Achieving energy performance-based design goals

Performance-based design allows design teams creative freedom when integrating energy efficiency strategies into a facility.

05/23/2017


This article is peer-reviewed.Learning objectives:

  • Illustrate the methods to evaluate building energy consumption.
  • Apply performance-based design methods to limit energy consumption via products or systems.
  • Learn about performance criteria that address building efficiency: energy utilization index and electric demand ratio.

Performance-based design criteria are gaining popularity in the United States as macroscopic means of improving building efficiency. This type of approach is used by some owners and supported by sustainable design guidelines and energy codes, to allow design teams creative freedom integrating energy efficiency strategies into a facility. The energy-control measures commonly proposed by these performance-based requirements focus on reducing overall energy consumption or limiting the peak electric demand. Architecture 2030 is a popular example of a holistic performance-based design criterion for the reduction of carbon emissions from the built environment.

Figure 1: Dodge City High School is one of the first K-12 facilities in Kansas that will use ice storage to supplement the chilled water system and reduce peak electric demand. Courtesy: GLMV ArchitectureBuilding energy consumption can be evaluated using an energy utilization index (EUI). The EUI is usually expressed in total energy consumed in a year divided by the floor area of the building. The sources in the EUI calculation include electricity, natural gas, fuel oil, and any other fuel used by the building. Energy created by renewable sources, such as photovoltaic energy, is usually not included in the EUI calculation. A common unit of measure for EUI is 1,000 x Btu (kBtu) per square foot per year. There are many resources, among them Commercial Buildings Energy Consumption Survey (CBECS), that catalog average EUI values for building types and locations.

An example of how the performance-based criteria can be applied is a stated requirement that limits the building consumption to 30% better than the 2012 CBECS average. In response, the design team uses this requirement when specifying energy-consuming components and systems related to the building, such as the envelope, HVAC, and lighting systems. An aggressive EUI performance requirement demands a holistic approach to design. The careful selection of building components that take advantage of the natural features available can reduce the need for energy-consuming systems.

An alternative approach to performance-based design can be achieved through limiting the peak electric demand of a facility. The techniques associated with this approach can be (and often are) coupled with the EUI strategy to maximize efficiency of the building.

Utility companies use cost strategies for electric demand to manage peak consumption and the effect that it can have on the electric infrastructure. These rate structures vary by region and utility company, but the premise is the same. For commercial customers, utility companies include the demand charge for maximum power used during a period, measured in dollars per kilowatt, in addition to the charge for electricity consumed, measured in dollars per kilowatt-hour. Design strategies that take advantage of electric demand rate structures to reduce operating costs are not new.

These designs have been most commonly applied in facilities that are very large power users, like hospitals and other mission critical facilities. Building types such as education facilities can experience costs associated with demand changes as high as 50% of the overall electric bills. The peak electric demand in education facilities is short-lived—30 minutes or less—and a result of building systems and food service simultaneously operating at the maximum level. As a result, a wider range of owners and developers are including peak electric demand performance criteria in their building designs.

 Table 1: An electric demand profile is shown for a facility that uses gas-fired boilers for heat, so much of the electricity is used for lighting, cooling, and plug loads. Courtesy: DLR GroupElectric demand control

The electric demand control limits are measured through an electric demand ratio (EDR) for a building. EDR is usually expressed in maximum electricity consumed in a defined interval for a billing cycle divided by the floor area of the building. The defined interval and billing cycle are normally consistent with the electric rate structure applied to the facility. A common unit of measure for EDR is watt per square foot. Demand rate structures vary widely depending on many factors, such as service provider, time of day, and month of year. The important criteria for the design team is to select systems, equipment, and control strategies that take advantage of these rate structures.

An example of how this approach can be applied is when an owner defines a maximum EDR for a building. In response, the design team uses this requirement when specifying building energy-consuming components and systems, such as HVAC, lighting, and equipment control systems. There are opportunities for architectural features of the building to affect this approach, but a bulk of the responsibility for success is associated with the engineered systems.

The approach to performance-based design strategies must include a verification stage where the actual energy consumption is compared with the prescribed guidelines once the project is complete and a history of quantifiable data is available. The commissioning of a building will play an important role in the success of achieving the performance-based design criteria. The design team often is required to evaluate these comparisons and make recommendations to resolve any inconsistencies that exist.

Electric demand control can be used to satisfy performance-based design criteria for new construction, renovation projects, and standalone applications. This approach is most popular in renovation or standalone projects, because the application tends to be engineered system-related and have attractive paybacks. The outcome for this type of project also is quantifiable, because the history of energy consumption and the utility rate structures are known. This approach focuses on electricity use, making it widely applicable to lighting, cooling systems, and buildings with electric heat. An approach to limiting electric demand can be accomplished using sophisticated control strategies or energy-storage equipment that manipulate system operation to avoid the peaks. A facility must be outfitted with a building automation system (BAS) that monitors an electric-power meter to automatically manage the operation.

Energy benchmarks

The project example used throughout this article is a 327,000-sq-ft high school located in a rural community west of the Kansas City metro area (see Figure 1). This building is located in ASHRAE Climate Zone 4.

The first step in creating performance-based design criteria limiting peak electrical consumption is to perform an energy benchmark using electric bills. This benchmark is used to understand how the rate structure is applied and the level of existing consumption. The peak electric values that trigger demand cost are of interest. From past analysis, it is known that reducing the peaks will also reduce overall electric consumption.

Table 1 shows an electric demand profile for the example K-12 education facility. This building uses gas-fired boilers for heat, so most of the electricity is used for lighting, cooling, and plug loads. The values provided in Table 1 for EDR and corresponding demand-cost ratio have an order of magnitude and pattern typical for this facility type in this climate.

Following benchmarking, the next step is to use data from the building power meter to understand the consumption profile and specific peak characteristics. Figure 2 shows an electric-consumption profile for a typical K-12 educational facility. This is useful for setting the limits for reduction of peak consumption.

Figure 2: The example shows a daily electric-consumption profile for a typical K-12 educational facility. Courtesy: DLR GroupThe source of the peak electric demand depends on the building personality, but there is evidence that peaks are not set by individual systems. Using the K-12 example, peak electric demand is rarely discovered when the building becomes occupied in the morning or during the heat of the afternoon. These times may require a big effort from the HVAC equipment, but the HVAC equipment alone does not set the consumption peak. It is more common for the peak electric demand to be set when all systems are fully operational and the occupants are present and engaged in activities. In K-12 facilities, peak electric demand typically occurs between 11 a.m. and 1 p.m. An office, hospital, or other building type may have a different energy personality.

There is some finesse required when determining the performance-based limits for peak electric demand consumption. The approach is represented in Figure 3 by drawing a line at the maximum allowable consumption point. If the line is drawn too high on the graph, then the opportunity to maximize cost avoidance is lost. An aggressively applied limit will result in decreased occupant comfort or other operational issues. The approach for any demand-control strategy should be so subtle that building occupants don’t detect the adjustments. The following guidelines should be used for applying electric demand limits:

  • Allow limits to be adjustable for future fine-tuning.
  • Vary the limits to take advantage of rate structures and building personality.
  • Apply the demand limits incrementally.
  • Use notification alerts to denote when limits are being applied.
  • Train building operators and educate occupants.

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