Like modern medicine, efficient HVAC design for lab buildings is moving away from a “one-size-fits-all” philosophy toward solutions tailored to individual situations. In particular, site-sensitive lab design continues to gain traction, as owners and their design teams achieve a more sophisticated understanding of the available strategies. A high-performance lab in a hot and humid climate will generally call for different tactics than a high-performance lab in a temperate or cold climate.
Regardless of the lab’s location, high-performance design should deliver certain key benefits, including reduced operating costs; improved environmental quality; expanded capacity; increased health, safety and worker productivity; improved maintenance and reliability; enhanced community relations; and superior retention and recruitment. The most energy-efficient and eco-friendly designs can reduce energy use by as much as 50% compared with a lab designed to comply with current baseline codes, such as ASHRAE Standard 90.1.
Energy-efficient lab design can be conceptualized as a pyramid, with the lowest cost and highest potential energy-reduction strategies at the bottom, and the highest cost, longest payback measures at the top (Figure1). This article will review basic strategies that are useful for virtually every lab project—those toward the bottom of the pyramid. Then, special attention will be given to energy-recovery methods and their performance characteristics for labs in hot and humid climates.
Minimizing airflow: The lowest-hanging fruit
A bedrock principle of efficient lab HVAC design is airflow minimization. Basic steps include the following:
- Use the most efficient lighting option. Lighting produces heat, so the amount of airflow required for cooling can be significantly impacted by lighting choices. These decisions involve lamps (T5s, T8s, LEDs), occupancy or daylighting sensors and dimming or bi-level switching to take advantage of daylighting.
- Minimize process and equipment energy use. A 2014 study done at Stanford Univ. revealed lab freezers, incubators, water baths, refrigerators and autoclaves/sterilizers represent nearly 50% of total equipment energy use, campus-wide. This finding highlights the importance of selecting efficient lab equipment, such as ENERGY STAR-rated refrigerators and desktop electronics, research-grade autoclaves (versus more resource-hungry medical-grade units) and ultra-low-temperature freezers using the more efficient Stirling Engine thermal cycle. Ideally, large, energy-consuming equipment will be shared and centralized to optimize use and reduce wasted energy.
- Minimize design airflow requirements. Airflow in labs is always driven by one of three factors: makeup air requirements (offsetting exhaust through fume hoods and other devices); client-stipulated air change rates (ACH); or cooling needs. Understanding which of these factors is driving the demand for air is critical to making correct design decisions—which may include high-performance fume hoods, ACH reductions and minimization of airflow required for cooling. Energy-efficient lighting and equipment are important tools for reducing cooling-related airflow demand. In some cases, it’s wise to consider decoupling the cooling load from the airflow altogether via water-based cooling (chilled beams, fan coil units). For animal labs, consider ventilated cage racks, which allow room air change rates to be reduced since general room air isn’t used to provide ventilation to the animals. All of these strategies can have important effects on both safety and comfort, so input from the project’s EH&S team is a must.
Controlling airflow: VAV, demand-based control, exhaust
The pyramid’s next-highest level encompasses tactics that are still relatively low cost and can deliver good energy savings. These strategies allow airflow to be modulated below the design maximum when labs are unoccupied, or when conditions otherwise make it suitable to deliver lower airflow. Strategies include:
- Selection of VAV fume hoods and lab exhaust. VAV devices allow airflow to be reduced when sashes aren’t fully open or when hoods are idle. Occupancy sensors and automatic sash closers can be helpful, as well as “close the sash” campaigns encouraging changes in user behavior. VAV can be used in combination with high-performance (low-flow) fume hoods.
- Selection of VAV makeup and supply air. The VAV devices mentioned above work hand-in-hand with VAV terminal units, such as Venturi valves. These are required on fume hoods, groups of snorkel exhaust devices, some biosafety cabinets and the supply air from the air-handling units.
- Installation of demand-based control technologies. This relatively new innovation actively measures air quality using a sophisticated suite of sensors, which interact with the lab’s control system to reduce air changes per hour if no contaminants are detected. When contaminants are detected, ACH is ramped up to clear the air.
- Reducing exhaust energy. Older conventional exhaust designs discharge large quantities of air at high velocities constantly, to avoid the possibility that contaminated exhaust air will be sucked back into the supply system (re-entrainment). Optimized exhaust system designs may include air quality sensors (in the exhaust airstreams), higher stacks, variable speed fans (allowing reduced airflows), wind-responsive controls and the reduction or elimination of bypass air to reduce energy usage while maintaining safe operation.
Reducing pressure drop: Go with the flow
The third highest level of the pyramid has mid-range cost and delivers good energy savings. It involves reducing “pressure drop”—power wasted due to high static pressure as air moves through filters and other ventilation system components. All things equal, higher static pressure means the lab HVAC system must work harder than in a system where design static pressure is lower.
Key points of low-pressure-drop design include:
- Use low-pressure-drop air-handling units (AHUs). Upsizing the cross section of the AHU will reduce face velocity and pressure drop across filters, cooling coils and so on. (In addition, there can be vast differences in pressure drop associated with filter design.) A traditional design might call for air moving through the AHU at 500 fpm; a low-pressure-drop design might be 400 fpm or lower. Admittedly, bigger AHUs involve net incremental costs, including a bigger sheet metal box, larger coils and filters and potential tradeoffs in mechanical room space. However, motors and variable frequency drives can be smaller, and sound attenuators and mist eliminators can often be omitted altogether. The result of these choices: Simple and reliable energy savings over the life of the AHU, which can never be overridden through control sequences.
