Energy Simulation In the Building Design Process

Published in November 1983 ASHRAE Journal

This article was published in ASHRAE Journal, July 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

Energy Simulation In the Building Design Process Mainframe computer simulation programs for energy analysis can provide important information throughout the building design process.

By Daniel H. Nall, P.E., Member ASHRAE; and Drury B. Crawley, Ph.D., Fellow ASHRAE

I

n the ten years since the first dramatic escalations in oil prices, energy consciousness in building design has gone through several phas-

es. Currently, energy conservation is understood to be one important, but not preeminent, concern in the design process. Conservation is seen as an almost purely economic matter. A specific investment has a specific savings on return which either does or does not merit that investment. Compromises of comfort, habitability, function, image, or marketability to achieve energy savings are typically not justifiable. The short period of the “energy building,” which made both grand gestures and sacrifices of other building functions to achieve low energy consumption, is probably gone forever. Energy concerns can be an important input, however, to any design decision, either architectural or engineering, given that no compromise of other important goals is entailed. In 36

ASHRAE Journal

order to make these decisions, either for conservation investments, or for design alternatives, specific energy consumption estimates are needed. These estimates must furnish sufficient information to calculate annual energy cost, so they must include not only consumption for various fuels, but also monthly demand and consumption estimates for electricity. The

calculations must be of sufficient completeness that detailed component, control, material or occupancy alternates may be tested against one another. Currently, only a “mainframe” building energy simulation program can meet these requirements.

Program Characteristics Building energy simulation programs have been in existence for approximately fifteen years. Initially, they were commissioned by electric and gas utility associations to influence designers in the selection of gas or electric systems. Subsequent development of programs was sponsored by About the Authors At the time of initial publication, Daniel H. Nall, P.E., and Drury B. Crawley, Ph.D., were with Heery Energy Consultants in Atlanta. Today, Crawley is director, Building Performance Products at Bentley Systems in Washington, D.C., and Nall is senior vice president at WSP Flack + Kurtz in New York.

a s h r a e . o r g

July 2011

30 Years Later, Modeling Process Remains the Same

The biggest change: from mainframe computers to desktops, laptops and tablets.

By Daniel H. Nall, P.E., and Drury B. Crawley, Ph.D. This article was written in 1983 when energy modeling was rarely a part of the architectural and engineering design process. Pursued by only a few die-hard energy conservation advocates, available only through large commercial time-sharing networks, and accessible only through cryptic text input (Building Description Language, [BDL] or even 80 column card images), these tools were far outside the practice mainstream. National laboratories, such as Lawrence Berkeley National Laboratory and the U.S. Army Corps of Engineers Construction Engineering Research Laboratory, were hard at work developing DOE-2 and BLAST. The private programs listed in the article competed for the attention of a handful of practitioners. Significant energy modeling exercises had been pursued in support of the abandoned Building Energy Performance Standards, and in the design of ground-breaking buildings such as the TVA Chattanooga office building, but no guidelines existed for the role of energy modeling at various stages of the design and construction process. This article was one of the first attempts to characterize those roles and to define specific procedures by which the relevancy of energy modeling to the design process could be maximized. Today, energy modeling is a mainstay of the design process. Lifecycle costing with energy modeling has been mandatory for public sector projects for more than 25 years. LEED certification almost always requires energy modeling, and many projects, exceeding the maximum glass percentage of ASHRAE/IES Standard 90.1, rely upon energy modeling for code compliance. Energy modeling is much more accessible than in 1983, available on anyone’s PC (or Mac), with computer graphic input, automated Help, and pro-active Wizards to expedite the process of creating simulations. Current desktop PCs are orders of magnitude more powerful than the IBM 370s or CDC 6600s that hosted building energy modeling programs when this article was written, enabling tens or hundreds of simulations to be performed in the amount of time it took to do a single run in the old days. No matter how much energy modeling has changed and improved over the years, the design and construction process remains much the same, with the notable exceptions of building information modeling (BIM) and integrated project delivery (IPD). This article attempted to define the role of energy modeling in the design process and to describe procedures to overcome budgeting and procurement obstacles to maximized energy efficiency. The process of defining an energy conservation line item for the building budget before the design, building systems or energy conservation measures are finalized is an important management tool for preserving flexibility in the pursuit of maximized energy efficiency. Similarly, the process of equipment or system pre-purchasing based upon first-cost bids and estimated operating cost enables proprietary components or disparate systems to be procured in a competitive fashion. These procedures are as relevant today as they were a quarter of a century ago. Other changes that have occurred in the interim include the greatly enhanced rigor of building energy standards, vast performance improvement for many components including glazing, lighting, and building automation, and the dramatic increase in building internal loads. Building energy modeling has been supplemented by additional

