Percent Energy Savings

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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.

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.

Varkie Thomas's picture
Joined: 2011-09-30
Reputation: 0

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

Varkie Thomas's picture
Joined: 2011-09-30
Reputation: 0

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 Hoover

Andy hoover's picture
Joined: 2011-10-02
Reputation: 0

I think that this issue has been brought up before, but in some cases, energy comparisons to ASHRAE 90.1 should also be looked at closely. In particular, for a ground source heat pump, or any heat pump with electricity as its heating source, the ASHRAE baseline system calls for electric resistance heat. This automatically makes the heat pump more attractive than it would, say, against a VAV system with hydronic heat and a high efficiency boiler, at least in terms of ASHRAE 90.1. The heat pump will get you more LEED points, but will not necessarily be the most energy efficient system for your customer.

Todd Lagus, EIT

Todd Lagus's picture
Joined: 2011-09-30
Reputation: 0

Indeed, varaible reference comparison methodologies (each building is
compared with a different reference and judged by its relative
performace compared with this variable reference) are far from an
objectives oriented methodology, and can easily introduce distorsions
when applied (like better qualifying solutions consuming more energy
than other solutions with worse qualification). They are 'easy' to
implement but not that useful to achieve quantified objectives in
reducing building energy consumption. On the other hand, fixed reference
comparison methodologies (where the building performance is compared
with an absolute indicator like kWh/m2-y or emissions) are a bit more
hard to implement since an apropriate scale for the abolute indicator
has to be choosen to properly represent each use cathegory of the
current building stock, but then ara completly oriented to objectives.
In the US you already have mechanisms to stablish the apprpriate scales
for the absolute indicator (e.i. Energy Star), and therefore it seems a
pity that Std. 90.1 and LEED rely on variable reference comparison

*Xavier Garc?a Casals*

Xavier Garcia Casals's picture
Joined: 2011-09-30
Reputation: 0

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

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

Nathan Miller's picture
Joined: 2011-09-30
Reputation: 200

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.

*Xavier Garc?a Casals*

Xavier Garcia Casals's picture
Joined: 2011-09-30
Reputation: 0

Professor Edna Shaviv and I have been corresponding on this subject and she has given me permission to post her comments on the Bldg-Sim website. Her views are expressed in two papers that are not attached.

Varkie Thomas's picture
Joined: 2011-09-30
Reputation: 0