What You Need To Know About Adaptive Thermal Comfort / by H

What is thermal comfort?

Thermal comfort means people's pleasant perception regarding their thermal environment. Although this personal comfort is subjective and varied by each individual, the thermal condition refers to whatever the majority feel the most comfortable with. 

What factors affect thermal comfort

There are six significant factors influence how we feel about our thermal environment.

At environmental level

  • Air temperature: the objective temperature measurement, usually given in degrees Fahrenheit (°F) or Celsius (°C).

  • Air velocity: the speed of air movement.

  • Mean radiant temperature: the average heat radiates from an object.

  • Humidity or relative humidity level: the amount of water vapor in the air as a percentage of the total amount it could hold at that temperature.

At personal level

  • Our clothing insulation: clothing insulation is the most intuitive factor in the comfort equation. It is often expressed in “clo.” A standard man's long-sleeve shirt with a suit jacket has been set at 0.96 clo. (See ASHRAE 55, table 5.2.2.2A)

  • Our metabolic rate: physical activities raise our body's energy production level, also called the metabolic rate. A metabolic rate is measured in units "MET" (metabolic equivalent). Table 5.2.1.2 from ASHRAE 55 shows how our metabolic value (Met units/ Wm² / Btu/h-ft² ) is equivalent to physical activity intensity levels.

What is adaptive comfort?

Adaptive thermal comfort suggests occupants connect to the outdoors and control their immediate environment to adapt to a broader range of thermal conditions. There are a few factors that influence our thermal comfort level:

  • Behavioral adaptation: we adjust our perception of the thermal surroundings based on conscious or unconscious actions. Examples include grabbing a pillow when watching TV (unconscious action) or opening a window (conscious activity).

  • Physiological adaptation: we adapt biological changes due to extended exposure to a particular thermal environment. For example, people who adapted to hot climates begin to sweat at a lower temperature.

  • Psychological adaptation: we alter our perception based on past experiences or expectations. These experiences and expectations are related to season, routine, or culture. For example, people who migrate to northern America from a tropical island are more prone to wear long sleeves in an air-conditioned space.

What does the adaptive comfort model mean for design and occupants?

 

"adaptive comfort model has become the global standard for designing and operating naturally ventilated building and has led to energy saving worldwide."

 

How To Measure Thermal Comfort?

In ASHRAE 55, there are three primary methods to see how space and system are designed according to standard thermal comfort: 

  • the Graphic Comfort Zone method (section 5.3.1),

  • the Analytical Comfort Zone method (section 5.3.2), and

  • the Occupant Controlled Naturally Conditioned Spaces (section 5.4)

Using the parameters listed in section 5.4 is how we determine acceptable thermal conditions in an adaptive thermal environment. "naturally conditioned space" applies to building without mechanical cooling and the heating system is not in operation. It also relates to spaces where occupants are nearly sedentary and have the freedom to adapt their clothing to thermal conditions. 

Analytical Comfort Zone method 

That method translated these six variables (mentioned in the first paragraph) into a single output called predictive mean vote (PMV). Then use PMV as the thermal control guidance. 

Although PMV's term is associated with "vote," the PMV is an index that predicts the mean value on a sensation scale expressed from -3 to +3. That scale corresponds to following categories "cold" (-3), "cool"(-2), "slightly cool"(-1), "neutral"(0), "slightly warm"(+1), "Warm"(+2), "hot"(+3). To make PMV easier to understand by the popularity of a satisfying temperature range, people use the Predicted Percentage Dissatisfied (PPD) curve to quantify any given PMV level. PMV of ±0.5, for example, means 90% of people in that designed space will vote for comfort satisfaction. 

RMI- Shifting focus from actively control to adaptable range

Use Rocky Mountain Institute Innovation Center in Basalt, Colorado, as an example. The owner recognized that setting the temperature uniformly throughout the building would require significant mechanical conditioning ductwork. Therefore, the owner is willing to accept temperature variation as all areas are within the acceptable operative temperature (ASHRAE 55, figure 5.4.2). In the events of extreme weather, the innovation center will request all employees work from home if the building cannot maintain its comfort level.

To make this 15,610 square feet office a naturally ventilated building, the design team set the upper(PMV=0.5) and lower bounds (PMV=-0.5) for each zone. Then define the required range of clothing, metabolic rate, and airspeed. Last, they set internal load assumption factors constant at their maximum value and ensure the building systems deliver under these conditions. At the end of the process, the project will see if the combination of assumptions and anticipated user responses will fall within the designed PMV range. (see Figure 4: Innovation Center Room Data Sheets, at RMI's Insight Brief)

As a result, RMI established an environment of air temperature ranging from 67F to 82F. This range is broader than most office buildings. they also purchased 70 Hyperchairs to enhance personal comfort. Their EUI (energy use intensity) has turned out to be 13.6 based on the data from the first year of occupancy.

During the design process, the comfort criteria was developed refer to ASHRAE55 appendix B: Computer Program For Calculation of PMV-PPD.

The space performance was calculated based on these two conditions. The condition on the left calculated the performance only with Met:1.2 with Hyperchairs during the Cooling condition. The condition on the right calculated with assumption of Met=1.2 (seated occupants) and 1.7 (for speakers).

What's the takeaway

We need to consider behavioral, physiological, and psychological adaptation to reach the desired adaptive thermal comfort. These adaptive actions lead to not only operational cost savings but energy efficiency. 

In the past, we often rely on building systems to provide thermal comfort to occupants. But to reach adaptive thermal comfort requires the design team, user, and the building system to all work together. Perhaps, with this adaptive comfort movement, we human beings and buildings can be more suitable for the future climate. 

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