Re: [Phys-l] modern light sources

[brian whatcott <betwys1@sbcglobal.net>]

On 1/18/2011 1:21 PM, Dr. Richard Tarara wrote:
> ----- Original Message -----
> From: "John Denker"<jsd@av8n.com>
> > We should also keep in mind that in an office building of any
> > reasonable size, there is no need for heating of any kind --
> > not from heat balls, heat pumps, or anything else -- because
> > the people and the office equipment produce more heat than is
> > needed.  They have to run the cooling equipment year-round.
> Not sure about this--certainly not in my 'neck of the woods'.  I can look at
> our Science building which is regularly occupied by many more people than
> would be in an office building, computers and the like in every office with
> extra computers around the building, lab and office equipment running
> constantly.  The building gets damn cold in the winter even though the
> building is heated.  Now to be sure, all our lights are flourescent and the
> building is not occupied (or barely so) in the evenings and weekends, but
> then neither are most office buildings.  Maybe I've missed something earlier
> in this thread though?
>
> Rick
>
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, Indiana
I wondered about this myself. Your observations may be saying something 
about the Science building's design standard, as well as the climate there.
I did a back of the envelope estimate to get a feel for the issue.

It is certainly the case that a bigger building ought to be easier to 
keep comfortable for the usual reason:  volume increases with L cubed 
while surface area grows with L squared,
and heat loss goes through the surface for some representative length L.
Still, its simpler to balance the least unit  case: one office cubicle: 
10 ft X 10ft with a ten ft ceiling. There's a total surface of 600 sq ft.
Suppose the whole shebang is insulated at 0.04 W/m.K, and suppose that 
the sole occupant whose dietary input is 2000 Cal, puts out 100 watts, 
and has a 60 watt lamp and a 60 watt appliance.     What temperature 
rise are we talking about here?
Say 600 sq ft =  56 sq m.   That thermal conductivity for a 15 cm 
insulation thickness
ought to transfer energy at this rate: 0.04W/m.K  X 56 m^2 / 0.15 m = 15W/K

The 220 W input should lift the inside temperature by 220/15 K = say15degC

But a window that provides an insolation of 700 W for 4 hrs/day on 
average and is shuttered at other times gives an extra 700/6 watt on 
average  say 120 W
which lifts the temperature an extra 8degC.
 23 deg C inside  from an OAT 0degC?
Now we're cooking!   That is, if I worked the numbers appropriately for 
my optimistic inputs  - - 6 inches of glass fiber insulation all round? 
Shuttered window?  Probably not!

Brian W