Have you ever wondered why it feels biting cold after a clear night in winter? Or, when you walk in your garden you see frost covered grass despite last night’s temperature being well above freezing temperature. Did that intrigue you? If so, you might have already come across concepts like radiative heat transfer and emissivity. If not,  I will try to explain.

There are three primary mechanisms for heat transfer. Conduction: what happens when you touch a boiling pot is conduction. Heat transfers from the pot to your body through physical contact. The second one is convection, which causes the water in the pot to boil. Convection is the transfer of heat energy through fluid motion (either gas or liquid). The third one is heat radiation, where heat energy is transmitted as electromagnetic waves. Where does heat radiation happen? Normally, any object at a temperature emits heat radiation proportional to the fourth power of its absolute temperature. Radiation is also how we receive energy from the Sun. The efficiency of radiative heat transfer by an object is measured by its emissivity. Emissivity is a unit less quantity and is usually measured between 0 and 1. An object emitting no radiation at all will have zero emissivity whereas the one emitting all its energy through thermal radiation will have emissivity 1. Also, emissivity is a wavelength-dependent property. That is, if you measure the emissivity of an object from, say 300 nm to 25 µm, and plot it against the wavelength,  you might get an uneven graph.

Now, how does heat radiation cause frost on grass? As I mentioned, any object at a temperature can emit thermal radiation. What does this mean- even grass can emit thermal radiation depending on its temperature. However, the emission can not happen to just its surroundings, because both the grass and its surroundings are nearly at the same temperature. But the grass is exposed to the open sky on a clear night. So, the grass can emit heat to the sky where the temperature can be well below the ambient temperature.  Thus, the grass, on a clear night, emits heat as EM radiation to the sky, causing its temperature to go below ambient. Therefore, despite having the ambient temperature above freezing point, the grass would reach sub-freezing point, leading to the formation of frost. So, next time you see frost cover on grass, you know how that happened.

The atmosphere is filled with water vapour,  gasses,  etc. which would block (actually absorb) some of this emitted radiation.  However, there is a specific range of wavelengths in which the blockage (absorption) by the atmospheric contents is minimal. This is called the atmospheric (infrared) window and it extends from 8 µm to 13 µm. This implies that any transmission through this window will be exposed to outer space, where the temperatures are extremely cold (~3 K/−270 °C). Therefore, if an object on the Earth emits EM waves in the atmospheric window, it will exchange energy with space, and as it gives up more energy than it absorbs, its temperature can go many Celsius below the ambient temperature. Therefore, if the grass has a (spectrally selective) high emissivity in the 8 µm to 13 µm, we would see a sharper drop in the temperature. Nonetheless, radiative heat transfer to space (or sky) is possible only when there are no radiation-blocking particles such as clouds, or water vapour (humidity) in the atmosphere. That is why you see frost only after a clear night.

Let’s look at a different scenario, i.e., daytime. Solar radiation emitted by the Sun is absorbed by earthly objects causing them to heat up. The amount of energy absorbed by an object is defined by its absorptivity. Again, absorptivity is a unitless wavelength-dependent quantity measured between 0 and 1. Around 99% of energy from the solar radiation is confined to the 0.15 to 4 μm wavelength. This contains the near ultraviolet, visible light and the near-infrared spectrums. So, an object with high absorptivity within this spectrum would absorb almost all the radiation from the Sun. This is what solar heaters exploit.  They are usually painted with a special coating with high absorptivity, not necessarily in the whole 0.15 to 4 μm spectrum, but a portion of it. Therefore, they can absorb a lot of radiation and heat up rapidly.  On the other hand, if a material has zero absorptivity, then, it will hardly absorb any radiation. Now, imagine that the material has zero absorptivity in the solar spectrum and high emissivity in the atmospheric window (If we plot the emissivity and absorptivity against wavelength, it would look something like in Fig. 1.)  Since the object has zero absorptivity, its temperature would hardly increase. But due to its high emissivity, the object will radiate heat to outer space and its temperature will subsequently go below ambient temperature. Well, what just happened? An object placed in the scorching sun has been cooled down below the ambient temperature, despite the sun shining at its peak! This is called passive (daytime) radiative cooling (PRC). It is passive because the cooling happens without any active energy source, only by emitting EM waves. Passive radiative cooling has become an important topic for research these days. And, researchers are in pursuit of developing materials and paints that can keep your house and office cool without using any energy source. Such inventions can dramatically reduce the energy requirement for air conditioning and cooling. This is particularly significant considering the threats global warming and climate change are posing. However, achieving PRC requires materials with selective emissivity as shown in Fig. 1.

While many researchers are looking at PRC as a solution to the challenges of global warming, few have discovered the potential of the PRC to assist in energy harvesting. If you have read about soil-air thermal energy harvesting, you know the temperature difference between soil and air can be used to harness the energy; a unique 24/7 available source. PRC offers a viable method to enhance the energy yield from soil-air thermal energy harvesting. To harvest soil-air thermal energy, we use a system with a TEG, copper rods at the soil side and a radiator plate at the ambient side. The radiator plate is painted black. So, it has high absorptivity and emissivity. However, if the radiator plate is built using a special material which has high absorptivity and zero emissivity in the daytime (Fig. 2a), its temperature would significantly increase, leading to an enormous temperature difference between the soil side and the ambient side. Then, if we could somehow change its properties at night so that it has high emissivity in the atmospheric window (Fig. 2ab), the radiator temperature can be reduced below the ambient temperature at night. Since the soil is at a higher temperature than the air at night, we will again see a considerable increase in temperature difference. Therefore, just by changing the spectral properties of the material dynamically, we can increase the energy yield of the harvester. This definitely looks like an interesting approach. Nonetheless, there aren’t any off-the-shelf materials available that can give you a spectral response as shown in Fig. 2. In fact, you don’t have any proper PRC material so far available in the market. Though there are numerous solutions presented in the scientific literature, developing them requires access to many tools and materials. However, there are some DIY solutions available that we might be able to use. I am currently experimenting with such a PRC film and hope to combine it with a high absorptivity film to achieve the required spectral properties. Stay tuned for the results!