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Re: [EnergyPlus_Support] Cooling load calculation E20 vs E++
I can give some theoretical background regarding these methods, but I do not
have practical experience applying them in the design process.
ASHRAE has published several methods for nonresidential load calculations over
the years. The current methods in the ASHRAE 2005 Handbook of Fundamentals are
Heat Balance (HB) and Radiant Time Series (RTS). EnergyPlus applies the heat
balance method. Carrier E20 applies the Transfer Function Method (TFM). TFM
is an older method, but it is not necessarily invalid.
The accuracy depends on the building. If it is a core dominated building,
almost any method will give reasonable results. If it is a high performance
green building or a heavily glazed building, then a heat balance method is
certainly the best choice. The transfer function method can differ from heat
balance by 100% in such cases.
A chart comparing HB, RTS, TFM and TETD (Total Equivalent Temperature
Difference) may be found at the end of Chapter 29 in the ASHRAE 2001 Handbook
of Fundamentals. (Note in the SI version of this chart HB is the small diamond
symbols, the legend is incorrect). In this example, for a building called "the
traditional little ASHRAE store building (used for example purposes since the
1940s)", the peak HB cooling load is approximately 160 kBtuh, while the TFM
peak cooling load is approximately 150 kBtuh. But the magnitude of differences
will vary significantly depending on the building characteristics.
Some quotes regarding these methods from the ASHRAE 2005 Handbook of
Fundamentals (IP Edition, p. 30.45) follow.
"Procedures described in this chapter are the most current and scientifically
derived means for estimating cooling load for a defined building space, but
methods in earlier editions of the ASHRAE Handbook are valid for many
applications. These earlier procedures are simplifications of the heat balance
principles, and their use requires experience to deal with atypical or unusual
circumstances. In fact, any cooling or heating load estimate is no better than
the assumptions used to define conditions and parameters such as physical
makeup of the various envelope surfaces, conditions of occupancy and use, and
ambient weather conditions. The experience of the practitioner can never be
ignored.
"The primary difference between the HB and RTS methods and the older methods is
the newer methods´ direct approach, compared to the simplifications
necessitated by the limited computer capability available previously.
"The transfer function method (TFM), for example, required many calculation
steps. It was originally designed for energy analysis with emphasis on daily,
monthly, and annual energy use, and thus was more oriented to average hourly
cooling loads than peak design loads."
And more from p. 30.50:
"Although the TFM, TETD/TA, and CLTD/CLF procedures are not republished in this
chapter, those methods are not invalidated or discredited. Experienced
engineers have successfully used them in millions of buildings around the
world. The accuracy of cooling load calculations in practice depends primarily
on the availability of accurate information and the design engineer´s judgment
in the assumptions made in interpreting the available data. Those factors have
much greater influence on a project´s success than does the choice of a
particular cooling load calculation method.
"The primary benefit of HB and RTS calculations is their somewhat reduced
dependency on purely subjective input (e.g., determining a proper time-
averaging period for TETD/TA; ascertaining appropriate safety factors to add to
the rounded-off TFM results; determining if CLTD/CLF factors are applicable to
a specific unique application). However, using the most up-to-date techniques
in real-world design still requires judgment on the part of the design engineer
and care in choosing appropriate assumptions, just as in applying older
calculation methods."
Mike
On 11 May 2006 at 5:54, ashishkadam78 wrote:
> Dear Friend,
>
> Have you tried doing cooling load calculation on carrier E20 shhet or
> any other similar format (may include any conventional method). What
> is your impression about the accuracy of these methods, which are
> currently being followed by most of the consultants and companies??
>
> What is percentage deviation in cooling or/and heating load oocurs
> when we use energy plus over these conventioal methods. Also, if you
> can give rough percentage reduction in installation capacity and
> operating cost then it will be more delighting.
>
> Thanks,
>
> Ashish Kadam
>
>
>
>
>
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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