Hybrid Car – More Fun with Less Gas

Seeing Through the Thermal Darkness

register ::  Login Password  :: Lost Password?
Posted by Nick Pine on September 16, 2008, 4:44 pm
 
Laren wrote:


.... the slow-moving indoor airfilm resistance is R2/3, and the solar
gain is (80-70)1ft^2/(R2/3) = 15 Btu/h-ft^2?

That could happen with an outdoor temp of 30 F and net solar insolation
I = 150 Btu/h-ft^2 and a single-pane window with R2/3 and R1/3 indoor
and outdoor air films, like this, viewed in a fixed font like courier:

                        R1/3  (=1-2/3)
Tb = 80 = 30+I/3 --------www---- 30 F
                    |
                    |    ---
                     ---|<--|---|
                         ---
                          I = 3(80-30) = 150 Btu/h-ft^2

The rest of the window would look like this:

                  R1
Ti = 70 F -------www---- 30 F
             |
             |    ---
              ---|<--|---|
                  ---
                   I = 150 Btu/h-ft^2

with a net solar gain of 150-((70-30)1ft^2/R1 = 110 vs 15 Btu/h-ft^2,
ie 7.3X more solar gain.

Alternatively, using your calc below vs your words above, the net gain
would be (80-70F)1ft^2/R1 = 10BTU/hr-ft^2...

That's an enormous (11X) difference. What gives?


An R2 window with an R2/3 slow-moving indoor airfilm and an 80 F box temp
on a 30 F day would transmit 38 Btu/h-F of sun:

                          R1.33
Tb = 80 = 30+1.33I --------www---- 30 F
                      |
                      |    ---
                       ---|<--|---|
                           ---
                            I = (80-30)/1.33 = 38 Btu/h-ft^2

with a net solar gain of 38-((70-30)1ft^2/R2 = 18 Btu/h-ft^2

                 R2
Ti = 70 F -------www---- 30 F
             |
             |    ---
              ---|<--|---|
                  ---
                   I = 38 Btu/h-ft^2

vs the 5 you would predict? And with 150 vs 38 Btu/h-F of net sun,

                         R1.33
Tb = 80 = T+1.33I --------www---- T = 80-1.33xI = -120 F :-)
                      |
                      |    ---
                       ---|<--|---|
                           ---
                            I = 150 Btu/h-ft^2,


with a net solar gain of 150-((70-(-120))/R2 = 55 Btu/h-ft^2

                 R2
Ti = 70 F -------www---- -120 F
             |
             |    ---
              ---|<--|---|
                  ---
                   I = 150 Btu/h-ft^2

vs the 5 you would predict, leaving a large thermal darkness...

Nick


This Thread
Bookmark this thread:
 
 
 
 
 
 
  •  
  • Subject
  • Author
  • Date
please rate this thread