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ASHRAE2010-85037
What’s My Baseline?
Susan Reilly,P.E.,LEED AP
ASHRAE Member
Aleka Pappas,EIT,LEED AP
ASHRAE Member
ABSTRACT
Energy modeling is used to compare efficiency strategies and show code
compliance with an energy code relative to a baseline. The baseline reflects
conventional design or a design compliant with the local energy code. To identify
cost-effective, efficiency alternatives, or demonstrate code compliance, the
baseline needs to be clearly defined and understood.
This paper will explore how baseline assumptions drive decisions. The
differences between the ANSI/ ASHRAE/IESNA 90.1-2004: Energy Standard for Buildings
Except Low-Rise Residential Buildings Energy Cost Budget method and Appendix G
baselines will be discussed, as well as real-world considerations when establishing
a baseline. Examples of how assumptions for windows, lighting, ventilation, and
HVAC system options affect design decisions are given.
INTRODUCTION
Energy analysis is most valuable when used to identify cost-effective efficiency
strategies, and is required for certification under Leadership in Energy and
Environmental Design (LEED) and other building rating systems. Energy analysis can
also be employed to demonstrate energy code compliance and qualification for
Federal tax deductions. All of these purposes require different baselines.
ANSI/ ASHRAE/IESNA 90.1: Energy Standard for Buildings Except Low-Rise
Residential Buildings is the most referenced standard by commercial building energy
codes in the United States.
The standard includes prescriptive and performance
paths for compliance.
The Energy Cost Budget Method (ECB) for compliance and
Appendix G: Performance Rating Method, rely on whole building energy simulations.
Baselines are explicitly defined for both of these methods. While the ASHRAE 90.1
baselines are appropriate for demonstrating code compliance or savings for a
building energy rating system, these baselines are not always optimum for making
design decisions regarding efficiency strategies.
For example, a design team specifies high-performance glazing and efficient
lighting at less than 11 W/m2 (1.0 W/sf); both of which are better than energy code
requirements, and will reduce cooling loads. The baseline for comparing mechanical
system options should include the glazing and lighting upgrades. The baseline for
comparison should not be the energy code baseline because the energy cost savings
Susan Reilly is president of Enermodal Engineering, Inc. in Denver, Colorado. Aleka Pappas is an energy engineer and project manager
for Enermodal Engineering, Inc..
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will be over-estimated using the code baseline.
The baseline model drives design decisions, so the underlying assumptions must
be well thought-out and transparent. For almost all energy analysis projects, there
will be multiple baselines serving various purposes. The following examples
demonstrate the importance of defining appropriate baselines, and highlight some
details to consider when developing baseline models.
WINDOWS AND INSULATION
In commercial buildings, windows generally have the greatest impact on building
energy consumption of all envelope components. In many climate zones, the energy
code prescribes minimum window performance of a U-factor of 3.2 W/m2-K (0.57
Btu/hr-ft2-F) and a solar heat gain coefficient (SHGC) of 0.39.
Windows with
thermally-broken metal frames and double glazing with a 12.7 mm (1/2 inch) air gap
will meet these performance levels. But, a higher performing low-emissivity (lowE) glazing system is standard practice in most parts of the country. A window with
low-E, double glazing has a U-factor of 2.3 to 2.6 W/m2-K (0.40 to 0.45 Btu/hr-ft2F), and an SHGC ranging from 0.1 to 0.6.
To identify the most cost-effective window, comparison to the code baseline
appears to make sense.
Yet, the Energy Cost Budget Method requires that the
baseline model have no more than 50% window area and no shading, and Appendix G
requires that the baseline model have no more than 40% window area and no shading.
However, the building design may have more than 50% window area.
The most costeffective glazing for a building with more than 50% window area and exterior
shading may differ from the most cost-effective option for the baseline with 50% or
less window area and no shading.
The baseline for selecting the glazing should
have the same area and window distribution, as well as shading. A high-rise office
building in downtown Denver has floor-to-ceiling glazing that comprises 65% of the
shell.
Because of shading from adjacent high rises, the most cost-effective
glazing for the south façade has a U-factor of 2.3 W/m2-K (0.40 Btu/hr-ft2-F), and
an SHGC of 0.38. Without the shading, glazing with a U-factor of 2.3 W/m2-K (0.40
Btu/hr-ft2-F), and an SHGC of 0.28 would have been recommended.
Another consideration is how to identify cost-effective wall and roof upgrades.
If the ECB baseline is used, the baseline assembly has the same “heat capacity” as
the proposed design.
In other words, the baseline walls and roof have the same
construction type as the proposed design. If the Appendix G baseline is used, the
baseline assembly has steel-framed walls and a roof with insulation above the deck.
