission = %80 & nt = 0.2m^-1

% Transmission - 100(Iz/I0) = 100e ^(-n)
 80 – 100(Iz/Io) = 100e^-0.2
80 – 100(Iz/Io) = 100*0.8187 = 81.87

80-81.87 = 100(Iz/Io)
-1.87 = 100(Iz/Io)

Iz/Io = -1.87/100 = - 0.0187

How much light is absorbed?

% Absorption - 100*(Io-Iz)/Io = 100(1-e ^-n)
% Absorption - 100*(1-Iz/Io) = 100(1-e ^-0.2)
% Absorption - 100*(1-(-0.0187)) = 100(1-0.8187)
% Absorption – 101.87= 18.13
% Absorption = 120%

120% is like impossible, so the real world results don't exactly match
the equations. I suspect, that ‘120%' absorption (basically 100%)
means that  about 100% of the light is absorbed in this layer of
water, and that if you were swimming in this layer of water, then 80%
Transmission means that 80% of the light from the light irradiance can
be seen. Hence, some (20%) of the light still filters through….

How thick is this layer?

Recall this equations from Lambert's law:
 Iz = Ioe ^(-nz)
Iz / Io  = e ^(-nz)
ln Io - ln Iz = nz
ln(Io/Iz) = nz
Io/Iz = e^(nz)

From above, Iz/Io = -0.0187  (but ignoring the ‘-‘)  we get:
0.0187 = e ^(-nz)

Taking the natural logarithm:
Ln(0.0187) =  -nz
-3.98 = -nz
z = 3.98/n
z = 3.98/0.2 = 20m

This seems reasonable becaue notice the article says: Thus, below
depths of about 60 m, the only visible light left is usually blue.

Posted by nicksanspam on July 22, 2004, 10:57 am
 

Negative light. Don't quit your day job...

Nick


Posted by Toby Anderson on July 22, 2004, 1:09 am
 Dear Solar HEads,

This is a follow up to the last posting. After my signature below, I
give my calculations.. Here are the results:

In summary, using a 3inch layer of water on the roof reduces the heat
entering the greenhouse due to the light by:   17+20 = 37%

Yet it let's 83% of the light through for the plants to ‘eat'.

a zoe lithoi,
  Toby

let's look at 3 inches of water, that is,

Z = 3 inches = 0.0762m

How much of the heat from the light does it absorb?

n*z = 0.2m^(-1) * 0.0762m = 0.01524

From Lambert's law:
Iz / Io  = e ^(-nz)
Iz / Io  = e ^(-0.01524)
Iz / Io  = -0.985

% Absorption - 100*(Io-Iz)/Io = 100(1-e ^-n)
% Absorption - 100*(1-Iz/Io) = 100(1-e ^-0.2)
% Absorption - 100*(1-(0.985)) = 100(1-0.8187)
 % Absorption – 1.5 .13
% Absorption .63

That is, the water absorbs about 20% of the heat from the sun in the
first 3 inches of water.

How much light is transmitted through the water?

% Transmission - 100(Iz/Io) = 100e ^(-n)
% Transmission – 100(-0.985) = 100e^-0.2
% Transmission + 98.5 = 100*0.8187 = 81.87
% Transmission = -16.63

I think this means, that 17% of the light photons are used
‘irradiating' the water, and thus 100-17= 83% of the light passes
through.

In summary, using a 3inch layer of water on the roof reduces the heat
from the light by:   17+20 = 37%

Yet it let's 83% of the light through for the plants to ‘eat'.

Posted by Toby Anderson on July 23, 2004, 12:30 am
 Dear Solar Heads,

Consider the following table showing the penetration of various light
spectra into the water (data is from
'http://www.lander.edu/rsfox/415lightLec.html '):

-------------------------------------------
color     penetration (m)
UV    70-80
blue    >100
green    80
yellow    70
orange    17
red    4
IR    <1

 *about 50% of incident energy is absorbed by first 2 meters
-------------------------------------------

Notice, that the IR light penetrates less than 1m deep into the water,
while UV light penetrates 70 or 80m deep.

This is more evidence for my proposal of having a layer of water, say
3inch thick, for the roof of a summertime greenhouse in a hot climate.
The water acts as an 'IR' (infrared) filter, that is, it filters out
the IR spectrum of light and passes the UV (ultraviolet light) ---
plants love UV light. The IR part of light carries the most 'energy'
with it, so, when the IR gets filtered into the water, the water will
heat up. Once the water is heated, alot of it will evaporate (which
takes heat out of the water in the work of changing the liquid water
into vapor)...i.e. 1000Btu per pound of evaporated water. Further, the
heated water can be circulated into your pool as a pool heater. Using
these methods, alot of the summertime heat can be kept out of the
greenhouse.

Below, after my signature, is more of the internet article mentioned
above,

Toby

b. Lambert's Law

*light intensity decays exponentially with depth

*intensity at any depth (Iz) is a function of surface intensity (IO),
depth (z), and the wavelength-specific extinction coefficient (h)

*the relationship is expressed by Lambert's law


  Fig 2.3 Extinction of white light by pure water (Horne/Goldman,
1994. This is a plot of Lambert's equation. Note the exponential
relationship between intensity and depth. Note also the very rapid
extinction of light in this typical lake. light7.gif

   Fig E Light extinction in Lake Secession, SC, 14jun1998. At 5
meters intensity is almost zero. light15.gif

 c. The Extinction Coefficient

*the extinction coefficient (h, eta) is wavelength specific

*a measure of the fraction of light absorbed by a meter of water

*short wavelengths usually have small coefficients and diminished
absorbance

*long wavelengths usually have large coefficients and enhanced
absorbance

*large coefficients are associated with rapid absorption and shallow
penetration

*small coefficients with slower absorption and deep penetration

*Lambert's equation is easily rearranged to solve for the extinction
coefficient (h)


  Table 41 Extinction coefficients for light in the visible and near
visible spectrum (Hutchinson, 1957).

Wave-   Extinction
length  Coefficient
(nm)      N
------------
820    2.42
800    2.24
780    2.31
760    2.45
740    2.16
720 (IR)1.04
700    .598
680 (red).455
660    .37
640    .31
620 (orange).273
600    .21
580 (yellow).078
560    .04
540    .03
520 (green).016
500    .0075
480    .0050
460 (blue).0054
440    .0078
420    .0088
400 (violet).0134
380 (UV) .0255
white light .1

 *minimum values are for blues

*values increase above and below blue

color     penetration (m)
UV    70-80
blue    >100
green    80
yellow    70
orange    17
red    4
IR    <1
 

 *about 50% of incident energy is absorbed by first 2 meters





kjvonly@usintouch.com (Toby Anderson) wrote in message


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