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Building energy simulation is the science of estimating the energy interactions within
a building. These interactions include the direct purchase of energy, such as
electricity for lighting or natural gas for heating, but also the exchange of energy
due to such things as the infiltration of air into a building or the heat generated by
a building's occupants. Simulation attempts to account for these factors, plus many
more, in determining the heating, cooling and ventilation loads within a building, the
equipment types and sizes needed to meet these loads, and the cost to operate this
equipment plus other non-HVAC (heating, ventilating and air-conditioning) equipment.
Building energy simulation is used as a tool in the design of buildings, for
determining compliance to building standards and for the economic optimization of
building components. It can be used on buildings of any size, from one zone
residential houses to multi-zone large commercial buildings, or any occupancy, such as
schools, offices, hospitals, supermarkets, etc.
There are different methods of building energy analysis which vary in complexity, but
all have three common elements, the calculation of space heating and cooling loads, the
load on secondary equipment, and the energy requirements of primary equipment.
Secondary equipment is that equipment which distributes the heating, cooling or
ventilating medium to the conditioned space, while primary equipment is central plant
equipment that converts fuel or electricity to the heating or cooling effect.
Generally, as a method becomes more complex it becomes more accurate. However, the
improved accuracy usually comes with increased effort and time to perform a
simulation.
Simpler Energy Analysis Methods
The simplest method of building energy analysis is the degree-day method. This method
provides a simple estimate of annual loads based on tabulated degree-day values for a
location and the balance point of the building (the outdoor temperature where space
conditioning is not required). The degree day method can only be used with accuracy
where building use, HVAC equipment efficiency, indoor temperature and internal gains
are relatively constant.
Where HVAC equipment efficiency or conditions of use vary with outdoor temperature,
energy consumption can be calculated for different values of outdoor temperature and
then multiplied by the number of hours at which that temperature occurred. This energy
analysis method is called a bin method and is often used where ventilation or internal
loads vary with time of day or where equipment such as air-source heat pumps have
efficiency as a function of outdoor air temperature.
Hour-by-Hour Energy Analysis
These methods, however, do not yield accurate estimates for commercial and
institutional buildings. Large buildings (i.e. larger than one zone residential) have
wide-ranging and constantly changing internal and external factors that determine space
heating and cooling loads. These factors need to be evaluated on a relatively small
time step, such as an hourly basis, to obtain accurate estimates. Such repetitive and
detailed calculations are best handled by a computer, and a number of software programs
are available. Most of these programs are based on ASHRAE endorsed heating and cooling
load algorithms. Hourly simulation software steps through each hour of a year
performing calculations. With this type of analysis the dynamic nature of large
buildings can be modelled.
Hour-by-hour energy programs, however, are only as good as the programmer using them
and the information supplied to them. This is their major disadvantage. It takes time
to learn the complex software, and it takes time to translate the input from building
drawings and schedules to the computer. Other disadvantages, common to all methods of
building energy analysis, include they idealize the control of HVAC systems; the
programs tend to yield demands more representative of an average for a building type
rather than a peak demand; and, for this last reason, often do not accurately size HVAC
equipment. The advantages of computer simulation include the ability to accurately
simulate the details of complex systems; to evaluate a large variety of system types;
they provide a standardized and repeatable method of calculation; and the flexibility
to optimize systems based on energy consumption or cost.
A C-2000 building is required to meet more stringent criteria for energy efficiency
than ASHRAE/IES Standard 90.1 - 1989, "Energy Efficient Design of New Buildings Except
Low-Rise Residential Buildings". ASHRAE Standard 90.1 is an internationally recognized
standard for good building energy performance. The C-2000 Program requires a builder
to demonstrate that their office building will use only 50% of the energy of a
reference building meeting ASHRAE 90.1, or 55% for multi-unit residentials.
In order to demonstrate compliance to the C-2000 guidelines, a builder must perform
hourly simulations using a software program specified by CANMET. All building projects
which have progressed through the C-2000 Program to-date have had simulations performed
using the DOE-2 building energy analysis program. DOE-2 is discussed further in the
following section.
