[BLDG-SIM] TMY2 weather data for design purpose simulation

Chip Barnaby cbarnaby at wrightsoft.com
Thu Mar 22 08:02:04 PDT 2007


Xiaobing,

There are at least 2 issues here:

1) TMYs are selected to be representative of long term conditions and 
are generally used for energy calculations, not sizing.  In theory, 
the TMY year has about the same overall distribution of some combined 
statistic as does the 30 year record.  However, that does not 
guarantee representative distribution of dry-bulb temperature, 
coincident dry-bulb / wet-bulb, etc.  There is probably an OK match, 
but to my knowledge there has not been a systematic study of how good 
the match is.  So there is a risk.

2) A completely separate issue is use of real day sequences for loads 
calcs.  ASHRAE loads methods (Heat Balance, RTS) simulate the design 
day over and over.  Thus the design condition is equivalent of a long 
sequence of hot, sunny days with the building fully occupied.  No 
cooler days, no weekends.  Depending on the climate and the building, 
a real day sequence simulation will predict lower peaks, often much 
lower.  If the right sequence is used, this result might be more 
realistic.  But selecting the right sequence is tricky.  For the 
present, the conservative approach is to avoid the problem and repeat 
the design day.

TC 4.2 has sponsored research to support a possible transition to 
real-sequence design procedures --


Colliver et al.  1998.  Sequences of Extreme Temperature and Humidity 
for Design Calculations.  828-RP.  This project produced the "

Design Weather Sequence Viewer" CD-ROM available from the ASHRAE bookstore.


Klein and

Reindl.  2000.

Automated Generation of Hourly Design Sequences.  962-RP.   This 
project produced "EXTREMES: Weather Sequence Generator" CD-ROM 
available from the ASHRAE bookstore.

These projects may have been a bit ahead of their time.  As 
simulation becomes more mainstream, real-sequence loads calcs may 
move into practice.  A very interesting project would be a comparison 
of equipment sizing results using traditional design-days, TMYs, and 
synthetic sequences (e.g. EXTREMES), and real sequences.

Chip Barnaby

At 05:51 PM 3/21/2007, Xiaobing Liu wrote:
>Dear all:
>
>I'm looking for information regarding the uncertainty of using TMY2 
>weather data in the simulation for sizing HVAC equipment. Does 
>anyone know anything about this? Thanks,
>
>Xiaobing
>
>
>
>
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---------------------------------------------------------
Chip Barnaby                   cbarnaby at wrightsoft.com
Vice President of Research
Wrightsoft Corp.               781-862-8719 x118 voice
394 Lowell St, Suite 12        781-861-2058 fax
Lexington, MA 02420            www.wrightsoft.com
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