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Straw-infilled-Pallet Winter Greenhouse. - Page 2

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Posted by zoe_lithoi on December 25, 2014, 11:10 pm
 
Straw-infilled-Pallet Winter Greenhouse.  
Greetings,
   I've taken all the postings, and condensed all the refinements into this
 one posting....

My son and I are building a 80" x 80" x 8'(tall) 'winter' Greenhouse made o
f pallets screwed together and stuffed with straw for insulation which I es
timate would be about R10, but which, based on temperature measurements, tu
rns out to be about R8.8.  It has plastic stapled to the inside which shoul
d prevent heat loss by convection. The Roof has masonite siding on it in ca
se of rain. We will have 200W of grow lights which will also serve as the p
rimary heat source. The 200W grow lights, based on the temperature data, yi
eld only about 90W of heat, while the rest of the electrical energy is conv
erted to light. The ground will be a heat source when the outside temperatu
re is lower than about 53degF, and the ground is a heat sink when the outsi
de temperature is greater than 53degF. The goal is to keep the temperature  
above freezing so the seedlings don't die.  

The surface area of the 4 walls, and ceiling would be:  

A = 4*7*8 + 7*7 = 273 sqft

I took some temperature readings with a usb-type data logger for 2 days. on
e day had some 200Watt grow lights on, while the other did not. The 2 logge
rs, unfortunately were not very accurate because to start with, I had them  
both inside a room next to each other, and one read 80degF while the other  
read 82degF. I put one outside, and the other inside the greenhouse. I'm pa
sting the spreadsheet data here, and am not sure how it will appear when it
's processed by Google.

            No Lights    200W Lights    Notes
            Tmp-I    Tmp-O    Tmp-I    Tmp-O    
Date        Time    Inside    Outside    Inside    Outside    
12/22/14    1616    80    82            Both Temp Probes
                            In House
12/22/14    1816    63                
12/23/14    0    57    56            
12/23/14    700    52    53            low Tmp-I & Tmp-O
                            Equalization: No Heat
                            Flow into or out of
                            Greenhouse
12/23/14    830    52    55            
12/23/14    1306        82            High Tmp-O
                            Heat into Ground
12/23/14    1420    62    69            High Tmp-I
12/23/14    1545    62    64            
12/24/14    0            50    35    
12/24/14    300                30    
12/24/14    500            43    30    
12/24/14    545            43    30    

I estimate the 'ground temperature' equals 53degF by noting that with the l
ights off on  12/23/14 for a period around 700 (7am), the outside temperatu
re and inside temperature were about equal. I call this equalization. There
 was no heat flowing into the greenhouse from the outside, and there was no
 heat flowing into or out of the greenhouse through the ground. This was an
other way to confirm my estimate in the previous posting on this thread whe
re I said the ground temperature about 4inches deep was about 50degF. IT's  
not quite that simple. The temperatures 7 hours earlier at 0am on 12/23/14  
show heat flowing from the greenhouse to the outside. This heat is being su
pplied by the ground. So what has happenned is that the ground temperature  
had heated up (charged up) prior to that, and now this thermal capacitor wa
s discharging. The ground temperature had to have been greater than the gre
enhouse temp (57). What this tells me is that the ground temperature cycles
 on that day from about 53 to 58F.

Let's make a better estimate of the the thermal resistance of the Greenhous
e by looking at the temperatures around 1420 to 1545 on 12/23/14. The green
house temperature, Trm, was 62F, and the outside air temperature was about  
65 to 66F (taking into account the 2deg temperature error mentioned above.
  

=========================
=  
Wall and Ceiling Rvalue (Straw-Infilled Pallets)  
  Wood Door Rvalue
=========================
=  

The Rvalue of a strawbale is somewhere around R50 for a 24inch wide or so b
ale. So a 5inch wide straw-infilled palet might be 1/5th of this, i.e. R10.
 In this case, it would be over a 273sqft surface area (walls and ceiling).
  Lets call this Rgv. Really, though, the 2'x7', i.e. 14Sqft door, has an R
value of about R0.5. Leaving the Wall area = 273 - 14 = 259sqft

Hdr =  heat entering the greenhouse room of Temperature, Trm, from the ou
tside air of temperature To, thru the 14sqft of R0.5 wood door
Hdr = (To - Trm)degF*14sqft / R0.5 hr-sqft-degF/Btu]  
Hdr = (To - Trm)*28 Btu/hr  

Hrm =  heat entering the greenhouse room of Temperature, Trm, from the ou
tside air of temperature To, thru the 259sqft of Rgv walls and ceiling
Hrm = (To - Trm)degF*259sqft / Rgv hr-sqft-degF/Btu]  
Hrm = (To - Trm)*259/Rgv Btu/hr

=========================
====  
Ground Heat and the 'Warm Air Still Resistance'
=========================
====  
Heat travels about 4" through the ground in 1 hour. (see the appendix after
 my signature). In 1 hour, the heat is supplied to the air by the 4" of dir
t. There is what is called a 'warm-still' air resistance in series with the
 ground. It is small, and normally does not need to be taken into acount, b
ut because the ground also has a 'small' R-value, we need to take it into a
ccount.
R0.67 sqft-hr-F/Btu for the warm air film