July 2011

Michael L. Umbricht, The Retro-Computing Society of R.I.

design tools, including computational fluid dynamics to evaluate airflow and temperature distribution patterns in spaces, ray-tracing programs to evaluate the daylighting and electric lighting designs, and three dimensional heat transfer analysis to evaluate window thermal performance of framing and façade construction. The building energy model still sums up the results of all of these analyses providing the bottom line in dollars, kilowatt-hours, therms, and tons of carbon. This is not to say that building energy software hasn’t changed significantly in nearly 30 years. Advances in simulation methods and flexibility in modeling solutions have greatly improved the simulation user’s portfolio of tools. The article presages major controversies and challenges surrounding energy modeling today. The issue of design phase energy modeling results versus later actual building performance is the center of a current controversy that has entered the courtroom. Cooler heads recognize that this controversy centers on the difference between standardized comparisons of energy conservation alternatives with attempted prediction of building energy consumption. As the article states, predicting actual consumption is not the strength of building energy modeling—not because of technical deficiency of the programs, but because the user cannot accurately predict how the building will be used before it is built. The article demonstrates the great importance of energy tariffs for the life-cycle cost performance of energy conservation measures. It demonstrates how the study of these tariffs can be as important to operating cost savings as studies of local weather patterns, or of the owner’s occupancy schedules. Another insight briefly and naively described is the use of building energy modeling to detect operational deviations through comparison of predicted and operational performance. Later pursuit of this aspect of energy modeling resulted in the sophisticated calibrated modeling procedures described in the LEED measurement and verification credit. The two innovations mentioned previously, BIM and IPD, are likely to enhance rather than diminish the role of energy modeling in the building design process. Correctly handled, BIM facilitates the creation of energy models, and through the equipment specification standard under development by ASHRAE will make system and component comparisons even more accurate. IPD, with design continuity between design and construction partners maintained by BIM, means an even greater role for energy modeling in the design and construction process. Energy performance goals established during the preliminary design phase will be verified as the construction team provides the final detailing for the building. Creation of a calibrated as-built energy model will be facilitated by reliance on the asconstructed BIM model for building systems and component performance definition, and the BIM defined measurement and verification system. The authors thank ASHRAE for republishing this article. We are gratified that some of its insights seem to have withstood the test of time.

ASHRAE Journal

37

Btu/gsf · yr (In Thousands)

