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Ventilation deflation: Reducing HVAC energy consumption in laboratory buildings

Fri, 04/05/2013 - 1:36pm
Brian Spangler, Andrea Love, AIA, LEED AP, and Charles Klee, AIA, LEED AP
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Fig 1. Energy use index by building sector and site energy use in laboratory buildings   

Like people, buildings require ventilation—and specifically fresh—outside air, to stay healthy. Conditioning and circulating that air, however, requires a tremendous amount of energy and tends to drive building operating costs. This problem is particularly significant in laboratory research facilities where health and safety concerns often push outside air ventilation rates up over eight and even ten air changes per hour. Of course, fume hood designs and HVAC controls have evolved substantially over the past few years, but we must acknowledge that research buildings continue to be among the top consumers of energy when compared to other building types (Fig. 1). Of the total annual energy consumption of a typical laboratory building, over 60% can be attributed HVAC systems (Fig 1).

In response, the design industry, in conjunction with building owners and other industry experts, has been gradually questioning the recommended minimum ventilation rates in lab buildings. Twenty years ago we may have blindly targeted 15 ACH for a conventional biochemistry facility while current designs might fall into a 6 to 12 ACH range today. To add to the confusion, it is now recognized that air change rates may not be a good metric for evaluating ventilation because it established that spaces with taller ceilings require more ventilation than others, and of course, the dilution that comes from the added volume would seem to suggest the opposite. With this in mind, many industry standard criteria are established around a one cubic foot per minute per square foot (CFM/SF) exhaust rate which is equivalent to six ACH in a room 10 ft.-0 in. tall.

Regardless of the metric, the challenge remains in applying this or some other standard. Different codes reference different standards and many institutions supplement existing codes. Without claiming to be specialists in environmental health, Payette, an architecture firm specialized in this building type, set out to identify the issues that drive different ventilation rates within laboratories and develop rational strategies that set HVAC energy loads based on need. This included a benchmarking analysis of past work and energy simulations to quantify the impact of different operational approaches.

Codes and standards
When determining ventilation performance requirements for research laboratories, it is fundamental to understand the underlying regulatory requirements and industry standards. Unfortunately, there is little agreement (Fig. 2) and in the absence of clarity, designers are often motivated to favor conservative principles. Although many standards have been developed to address specific safety considerations surrounding normal lab operation, there is a common misconception that the ventilation rates are set to mitigate contaminant spills or other accidents. It might then also follow that ever increasing ventilation rates lead to heightened levels of protection, but there is an increasing body of research showing this to be misleading (Bell, G. Laboratories for the 21st Century: Best Practice Guide. Lawrence Berkeley National Laboratory; 2008). In fact, increased ventilation can actually interfere with fume hood operation causing it to release contaminants back into the lab environment. Well placed primary containment devices (such as fume hoods) with directed air flow paths and reduced turbulence can be more effective at mitigating contaminants and protecting laboratory occupants than simply ratcheting up air exchanges.

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Fig. 2. Standards and codes addressing outdoor air ventilation rates in laboratories   

If we are serious about optimizing the amount of outside air used within laboratories, we need to be more aggressive in understanding when the ventilation is meaningful and when it is simply the path of least resistance. As designers, we need to work with our clients to understand which facilities are truly laboratories processing chemicals and airborne hazards and which might be characterized as more benign operations that can be subjected to different interpretations. Taking it one step further, when we identify the spaces that process chemicals, we need to be rigorous in understanding how many fume hoods are truly required and how the distribution within a facility and laboratory can drive overall air change rates above a target that is set purely based on room size. Using technologies such as variable-air-volume, low-flow fume hoods with automatic sash closers, we can make aggressive reductions in the minimum air demand of chemically intense laboratories.

Defining a laboratory
As noted above, the energy consumption attributed to a laboratory ventilation system is directly related to the amount of outside air the system must deliver and exhaust. The International Mechanical Code (IMC), Table 403.3, lists minimum ventilation rates for science laboratories within an educational setting but it is unspecific about professional laboratories. In section 502 of the IMC, it becomes clear that spaces dealing with substances that are “harmful or injurious to health or safety” generally require an exhaust rate of one CFM per square foot of occupied area. Consistent with Table 403, however, it is not required that all of this exhaust air come directly from outside. It could, for example, come from air that has already circulated through other, non-laboratory parts of the building.

In an educational setting, this might work well as there are frequently substantial nonlaboratory programs adjacent to lab areas. In a research facility, however, this is not the case and whatever exhaust is needed tends to be supplied via outside air. As a result, it becomes increasingly important to understand which spaces truly use the sorts of materials that trigger the exhaust requirements and which may be explicitly exempt.

