Building Thermal Comfort Analysis
The global footprint of multi-storied commercial buildings is growing by the day. These buildings are not only limited to offices but also include shopping malls, hotels, museums, theatres and hospitals. The increased concentration of such buildings in large cities has resulted in the ‘Urban Heat Island Effect’ which increases the amount of heat absorbed, thereby driving up air-conditioning costs.
While air-conditioning costs have burgeoned, this has not always ensured the thermal comfort of a building’s occupants. Although centrally controlled air-conditioning systems exist in most huge buildings, occupants often do not have control over the thermal comfort of individual spaces.
Consequently, air-conditioning is often turned on & off in many spaces. This leads to unnecessary wastage of energy & is also not optimal for Thermal Comfort.
Human Thermal Comfort
Human thermal comfort is driven by multiple factors. These include:
· Surface Temperature
· Air Temperature
· Air Movement
· Metabolic Rate
It is important for the architects, engineers and consultants to work together to ensure that buildings are optimized for thermal comfort. The above-mentioned factors can be analyzed using a thermal comfort simulation.
Thermal Comfort Simulation
Today, a thermal comfort simulation can be used to create ideal indoor comfort conditions for occupants. The strong green building movement around the world is encouraging building owners to design thermal comfort optimized indoor spaces. Green Building Rating systems such as LEED & BREEAM have credits that are dedicated solely for thermal comfort.
Thermal comfort analysis follows different standards such as ASHRAE 55, ISO 7730 & EN 15251. The referenced standards use two indices for this analysis namely the PMV (Predicted Mean Vote) & the PPD (Predicted Percentage Dissatisfied).
PMV was created by placing occupants in special climate chambers and asking them to rate their level of comfort on a seven-point thermal sensation scale. The scale runs from +3 (Too Hot) to –3 (Too Cold); with 0 representing neutral. This scale is based on the heat balance of the human body. Thermal balance is achieved when the heat production of the body is equal to the loss of heat to the environment.
The PPD Index is determined from the PMV. It predicts the percentage of people who are likely to be dissatisfied with a given thermal condition. This would indicate the percentage of people who are likely to feel that the prevailing thermal conditions are too hot or too cold.
Passive Design Strategies
Thermal comfort in indoor spaces can be enhanced through the use of passive design strategies. Passive design uses the orientation, form and construction materials to minimize mechanical cooling, heating, ventilation & lighting loads. These aspects can be used to minimize solar heat gain, maximize daylighting, increase natural ventilation and use the thermal mass to reduce peak internal temperatures.
Thermal Comfort Controls
Buildings are often designed in such a way that its occupants have limited control over their indoor thermal comfort. This can potentially lead to huge energy wastage or very uncomfortable conditions for its occupants.
Building designs can incorporate individual thermal comfort controls by providing thermostats, ceiling fans, plug-in desktop fans, operable windows etc.
Designing for the Future
The effects of climate change have placed a great emphasis on buildings being designed for climatic variations in the future. A thermal comfort analysis can be performed for projected climate change scenarios using weather data from the future.
These projected variations in weather data will occur during the building’s life cycle. The time period over which these changes occur is usually considered as 15 years after construction is completed for mechanically ventilated buildings. This time period is chosen due to the consideration of mechanical servicing equipment life span before a major retrofit is required.
Performing a thermal comfort analysis can have many benefits for building occupants as well as reduce capital & operating costs for the owner. All stakeholders involved in the design & construction of a new building could benefit immensely by making this an integral part of the design process.