[Bldg-sim] Percent Energy Savings

Xavier Garcia Casals xgcasals at telefonica.net
Fri Mar 6 02:18:56 PST 2009


I do not think that the 'modelling accuracy' argument supports the 
variable reference comparison methodologies (VRCM: percent better 
methodology) in front of the fixed reference comparison methodologies 
(FRCM: EUI as kbtu/sf/yr). The building simulation tools we have 
available nowadays are good enough to benchmark our buiding against a 
standar use (schedules, TMY,...) as it is done with other equipments 
(indeed calibrating the model with an existing building requieres more 
dedication, but this is not the object of a FRCM). Of course they still 
have innacuracies (some more than others), but these ones reflect as 
well into an absolute indicator (FRCM) as into a relative indicator 
(VRCM). In fact often the more energy conscious strategies, design 
features or equipments are the ones that scape from the possibilities of 
our modelling tools, and therefore, with a VRCM you do not cancel those 
inaccuracies between numerator and denominator.
On the other hand, with the VRCM you leave out of the score given to 
your building important design parameters (like compacity) by canceling 
them between baseline and proposed building, which can lead to the 
senseless (from my point of view) situaton that a design option for a 
building with higher EUI gets more LEED EAcr1 points than another design 
option with less EUI. The reference HVAC you assign for each case can 
also introduce distorssions as pointed out in former mails.
Thats why I think that nowadays, when we are strugling to bring the 
building sector (with its huge inertias to change) towards a 
sustainability path with a time urgency (transiton finished in 2050 
worlwide?), we can not afford to use indicators which are not oriented 
directly towards this objective (like VRCM). If building simulation can 
not contribute now (with its available capabilities) to this transition 
(and I believe this strongly depends on adopting FRCM), then I am afraid 
it will just have failed.
Regards,

En/na Nathan Miller ha escrit:
>
> Of course this gets into the whole debate about the absolute accuracy 
> of energy modeling. Appendix G makes it clear that it does not predict 
> actual energy usage, which is what you would be trying to compare to 
> if you use energy use intensities (ie- kbtu/sf/yr). It is much more 
> difficult and intensive to set up an energy model intending to show 
> you are better than a real-world EUI value rather than trying to be X% 
> better than ASHRAE baseline.
>
>  
>
> At this point, given the pressure to make energy modeling more 
> affordable (not to spend many extra hours trying to nail down 
> absolutes), it seems reasonable to me to use the percent better 
> methodology.
>
>  
>
> Of course we all look forward to the day when we have cheap, easy to 
> use tools that can actually predict total building energy consumption, 
> but I'm not holding my breath.
>
>  
>
> Nathan Miller
>
> /Senior Energy Engineer/Mechanical Engineer/
>
> direct: 206.788.4577
>
> fax: 206.285.7111
>
> *From:* bldg-sim-bounces at lists.onebuilding.org 
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Andy hoover
> *Sent:* Wednesday, March 04, 2009 12:20 PM
> *To:* 'Varkie C Thomas'
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] Percent Energy Savings
>
>  
>
> The highlighted yellow and that entire paragraph nail it for me.  It 
> results in showing/taking the most effective steps to decrease energy 
> usage which should be the objective, regardless of percentage.  
> Percent savings is always very very subject to manipulation.
>
>  
>
> Andy  
>
>  
>
> Andy Hoover
>
> Principal
>
> The BEST Consultant, Inc.
>
> Office: 678-200-7648
>
> Fax:678-827-0574
>
> Cell: 678-793-1159
>
>  
>
> www.thebestconsultant.com <http://www.thebestconsultant.com>
>
>  
>
>  
>
> ------------------------------------------------------------------------
>
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>
> *From:* bldg-sim-bounces at lists.onebuilding.org 
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Varkie 
> C Thomas
> *Sent:* Wednesday, March 04, 2009 11:04 AM
> *To:* Varkie C Thomas
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] Percent Energy Savings
>
>  
>
> I received a few responses to this message from outside the USA sent 
> directly to me.  Below is the message again with some additional 
> points in red.  The sections highlighted in yellow is from one of the 
> responses
>
> ------------------------------------------------------------------------
>
> *From:* bldg-sim-bounces at lists.onebuilding.org 
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Varkie 
> C Thomas
> *Sent:* 2009-03-03 18:26
> *To:* bldg-sim at lists.onebuilding.org
> *Subject:* [Bldg-sim] Percent Energy Savings
>
>  
>
> ASHRAE Std90 for the baseline is already stringent and specifies the 
> commonly used systems & plants based on building type and size.  Based 
> on the CCGT building, high percentage energy savings for LEED 
> certification can be achieved with low-rise buildings with a high 
> exposure to floor surface areas to locate PV panels, high but 
> efficient glass to wall area for day-lighting, the use of GSHP (which 
> requires a large site area which may not be available in city blocks), 
> and low common energy consuming systems in baseline and proposed which 
> increases "percentage" energy savings. 
>
>  
>
> A school building is low-rise, is low in process loads (increases % 
> savings) and is high in occupancy ventilation which allows for 
> air-to-air heat recovery. It operates only during the day increasing 
> the impact of day-lighting. It is closed in summer, reducing the 
> annual cooling load and increases the impact of solar heating during 
> the rest of the year in cold climates.  So it is possible to achieve 
> zero energy usage. 
>
>  
>
> The choices are limited in the case of inner city high-rise 
> buildings.  Today's high-rise office building design tries to maximize 
> the glass percentage which increases the impact of day-lighting.  The 
> glass could have PV properties with a very small overall efficiency of 
> converting light to 110V electricity.  The ASHRAE Std90 baseline for 
> Systems and Plants for such a building is considered normal design.  
> UFAD and other options are not typical.  The proposed envelope has to 
> offset the 40% Window-Wall-Ratio specified by ASHRAE.  If the office 
> building envelope is used for a hospital or hotel, which operate 8760 
> hours/year, then the percent energy savings will go down. 
>
>  
>
> High percent energy savings does not therefore necessarily mean a 
> better or optimized designed building in terms of the client's 
> interests.  Percent energy savings should therefore not be the 
> criteria for energy efficient building design.  It should be based on 
> Energy Conservation Measures (ECM) used for the given building that 
> are more energy efficient compared to ASHRAE Std90 for the given 
> building type and size which usually results in increased first costs. 
> The ECMs are going to be different for different types of buildings in 
> different locations.  If the ECM used is inappropriate, then the 
> client pays a price for the high percent energy savings.
>
>  
>
> In my opinion the way to express energy efficiency would be energy per 
> unit area.  Occupancy per unit area has to be defined for different 
> types of buildings (office, hotel, retail, hospital, etc.) and energy 
> efficiency should also take into account location based on HDD & CDD.  
> Energy per Person will account for overall energy efficiency of 
> buildings & transportation when designing sustainable townships.
>
> ------------------------------------------------------------------------
>
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-- 

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*Xavier García Casals*

 

Tel: (+34) 91 843 19 85

Móvil: 660 22 94 45

xgcasals at telefonica.net <mailto:xgcasals at telefonica.net>

 

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