The reference building is defined using ASHRAE 90.1 which details lighting and
equipment levels, occupancy, infiltration and ventilation, envelope insulation levels,
percentage glazing, hot water use, HVAC system type, thermostat setpoints, etc. plus
all related schedules. Schedules and occupancy are consistent between the reference
building and the C-2000 design. This ensures that the energy efficiency improvement of
the C-2000 design over the ASHRAE 90.1 reflects design changes only. A third party
independently reviewed the simulations to maintain quality control, to assist designers
in identifying problems or errors with their simulations, and to confirm that C-2000
criteria were met.
DOE-2 is a public domain PC-based program for energy analysis of residential,
commercial and institutional buildings. The program was developed by Lawrence Berkeley
Laboratory with support from the U.S. Department of Energy. DOE-2 calculates the
hourly heating and cooling loads, and the total energy use of a building given
information on the building's location, construction, operation, and HVAC system. The
most recent version of DOE-2 is DOE-2.1E.
A listing of the "engine" behind the DOE-2 program would consist of well over 70,000
lines of FORTRAN source code. To interface the details of the building and its system
with the source code, a Building Description Language (BDL) was developed. The BDL
uses words and numbers to describe the building and includes coordinate systems to
relate the positions of zones, walls and windows to each other.
DOE-2 consists of four sequential subprograms: loads, systems, plant, and economics.
The function of each subprogram is as follows:
The loads subprogram calculates hourly heating and cooling loads based on heat
gains and losses through the building envelope and internal gains. It is a dynamic
modelling which uses hourly weather data and takes into account the thermal storage
effects of the building elements. Internal lighting, equipment and occupant loads are
calculated based on user-defined schedules. Simulation of daylighting and electric
lighting controls is an option.
The systems subprogram simulates the operation of secondary HVAC (airside)
distribution systems used to control the temperature and humidity within each zone of a
building. The subprogram takes into account outside air requirements, operating and
control schedules, and transient building responses. It uses the hourly output from
the loads subprogram to calculate the hourly thermal and electrical energy requirements
of the HVAC system.
The plant subprogram calculates the performance of the primary energy conversion
equipment based on hourly operating conditions and part-load performance
characteristics. The program models conventional central plants, and plants with
on-site generation, waste heat recovery and sell-back of electricity. It also permits
load management of plant equipment and energy storage. It calculates the monthly and
annual amount of each type of fuel used and the daily electrical load profile. The
user selects the type and size of equipment or can allow the program to automatically
size the equipment based on either peak heating or cooling loads or design day
conditions.
The economics subprogram calculates the cost of consumed energy based on
user-defined rates. The subprogram can also be used to compute the present value of
the life cycle cost of the building, including fuel and electricity, equipment,
operation and maintenance.
There are a number of vendors of the DOE-2 program. A listing and contact information
for these companies is contained in Attachment A to this document.
The goal of any simulation is to take something that is extremely complex (a building)
and to model it as simply as possible yet as accurately as necessary. The initial step
and perhaps one of the most important, is the zoning of the building. The more complex
the building the more important this step becomes.
HVAC Zone Description
A modelling zone is similar to an HVAC zone. An HVAC zone is defined by an individual
thermostat and that part of the air distribution system that responds to that
thermostat. Often different occupants of an area will have individual thermostatic
control even if the area is expected to be relatively homogenous in temperature. A
model zone, however, would represent this area as one zone on which an energy balance
is performed. Often there is more than one HVAC zone per modelling zone, but rarely is
there more than one modelling zone per HVAC zone.
Zoning is usually based on blueprints or design drawings. Since simulations are often
performed before a design is finalized, blueprints may not be available and the analyst
must use the best information available. The analyst positions each zone relative to
other zones and relative to the building as a whole using a space coordinate system.
This step, to ensure correct orientation, is most important with an option such as
daylighting control, or where unconditioned zones or disproportionate amounts of
glazing on one building face exist.
Building Envelope Details
The next step in the construction of the input file is to describe the building
envelope components. This includes detailing the thermal resistance, thermal capacity
and reflectance for all walls, roofs and floors. The DOE-2 program provides a library
of materials (such as brick facing, concrete blocks and insulation materials) from
which a user can construct the envelope component. It is also necessary to describe
for each window the thermal resistance, shading coefficient and frame effect. Again,
the DOE-2 program has a library of different glazings that can be used if one does not
have specific details. For C-2000 validation, all envelope characteristics are
detailed for the reference building in the ASHRAE 90.1 standard.