Dirt has an Rvalue of R0.083/inch  

R0.083/inch * 4 = R0.33 sqft-hr-F/Btu for 4 inches of dirt  
plus  
R0.67 sqft-hr-F/Btu for the warm air film  
= R1 sqft-hr-F/Btu

Hgnd = ground heat  entering the greenhouse room of Temperature, Trm, thr
u the 50sqft of R0.33 dirt with temperature Tg  
Hgnd = (Tg - Trm)*50sqft / [R1 hr-sqft-F/Btu]
Hgnd = (Tg - Trm)*50 Btu/hr

If the room gets new air each hour equivalent to it's volume, then the air-
exchange heat loss for the amount of heat the air in the 400cuft (8'*7'*7')
 room  absorbs to go from To degF to Trm is:  

Hair = (To - Trm)degF * 1/55 Btu/F /cuft  * 400cuft  
Hair = (To - Trm)* 5 Btu/hr

Now let's look at the data at 1420 to 1545 on 12/23/14 as stipulated:
Trm = 62F
To = 66F (The temperature range wa sfrom 69 to 64, but remember that this
 temperature probe recorded a 2degF higher temperature at the same location
 and time as the other probe, so the temp range was really 67 to 62. AT 62,
 it would be the same temperature as the other probe. So we will look at th
e 66F.)
Tg = 57F (in reality it could be anywhere between 55 to 58F, but since it
's at the hottest part of the day and still charging up, 57F is a reasonabl
e estimate IMO.)

Kierkoff's Current (Heat) flow equation is now :  
  Hrm + Hdr + Hgnd + Hair = 0  
  (To - Trm)*259/Rgv + (To-Trm)* 28 + (Tg - Trm)*50 + (To - Trm)*5  = 0
  
  (To - Trm)*(259/Rgv + 28 + 5) + (Tg - Trm)*50  = 0  
  (To - Trm)*(259/Rgv + 33) + (Tg - Trm)*50  = 0  

Now  plugging in the temperature values just given above...
(66 - 62)*(259/Rgv + 33) + (57 - 62)*50  = 0  
(4)*(259/Rgv + 33) - (5)*50  = 0  
1092/Rgv + 132 - 250 = 0  
1036/Rgv = 118  
Rgv = 1036/118 = R8.8

Kierkoff's Current (Heat) flow equation is now :  
  Hrm + Hdr + Hgnd + Hair = 0  
  (To - Trm)*259/Rgv + (To-Trm)* 28 + (Tg - Trm)*50 + (To - Trm)*5  = 0
  
  (To - Trm)*(259/R8.8 + 28 + 5) + (Tg - Trm)*50   = 0  
  (To - Trm)*(29 + 33) + (Tg - Trm)*50    = 0  
  (To - Trm)*(62) + (Tg - Trm)*50   = 0  
=========================
====================  

Now, let's add a 200W growlite.
The heat from the growlights  
Hgl = 200W * 3.41 Btu/1Watt-hr = 682 Btu/hr

The problem is, that not all of this 200W is going towards 'heat'. Some of  
it is going towards light. We will have to compare theory with real data to
 make a better thermal model.

Kierkoff's Current (Heat) flow equation is now :  
  Hrm + Hdr + Hgnd + Hair + Hgl = 0  
  (To - Trm)*259/Rgv + (To-Trm)* 28 + (Tg - Trm)*50 + (To - Trm)*5 + 682  
= 0  
  (To - Trm)*(259/R8.8 + 28 + 5) + (Tg - Trm)*50 + 682 = 0  
  (To - Trm)*(29 + 33) + (Tg - Trm)*50 + 682  = 0  
  (To - Trm)*(62) + (Tg - Trm)*50 + 682  = 0  

The data shows that Tg varies from 53 to 57degF, 53 being the coldest part  
of the night. If the outside air, To,  is 30degF, what would Trm be?

  (30 - Trm)*(62) + (53 - Trm)*50 + 682  = 0  
  1860 - Trm*62 + 2650 - Trm*50 + 682  = 0  
  5192  = 112*Trm
  Trm = 5192/112 = 46degF
The data between 5am and 5:40am on 12/24/14 shows that Trm = 43degF when  
To = 30.
The 'theory' now matches the real data fairly well.

What could be the source of the 3degF difference (46degF - 43degF)?

Perhaps, it is our grow lites are not producing 200W of heat.

