Mean radiant temperature

Mean Radiant Temperature (MRT) is a concept used in the fields of building physics, thermal comfort, and environmental engineering to describe the average temperature of all the objects surrounding a person, considering the radiant heat exchange between that person and the surroundings. The MRT is significant in determining how a person perceives their thermal environment, influencing comfort levels in both indoor and outdoor spaces.

Overview

The Mean Radiant Temperature is a weighted average of the temperatures of all surfaces within a space, with the weighting based on the angle of view (solid angle) of each surface from the point of interest. In essence, it quantifies the influence of the thermal radiation emitted by surrounding surfaces on an individual's sensation of warmth or coolness. Unlike air temperature, which measures the kinetic energy of moving air molecules, MRT accounts for the radiant heat transfer, which does not require air as a medium.

Calculation

The calculation of MRT is complex, as it involves understanding the thermal interactions between a person and the surrounding surfaces. One common method to estimate MRT in a space is through the use of a globe thermometer, a device that measures the steady-state temperature reached by a blackened sphere due to both convective and radiant heat transfers. The globe temperature, along with air velocity and air temperature, can be used to approximate the MRT experienced in the environment.

Importance in Thermal Comfort

MRT is a critical factor in the assessment of thermal comfort, which is the condition of mind that expresses satisfaction with the thermal environment. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 55 and the international standard ISO 7730 outline the importance of MRT in creating comfortable thermal environments. These standards recognize that for individuals to achieve thermal comfort, the MRT, along with air temperature, humidity, and air velocity, must be within certain acceptable ranges.

Applications

In the design of buildings and public spaces, understanding and controlling MRT can significantly impact energy efficiency and occupant comfort. For example, in hot climates, minimizing exposure to direct sunlight and using materials that do not absorb and reradiate heat can help reduce MRT, thereby enhancing comfort and reducing the need for air conditioning. Conversely, in cooler climates, maximizing exposure to sunlight and using materials that store and radiate heat can increase MRT, contributing to warmer perceived temperatures and reducing heating demands.

Challenges

One of the challenges in applying MRT in building design and environmental engineering is the dynamic nature of the thermal environment. Factors such as changes in sun position, cloud cover, and the presence of heat sources can significantly alter the MRT over time. Additionally, the subjective nature of thermal comfort, which can vary widely among individuals, makes it difficult to design spaces that are universally comfortable.

Conclusion

Mean Radiant Temperature is a fundamental concept in understanding and designing comfortable thermal environments. By considering the radiant heat exchange between individuals and their surroundings, architects, engineers, and environmental designers can create spaces that are more energy-efficient and comfortable for occupants.