Julie,
Thanks for sending me these links. However, I think it's best to
take this discussion offline, since it doesn't have much to do
with EnergyPlus.
Joe
On 10/11/2014 7:42 AM, 'julie, julie futcher, futcher'
julieannfutcher@xxxxxxxxx [EnergyPlus_Support] wrote:
Joe � I am happy to send
you a copy of my PhD thesis ‘The Role of the Urban
Setting as an Energy
Management Parameter’. My supervisor was Dr Gerald
Mills, my external examiner
was Professor Rohinton Emmanuel. Chapter two
examines the importance of the
role of urban form as a climate modifier, the
following 3 chapters are published
papers that demonstrate the role of form�. My email
is julie@xxxxxxxxxxxxx
Also, below are two links
to articles around this theme that we had published
in the CIBSE journal, earlier
this year� hope this helps to keep the discussion
going!
regards
Julie
As one of the founding members of
the Urban Heat Island Project in LBNL
back in the mid-1980's, I believe I have
some understanding of the Urban Heat
Island. The reason I do not consider
reduced night sky radiation as a major
cause is that the amount of night sky
radiation per surface area from an UHI
is roughly the same, and actually a
little more because its elevated
temperatures, than that of the
surrounding area. However, due to the
thermal mass of the buildings, there is
more heat to be rejected, and thus it
takes several hours for the UHI to
dissipate. In other words, it's the
increased thermal mass and low albedo
that are the prime drivers in creating
the characteristic UHI temperature
profile - no difference from the
countryside at sunrise, slowly
increasing through the day, peaking
right after sunset, and then slowly
dissipating through the night. What you
call "trapping outgoing radiation" is to
me more a fundamental thermal
characteristic of buildings.
During the time when I was most involved
with the UHI Project in the early
1990's, we used several mesoscale
climate models, esp. MM5, to simulate
UHIs over entire metropolitan areas,
such as the whole LA Basin or
Sacramento, and then used those results
as input weather data for BES programs
like DOE-2 or EnergyPlus.
The two types of programs operate in
different domains, and are each so
complicated that we never thought of
combining them together.
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com/ for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
On 10/10/2014 12:57 PM, 'julie, julie
futcher, futcher' julieannfutcher@xxxxxxxxx
[EnergyPlus_Support] wrote:
there are many different urban
heat islands - the most commonly
talked about effect is the
nocturnal canopy layer urban heat
island...(CLUHI), that is the ait
temperature difference between a
non-urban setting and a urban
setting... you are right these
nocturnal air temperature
differences are found to be more
intense in the down town area...
where the buildings are usually
taller, blocking the transport of
stored heat to the cold night time
sky. this is governed by the
amount of visible that a point of
the urban surface can see...this
parameter is called the Sky view
factor (SVF)... Urban Geometry is
the prime mechanism for the amount
of heat that can escape the
system....
At the same location the
daytime air temperatures are often
found to be lower. as a result of
the same parameter URBAN GEOMERTY
the parameters you describe (1)
and (3) are found to make a
contribution, but these are not as
significant as the role of urban
geometry...
Whilst materials & thermal
storage plays a role, it is the
trapping of this outgoing
radiation that is the main
factor...
The UHI are a significant and
miss understood phenomena....
unfortunately, energy+ other such
tools are limited in their ability
to report on these effects, which
cannot be accounted for entirely
with an air temperature uplift and
the lowering of wind speeds... BUT
this is something we are working
on!
I think its worth running
the thing again with the blinds
open at night!
J
I doubt very
much that blocking
heat loss to the
night sky is the
prime cause of the
urban heat island.
From my readings
and research, the
causes of urban
heat islands are
(1) the increased
amount of thermal
mass due to
buildings,
especially
downtown, (2) the
reduced albedo due
to the combination
of dark man-made
surfaces (asphalt,
concrete, dark
roofs) and
multiple
reflections
between buildings
and the ground,
and (3) reduced
wind speed due to
greater surface
roughness. The
urban heat island
is most intense
after sunset as
the countryside
cools quicker than
does the city, but
that's because
cities retain more
of the daytime
heat, rather than
have less capacity
to shed it to the
night sky.
I also remain
skeptical about
reduced night sky
radiation causing
blinds to raise
mean zone air
temperatures, and
if EnergyPlus
indeed does show
that, I would look
into its
calculation of
night sky
radiation.
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com/ for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
On 10/9/2014 3:21
PM, 'julie, julie
futcher, futcher' julieannfutcher@xxxxxxxxx
[EnergyPlus_Support]
wrote:
I'd run
the simulation
again - but
opening the
blinds at
night... as
suspect that
the heat is
trapped within
the
room...blocking
heat loss to
the night time
sky is (dare I
say it) the
prime cause of
the urban heat
island
effect.... it
would be an
interesting result
if open
the blinds
after sunset
reduced
overheating
risks!!
J
Good
analysis, Joe!