the federal government with NBSLD as a re90 85,600 search tool and the U.S. Post Office Program as a compliance tool for design guidelines. 80 These federal efforts have culminated in the 72,540 BLAST and DOE-2 programs available to69,685 70 66,680 day. Private sector program development was 66,000 64,415 also spurred by the “energy crisis.” Most of 60 these programs have borrowed significantly 56,830 54,410 from the government-sponsored develop50,865 50 ment efforts or have utilized consultants who were instrumental in the federal efforts. Some 42,290 of the private sector programs available today 40 include: ECUBE III from the American Gas Association; ESP-II from APEC, TRACE 30 from the Trane Company; ESAS from Ross Meriweather and Associates; and various 20 forms of AXCESS available from Edison Electric Institute or Syska and Hennessy. 10 Each of these programs has specific characteristics, strong points and weak points. The 0 major shortcomings of current simulation Baseline Schematic SD Design DD DD DD Construction CD HQ Energy Design Energy Development Energy Phase Energy Documents Energy Without methodologies are recognized and various Target Phase Code Phase Code Without Code Phase Code Process efforts are under way to over-come them. With With Computer Without Loads Computer Computer Computer Indeed, the science of building simulation is far more advanced than the art of utilizHeating Cooling HVAC Auxiliaries Lighting ing these programs in the design process. Many practitioners do not understand Equipment Elevators, Escalators, and DHW exactly what kind of information is provided by building energy simulation models. Figure 1: Energy analysis results for a corporate headquarters building in Cleveland. The cardinal rule of using these models is that of any computer analysis: the quality of the output is only as operating procedures are followed perfectly. Even with acgood as the quality of the input. The simulation program merely curate input data, savings estimates for conservation measures will tend to be somewhat low, so that returns on investment manipulates numbers. It is an accounting program which sums calculated using these savings will be conservative. Breakdowns many numbers to arrive at a concise result. It cannot provide into energy end use components will be proportionally accuinsights beyond the depth and range of its input data. It cannot rate, assuming that the operating procedures are representative provide an unsophisticated user with a sophisticated answer. of actual procedures. The usage, therefore, to which programs Using building simulations to provide estimates of building are currently put is possibly the least valid of these three posenergy consumption is a risky business. For the estimate to have sible uses. The program is best used calculating conservative any validity at all, the input must be well researched and docusavings between physical alternatives in a given building. The mented, not only for the physical configuration of the building and next most accurate use is to understand the fraction of total its systems, but also for the occupancy patterns and the operating energy consumption which might go into each end-use. The procedures and schedules. Those persons who use these estimates should realize that these intangible variables have more influence on riskiest use of the program is the prediction of actual annual energy cost for a fully designed building. The caveat to the EPA building energy consumption than does the physical configuration. mileage ratings also applies here, “Your mileage may vary ...” Building energy simulations have a great resemblance to the Environmental Protection Agency mileage ratings for automoBuilding Design biles. They isolate the effect of the physical configuration from Building design is a very fluid, even amorphous process. Despite obscuring operational variables. If the operational variables are segmentation of the process in commonly used contracts, the segheld constant, and these variables are a good representation ments themselves and the tasks therein are poorly defined. Definiof the projected usage of the building, then comparisons made between physical variables using the simulation tool have validity. tion of a rigorous schedule by which energy simulation tools are to be used in this process would be unreasonable. For these tools to In general, energy consumption estimates made with building be effective, however, they must be used so that they provide insimulation tools will tend to be low, if only because occasional formation exactly when it is needed for decisions. The issues that lapses in standard operating procedures in the building will should be addressed are not confined to mechanical engineering, increase consumption. The simulation, of course, assumes that 38

ASHRAE Journal

a s h r a e . o r g

Febru Ja u rl y 2 0 1 1

but extend throughout most disciplines in the design effort. Although the ultimate intent of using these tools is rigorous economic optimization of the building, initial studies, performed without detailed costing, must be concerned with the potential of various alternatives. The building program often will change during the process of the design, so that initial studies should not reach definitive answers which might be invalidated later. In Figure 1, Page 38, the estimated energy consumption of a large office building and the state energy code requirements are traced throughout the process of design. Much of the variation is caused by changes in the building program. The conceptual process, therefore, is to develop qualitatively the potentials for energy conservation early in the design phases and to evaluate quantitatively energy cost avoidance measures in the later design phases. Included here is a discussion of the various studies that are performed with energy simulation programs at various phases of the design process. Not all of them will be useful in every design project. Engineering experience with a particular building type in a specific location can often substitute for the most qualitative studies. The experience should be extensive and broad based, however, rather than a mere extension of intuition. The effects of conservation techniques often seem counter-intuitive at a superficial level and only make sense when the interactions of all of the affected components and systems are considered in each of the applicable operating modes.