Let's make TrmC, and solve for the grow lite Heat, Hgl

 1860 - Trm*62 + 2650 - Trm*50 + Hgl  = 0  
4510 - Trm*(62 + 50) + Hgl = 0
4510 - 43*112 + Hgl = 0
4510 - 4816 + Hgl = 0
Hgl = 306 Btu/hr
Hgl = 306 Btu/hr * 1 Watt/3.41Btu/hr = 90watts

So, of the 200W electrical input, 90Watts make heat, and 110 Watts go towar
ds making light. So, from a heat standpoint, 90/200 = 45% efficiency.
From a light perspective, 110/200 = 55% efficiency.
 Hgl really equals:

Hgl = 0.45 * 200W  *3.41 Btu/1Watt-hr = 306 Btu/hr

Now, redoing the model:

Kierkoff's Current (Heat) flow equation is now :  
  Hrm + Hdr + Hgnd + Hair + Hgl = 0  
  (To - Trm)*259/Rgv + (To-Trm)* 28 + (Tg - Trm)*50 + (To - Trm)*5 + 306  
= 0  
  (To - Trm)*(259/R8.8 + 28 + 5) + (Tg - Trm)*50 + 306 = 0  
  (To - Trm)*(29 + 33) + (Tg - Trm)*50 + 306
  (To - Trm)*(62) + (Tg - Trm)*50 + 306  = 0  

If the outside air, To,  is 30degF, and the Ground Temp, Tg = 53degF, wha
t would Trm be?

  (30 - Trm)*(62) + (53 - Trm)*50 + 306  = 0  
  1860 - Trm*62 + 2650 - Trm*50 + 306  = 0  
  4816 = 112Trm
  Trm = 4816/112 = 43degF

If the outside air, To,  is 20degF, and the Ground Temp, Tg = 53degF, wha
t would Trm be?
  (20 - Trm)*(62) + (53 - Trm)*50 + 306  = 0  
  1240 - Trm*62 + 2650 - Trm*50 + 306  = 0  
  4196 = 112Trm
  Trm = 4816/112 = 37 degF

What would be the coldest outside air, To, such that the room temp, Trm =
 just above freezing, i.e. 33DegF? ( Ground Temp, Tg = 53degF)
  (To - 33)*(62) + (53 - 33)*50 + 306  = 0  
 62To - 2046 + 1000 + 306  = 0  
  To = 740/62
  To = 12 degF

If the coldest we want the room to be = 45degF, and suppose the coldest i
t gets outside is 0degF, then what wattage of grow lights would we need?

(To - Trm)*(62) + (Tg - Trm)*50 + Hgl  = 0  
(0 - 45)*(62) + (53 - 45)*50 + Hgl  = 0  
-2790 + 400 +Hgl = 0
Hgl = 2390 Btu/hr
Hgl = 2390 Btu/hr * 1Watt/3.41 Btu/1Watt = 700W

Since the grow lights are 45% efficient from a heat perspective.
700W/0.45 = 1560Watts of Grow Lights are needed.

Since the existing Grow Lights (200W light bulbs) produce 90W of 'heat', on
e could just get a 610Watt heater.

Toby

Appendix: Heat travels about 4inches into the ground in 1 hour.  
Let's look at the depth, D, below 1 sqft of slab surface, As, heat  
will travel in 1 hr, t:  
  t = time = 1hr  
  As = 1 sqft  
  tr = thermal resistivity = 1.7 hr*ft*F/Btu  
          (fig 11-1 of  my earth-coupled heat transfer book)  
  Cv = 30 Btu/F/cuft  
  td = thermal diffusivity = 1/tr*C  
-*-*-*-*-*-*-*-*-*  
C = 30 Btu/F/cuft * D*1sqft = 30D Btu/F  
  td = 1/[(1.7 hr*ft*F/Btu)*(30D Btu/F)] = 1/51D  sqft/hr  

D = (1/51D sqft/hr)/(D ft) * 1 hr  
D = 1/[51*D^2]  
D^3 = 1/51 = 0.31 ft  
D = (1/51)^(1/3) = 0.307' = 3.7"

Posted by rakeshmishra36 on August 28, 2015, 6:19 am
 
On Sunday, 30 November 2014 12:28:27 UTC-5, zoe_lithoi  wrote:

 of pallets screwed together and stuffed with straw for insulation (R10?).  
 It has plastic stapled to the inside which should prevent heat loss by con
vection. The Roof has masonite siding on it in case of rain. We will have 2
00 of grow lights which will also serve as the primary heat source. The gro
und will be either a heat source or a heat sink (not sure yet.....). The go
al is to keep the temperature above freezing so the seedlings don't die.

10 walls and ceiling to the 20degF outside  is:

 Btu/hr

r-exchange heat loss for the amount of heat the air in the 400cuft (8'*7'*7
') room  absorbs to go from 20degF to Trm is:

 Btu/hr

, for the southwest (NEvada, Utah, Arizona), 4inches deep (the approix dept
h heat can travel in the ground in 1 hour --- see the 'daycreek' thread in  
this group), is about 50degF. So as long as the greenhouse temperature, Trm
, is below 50degF, then the ground is a heat source and supplies heat to th
e greenhouse.

 travels about 4inches per hour, so:

 40degF

e dropped below 45degF.... AND Further, put the growlites on a timer to giv
e it 12 hours each day (so the plants can get light) during night-time hour
s when it is coldest outside.


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