Zone Conditions
The conditions within each space must be determined based on the best information
available at the time of simulation. These conditions include lighting and equipment
levels, the number of people, infiltration, whether daylighting control is to be used
and if the zone is to have space conditioning. Along with the levels describing the
peak conditions in each space there are associated schedules which dictate how these
levels vary with time of day, day of the week or by the time of year.
Once the zones and the conditions in them have been defined, the physical dimensions ofeach zone must be described. This includes the floor area and volume of each zone, the
type (i.e. construction) and location of each wall, floor, ceiling and window, as well
as the length and height of each envelope component. Internal walls between zones,
especially where one zone is unconditioned, should be defined and can include "air
walls" or air partitions between zones.
HVAC System Description
This generally completes the input relating to the loads subprogram of DOE-2. Now it
is necessary to describe the system that serves these loads. Initially, the schedules
of when heating or cooling are available, when ventilation is to occur and how the
space heating and cooling thermostat setpoints vary with time are defined. The
characteristics of each zone, such as fan air volumes, which schedules apply, heating
or cooling capacities, etc., are inputted. Many of these variables can be sized
automatically by DOE-2 if the design values are not known.
The system supplying the heating or cooling medium to each zone is now defined. The
important parameters are the system type (i.e. variable air volume, water-loop, etc.),
supply air temperatures in heating and cooling, whether ventilation heat recovery
equipment is present, preheat temperature setpoints and other system controls. Again,
for the C-2000 reference building many of these inputs are outlined in the ASHRAE 90.1
standard. Also detailed in the systems section are energy uses that do not influence
the HVAC energy, such as outdoor lighting, elevator usage and service hot water
requirements.
Central HVAC Plant Description
The characteristics of the primary HVAC plant need to be determined from design
information, or from the ASHRAE 90.1 standard in the case of the C-2000 reference
building. These details include the efficiency of plant equipment (boilers, chillers,
cooling towers) and how it is influenced by part loads, entering air or water
temperatures, etc., the size and performance of circulating pumps, and the load
management of all plant components. Thermal storage and cogeneration equipment, if
present, are also defined here.
The final section in constructing an input file deals with the economics for the
building. This includes obtaining the local utility rates for electricity and any
other fuel to be consumed. The DOE-2 program is very flexible in handling a variety of
utility rate structures. If a more detailed economic analysis other than annual
operating costs is desired, the user must input first costs, replacement costs, annual
costs for non-plant items and baseline data for comparative runs.
Extensive documentation is available to a DOE-2 user who is attempting to complete a
simulation input file. The user, however, needs to have a good understanding of the
simulation program and how it models the various components, as well as a firm
knowledge of building components and energy analysis. Learning the DOE-2 program is an
investment in time and effort that a designer needs to consider. The modelling of
complicated buildings, as exist in the C-2000 Program, will take an experienced user of
the program at least a week to complete. Any design optimization, changes or debugging
may add considerably to this time.
Other simulation programs that are easier to use than DOE-2 are available. In fact,
the developers of the DOE-2 program are currently working on a more user friendly
program. In any case, a analyst must have knowledge of the tool being used and of the
real life applications and physics being modelled. While other programs are available,
DOE-2 is the industry benchmark program to which all others are compared and has had
more development, research, background support and public use than the others. It is
not perfect and, at times difficult to work with, but it is the most accurate,
versatile and reliable tool available.
All simulation files contain errors regardless of the number of times the file has been
reviewed and checked. These errors arise due to analyst error while inputting data,
misinterpretation of design information, design information that changes over time,
poor assumptions or judgements, or errors in the code of the simulation program. This
final error source can be minimized by selecting a program that has been used and
tested over many years. The other errors cannot be eliminated but can be minimized
with modelling experience, careful choice of assumptions and having a second
experienced analyst review the input file. This is the path that was taken in the
C-2000 program. An experienced analyst was to be part of the design team and input
files from all teams were reviewed by another independent analyst designated by the
C-2000 Program. This assured conformity to the Program requirements and also minimized
human error.
