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 GEOMERTYthe 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
From: "Joe Huang YJHuang@xxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Friday, 10 October 2014, 20:30
Subject: Re: [EnergyPlus_Support] why blinds not reducing heat gain through window?
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
From: "'Dr Yi Zhang' yi@xxxxxxxxxx [EnergyPlus_Support]" mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Thursday, 9 October 2014, 22:54
Subject: RE: [EnergyPlus_Support] why blinds not reducing heat gain through window?
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�YiFrom: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: 09 October 2014 19:17
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] why blinds not reducing heat gain through window?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
JoeJoe HuangWhite Box Technologies, Inc.346 Rheem Blvd., Suite 205AMoraga CA 94556yjhuang@xxxxxxxxxxxxxxxxxxxxxxxxhttp://weather.whiteboxtechnologies.com/ for simulation-ready weather data(o) (925)388-0265(c) (510)928-2683"building energy simulations at your fingertips"
On 10/8/2014 10:58 PM, Almofeez almofeez@xxxxxxxxx [EnergyPlus_Support] wrote: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
في ٠٩/١٠/٢٠١٤، الساعة ٨:٣١ ص، كتب "'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>: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.Jean
On 09.10.2014, at 03:04, "Joe Huang YJHuang@xxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote: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)
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Sent: Wednesday, October 08, 2014 7:07 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] why blinds not reducing heat gain through window?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.
JoeJoe HuangWhite Box Technologies, Inc.346 Rheem Blvd., Suite 205AMoraga CA 94556yjhuang@xxxxxxxxxxxxxxxxxxxxxxxxhttp://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)i. A.Jean Maraisb.i.g. bechtoldTel. +49 30 6706662-23
On 06.10.2014, at 21:53, "annavenediki@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote: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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