Designing Efficient Schools

Working to design a LEED-certified school? This article highlights some MEP trends, including the latest HVAC trends.

09/03/2010


Main Story                Design Strategies                LEED MEP Related Credits                Case Study


I have read articles with similar titles as this article in numerous trade and technical journals. I typically read the first few paragraphs but then lose interest because the articles invariably expound that such-and-such system or “trend” is the absolute best way to design an HVAC system to obtain the most “green” system, period.

This article discusses various design solutions that my firm and I have successfully integrated in various U.S. Green Building Council (USBGC) LEED-certified school designs. However, what the article avoids is to suggest that any one of these design solutions is the best approach for every project, wherever they may be. Quite to the contrary, the article will be successful if engineers read it and take away that the following solutions worked well for schools of particular climatic, geographic, budgetary, and project constraints and may perform equally well under similar circumstances.

The only common thread in all of these systems is that the schools were all built in the Chicago metropolitan area. Readers should consider the various solutions described below as colors in a design palate at their disposal, and craft their own designs to suit the design conditions that best define their project with a healthy dose of their own experience and expertise to judge what will and won’t work.

Holistic Building Design Approach

Although hardly a new design trend, marketing has caught up with engineering, and the “old school” approach of understanding the entire building and designing accordingly has now been labeled by various entities. The leading “branded” design approaches are currently Whole Building Design Guide (WBDG) developed under the National Institute of Building Sciences (NIBS), and the Standard 189.1, Standard for the Design of High- Performance, Green Buildings, developed under the American National Standards Institute (ANSI).

The WBDG was developed by a plethora of federal agencies, including the Dept. of Defense, the General Services Administration, the Dept. of Energy, the Dept. of Veterans Affairs, and the U.S. Environmental Protection Agency, along with a number of other agencies. Standard 189.1 was developed by ASHRAE, USGBC, and Illuminating Engineering Society of North America (IESNA). Both design approaches follow the core principle that a body whose left hand knows what the right hand is doing will perform best. In this case, one hand is the architectural professionals and the other is, broadly speaking, the engineering professionals.

The fast-growing acceptance of the whole building design concept—whether in the specific steps defined under the WBDG or through coincident application of the key concepts as generally good design practices—is the common thread of most high-efficiency MEP systems. Similarly, the concepts espoused under High-Performance Green Building Design, whether specifically under the ASHRAE design approach or again through coincident application of the key concepts as generally good design practices, lead to similar results in extremely high efficiency and high efficacy MEP systems.

The primary reason to follow one of the documented design approaches, WBDG or Standard 189.1, is that following all the time-proven steps of either approach typically results in a higher performance building than a piecemeal approach that misses one or more of the various design steps or performs them out of a logical sequence.

Left Hand, Meet Right Hand

Due to the unfortunately accurate adage that most engineers are good thinkers but poor presenters of that knowledge, many architects are unaware of the impact their design decisions have on the overall efficiency of a building. On project after project in the past, we have met with owner’s reps and architects at the kick-off meeting and the conversation goes something like this:

“We want to design a high-performance, sustainable building that will exceed the status quo by __% (fill in the blanks with your wildest imagination). The client is committed to obtaining the highest LEED certification. What kind of systems are you considering?”

Once the rep is done throwing out every buzzword he can think of, my usual response is, “We will design the best MEP systems to satisfy the building envelope and the project constraints. What are your plans for the walls, windows, and roof?” Usually, there are a lot of blank stares and then the architect states, “whatever is required to meet the energy code.”

This is when a primer on the holistic design process is beneficial, and we explain that a high-performance building isn’t an engineering solution, but a team solution where all team members will need to collaborate to determine the best wall insulation and fenestration parameters (U-value and SGHF) that the project budget can support, and how it will directly affect HVAC and lighting system choices and sizing.

By educating architects about how their building systems and components affect the way MEP engineers design systems, and vice versa, along with the general lifecycle cost implications of their and our designs, it is possible to develop complementary systems that work with each other instead of working in isolation from each other at best or working contrary to each other at worst. Fortunately, more of the architects we are dealing with are LEED and HPB savvy, and the end result has been a series of LEED Gold certified and targeted buildings.