www.info.hotims.com/37988-31

Fu J e lbyr u2a0r1y1 2 011

Pre-Design The groundwork for an energy conscious building must be laid early in the design process. The most important consideration is that the building construction budget be so configured as to make allowance for extra first cost conservation measures which may be identified well after the budget is finalized. One management procedure for making this allowance is to identify the conservation potential in a particular building and to capitalize on that potential. A simulation is performed of a simple unarticulated building conforming to the site and conforming to the client’s program on building size, height and appearance, function, internal loads, and operating schedule. Mechanical and electrical systems chosen are those commonly used in a building of that size, type, and budget. The appropriate utility rate structures are applied to the results of that simulation to develop an annual energy cost baseline. A modified reversion of this building description is then prepared, in which state of the marketplace energy conservation measures are applied. A simulation is performed on this building and annual energy costs computed. The difference in energy costs between these two cases is the maximum savings that can be realized for this building program using off-the-shelf components. Not all these measures will prove cost-effective, and other measures may be identified later in the design, but what is sought is a budget allowance rather than a decision on exact measures to be included. A capitalization factor is


ASHRAE Journal

39

100

SCHEME A

97,090

System 1 – Interior zone VAV induction system, exterior zone four pipe fan-coil system, natural gas boiler, open drive centrifugal chiller

Annual Energy Performance (Btu/ft2 · yr) (In Thousands)

95 90 85 80

SCHEME B

75

System 1 – Interior zone VAV induction system, exterior zone VAV with baseboard heat, natural gas boiler, open drive centrifugal chiller

70 65 60 55

55,900

58,840

56,130

56,040

60,630

53,080

50,415

50

System 3A – Interior zone VAV induction system, exterior zone four pipe fan-coil system, natural gas boiler, double bundled chiller

45 40 35

System 3B – Interior zone VAV induction system, exterior zone four pipe fan-coil system, natural gas boiler, open drive centrifugal chiller

30 25 20

System 4 – Dual conduit system, interior zone VAV system, exterior zone constant volume outside temperature reset, interior zone VAV system, open drive centrifugal chiller

15 10 5 0

System 2 – Interior zone VAV induction system, exterior zone powered induction system, natural gas boiler, open drive centrifugal chiller

Scheme A HVAC System 1

Scheme B HVAC System 1


Cooling

Scheme B HVAC System 3A

Heating

Scheme B HVAC System 3B



Other Components


(DHW, Lights, Vertical Transportation, Miscellaneous Equipment)

System 5 – Constant volume reheat system, natural gas boiler, double bundled chiller System 6 – VAV with plenum hallway on south, east and west sides, interior zone VAV induction system, exterior zone on north four pipe fan-coil natural gas boiler, open drive centrifugal chiller

Figure 2: Energy analysis results for comparison of HVAC system alternatives. developed, using the client’s economic parameters including his tax situation and his desired investment performance. Capitalizing the maximum savings using this factor yields the maximum cost-effective conservation investment possible in this building. While this quantity may be easy to quantify without simulation for a simple building program, it is almost indispensable for a complex program. At this same stage, two additional tasks can be accomplished. The building description can be modified to comply exactly with the state energy code (some version of ASHRAE Standard 90-75 in over 40 states) to determine the approximate consumption level required by the code performance path. Studying the results of these three simulations (two for energy costs and one for the energy code) can shed some light on the conservation strategies that should be pursued in a specific building. First of all, the energy end-use components (lighting, heating, cooling, air movement, etc.) should be reviewed and compared with other buildings to identify which components offer the most potential for conservation. Daily and monthly electric demand profiles, and cooling load profiles, should be examined to determine whether demand shaving or cooling storage (either mechanical or architectural) might be useful. Average annual part load ratios for major mechanical components will indicate whether or not enhanced part load performance should be sought for these components. Studying the breakdown of peak cooling load components will reveal where solar control may be useful and whether or not night cooling or reduced insulation levels (to avoid pull down loads) should be investigated. 40

