How to Ensure Thermal Comfort in a Building
In the design phase of building construction, engineers have an increasing amount of factors to account for. Some of the more modern considerations include everything from evaluating the surrounding area for wind conditions, assessing the ecofriendliness and determining the carbon footprint, to the less quantitative factors such as ensuring thermal comfort for building occupants.
This more qualitative determinant of thermal comfort is defined by internationally recognized standards across the globe such as ASHRAE 55 and ISO 7730 as “a condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation.”
While comfort may be a subjective state, in architecture, engineering and construction design, thermal comfort is actually a measurable factor needed to ensure the safety and satisfaction of occupants. This quantity is based on the calculated predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) of a given space.
PMV and PPD
To quantify thermal comfort, the PMV and then PPD must be calculated. The PMV is an index
that determines the mean value of votes of a group of occupants on a seven-point thermal sensation scale. To obtain the PMV, predicted occupant metabolic rate, clothing insulation, temperature, airspeed, mean radiant temperature and relative humidity must be assessed.
Once the PMV is ascertained, the PPD, or index that establishes a quantitative prediction of the percentage of thermally dissatisfied occupants (i.e., people that may feel too warm or too cold), can be defined. The PPD gives the percentage of people predicted to experience a condition called local discomfort. There are a few factors causing local discomfort, including draft or lack of airflow, but the resulting consequence is the undesired cooling or heating of an occupant’s body. In the presented case, these calculations and how they can help ensure thermal comfort inside a building will be discussed.
Analysis of an Office Space: Ensuring Thermal Comfort in Extreme Conditions (Qatar)
As thermal comfort can be an innately difficult achievement, this case examines a small office as an example. The model is an office space located in Qatar, where the average outdoor summer temperature is around 42°C. The case assumes that the space will contain four occupants each emitting 80 W while their respective work stations emit 72W, wearing short sleeve clothing (0.65 clo), and each has a working metabolic rate of 1.15 met. Additionally, the relative air humidity is 65%.
The project’s CAD model
To achieve thermal comfort for all occupants, an ideal setup, using the least amount of energy, and where the predicted mean vote (PMV) is between -0.5 to 0.5 must be achieved. This study will examine how variables such as wall insulation, cool air supply rate, and solar loading play a role in creating (or hindering) a space where occupant comfort is achieved.
This animation shows airflow velocity streamlines from an HVAC diffuser in an office building (Source: SimScale)
The thermal insulation of the external-facing walls, highlighted in blue, needs to be a low value, as this will allow less heat to be transferred through. Solar radiation through the window, highlighted in orange, must also be taken into consideration. The remaining internal walls are considered to be adiabatic, while the well-insulated ceiling and floor have a U value of 0.13-0.18.
In this case, one air supply inlet is used for simplicity (dispersing air at 18 °C), with a flow rate of 0.304m³/s to ensure indoor air quality. Additionally, two outlets in the room corners can be visualized below.
In the design phase, different ceiling diffusers can be tried and tested to ensure thermal comfort by generating different airflow patterns. When the airflow comes from a ceiling diffuser, it is projected sideways along the ceiling walls, creating a layer of low pressure between the wall and the airstream making it “stick” to that wall, this is known as the Coandă effect. Diffusers usually make use of the Coandă effect distributing the air around the given space. In the presented case, a round diffuser was used.
The result shown below illustrates how the wall insulation, cool air supply rate, and external temperature were evaluated alongside the given variables, to find out how to ensure thermal comfort for an extreme building condition.
This case also demonstrates how computational fluid dynamics (CFD) can be an extremely useful tool to assess the thermal comfort of buildings, determine energy consumption, and be used to iterate and optimize an early design. Using cloud-based CFD software, engineers can run a multitude of simulations in parallel to observe the impact of different types of insulation increasing or lowering the supply flow rate, as well as modifying the supply temperature, to make sure thermal comfort is achieved in their designs.