In climates where the energy code prescribes exterior insulating sheathing on metal
stud walls, a design with metal studs and no exterior insulating sheathing will
perform worse than the ECB and Appendix G baselines.
However, if the proposed
design has higher performance windows than the baselines, the proposed design will
almost always perform better than the ECB and Appendix G baselines, even without
exterior insulating sheathing.
Two important conclusions should be drawn from
these results.
First, the building without exterior insulating sheathing and
higher performance windows complies with the ECB method.
Second, the cost
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effectiveness of
consideration.
exterior
insulating
sheathing
on
the
walls
deserves
further
LIGHTING
ASHRAE 90.1 offers two methods for determining lighting power allowances: the
building-area method, and the space-by-space method.
The space-by-space method
generally results in a slightly higher lighting power allowance for the building
than the building-area method. To determine if a lighting design complies with the
energy code, the power for all fixtures in a design is summed.
From this, the
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lighting power density (Watts/m or Watts/ ft ) is calculated by dividing by the
building area.
If this building average lighting power density is used in the
baseline energy model, energy consumption is miscalculated and efficiency
opportunities are missed.
For instance, a building is designed with an average lighting power density of
11 W/m2 (1.0 W/ ft2). There are classrooms with a lighting power density of 19 W/m2
(1.8 W/ ft2).
If the energy model assumes 11 W/m2 (1.0 W/ ft2) throughout the
building, lighting, cooling and heating energy consumption may all be underpredicted.
(Heating energy is under-predicted in a VAV system when the warmest
zone is calling for cooling and the coldest zone requires reheat. This interaction
is missed when the average lighting power density is applied.) As for peak design
conditions, zone air flow and cooling loads will also be under-predicted.
OUTSIDE AIR REQUIREMENTS
ASHRAE 90.1 states that the baseline model must have the same ventilation
airflow rate as the proposed building.
So, how do you take credit for energy
savings from reduced ventilation air requirements with multiple air handlers or
dedicated outdoor air systems?
Consider a two-story, 50,000 ft2 office building
with a significant fraction of multi-purpose rooms with variable occupancy:
•
•
•
A single, variable air volume air handler is planned for the building. Initial
outside air calculations show 8500 l/s (18,000 cfm) of outside air is required
by ASHRAE 62.1-2007.
The energy code baseline has a packaged variable air-volume system serving each
floor.
The total design outside air would be reduced to 6370 l/s (13,500 cfm)
with two air handlers because of increased system ventilation efficiency.
A dedicated outside air system with 1.0 ventilation efficiency would only
require 4250 l/s (9,000 cfm) of design outside air.
In Denver, the dedicated outdoor air system reduces energy costs by an estimated
$15,000/yr and reduces peak cooling load by 20 tons. Neutralizing the savings from
reduced outdoor air requirements does not provide design teams with an accurate
comparison of different systems.
ENERGY RECOVERY VENTILATION
In many applications, energy recovery ventilation (ERV) is not cost effective
relative to the ASHRAE 90.1 baseline. ASHRAE 90.1 states that supply fan systems
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over 2400 l/s (5,000 cfm) with 70% or greater outside air shall have energy
recovery ventilation. There are exceptions for labs, kitchen hoods, mild climates,
and systems where the largest exhaust is 75% of the total design outside air.
However, if energy recovery ventilation is included in the proposed design with a
system over 2400 l/s (5,000 cfm) and 70% outside air or more, it must be included
in the baseline as well, unless there are fume hoods meeting the fume hood
requirements.
So, the only potential energy cost savings with energy recovery
ventilation are from an improvement in the system effectiveness over the 0.5
effectiveness in the baseline.
(In ASHRAE 90.1-2007 Appendix G, this has been
addressed.)
In some applications, this is a disincentive to pursue energy recovery
ventilation.
Dormitories and some apartment buildings have central ventilation
systems.
Exhaust is typically through bathroom and kitchen fans that are vented
through a sidewall, and in this case energy recovery ventilation is not required.
An alternative would be to have a central exhaust system.
Per the ECB method,
the central ventilation and exhaust system would have energy recovery in the
baseline.
While the energy analysis can predict cost savings relative to a
building without energy recovery ventilation, under ASHRAE 90.1 the project will
not receive credit for a central system with energy recovery ventilation over a
design with distributed exhaust.
Under ASHRAE 90.1 Appendix G, the baseline system for an apartment building is
packaged terminal air conditioners. This system has distributed supply and exhaust
systems rather than a central supply and exhaust system, so arguably a proposed
design with central supply and exhaust would be credited for energy recovery
ventilation.
While energy savings in laboratories can be significant, it is difficult to
achieve more than 20% energy savings under ASHRAE 90.1 Appendix G.