ASHRAE Journal

Analysis of these preliminary simulations becomes extremely critical in locations where the utility rate structure contains a demand ratchet. For example, one utility has the following demand determination for summer periods (May through September) and winter periods (October through April): A. Billing demand in a summer month is the greater of (1) Actual demand in the month, (2) 95% of the greatest summer demand for previous year, or (3) 60% of the greatest winter demand during the previous year. B. Billing demand in a winter month is the greater of (1) 95% of the greatest summer demand of the previous year or (2) 60% of the greatest winter demand during the previous year. For a building operating under this rate structure, the optimal strategy is to seek a summer peak electrical demand which is a little over 60 percent (60%/95%) of the peak winter demand. For a building which naturally has a ratio somewhat greater than this, summer peak demand shaving (cooling storage, electrical generation, absorption cooling) will probably be cost effective until this ratio is reached. For office buildings under this rate structure, electric resistance heating is often the cost-effective choice because it does not affect the monthly billing demand and, thus, can often be bought for the same approximate price, per unit of delivered heat, as fossil fuels. Rate structures which tier energy charges based on monthly “hours of demand” (total monthly consumption divided by billing demand) offer potential savings which can be identified in this early analysis. Typically, moving to greater hours of demand results in a reduction of the energy charge component of the a s h r a e . o r g

July 2011

electric bill. Thus, reduction of billing demand Energy Discounted Internal Energy Estimated may not only save demand charge, but also enInvestment Payback Rate of Cost Savings First Cost Target Period Return ergy charges, by moving some of the consumpEmergency Generator tion into a lower cost category. $24,400 $97,600 $50,000 2.5 Years 60% Peak Shaving Finally, time of day rate structures also indi0.60 kW/ton Chillers $7,900 $31,600 $22,000 3.5 Years 47% cate extensive attention to these early simula45°F Chilled Water $1,800 $7,200 $4,000 2.7 Years 56% tions. Peak electrical demands and peak Cooling Two-Speed loads in the various time slots will indicate the $1,500 $6,200 $2,100 1.6 Years 85% Cooling Tower Fans potential for moving loads into adjacent nonAll Strategies $35,600 $142,600 $78,100 2.6 Years 57% peak time slots. Despite the fact that the building form may Table 1: Preliminary economic analysis of central plant energy conservation meanot yet have been set, the building program, site sures for an office building in Atlanta. constraints on the form, and operating schedules will largely determine the pattern of energy consumption for the the architectural impact of the system selection can be included project. This initial analysis will indicate general strategies which in the schematic design. The location of occupancies with opwill be applicable not only to systems design but also to architecerating schedules more extensive than the rest of the building tural design. Some of the issues accessible through this analysis may also have significant impact on the total consumption of the building. These decisions can utilize information supplied include: by energy simulations. The system selection, in particular, can • The relative importance of sun control • The relative importance of overall envelope heat transmis- benefit from comparisons in which generic energy conservation measures are included with the various systems. sion (U-value) For example, a hydronic heat pump system might be more cost • The potential for daylighting effective with Single glazing in a mild climate than would a VAV • Whether airside economizer might offer significant savings system in a double glazed building. An airside economizer then • Whether a massive building might be beneficial might make the VAV system more attractive. Daylighting con• Whether space should be allocated for thermal storage or electric generation • Whether any spaces might not require mechanical cooling • Whether the most likely cost effective method of space heating will be electric resistance heat or hydronicalIy conveyed heat from fossil fuel combustion or refrigeration recovery • Whether a central system is likely to be more cost effective than packaged units. The proper analysis of these predesign simulations can furnish the building design team with guidelines for the pursuit of energy conservation in the project from the first day of design.

