What is Radiant Heat Transfer?
There are three main mechanisms by which heat can transfer from one thing to another.
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Conduction: the transfer of energy through the direct contact. On a molecular level, energy moves from more energetic particles to adjacent, less energetic particles. For example, if you touch something warm, it will warm you.
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Convection: the transfer of energy between a solid surface and a moving fluid. On a molecular level, the interaction is the result of a combination of conduction and fluid motion. For example, your air conditioning might blow cold air past you, thereby cooling you.
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Radiation: The transfer of energy due to the emission of electromagnetic waves. The energy levels of particles on the surface change as they emit and absorb photons. For example, the sun heats you via electromagnetic radiation when you stand in the sunlight.
Types of Radiation
Broadly speaking, electromagnetic radiation is classified into two main categories.
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Shortwave radiation refers to light with shorter wavelengths, including the ultraviolet, visible, and a portion of the near-infrared spectra. In daily life, you would typically encounter shortwave radiation from direct or reflected sunlight.
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Longwave radiation refers to infrared light with longer wavelengths. All objects emit some longwave radiation, the degree of which is dependent on their temperature and material.
The Importance of Materials
Of course, not all materials radiate equally. Walking on asphalt feels significantly different than walking on grass on an equally hot day. The amount of energy emitted by two surfaces of equal temperature depends on a scaling factor called emissivity. Most dark-colored stone materials have relatively high emissivity (close to 1), whereas reflective materials have relatively low emissivity. This emphasizes the importance of material choice when designing the surfaces around us.
Radiation and Comfort
The forgotten side of thermal comfort
The traditional thermal comfort industry has been dominated by Heating, Ventilation, and Air Conditioning (HVAC) systems. These are entirely based on convection, circulating heated or cooled air to regulate occupants. Because of this, most people equate the air temperature with their thermal comfort. However, convection is just a small piece of the thermal comfort puzzle. Radiation is often responsible for upwards of 50% of human thermal comfort, whereas convection typically sits below 30%. Because of this, knowledge of the radiant environment is paramount to understanding our thermal comfort in everyday life. We should be thinking about the radiant environment as much as we think about the air temperature. Learn more about thermal comfort modelling here.
The Radiant Measurement Problem
While it is fairly easy to conceptualize radiant heat transfer, quantifying it is another matter. To accurately categorize the total amount of thermal radiation to or from your body, you would need to know the surface temperature of everything around you and individually calculate your heat exchange with each surface. This is an impossibly tall order in most scenarios. Instead, we use a metric called Mean Radiant Temperature (MRT), which is an average of all of the surrounding surface temperatures, accounting for how much of your field of view they take up.
The Failure of the Globe Thermometer
Currently, the most common device used for measuring MRT is called a globe thermometer. The idea is that by placing a high-emissivity globe in a space, it will equilibrate with the surrounding radiant environment. Placing a thermometer inside of this globe therefore allows one to measure the MRT.
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The main problem with this method is it assumes that globes are not affected by convection. In reality, however, natural buoyancy effects will cause the globe to exchange heat through both convection and radiation, meaning the MRT measurement is not accurate. Consider a room where the surfaces are warmer than the air. A globe placed in this space will warm due to radiation with the surfaces. However, this heating will also cause the globe to warm the air around it, which will rise due to natural buoyancy, creating an airflow around the globe that will convect heat away from it, thus cooling it. This effect was understood when the globe thermometer was developed in the early 1900s, and a separate apparatus was needed to account for it. In modern times, however, this process has long been neglected due to the complexity of the setup. As such, globe thermometers on their own are not an accurate method of measuring MRT, but they are commonly used erroneously throughout industry and academia. If you want to delve deeper into the research regarding the convective bias of these devices, refer to this paper: https://doi.org/10.1038/s41598-022-10172-5.
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Another problem with globe thermometers is their latency. They take a long time (>5 minutes) to equilibrate with the radiant environment. This makes them unsuitable for mobile applications, like climate walks, or rapidly-changing environments. They often fail to capture the whole radiant picture.
Enter: CHAOSense
We seek to address this gap in radiant instrumentation and pave the way to a more complete understanding of our radiant environment. We are the only company that offers accessible, integrated digital devices for measuring radiant heat transfer. Having fast and accurate radiant measurement tools is the first step to researching and developing systems that allow us to more fully understand and harness the potential of radiant heat transfer.
