Natural ventilation not only has the perk of being free, but it also has the potential to improve occupant health and performance. Natural ventilation systems are an effective strategy to reduce energy consumption and meet strict energy goals. In this blog, we discuss how naturally conditioned spaces are assessed for comfort and how they played a crucial role in the US Virgin Islands Arthur Richards School project.

Project Background

The US Virgin Islands (USVI) Arthur Richards School is an ongoing project involving a 7-building all-electric campus spanning 141,479 square feet in the US Virgin Islands. It aims to replace the school and emergency infrastructure damaged by Hurricanes Irma and Maria in 2017. This pre-K through 8^th-grade educational complex has bold resiliency and efficiency goals. The project serves as a leading example for the USVI’s master plan to construct 18 new schools in the next five years.

The Virgin Islands Department of Education (VIDE) requires the project to achieve 2-3 weeks of off-grid functionality after a hazard event and be net-zero energy-ready. This means the campus must have the space and infrastructure to support enough on-site renewable energy to offset the annual campus energy use. The Teachers’ Union also requires the classrooms to have strict maximum temperature limits.

The Arthur Richards project team uses passive strategies to support resiliency and net-zero goals. These strategies maintain an indoor environment without using energy. The primary passive building features are the atria, which are not subjected to the maximum classroom temperature limits. The atria have openings in the walls and roof to allow for natural ventilation and daylight. In collaboration with the team, Baumann Consulting (Baumann) utilizes Computational Fluid Dynamics (CFD) to simulate the USVI Arthur Richards School K5 building atrium. The goal is to improve the building’s architectural and HVAC design for energy efficiency and occupant comfort.

The design incorporates semi-conditioned atria to use natural ventilation, with additional air transfer from classrooms to condition the space. In the original concept, energy recovery ventilation (ERV) is not utilized. This lack of energy recovery significantly complicates the project’s ability to achieve its net-zero-ready goal. Additionally, from a resiliency perspective, this raises concern about relying on mechanically cooled transfer air to maintain thermal comfort in the atria. For these reasons, the project team desires the exploration of conditioning the space solely with natural ventilation, considering optimization strategies such as tuned building orientation, increased ventilation openings, and high-volume low-speed (HVLS) fans.

Baumann’s models show that removing the air transfer from classrooms to the atrium space for energy recovery ventilation would be worthwhile and would not hinder occupant comfort in the atrium. It also identifies and validates potential optimization strategies where the alternate design concepts fail to outperform the original. Baumann’s expertise in CFD modeling allows an assessment of all options and provides an analysis of thermal comfort in summertime under each condition.

Assessing Thermal Comfort

As mentioned above, the atrium is not subject to the classroom temperature limits provided by the Teachers Union, so the design team was unclear on how to define the space requirements for thermal comfort. Baumann, with the design-build contractor, outlines the required interior space temperature as dictated by ASHRAE Standard 55-2020: Thermal Environmental Conditions for Human Occupancy (ASHRAE 55).

Thermal Comfort, defined by ASHRAE 55, is „that condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.“ It is a holistic assessment used to determine occupant comfort, considering factors like metabolic rate, clothing insulation, air temperature, radiant temperature, airspeed, and humidity.

ASHRAE 55 provides the adaptive model to determine acceptable thermal conditions in occupant-controlled naturally conditioned spaces, like the atria. The basis of this model is that humans adapt their perception of thermal comfort to seasonal climate changes. The adaptive model considers the outdoor air temperature from the past 30 days and the indoor airspeed.

During September, the hottest month of the year based on historical typical meteorological hourly weather data from Charlotte Amalie-Harry S Trumann Airport, the prevailing mean outdoor air temperature is 84.2°F. Our model also looks at a specific design condition of 91.9°F. Using ASHRAE 55, we found the max operative temperature that 90% of occupants would find comfortable ranges from 81.0°F-88.7°F. See Figure 4.

Just as a breeze makes a sweltering day more bearable, higher interior air speed makes warmer temperatures feel more comfortable. A useful tool when working with thermal comfort is the CBE Thermal Comfort Tool [2]. This tool allows the user to assess thermal comfort in different simulated conditions.

CFD Modeling

With the maximum allowable operative temperatures at various internal airspeeds identified, Baumann utilizes multiple CFD models. Baumann utilizes an exterior site analysis to determine the inlet velocity at the natural ventilation openings of the K5 building based on an average prevailing wind speed and direction. This was necessary as the entering air speed and direction determine the interior airspeed.

Based on the local weather data, we conduct the exterior analysis at high (12 mph) and low (6 mph) wind speeds entering at 80° northeast. The image below shows the velocity profile for the low wind speed case in the exterior CFD analysis.

After identifying the exterior conditions at all openings, interior analyses for the original concept and all alternate concepts are generated. In all scenarios, air enters the space through exterior breezeblocks and exits through roof ridge vents.

We conduct interior analyses for the following scenarios:

1. As designed with classroom ventilation relief
2. Removal of the classroom ventilation relief
3. Increased louvered areas
4. HVLS Fans

Scenario 1

The design does not recover energy when ventilating between classrooms and the atrium. In this scenario, the atrium has the coolest temperatures compared to all the other cases. However, some areas have still air that moves slower than 40 fpm. People may find these areas uncomfortable.