Schematic Design With the commencement of the design phase, simulation can be used to provide input for specific design decisions. Parametric studies have been used on many projects to test the sensitivity of energy consumption to specific design variables. Orientation, aspect ratio, percentage of glass, over- hang depth, all have been tested on a number of projects to give guidance in the selection of an architectural scheme. Such high volume parametric studies, however, overemphasize the importance of energy and may be misleading when applied only to specific characteristics of the building envelope. In the perimeter zones of a building, the building envelope, the lighting strategy, the glazing type and the air terminal control scheme interact to the extent that a determination of one variable should not be made without consideration of the others. The major energy related decisions that will be made during the schematic design phase will be the selection of basic mechanical system types. The selection will be made so that Fu J e lbyr u2a0r1y1 2 011


ASHRAE Journal

41

trols might make a bypass multizone more Peak Energy Discounted Internal kW Additional kW Cost Payback Rate of attractive because they would tend to reduce Savings First Cost Savings Savings Period Return the peak required air volume. At this stage Forward Curved Fans of the design process, conservation measures With Discharge — — — — — — are dealt with at the default level rather than Dampers (Base) the engineered level. For a firm which utilizes Forward Curved Fans 48,245 6 $1,856 $5,628 60. Years 40% With Inlet Vanes this process extensively, preparation of a set Mechanical Variable of default input valves for different build70,460 20 $3,875 $9,377 4.3 Years 49% Speed Drive ing types, systems, or components would be Variable Frequency useful. Examples would be a default of 4.0 61,320 22 $3,844 $13,976 8.0 Years 34% Motor Speed Drive inches total static pressure for a distributed Note: Peak electric savings assume demand management so that billing demand is not set during a VAV system, or 1.5 watts per square foot for a nonsummer billing month. high efficiency general office lighting system, or Table 2: Economic analysis of vendor submittals of air-handling units for a speculaa shading coefficient of 0.30 for high perfortive office building in Atlanta. mance glazing. The purpose of these analyses is to test the sensitivity of system selection to conservation measures to the end of design development on projects which are bid for which will be selected by rigorous economic analysis later in the guaranteed maximum price at the end of design development. Significant constructed conservation measures may be put design process. The systems selected should have the best potential into the pricing documents as alternates and the decisions cost/benefit relationship, including possible conservation oppormay be made after bids are received. In addition to energy tunities. An example of a system selection study for a corporate cost estimates, simulation programs can provide some very headquarters facility in Cleveland is shown in Figure 2, Page 40. detailed functional performance data for building systems. The bulk of this data involves comfort and control. Mean radiDesign Development ant temperature profiles provided by some programs can pinpoint The first attempts at economic analysis of conservation meacomfort problems which may exist even though air temperatures sures are made during the design development phase. The availare held within thermostatic limits. Histograms of air exchange ability of firm cost data will determine which measures should rates in rooms can locate problems of poor circulation which be rigorously analyzed and which should not. Table 1, Page 41, might ultimately result in “tight building syndrome.” Relative shows a summary of economic analysis for several central plant humidity histograms can show the need for humidification or the alternatives for an office building in Atlanta. The energy investment target is the capitalization of the annual energy savings. The limits for cold deck reset. Programs can also be used to test the estimated first costs were provided by a cost control consultant. effects of deliberately undersizing equipment and the temperature In defining firm cost data, it is necessary to differentiate between excursions in unconditioned spaces. All of these types of informaconstructed alternates and component alternates. Constructed tion can be useful in the detailed design of building systems. alternates are those which are built up by a contractor from general building materials. Some examples of constructed conserConstruction Documents vation measures include an airside economizer, shading devices, Simulation programs can be used in construction documents and chilled water storage tanks. Component alternatives involve in conjunction with a pre-purchasing procedure to compare and a substitute of one component for another or the addition of a procure proprietary or dissimilar pieces of equipment. A typicomponent. Such components are available from a single vendor cal process might work as follows: Vendors are furnished with and have easily identifiable, relatively invariant installation costs. a specification of the piece of equipment to be tested, including Obvious examples include high efficiency motors, reflective glass capacities, minimum efficiencies, etc. In many cases, the specificaand variable frequency inverters for motor speed modulation. tion may be open, allowing a number of means of achieving the Relatively firm cost data for component alternatives can usually be same functional end. For example, the specification for factory obtained from a vendor or a representative as soon as exact sizes assembled air handling units might allow centrifugal or vane axial and quantities are known. Unfortunately, for constructed alternafans, and might only specify that the volume be controllable to 30 tives firm cost data can usually only be obtained as a result of a percent of full delivery. Noise and vibration specifications would contractual agreement. For example, on a project with a construcbe normal. Vendors are asked to fill out a form which specifies tion manager, firm prices for alternates, both constructed and the exact energy consumption characteristics of the component component will be available during this phase. On jobs which will in a variety of operating conditions. The format of this informago out to bid, relatively firm cost data for component alternation is compatible with the component description format of the tives can be obtained from vendors, but only cost estimates will simulation program to be used. This form is a part of a formal be available for constructed measures. Despite this usual lack of submittal which includes a guaranteed price for the component. firm cost data, decisions on constructed conservation measures Simulations are run for each component alternative and an must be made during this phase because of their impacts on other economic analysis performed, taking into account purchase price aspects of the building design. These decisions may be delayed and operating cost differences. For some components, quotations 42

