Modelling Thermal Comfort
Accurate thermal comfort modelling is critical for creating and assessing efficient, comfortable environments for people to live and work. The subject has plagued researchers for centuries. "Thermal comfort" is an elusive and subjective concept, and encompasses a plethora of metrics, both external, like radiant heat transfer, and internal, like metabolic rate. Now, we have settled on two primary thermal comfort models defined by major organizations in the field: ASHRAE 55 and ISO 7730.
What Are ASHRAE and ISO?
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) is a professional association seeking to advance the Heating, Cooling and Air-Conditioning (HVAC) industry worldwide. They have over 50,000 members in 130 countries worldwide.
The International Organization for Standardization (ISO) is an independent organization that establishes technical and nontechnical standards worldwide. It has published over 25,000 standards covering everything from manufacturing to healthcare.
What Metrics Go Into These Models?
Some of the primary factors involved in both ASHRAE 55 and ISO 7730 are:
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Air temperature
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Air speed
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Humidity
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Mean radiant temperature (MRT): an average of the surrounding surface temperatures. Learn more here
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Radiant asymmetry: the radiant heat transfer gradient felt due to large differences in surrounding surface temperatures
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Metabolic rate: the heat energy produced by a person per unit skin surface area
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Clothing insulation: the amount of thermal insulation provided by a person's clothing
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Skin wetness: the fraction of skin covered in liquid, namely sweat
The models then attempt to aggregate these factors to predict how comfortable someone might be. These calculations produce two comfort estimations: PMV and PPD.
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Predicted mean vote (PMV) refers to a seven-point scale where participants rank their comfort from -3 (cold) to +3 (hot). The models combine the thermal comfort metrics to estimate the average value of these votes.
Predicted percentage dissatisfied (PPD) is calculated from PMV and refers to the estimated fraction of participants that are thermally uncomfortable. Both standards go on to specify maximum PPD within a space. For a building to be in compliance with the standard, it cannot exceed this limit at any occupied point.
Data Collection
With so many metrics needed to model thermal comfort, collecting the data necessary to make these predictions is a tall order. At CHAOSense, we seek to ease this process in 2 ways. First, we advance radiant sensing and instrumentation to make more accurate and accessible measurements. Second, we strive to integrate as many of these metrics as we can into a single platform to reduce the number of instruments needed and the complexity of a setup.