Show Me the Data

An interesting data set is from Table 3.9.1 in the 2009 Buildings Energy Data Book (PDF). The table provides the statistical breakdown of energy usage in a large sample of K-12 as well as higher education buildings. Knowing this helps prioritize where the design team’s energies should be directed to obtain the best operational results. Figure 1 illustrates the table’s data where energy is being used in a typical school.

The focus of the design team should be on reducing heating and cooling loads through tight building envelope with high-performing (low U-value) walls, roof, and windows; minimizing lighting loads through daylighting, high-efficiency lighting sources, dimming or step controls to avoid over-lighting of spaces, task lighting, and other similar techniques; light shelves and north-facing clerestory to allow daylighting in while minimizing direct solar heat gain in the occupied spaces and reduced ventilation while maintaining high air quality control through demand control ventilation, economizers, and energy recovery wheels.

The most efficacious design solutions will be those where the MEP system thermal output closely matches thermal input across the building envelope. The most energy-efficient design solution will be one where the net energy input (natural gas and electricity) most closely matches the net MEP system thermal output. The rub is that these two criteria may not result in the same design solutions. This is where climatic, geographic, budgetary, and project constraints factor heavily in the final system solutions.

The best way to determine the impact of the architectural components of a building is to perform an energy model of building components using eQuest, EnergyPlus, Trane Trace, or a host of other industry-accepted energy modeling programs.

 

Table 1: Energy input variation for various design options

 

 

Overall exterior wall U-value

 

Fenestration SGHF

 

Average HVAC chiller efficiency

 

Average classroom lighting power density

 

90.1

0.084

 

D1

0.060

 

D2

0.050

 

90.1

0.29

 

D1

0.39

 

D2

0.45

 

90.1

9.0 EER

 

D1

11.1 EER

 

D2

12.0 EER

 

90.1

1.4

 

D1

1.32

 

D2

1.24

 

Average KBtu/sq ft

 

36.3

 

36.0

 

35.9

 

36.3

 

35.8

 

35.4

 

36.3

 

35.6

 

35.3

 

36.3

 

36.1

 

35.9

 

Notes:
1.         90.1 = ASHRAE 90.1 baseline, D1 = Design Option 1, D2 = Design Option 2
2.        The values for each category (exterior wall, fenestration, etc.) are isolated variables with all other factors being equal

 

As shown in Table 1, the large improvement (percent deviation) for the building envelope U-value relative to ASHRAE 90.1 baselines is one of the easiest attainable design strategies (D2 is almost 40% less than 90.1 improvement over ASHRAE 90.1 in thermal performance) and results in a 1% reduction in energy usage. The case is similar for fenestration SGHF (D2 is 55% improvement over ASHRAE 90.1) and results in a 2.5% reduction. The relative improvement for the improved chiller plant efficiency is 33% and results in a 3% reduction. Lowering lighting power density by just over 11% yields an additional 1% reduction in energy usage.

The impact of improved building envelope construction is twofold. First, this enables a smaller HVAC plant that results in lower first cost for the HVAC plant. Second, the lifecycle energy costs of the smaller plant are also lowered. Combining these savings with a higher efficiency HVAC plant leads to significant overall lifecycle cost reductions. Although the individual resultant energy savings may appear small, they are all additive.

Keep in mind that the design options listed above are not a comprehensive collection of energy saving strategies available to the design team. In a comprehensive design strategy these savings can total in the mid- to upper-20s in energy reduction relative to ASHRAE 90.1.

 

 

 

 


--Roy is vice president with CCJM Engineers. He is a cross-disciplinary mechanical engineer who has successfully designed integrated mechanical/electrical systems for LEED-certified schools, as well as commercial, aviation, industrial, and institutional facilities for the past 22 years.
Continue to:

Designing Efficient Schools: Design Strategies

Designing Efficient Schools: LEED MEP Related Credits

Designing Efficient Schools: Case Study



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