One possible
explanation to
the higher
room
temperature
may be that
shortwave
radiation
would have
fell on the
concrete floor
without the
blinds. So the
internal
blinds in
effect quicken
the thermal
response of
room�
Yi
What you
say is true,
but it's not
the only
effect of
window
blinds. Of
the solar
radiation
that's
intercepted by
window blinds,
most of it
is reflected
back out of
the window
depending on
its
reflectance, a
small fraction
(< 5%) is
transmitted,
and the
remainder
absorbed and
reradiated as
long-wave
radiation.
So, when I
hear that
window blinds
actually
increase the
indoor air
temperature,
this would
imply that the
reradiated
long-wave is
significantly
greater than
the reflected
short-wave,
which would
happen only if
the blinds are
heavy and
dark. However,
all the blinds
I've seen,
especially
those used for
solar control,
are either
white or very
light in
color.
It's pretty
well-known
that external
blinds are
more effective
than internal
blinds for
reducing
unwanted solar
gain. However,
this is the
first I've
heard that
internal
blinds are not
just
ineffective,
but
counter-productive.
In rereading
the original
post, I
noticed it
said "Zone
mean air
temperature"
and "Surface
inside face
temperature"
have both gone
up. I'm not
particularly
surprised at
the latter,
because blinds
tend to have
higher
absorptivity
than glazing
(although I
would caution
not to ignore
the
transmitted
short-wave
through the
window in
comparing
surface
temperatures),
but I am
surprised at
the former. I
don't want to
speculate
further on
this until I
know more how
the blinds are
being modeled,
how the zone
is being
controlled,
and how the
HVAC, if there
is one, is
being modeled.
The following
link is for a
paper done in
2004 studying
the same issue
in a similar
building (80%
glazing on the
facades) and
comparable
climate
(Canada).
That study
concluded that
internal
blinds reduced
cooling loads
on a continuum
depending on
their
reflectance,
with the best
(90%
reflectance)
approaching
but still
significantly
less effective
than external
blinds.
http://sbrn.solarbuildings.ca/c/sbn/file_db/Doc_File_e/Simulation%20design%20study%20for%20the%20facade%20renovation.pdf
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com/ for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
It may be
explained
that, the
blinds absorb
short wave
solar heat and
radiate long
wave heat. We
know glass is
opaque to
short wave
radiant heat.
Therefore
heat is
trapped inside
room.
من
جهاز الـ iPhone
الخاص بي
I can't
point to a
study, but the
guys at trnsys
have told me
that studies
have shown
that internal
blinds bring
little to no
positive
effect on room
heat gain.
However,
as we're on
the topic of
blinds, could
someone please
draw up a
table showing
which controls
are effected
by which input
parameters,
e.g. Control
type glare -
does it go the
the "fixed
slat" position
on trigger or
does it change
possition
between
min/max slat
angles to
maintain a
setpoint?
There are
a lot of
control
options...a
table would be
a great
addition to
the
documentaion.
Hmmm,
interesting.
Wouldn't the
blinds reflect
a fair amount
of the solar
back out
through the
window? Seems
counter-intuitive
that deploying
blinds behind
a window will
increase
rather than
decrease the
mean air
temperature of
the space.
Have we been
wrong all
these
decades? I'd
still
like to know
the
thermal/optical
characteristics
of the window
and the
blinds. I
suppose having
a black
blind behind a
window with
very low IR
transmissivity
might create
problems.
Joe
On 10/8/2014
4:45 PM,
'Edward G.
Lyon' eglyon@xxxxxxx [EnergyPlus_Support]
wrote:
The low
mass blinds
will also
absorb all the
solar and
transfer it to
the interior
air. Without
blinds, the
solar hits a
more massive
floor/walls
with much less
immediate
transfer to
the air. This
could give you
higher cooling
peak demand.
Ned
Lyon, P.E.
(MA, WV)
Staff
Consultant
SIMPSON
GUMPERTZ &
HEGER
781.907.9000 main
781.907.9350
direct
617.285.2162
mobile
781.907.9009 fax
http://www.sgh.com/
I suspect
there's
something
wrong in the
modeling of
the blinds.
The effect of
night sky
radiation is
not so
pronounced on
a window,
because (1)
half of what
it sees is
ground, (2)
the other half
is the sky at
low angles
where the sky
temperatures
are much less
depressed as
they are
looking
straight up
the sky.
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
http://weather.whiteboxtechnologies.com/ for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
If you
have a
building with
lots of glass,
don't forget
the radiant
exchange with
the night sky
at -60 C. The
"radiant night
cooling" is
possibly being
blocked by the
blinds, if the
blinds are
there during
the night.
Mit
freundlichen
Grüßen- Sent
from my
iPhone (excuse
the brevity)
Hello,
I've
designed a
south facing
room with a
floor-
to-ceiling
window and
blinds
covering the
whole surface
of the window.
I run two
simulations,
one with no
blinds on the
window and one
with blinds
covering the
window in
order to
compare the
results. The
building's
location is in
Eastern
Europe. The
run period is
from July to
August (sunny
and
hot period).
Both output
variables
"Zone mean air
temperature"
and "Surface
inside face
temperature",
have much
higher values
when the
simulation is
run with the
blinds on the
window. How
can this be
possible? Can
it be
explained in a
way or is
there
something
wrong with the
data?
Thanks in
advance,
Anna V.
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