- Size ducts and pipes for low pressure drop. This strategy works hand-in-hand with the AHU decisions made above, resulting in an efficient system. In general, designing for 400 fpm is a no-brainer, and further reduction (down to 350 fpm, for instance) typically has a good payback as well. In some cases, utility incentives are available to help cover the initial costs of a low-pressure-drop system.
Incorporating energy recovery: Hot/humid climate choices
Regardless of climate, most lab facilities can benefit from incorporating energy-recovery systems. However, the most appropriate choice of equipment may vary depending on whether a building is in a hot/humid, cold or temperate zone. Building layout is another key factor, since some recovery technologies require side-by-side exhaust and supply airstreams.
Air-to-air energy recovery is now required by the International Energy Conservation Code (IECC) for many projects, with rules based on climate zone, building ventilation requirements and outside air percentage. Because labs tend to use 100% outside air, energy recovery is a code requirement in many cases, rather than a “nice to have” sustainability option.
Below are the available equipment choices, followed by a discussion of performance patterns in hot and humid climates:
- Enthalpy and desiccant wheels. These exchangers have the highest effective energy recovery, but can’t be used for hazardous exhaust (for instance, fume hoods), due to the potential for cross-contamination. They require adjacent streams of supply and exhaust air.
- Heat pipes. Effective energy recovery and ease of maintenance—heat pipes have no moving parts—are key benefits of these systems. They also require less space than heat wheels.
- Plate heat exchangers. Like heat pipes, these systems boast effective heat recovery and minimal maintenance. They are large and require adjacent airflows.
- Pumped run-around systems. Though somewhat less effective at recovery, pumped run-around systems have an important benefit: The airstreams can be far apart. Glycol (or, in some cases, a refrigerant) circulates in a piping loop to transfer energy between air passing through the equipment. Maintenance is required, but the incoming and outgoing airflow need not be adjacent. Thus, these systems can be particularly useful for retrofits.
Given these choices, what energy-recovery option makes the most sense for your lab? Our firm recently analyzed eight options for lab HVAC systems in Singapore, Houston, Atlanta and Chicago. We compared these choices to a base case involving cooling via chilled water coil (adequate for dehumidification) and electric reheat for space comfort. The energy-recovery options we studied were as follows:
- Enthalpy wheel with electric reheat (100% exhaust through wheel).
- Heat pipe with electric reheat.
- Heat pipe with evaporative pre-cool and electric reheat.
- Wrap-around heat pipe (no reheat).
- Enthalpy wheel with passive desiccant wheel and no reheat (100% exhaust through wheel).
- Enthalpy wheel with passive desiccant wheel and no reheat (70% exhaust through wheel).
- Enthalpy wheel with passive desiccant wheel and no reheat (45% exhaust through wheel).
- Enthalpy wheel with heat recovery chiller for pre-cool and reheat.
Interestingly, though the locations we analyzed were diverse in terms of climate, the options we identified as “highly recommended,” based on energy-reduction potential, were the same for all four areas we studied (Figure 2). Schematics for the four best-rated options are shown in Figures 3 through 6.
The enthalpy wheel energy recovery with a passive desiccant wheel and no reheat (100% exhaust through the wheel, shown in Figure 3) was rated as highly recommended for all four climates. For the hot and humid weather of Singapore, this choice yielded more than 144,000 kWh/yr in electric energy savings versus the base case. For Houston, annual modeled savings were 91,254 kWh/yr. The dual-wheel system actually offered energy savings of around 60% per year for all four areas modeled, and nearly 70% in Chicago, making it the top choice across the board.
Also highly recommended in our analysis was enthalpy wheel energy recovery with a heat-recovery chiller for pre-cooling and reheat (Figure 4). In Singapore, this configuration saved nearly 130,000 kWh/yr compared with the base case, though it did also have the highest first-cost ranking. In Houston, annual kWh savings were modeled at 83,982. The design delivered slightly better energy savings in Singapore and Chicago than in Atlanta and Houston, but still outpaced all the remaining options for the four zones studied, delivering energy savings of more than 50% versus base case.
Recommended options were an enthalpy wheel energy-recovery system with electric reheat (100% exhaust through the wheel) and a wrap-around heat pipe system with no reheat. Both designs resulted in annual savings of more than 72,000 kWh in Singapore versus the base case, and yielded an energy reduction of about 35% a year for that location with fairly low first costs. Of these two recommended options, the wheel with electric reheat (Figure 5) was superior to the wrap-around heat pipe (Figure 6) in Chicago and Atlanta, where the cooling season is shorter and less severe. Performance of the heat pipe was fairly similar to the wheel-plus-reheat configuration in both Singapore and Houston. For instance, in Houston, the wheel-plus-reheat combination yielded annual electric energy savings of 44,759 kWh, compared with 43,254 kWh for the wrap-around heat pipe system.
In conclusion, it’s generally best to explore all lower levels of the pyramid before reaching for higher levels. The top level, renewable energy, isn’t discussed in this article, but may be an option for some labs. In particular, solar power is growing in feasibility as photovoltaic technology continues to improve and costs continue to drop. Energy recovery is now code-mandated in many instances, and the best solutions in this category will vary. Climate remains an important factor affecting HVAC design decisions for lab facilities. Careful research and analysis will pay off in long-term energy and cost savings.
Daniel L. Doyle, PE, is chairman of Grumman/Butkus Associates, a firm of energy-efficiency consultants and sustainability design engineers based in Evanston, Ill.