[Bldg-sim] Rooftop Snow Cover???

[Crawley, Drury]

Yes, EnergyPlus uses the albedo in the TMY3 or snow cover in other files
to adjust the diffuse radiation.

________________________________

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Wednesday, December 02, 2009 5:48 PM
To: muehleisen at iit.edu; Christopher Schaffner
Cc: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Rooftop Snow Cover???



I'm starting a new "thread" as this is a definite tangent from the
emissivity discussion of the past few days.

 

A question occurred to me on reading Ralph / Chris's comments below...
does ANY energy modeling program/engine today use the snow cover data
present in TMY2 files?  There's obvious implications for things like
roof surface thermal behavior and hourly photovoltaic array performance
when you consider the reality of significant snow cover presence through
the day...

 

Of note... I'm no expert, but for others' reference I found a nice
resource this afternoon clarifying the what/why/how of TMY files here:  
http://rredc.nrel.gov/solar/pubs/tmy2/

 

~Nick

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Ralph
Muehleisen
Sent: Wednesday, December 02, 2009 10:48 AM
To: Christopher Schaffner
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Why should roofs have high emissivity?

 

True indeed.

Except when your roofs are not covered with snow.

Here in Chicago, at least the past few years,  snow has come in bigger
chunks with more warmi and cold periods in between.  And rooftop snow
has been melting during the warm periods so  rooftops have been bare for
more of the winter.

It would be an interesting study (maybe I can find an undergrad or
interested masters student) to actually look at a typical flat roof
small commercial building and see if the increased summer efficiency of
cooling equipment from a cool roof offsets the increased winter heating
load.

Ralph

Ralph Muehleisen, Ph.D., P.E., LEED AP, FASA
Assistant Professor and Director of the Miller Acoustics Lab
Civil, Architectural and Environmental Engineering
Illinois Institute of Technology
Chicago, IL 60616
muehleisen at iit.edu
tel: 312-567-3545  fax:312-567-3519



On Wed, Dec 2, 2009 at 5:47 AM, Christopher Schaffner <
chris at greenengineer.com> wrote:

Of course, if your roof is well insulated, it will be covered with snow.
I doubt very much that your "warm" roof will really help much. Go with
the cool roof. 

--

Chris Schaffner, PE, LEED AP, LEED Faculty(tm)
Founder and Principal

The Green Engineer, LLP
Sustainable Design Consulting
50 Beharrell Street
Concord, MA 01742
T: 978.369.8978
M:978.844.1464
chris at greenengineer.com
www.greenengineer.com




On 12/2/09 12:26 AM, "Ralph Muehleisen" <muehleisen at gmail.com> wrote:

	Chris makes a good point to consider with cool roofs.
	
	A cool roof will indeed reduce solar heat gain to the roof which
reduces its temp (good in summer) and reduces cooling requirements in
summer but will increase heating requirements in the winter.
	
	Until someone develops a material where the emissivity changes
with temperature (and not just wavelength) a cool roof that is good in
summer will be bad in winter.
	
	So, in colder climates, a cool roof can indeed increase the
overall energy use of a building.
	
	Even so, some northern cities like Chicago, mandate cool roofs
in building code.  Why?  Because the cool roof will reduce the urban
heat island effect where the city has increased temperatures compared to
the surrounding areas.  
	
	The thought is that the overall benefits of the  reduction in
urban heat island effect in summer is more important than the increased
energy use that comes from increased winter cooling load.
	
	
	Ralph
	
	Ralph Muehleisen, Ph.D., P.E., LEED AP, FASA
	Assistant Professor and Director of the Miller Acoustics Lab
	Civil, Architectural and Environmental Engineering
	Illinois Institute of Technology
	Chicago, IL 60616
	muehleisen at iit.edu
	tel: 312-567-3545  fax:312-567-3519
	
	
	On Tue, Dec 1, 2009 at 6:28 PM, Chris Jones <cj at cr-jay.ca>
wrote:

	In some cases it may be counter productive to use a high
emissivity roof.  I have worked on uncooled warehouses where the team
used an approved roofing product to get that point but the heating
energy increased enough to lower the savings enough to lose an EAc1
point.  
	
	
	
	
	
	
	Chris Jones
	14 Oneida Avenue
	Toronto, ON M5J 2E3.
	Tel.  416-203-7465
	Fax. 416-946-1005
	
	
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