Share thisVivariums present many unique challenges to design and construction professionals. Due to the stringent environmental and construction requirements, designers and builders have their hands full ensuring the facility meets the numerous standards regulating these spaces.
In the struggle to meet these challenges, compromises are often required to stay within budget, meet the physical constraints of the building, or satisfy the schedule. All too often, one of these compromises is energy efficiency; however, given today's rising fuel costs and concern for greenhouse gas emissions, energy efficiency merits a higher priority. Therefore, it is essential to examine all established design standards and "rules of thumb" and look for every opportunity to reduce energy consumption, while still maintaining a safe and tenable environment in the vivarium.
Odor Control
The generally accepted method for control of toxic or odor causing gases, such as ammonia, is dilution of spaces using non-recirculated, single-pass air, at a rate of 10-15 air changes per hour (ACH).1 Based on this criteria, a common design for a typical holding room, without regard to animal population or caging system, is constant volume reheat at a rate of 15 ACH. There are a number of ways to challenge these criteria and maintain acceptable conditions within the space. By designing the space as variable volume, the air change rate can be easily raised or lowered based on occupancy, thus reducing the air change to a minimum rate in spaces that are not in use or in spaces with small populations. Furthermore, spaces with ventilated rodent cages, "effectively address the ventilation requirements of animals without the need to ventilate secondary enclosures [holding rooms] to the extent that would be needed if there were no independent primary-enclosure [ventilated cage]."¹ Therefore, it should be possible to reduce air change rates in spaces with ventilated cabinets to a much lower level, perhaps 6 or 8 ACH. Another approach to reducing vivarium air change rates is demand controlled ventilation. See Gordon P. Sharp's "Dynamic Variation of Laboratory Air Change Rates" (ALN vol.7, no. 8) for an excellent discussion on this topic.
Heat Recovery
Reducing air change rates, either strategically or dynamically, is only part of the energy efficiency puzzle in the vivarium. As previously noted, supply air to vivariums is not recirculated, so all the energy spent conditioning this air is exhausted out of the building. There aremany heat recovery solutions on themarket to recover energy fromthis exhaust in order to preheat or precool supply air streams. It is essential to recognize that any solution with the potential for cross contamination, such as energy recovery wheels, should be avoided. Unfortunately, in eliminating energy recovery wheels, energy recovery effectiveness is also severely limited, by almost 50%, since other types of energy recovery only recover sensible heat. However, even with limited efficiency, with high airflows and 24 hour per day operation, sensible only heat recovery should be utilized to reduce energy usage and shave peak demand loads. Reducing peak demand has the added benefit of reducing required capacity of central cooling and heating equipment or the impact on campus chilled water and steamsystems.
Environmental Conditions
The environmental conditions required for vivariums pose additional energy challenges. The Guide for the Care and Use of Laboratory Animals and NIH guidelines require space temperatures to be adjustable between 64° F and 79° F while maintaining relative humidity between 40% and 70%. The low range is a particular problem. In order to maintain 64° F with a reasonable relative humidity (40-50%), supply air needs to be dehumidified to a dewpoint of approximately 40° F. This level of dehumidification is impossible with conventional chilled water systems. One method to achieve this performance is to use glycol chilled water systems, which cool the air to 40° F using water temperatures in the low 30° Fahrenheits, and then reheating this air to maintain space temperature (Figure 1). This approach results in 25% to 35% greater energy use over a conventional system. A second option is to use a side stream desiccant dehumidifier to significantly dehumidify a portion of the airstream to maintain design conditions (Figure 2). This option is also energy intensive, using approximately 15% to 25% more energy than the base design. Manufacturers are currently working to address this energy use challenge, too. One such approach is the Trane CDQ unit, which uses a series desiccant wheel to improve the dehumidification ability of a cooling coil (Figure 3). The CDQ wheel transfers the latent load off the cooling coil and passes it through the cooling coil again, essentially dehumidifying the air twice. This option uses approximately 20% less energy than the conventional system, while maintaining conditions at AAALAC standards, which the conventional system cannot do.