Typically,
laboratories require 100% outside air, 24 hours per day. In many labs, the outside
air can be setback from 8 ACH during occupied periods down to 4 ACH during
unoccupied periods. In the ASHRAE 90.1 Energy Cost Budget Method, the baseline can
have either variable flow or energy recovery ventilation, so if a lab includes both
efficiency measures it can claim energy savings from one of the measures.
In
Appendix G, if the proposed design has energy recovery ventilation than the
baseline has it too. The baseline and proposed designs have the same outside air
quantity and schedules as well.
So, while the laboratory may comply under the
Energy Cost Budget method, it may not show any savings under Appendix G.
For the sake of evaluating whether energy recovery ventilation is costeffective, compare the proposed design to the case without energy recovery
ventilation.
The simulations must account for increased static pressure drop
across the supply and exhaust fans as well as freeze protection.
Both of these
factors reduce the potential savings from energy recovery ventilation.
The
simulations must also accurately schedule the supply of outside air.
Energy
recovery ventilation is often very cost effective in 24/7 facilities; it may not
save any money in day-use facilities in milder climates. The analysis should also
account for the reduction in peak cooling and peak heating loads with energy
recovery ventilation.
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VENTILATION CONTROLS
ASHRAE 90.1 requires ventilation controls that will reduce outside air in spaces
designed for more than 100 people per 100 m2 (1000 ft2) and served by systems with
1400 l/s (3000 cfm) of outside air or more. The only exception to this is if the
system has energy recovery ventilation.
If the design includes ventilation
controls and energy recovery ventilation, the baseline would have energy recovery
ventilation and the proposed design model would reflect energy savings only from
the ventilation controls.
In a building with high outdoor air requirements and variable occupancy, energy
recovery ventilation and ventilation controls, such as demand control ventilation,
are both worth considering. Demand control ventilation can be more cost effective
than energy recovery ventilation, and the analysis should also consider combining
the strategies.
When combining the strategies, the energy cost savings will be
less than the sum of the savings estimated for the strategies individually.
Creating a baseline with neither demand control ventilation nor energy recovery
ventilation will allow the results of the energy analysis to identify which
strategy or both will be sufficiently cost-effective to incorporate into the
design.
With demand control ventilation, there are a number of important modeling and
design details that need to be addressed in the analysis:
•
•
•
•
•
A minimum outside air level must be set during occupied hours to maintain
building pressurization and meet ASHRAE 62.1 minimum requirements, which are
0.06 cfm/sf for most space types when no one is in the space. Typically, makeup air requirements for building pressurization are greater than the minimum
outside air requirements when the space is unoccupied.
Without including this
detail in the baseline and proposed models, savings from demand control
ventilation will likely be overestimated.
While the modeling software can reduce outside air delivered to individual
spaces based on occupancy and predict savings, it is not that simple in the
field.
The functionality depends on whether it’s a single-zone system, multizone system, or dedicated outdoor air system.
Control is very simple with a single-zone system. Carbon dioxide sensors can be
used in the return air to modulate the outside air damper.
Multi-zone systems require carbon dioxide sensors in densely occupied spaces and
in the return air, at a minimum. The building automation system will modulate
the outside air damper based on the zone with the highest outside air
requirements.
With a dedicated outdoor air system, dampers are needed at each zone to vary the
outside air in response to either occupancy or carbon dioxide sensors in the
space. This will increase first costs.
The baseline model should have the HVAC system type in the proposed design in
order to analyze the feasibility of a demand control ventilation strategy.
This
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will allow for greater confidence in the predicted energy savings.
GROUND SOURCE HEAT PUMPS
Understanding the benefits of a ground source heat pump (GSHP) system depends
largely on the chosen baseline, and on metrics used in the analysis. We recently
analyzed this HVAC system type for a multi-family residential building in Denver.
The analysis was used for determining energy savings for LEED certification,
calculating incentives from the local utility, and to show energy savings to bring
in funding for the affordable housing project. Energy cost savings were also used
to calculate paybacks for a number of HVAC system alternatives to determine the
most cost-effective system type.
Per ASHRAE 90.1 Appendix G, the GSHP system is compared to an all-electric
baseline system. The GSHP, in conjunction with a number of building envelope and
lighting efficiency strategies selected for the project, achieves 52% energy cost
savings relative to this baseline.
Another HVAC system type considered for the project is a hot-water loop fed by
gas-fired, condensing boilers to serve fan coil units in the residences.
The cost
of energy with the fan coil units is only about $100 higher per residential unit
per year than for the GSHP system.
With first cost information from the
contractor, the GSHP system shows a payback of nearly 65 years relative to the fan
coil units. However, the fan coil unit system shows only 20% energy cost savings
relative to the fossil fuel baseline required by Appendix G.
