I think Dennis was speaking more of the angle of the glass relative to
the sun rather than the type of glass. Certainly, if a pane of glass was
parallel to the rays of sunlight, it would transfer virtually no heat or
light (only that reflected from other surfaces).
But they type of glass does make a difference. In residential and
commercial glass, there's a rating called Solar Heat Gain Coefficient
(SHGC) and there are treatments that have quite a spread of effects -
without tinting. I have no idea if any of these are allowed for
windshields. Or, perhaps, they are already used.
Second, insulation makes a huge difference. True, nothing will stop
heat from entering over time. But slowing down the rate of energy
absorbed directly relates to the energy needed for cooling. If you
slow the heat absorption down by 50%, you would need 50% less cooling
capacity (for the heat coming through non glass areas).
Peri
------ Original Message ------
From: "Michael Ross via EV" <[email protected]>
To: "EVDL Administrator" <[email protected]>; "Electric Vehicle Discussion
List" <[email protected]>
Sent: 02-Aug-14 7:41:47 AM
Subject: Re: [EVDL] Hybrid Mustang: AC or DC?
I think the solar gain has less to do with the glass than the small
cabin
size.
While involved with designing climate controlled cabins (at
Caterpillar) I
learned that that any glass you have - whatever tint level -
re-radiates
the absorbed energy into the cabin. The main effect of tint is to
reduce
the heat felt on the skin - the AC load remains the same. Likewise, the
roof, walls, everything basically sinks heat until equilibrium. No
amount
of insulation mitigates this when you are just out in the sun for long
periods. Insulation slows the conduction, it can't really stop it.
Convection from motion helps, but absorption and conduction carry on
as
always. If you could make the cab into a vacuum bottle...
The load on the AC ends up being pretty simple to estimate - how much
area
is in the sun. It is hard to win this game.
Reflecting light back out when not moving is always effective.
Reflective
tints (not generally legal in US) don't actually reflect much energy -
the
glass still absorbs the bulk of it, and it is radiated in as IR.
On Sat, Aug 2, 2014 at 3:26 AM, EVDL Administrator via EV
<[email protected]
wrote:
On 2 Aug 2014 at 2:03, Dennis Miles via EV wrote:
> On the other hand a friend in St Petersburg, FL. uses a 7,500 BTU
ac in
his
> S-10 Pickup (About a 400 watt load) and runs it from a 1,000 Watt
110v.
> inverter...
I can absolutely believe that. Pickup cabs are typically fairly
small.
They also usually have relatively vertical glass, for less solar gain
than
most cars.
Besides ...
http://evdl.org/images/aircon-daewoo.jpg
;-)
> Your 25000 to 30000 btu/hr are two to two and a half tons (Same as
HP
> or similar to Kw. ) and that is as much a a small house.
Exactly! A passenger car with lots of glass area and poor insulation
can
easily take that much capacity to cool.
However, now that I think about it, I'd guess that much of that
capacity is
there so the car cools down quickly after it's been sitting in the
hot sun.
You could no doubt use a much lower capacity aircon unit if you
allowed
plenty of time to pre-cool it. (Or always parked in the shade. ;-)
David Roden - Akron, Ohio, USA
EVDL Administrator
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