Following this line of reasoning, it is most important to define what constitutes a space utilizing harmful substances. Tell-tale equipment such as fume hoods, down-draft tables, and biosafety cabinets would be easy identifiers of a lab environment. On the other hand, if a lab space is devoid of hazardous chemicals and special containment equipment, there may be an opportunity for a different ventilation strategy. An example of this might include a lab supporting computationally-based research. The point is that some spaces that look like labs may not require laboratory ventilation systems. It may be necessary to install the systems for future adaptability, but providing the flexibility to operate with and without elevated ventilation rates can have a substantial impact on building operation cost and performance.

Taking this concept further, it may make sense in certain conditions to subdivide laboratories so that only those areas using chemicals are included. Partitioning off areas for non-chemical use may reduce a level of adaptability but could have a significant impact on the required capacity and operation of air systems. For example, a typical open lab layout might include a write-up area for desk work. If that area is separated and classified as office space the required ventilation rate could be reduced by 90%. The air in that office space might even be able to be used to feed the mechanical equipment serving the laboratories thereby further trimming the energy load on the laboratory systems.

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Fig. 3. Digital simulation analysis for a variable air column system   

Evaluating strategies
In order to compare and contrast the energy consumption of various ventilation systems, a method for quantifying energy use needs to be established. Utilizing energy simulation software, Payette constructed a digital model consisting of a typical laboratory bay (22 ft. by 30 ft. by 10 ft.) with a south-facing wall containing 40% standard glazing. Furthermore, the researchers used benchmark data from the Labs21 program for plug loads, lighting power density, and occupancy to represent a typical use of the space.

An initial series of simulations were executed varying the air supply to quantify the energy impact of the range of rates that was observed from the benchmarked projects. The results of the study reinforce the idea that small changes to the air supply rate have a large effect on energy use (Fig. 3) and operational cost.

Because the IMC requires a minimum exhaust rate of one CFM/SF for spaces with hazardous materials, the remainder of the simulations used this rate. Various systems and methods were tested that would allow the space to reduce energy associated with the ventilation.

The strategies investigated included: enthalpy energy recovery wheel; glycol loop heat recovery coils; HVAC occupancy sensors; displacement ventilation; active chilled beams; and space zoning.

The analyses emphasize the significance of utilizing these strategies to reduce the energy associated with ventilation (Fig. 4). For example, the incorporation of the enthalpy wheel reduced the energy use intensity (EUI) of the model by close to 40%. While certain strategies seem feasible in an isolated simulation, realistic implementation of the strategy may not be possible. For example, because in displacement ventilation the air is introduced at a low velocity, large plenum spaces would likely be required within floor and/or wall constructions and this can have negative impacts for contaminants and spills. Furthermore, for spaces relying upon wall registers for air delivery, ample wall area is required. This may not be feasible in large, open layout laboratory spaces where wall area exists only at the perimeter, however the exploration was undertaken to better its energy impact in a research environment. While strategies, such as the cost of installing occupancy controls may increase the first-cost of a project, weighing that with the 10% reduction in operational energy seen in the simulation could make the option more financially viable.

While strategies can be considered individually, bundling multiple strategies together will lead to an even more efficient design. Two options of combined strategies were considered. Both options included the enthalpy wheel, occupancy sensors, and separately zoned lab and write-up area. However, they differed by one alternate including a chilled beam and the other the displacement ventilation. The option with the chilled beam showed over 55% reduction in energy usage, and the displacement ventilation showed over 45% reduction. These options show the magnitude of savings that can be achieved while still maintaining the ventilation required for health and safety.

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Fig. 4. Digital simulation analysis results for multiple ventilation systems and strategies in a laboratory   

Conclusions
The five-step approach employed for this investigation can be useful for other projects looking to identifying ventilation rates and systems to minimize energy:

1. Define space definitions and hazards
2. Determine applicable codes and standards
3. Define code minimum ventilation requirements
4. Evaluate strategies via energy simulation to understand influence
5. Select a combination of strategies that allows for a reduction in energy usage while meeting the demands of the project

This investigation demonstrates that small changes to the ventilation rate can have a large impact on energy use and operational cost. Therefore during early stages of design, it is highly beneficial to define spaces in such a way that only the areas that require a high ventilation rates by code be treated that way. This sort of space classification relies heavily on the owner’s EH&S officer who can speak authoritatively to the hazards that will be present within the facility and will also be charged with ensuring that the research activities follow the standards set within the design.

Having established those areas that will require special laboratory air systems, it is then critical to design them so as to afford the highest level of performance. Air supply rates should be monitored and should adjust based on actual occupancy. Elevated rates should not be used to mitigate fugitive emissions and hazardous releases should be contained within specialized equipment, such as fume hoods, at the source of the release.

Furthermore, methods for distribution, conditioning and operation of a ventilation system provide a significant path to minimize the magnitude energy associated with operating laboratories. Combining an understanding of the ventilation rate needed with these strategies could help to dramatically reduce the environmental impact from this industry.

Brian Spangler is a Designer, Andrea Love, AIA, LEED AP is a Building Scientist, and Charles Klee, AIA, LEED AP is a Principal with Payette, Boston.

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