ASHRAE Journal

a s h r a e . o r g

July 2011

for maintenance contracts or insurance may also be requested. The client may then pre-purchase the most attractive alternative and, in the case of conventional bid projects, assign the component to be delivered to a contractor yet to be designated. An example of the results of such a study, for a speculative office building with manufacturers’ names deleted, is shown in Table 2. Purchasing of air handling units and water chillers, and in some cases, lighting systems and controls, are most effectively handled in this fashion. Finally, at the end of construction documents phase, the as-designed energy consumption of the building is estimated using a simulation program.

Conclusion

Following construction of the building, an estimate of the as-built energy consumption can be prepared to be used as a management tool in the operation of the building. By referring to the monthly demands and consumption figures in this estimate and comparing the actual figures, the building operator can determine whether the assumed occupancy patterns are maintained. In one case, such comparison allowed a building operator to pinpoint that the cleaning crew was leaving on all the lights in the building during the nightly cleaning period rather than progressing from floor to floor. In another case with a leased building, the owner was able to discover a tenant had installed some special 24-hour equipment which was significantly increasing the energy costs for the building.

Building energy simulations are only as good as the design methodology which accompanies their use. In the early days of the usage of this tool some projects included vast optimization efforts early in the design process. In many cases, the results of these optimization efforts were not followed up later in the design, and expensively wrought energy features were watered down or totally lost in the final design. For other projects, the use of simulation programs is limited to a single as designed study, perhaps the least effective use for such programs. In the last few years, however, designers have become more sophisticated in how they use simulation programs. They realize that they have a powerful comparative tool, which can not only be used to perform economic comparisons, but can also report summaries of system performance during periods of part load operation. The information which can be provided by this tool can only be useful if it is provided in a timely fashion during the design process. Definitive solutions offered during schematic design will be watered down or lost by the end of the design period. Without proper planning, the construction budget will be “spoken for” by the time sufficient specific information is available to make definitive decisions. Although simulation programs are a powerful analytical tool, certain institutional barriers in the conventional design process impede their usefulness. This article has suggested some techniques for using simulation programs which will overcome such problems.



Post·Construction

July 2011

ASHRAE Journal

43