Journal Review | Estimation of Natural Ventilation Rates in an Office Room Using the Occupant-GeneratedTracer-Gas Method (1)

Seol, Hyeonji. Arztmann, Daniel. Kim, Naree. Balderrama, Alvaro. 2023. "Estimation of Natural Ventilation Rates in an Office Room with 145 mm-Diameter Circular Openings Using the Occupant-Generated Tracer-Gas Method." Sustainability. Vol 15. Issue 13. PDF

 

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 This article was mainly written by Hyeonji Seol, a temporal researcher of Ublo.Inc. It is a follow-up study of an experimental project to test the ventilation rate of the circular opening window named Ublo, in which I was also involved two years ago. Thus, it was a pleasure for me to read this study while I recalled my struggles and achievements.

 

The experiment method

 This research was implemented in the Ublo office, tracking the decay of CO2 the occupants generated on-site. By CO2 gas tracking, the author could calculate the ventilation and infiltration rate in ACH(Air Change per Hour) and analyze the given conditions to explain natural ventilation. Although the description of the Ublo product or the sensor design that I participated in was merely written, this study was devotedly based on previous ventilation-related studies of Ublo and would take a role as the foundation of the following steps.

 

 The author selected to implement the occupant-generated CO2 tracer gas method. It required adequate CO2 Concentration levels and long-term testing for collecting relevant datasets. These two critical factors were easily fulfilled at the Ublo office. First, the Ublo openings were already installed. Also, the tester resided in so effortlessly carried out the estimation for 26 days over three seasons. Third, the office offered also an exhibition place, which was a good reason to invite guests for collecting higher CO2 concentrations. Additionally, four pieces of all-in-one onboard sensors estimating temperature, humidity, CO2 concentration, and PM were ready, which was a revolution considering the ex-study used of separated off-the-shelf sensors by the way. The author calibrated each component value of the onboard sensor one by one, from Korean Laboratory Accreditation Scheme to the outdoor meteorological station, usually 2-3.3km away from the test building. Lastly, former studies on Ublo’s ventilation conditions probably served as a hint.

 

 To secure credibility and feasibility, the author referred to the appropriate ventilation standards of Korea, the United States, and Europe and established a feasible guide for its test. For instance, Korea didn’t indicate any specific guidelines for natural ventilation in existing or small-sized new buildings. It enforced mechanical ventilation systems and air-tightness for energy efficiency in offices or other types of buildings with more than a specified floor area.

 

 On the other hand, the US and Europe suggested similar criteria. The European guidelines had three types of guidelines, each of which contained four categories of expected dissatisfaction degrees(15% as the highest standard - 40% as the lowest standard): (1) Design ventilation rate for a 5-person office with 84sqm, 0.58ACH as a minimum; (2) 1790ppm as a maximum indoor CO2 concentration; (3) Total design ventilation rate for a 5-person office with 235sqm, 0.66ACH as a minimum. The US suggested: (1) 0.58 ACH as a minimum for office space; (2) 5000ppm CO2 concentration as a maximum; (3) Guidance, the minimum operable areas of window openings, the arrangement of windows.

 

Precedent studies regarding Ublo’s ventilation rate

  When it comes to Ublo product development, we came to realize Ublo needs underpinning studies to convince potential consumers, unfamiliar with circular openings on fixed glass. We had to get the hang of Indoor Air Quality concepts in combination with natural ventilation determinants like the total opening size or the proportion of circular openings. These openings would not only decide the efficiency of ventilation but also affect the building's appearance. Here, a tricky question arose because Ublo's market positioning was not precisely for those who want a beautiful window regardless of its function, nor for those interested in sustainability realizing natural ventilation no matter how awful the look is. Even if we narrowed down our goal to focus on the product function first, ventilation for openings and energy efficiency for air-tightness were still in conflict. We had to find an agreement where aesthetics, natural ventilation, and low energy consumption meet. As a starting point, Ublo team conducted ventilation experiments and CFD simulations via Star CCM+, SOLIDWORKS.

 

  Basically, the ventilation tests of Ublo varied with potential approaches. For example, one field experiment estimated ventilation rates via the fan pressurization technique to optimize the number of Ublo openings. In this experiment, the room was filled with 8000 ppm of CO2, and tracked the decay of CO2 concentration until 5000 ppm. Another test was sensing the indoor(t2) and outdoor temperatures(t1) to calculate ventilation rates(㎥/h) as a formula: H / (γ・Cp・(t2-t1)) (H: Heat, γ: air density, Cp: Heat capacity). The latter experiment was in purpose to calibrate CFD simulation by imitating the field environment and test various conditions in 3D for buoyancy ventilation depending on the window types and the height difference of Ublo openings on the computer.

 

  There are three categories of experimental design: true experimental design; quasi-experimental design; pre-experimental design. Its classification criteria are [if a control group of the experiment is affected by changes or not] and [whether a variable could be managed by a researcher.] Mostly, our ventilation estimate experiments were considered to be pre-experimental designs, meaning there were seldom controllable groups or variables that could result in the output. This is because the control and experimental groups were subjected to micro-climate variables like wind speed and temperature. For instance, even when researchers deal with a single test room for both groups, the surroundings of the room could not be in the same condition due to different testing days. Additionally, our previous studies attempted to control variables by taping all the gaps to prevent infiltration and get more accurate ventilation rates but, it was what it was. The researchers had no choice but to make too many assumptions or ignored uncontrollable conditions to result in optimistic interpretations of given conditions.

 

 

 In the next chapter, I would like to explain how the author confronted the inevitable experimental conditions to implement and explain a comparably credible study.