Right-sizing energy-efficient cleanrooms: Lessons learned from Harvard LISE and other peer institutions
Cleanrooms are energy hogs. But cleanroom energy use serves direct experimental needs. How do we balance these demanding requirements against institutional goals for greater sustainability?
The Harvard Univ. Laboratory for Integrated Science and Engineering (LISE) cleanroom began operation in 2006. Following Harvard’s desire for maximum research capability and future flexibility, the design team, including Wilson Architects (WA) and BR+A mechanical engineers, created an enormously robust facility. But almost immediately upon opening, Harvard realized actual ongoing experiments didn’t demand the full capacity of the design. Harvard’s on-site building management began a program of energy-efficiency improvements designed to “set back” the building’s environmental controls.
Through a process of post-occupancy evaluation and targeted research, WA, BR+A and Harvard have collaborated on a research case study of the LISE energy-efficiency projects, using metered data and input from the process of fine-tuning. WA and BR+A have extended that research to an extensive survey of cleanroom facilities across the U.S.
Since cleanrooms are widely varied and often part of larger buildings, it’s misleading to compare cleanrooms with broad metrics, such as total energy use per square foot (EUI). Our study evaluates several finer-grain metrics, including tool load, supply air temperature, recirculation air change rate, recirculation airflow efficiency, make-up air change rate and dewpoint.
Designs for electrical power and cooling capacity tend to be conservatively high. Some cleanrooms are designed for 70 to 110 W/sf, whereas actual tool loads are in the range of 4 to 36 W/sf. At the fully fit-out Harvard LISE cleanroom, tools only result in roughly 3 W/sf of air-side cooling load, once process chilled water is accounted for.
Supply air temperature
In summer, cleanroom make-up air-handling units (MAHUs) typically sub-cool the air to extract moisture, then reheat it to neutral temperature (60 to 68 F). The recirculating AHUs then re-cool the air to offset the internal heat in the cleanroom. This inefficient cool-reheat-recool sequence can be improved by allowing the make-up air to reset to a cooler temperature, such as 50 to 55 F.
Recirculation air change rate
Although Class 100 cleanrooms are often designed for well over 200 ACH, the Class 100 cleanrooms surveyed typically operate between 130 and 175 ACH. To allow lower ACH, the Harvard LISE facilities team connected the particle counters to the building automation system, creating a demand-based control loop. Some facilities rely on occupancy sensors for the same purpose.
Recirculation airflow efficiency
The efficiency of recirculating AHUs (RAHUs) is measured in terms of airflow per unit of energy (cfm/kW). RAHUs, the pressurized plenum and the ceiling grid of filters can be designed to minimize pressure drop. Additionally, turning down the recirculation ACH results in significantly improved cfm/kW. Based on the survey results, RAHUs are often designed for 2,000 to 4,000 cfm/kW (at peak airflow). But, when turned-down, the operating performance improves to 4,000 to 7,000 cfm/kW. For facilities designed with very low pressure drop, including 100% filter coverage in the ceiling grid, plus low airflow (such as 130 ACH), 10,000 cfm/kW can be achieved.
Make-up air change rate (ACH)
MAHUs are often designed for 20 to 45 ACH, whereas actual operation tends to fall in the 15 to 25 ACH range. In the case of Harvard LISE, the facility managers turned down the exhaust airflow and added pressure-differential sensors to the cleanroom envelope to allow the make-up airflow to turn down, while maintaining a positively pressurized environment. The latest trend in cleanroom energy efficiency is true variable-air-volume operation, relying on specialty-coated venturi air valves for corrosive exhaust airstreams.
Most cleanroom specialists would agree the standard cleanroom target dewpoint is 46 to 48 F (particularly for programs such as photolithography). But, based on our survey, designs range from 40 to 51 F, and actual operation is even broader. One cleanroom has essentially no operating dewpoint control. Given the high energy consumption associated with strict dewpoint control, further investigation is warranted.
This research bears-out the theory that cleanrooms vary significantly. But it also illustrates some key parameters for designing an energy-efficient cleanroom. A philosophy of “right-sizing + scalability” is informing the next generation of efficient cleanroom designs.
Jacob Werner specializes in the planning, design and construction of lab buildings. He’s the leader of Wilson Architects’ sustainability initiative, for which he conducts research on energy-efficiency strategies for labs. As Director of Sustainable Design at BR+A Consulting Engineers, Jacob Knowles heads the NET+ sustainability consulting team. Over the past decade, he has championed the sustainability agenda for over 20 million sf of healthcare, research, commercial and institutional projects.