As some funding
opportunities were dependant on achieving at least 50% energy cost savings relative
to ASHRAE 90.1-2001, this drove the selection of the GSHP.
When comparing the two systems to each other, the fan coil unit system is much
more cost-effective than the GSHP.
Furthermore, the fan coil unit system with
heating by natural gas instead of electricity results in lower annual carbon
emissions in Colorado.
The funding opportunities outweighed these two important
issues and the GSHP system was selected.
CAMPUS STEAM AND CHILLED WATER SYSTEMS
Campus steam and chilled water systems present an especially cumbersome analysis
when calculating energy or energy cost savings relative to an ASHRAE 90.1 baseline.
Per ASHRAE 90.1 Appendix G, the baseline model should have a code-compliant chilled
water plant, and is compared to the actual cost of chilled water in the proposed
building (Section G3.1.3.7).
As campus chilled water costs are often either
subsidized by the campus, or inflated to include plant equipment and maintenance
costs, this energy cost comparison is usually quite unrealistic.
Conversely,
Appendix G requires projects using campus steam to be compared to a baseline model
with the same campus steam system and same cost of steam (Section G3.1.1.1).
Following these two baseline requirements in an energy analysis will likely lead to
some less-than-useful results.
If the energy analysis is used to determine the
cost-effectiveness of strategies that will reduce the heating and cooling loads in
the building, it is important to use actual steam and chilled water rates.
However, if minimizing carbon emissions is the goal, modeling the actual plants in
the baseline is more appropriate.
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The United States Green Building Council defines a modeling procedure for LEED
that requires the Appendix G baseline chilled water or steam plant to be compared
to the proposed building’s campus chilled water or steam plant with actual plant
efficiencies and losses modeled, and using the same electricity rates in both
models (USGBC 2008). This seems to be a more appropriate baseline, but brings up
another issue: what electricity rate should be used? If the whole campus is on a
single meter with one electric demand charge, how will each building affect the
campus’s peak electric demand? If the peak demand of the building being analyzed
is not coincident with the campus peak, should just the electric per kWh charge be
used in the baseline? This may be a more realistic model of how the building will
impact the campus’s total electricity charge, but scoping the analysis this way is
likely to make efficiency strategies that reduce the building’s electricity use
look much less cost-effective than if the electric demand charge is used. If the
building will impact the campus peak demand, modeling a demand charge in the
baseline is probably appropriate; otherwise, consider using a blended per kWh
electricity rate for the baseline.
Of course, the same utility rates that are
selected for the baseline model must also be used in the proposed.
CONCLUSION
Using either of the ASHRAE 90.1 ECB or Appendix G baselines to compare energy
efficiency strategies for a project can result in an unrealistic analysis, showing
efficiency strategies to be more or less cost-effective than they may be. For this
reason, it is usually necessary to re-define a baseline model to be used for a
particular analysis. Often, an energy analysis will include multiple baselines to
analyze various systems in the building.
We anticipate that as ASHRAE 90.1 is
updated and the COMNET Energy Modeling Rules (NBI 2009) and Procedures are adopted,
that some of the ambiguity with defining baselines will disappear.
We recommend defining alternative baselines for calculating energy cost savings
and payback periods for energy efficiency strategies. We refer to these
alternatives as the Cost Base. For the envelope analysis, the alternative baseline
reflects the proposed construction type – whether concrete, steel framing or wood
framing, and the proposed glazing.
For the lighting analysis, include the most
likely envelope construction in the Cost Base.
As for the HVAC system in these
alternative baselines, select either the system that is likely to be designed or
that that is most commonly used for the proposed building type and model it as
meeting the minimum efficiency requirements of ASHRAE 90.1 or the local energy
code.
For analyzing HVAC alternatives, include the most likely envelope and
lighting design.
The Cost Base evolves with the project throughout the design
process. Taking care to develop an appropriate baseline model is a critical part
in scoping a building energy analysis that will bring useful results.
As for achieving energy goals for a project, whether for energy code compliance
or a building rating system, the baseline as defined in the referenced energy
standard or code must be used. Stay cognizant of how the Cost Base and energy code
baselines differ, and how priorities will reconcile budget constraints and energy
goals.
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REFERENCES
ANSI/ ASHRAE/IESNA 90.1: Energy Standard for Buildings Except Low-Rise Residential
Buildings
USGBC (2008).
Required Treatment of District Thermal Energy in LEED NC-2.2 and
LEED for Schools. United States Green Building Council, May 28, 2008.
NBI (2009).
COMNET Manual: Energy modeling rules and procedures. New Buildings
Institute and Architectural Energy Corporation, September 18, 2009 (Draft
Submission)
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