What s My Baseline? ASHRAE

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1 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Published in ASHRAE Transactions (2010, vol 116,part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE s prior written permission ASHRAE. THIS PREPRINT MAY NOT BE DISTRIBUTED IN PAPER OR DIGITAL FORM IN WHOLE OR IN PART. IT IS FOR DISCUSSION PURPOSES ONLYAT THE 2010 ASHRAE ANNUAL CONFERENCE. The archival version of this paper will be published in ASHRAE Transactions, Volume 116, Part 2. ASHRAE 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 : 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/m 2 (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 ASHRAE 1

2 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 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 (low- E) 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-ft2- F), 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 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 ASHRAE

3 effectiveness of exterior insulating sheathing on the walls deserves further consideration. 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 lighting power density (Watts/m 2 or Watts/ ft 2 ) 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/m 2 (1.0 W/ ft 2 ). There are classrooms with a lighting power density of 19 W/m 2 (1.8 W/ ft 2 ). If the energy model assumes 11 W/m 2 (1.0 W/ ft 2 ) 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 ft 2 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 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 2010 ASHRAE 3

4 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 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 ASHRAE

5 VENTILATION CONTROLS ASHRAE 90.1 requires ventilation controls that will reduce outside air in spaces designed for more than 100 people per 100 m 2 (1000 ft 2 ) 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 2010 ASHRAE 5

6 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 , 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 G ). 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 G ). 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 ASHRAE

7 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 ASHRAE 7

8 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, NBI (2009). COMNET Manual: Energy modeling rules and procedures. New Buildings Institute and Architectural Energy Corporation, September 18, 2009 (Draft Submission) ASHRAE