There are a number of building components which cannot be modelled within current
building energy analysis programs. The analyst has to use a method outside the energy
analysis program to estimate the impact of these components on overall energy use. The
following is a list of such components:
There are some components within the current DOE-2 program that are limited in how they
are modelled. The analyst should recognize these limitations and check the output to
be certain it reflects reality and, if necessary, modify the energy results outside the
program. Such components include:
If the building that is being modelled is small, the analyst should focus on the
envelope U-values, infiltration and the heat loss to ground. These are the more
important factors in overall energy use of small buildings. If the building is large,
one should focus instead on the zoning of the building, the economizer, system controls
and system type. These factors are more important in larger buildings because of the
greater need for cooling in the building core, sometimes year round. In the case of a
retrofit the analyst needs to determine how the existing equipment is controlled and
what the current airflow rates are, capacities, fan use and circulating pump use.
Table 1 displays the results of the simulation efforts for five C-2000 designs. The
designs vary from 2,174 m2 to 22,860 m2 in floor area. The predicted energy
consumption for the C-2000 designs vary from 82 ekWh/m2 to 124 ekWh/m2. This energy
consumption represents anywhere from 44% to 55% of the predicted energy consumption of
a building just meeting the ASHRAE 90.1 requirements.
Of the five buildings shown in Table 1, three are definitely going ahead with
construction (Bentall 8, Kamloops and Green on the Grand). Interestingly, these three
buildings are the three smallest shown in Table 1. The decisions to proceed with
construction of the other two are currently on hold.
The simulations that were performed in determining the energy consumptions shown in
Table 1 were performed by four different C-2000 teams (one organization was responsible
for simulating both Q-Lot and Bentall 8). These companies varied in their abilities
and experiences with the DOE-2 program and, as a result, the simulation input files
varied in accuracy and amount of detail. The following describes some of the errors
encountered while reviewing the input files:
Of the five buildings listed in Table 1, two had simulations performed by people who
had little or no DOE-2 simulation experience. This lack of experience was quite
evident in how they attempted to construct their simulation input files. In any future
C-2000 or similar program, it should be made mandatory that only experienced users of
the simulation program be involved with the design team. This would decrease the time
required to construct and review files, as well as ensure higher quality
simulations.
At one point during the early planning stages of the C-2000 Program consideration was
given to using one experienced person to perform the simulations for all the teams.
This would have ensured a high degree of quality as well as consistency between the
different designs and adherence to the C-2000 guidelines. However, such simulation
support does not allow full interaction between the analyst and the design team as they
try to optimize the design, nor does it contribute to the teamwork process that C-2000
endorses.
The C-2000 Program was designed to be a small demonstration program for
high-performance office buildings. So what is the role for building simulation when
the C-2000 Program is no longer on the Canadian building scene?
It is expected that building simulation will see increased use as a design tool for two
reasons:
The C-2000 Program has encouraged the use of building simulation in some organizations
that have never used it before. But with the adoption of the National Energy Code and
the use of compliance software, many more organizations will have to become familiar
and comfortable with energy analysis. The Program should view this as an opportunity
to encourage building energy efficiency. A series of promotional workshops could be
conducted to encourage the use of building energy analysis software. At the same time
the results of the C-2000 Program could be promoted, encouraging the designers to go
beyond the National Energy Code criteria to develop truly energy efficient
buildings.
To further encourage the use of energy simulation in efficient building design, C-2000
needs to provide evidence of the accuracy of energy simulation by comparisons of
simulated energy use and actual monitored results. The monitoring of C-2000 pilot
projects provides an excellent opportunity. A comparison could be made between
monitored results, initial simulations which were based on assumed conditions and
ASHRAE 90.1 schedules, and final simulations taking into account as-built and
as-occupied conditions. Presenting validation results for C-2000 buildings at
promotional workshops will advance the C-2000 goal of high-performance innovative
building design by encouraging designers to use energy analysis software.
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 1989.
"Energy efficient design of new buildings except new low-rise residential buildings,"
ASHRAE/IES standard 90.1-1989, Atlanta, GA.
Lawrence Berkeley Laboratory. 1981. "DOE-2 reference manual," report no. LBL-8706,
Berkeley, CA.
Kaplan Engineering. 1992. "Guidelines for energy simulation of commercial buildings,"
prepared for Bonneville Power Administration, under cooperative agreement no.
DE-FC79-85BP26683, Portland, OR.
Natural Resources Canada
Caneta Research Inc.
March 1996 |
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