Heat (i.e., thermal energy) is defined as the energy that is transferred spontaneously between two bodies due to difference in their temperature; heat flows from high to low temperature. There are three main well-known mechanisms for heat transfer: conduction through solid and fluids, convection through fluid, and radiation through solid, fluid or even vacuum.
The fact that thermal radiation can flow in vacuum can be understood when we know that thermal radiation is basically electromagnetic radiation. Therefore, the exchange of heat between bodies takes the form of exchange in electromagnetic energy. According to the laws of physics, electromagnetic radiation can only be generated by accelerating charge; either electric or conceptually magnetic charges.
In the case of thermal radiation, the source of charge can be found is any kind of materials; since they are all comprised of electrons and nuclei with negative and positive charges, respectively. Even if the material is neutral on average, the charges can still emit radiation. These charges are accelerated while they vibrate randomly (in amplitude and direction) due to the internal energy they possess. These mechanical vibrations are the result of thermal fluctuations, customarily termed thermal motion.
This fluctuation is one of the essences of statistical mechanics; at a finite temperature above zero kelvin, the value of each microscopic property of the system fluctuates around its macroscopic average; and velocity of particles comprising the material is not an exception. Therefore, any matter at a temperature above zero kelvin is a source of electromagnetic radiation. Since the driving force for this radiation is the temperature of the matter, this kind of spontaneous electromagnetic radiation is called thermal electromagnetic radiation, or thermal radiation for short. The thermal fluctuation of charges is a mechanism to exchange energy (via thermal radiative heat transfer), and momentum as well (i.e., Casimir forces)
It is known from classical physics that the maximum possible intensity of thermal radiation can be emitted by a blackbody; a body that absorbs and emits electromagnetic radiation perfectly at all wavelengths and all angles. The blackbody radiation concept was cleverly theorized by Max Planck.
In radiant heat exchange, electromagnetic energy is radiated from a warm body to a cooler one. This kind of heat exchange can, for example, power a thermophotovoltaic cell. A ‘blackbody’—a structure that absorbs and emits optical rays with perfect efficiency—is the thermodynamic ideal of such a radiator.
Near Field and Far Field Radiation
According to the canonical Stefan–Boltzmann law, which arises from the blackbody concept, the thermal radiation emanating from any large body is limited. The law also describes the maximum radiative-energy exchange rate between bodies that are separated by macroscopic distances.
Far-field radiative heat transfer between large bodies is limited by blackbody physics, for which the absorption/emission properties of each body in isolation determine the energy transfer rate. The far-field energy exchange is limited by the optical scattering properties of each individual body in isolation, and is bounded by the heat that radiates from the hot body at a rate of σT4A (where σ is the Stefan–Boltzmann constant, T is the temperature of the hot body, and A is the surface area).
As the separation between the bodies shrinks to microscopic distances, however, the concept of a blackbody loses meaning, i.e., light can no longer be treated as a bundle of rays that intersect/bypass a scattering body.
In contrast, for the near-field case, a significant evanescent-wave intensity can build up between the bodies when they are close enough. This result is proportional to the far-field blackbody expression, but has an additional near-field enhancement factor, i.e. (λT/d)2, where λT is the peak emission wavelength of the thermal emitter. The near-field expression also includes a material enhancement factor—|χ3|/Imχ (where Im denotes the imaginary component)—which corresponds to the potential for field amplification (as seen in plasmonic resonators).
There are three main attractive characteristics for near-field thermal radiation: (1) the increased radiation intensity is one of the obvious advantages over farfield thermal radiation, which is mediated by the extra evanescent waves that participate in transferring the heat. spectral selectivity (i.e., high intensity emission within selected frequencies), which is achieved by the resonant surface waves such as surface phonon/plasmon polaritons. And the exponential increase in the radiation intensity with the decrease in the separation distance, which permits localized heating and manipulation of heat flow. Due to these qualities, near-field thermal radiation finds application in a number of fields such as energy conversion, microscopy, spectroscopy and thermal computing.
Near Field Radiation Applications
The spectral selectivity and high intensity of near-field thermal radiation can increase conversion efficiency and output power density of thermophotovoltaics systems, respectively. Both of these qualities are lacked in current thermophotovoltaic systems.
Thermophotovoltaics utilize semiconductor pn-junction (i.e., photovoltaic cell) to produce electrical energy. The photovoltaic cell is illuminated by thermal radiation emitted by an emitter connected to a heat source. The efficiency of thermophotovoltaic can be pumped by near-field thermal radiation up to ~60% and power density of 24 𝑊𝑊/𝑐𝑐𝑚𝑚2 for an emitter kept at 2100 K. For thermoelectric generators, near-field thermal radiation has the potential to increase their conversion efficiency to 9% by separating the heat source from the thermoelectric generator by a small vacuum gap.
The quality of near-field to achieve localized heating can result in new applications in the field of magnetic recording, microscopy, spectroscopy and lithography. Near-field thermal radiation can increase the resolution of heat-assisted magnetic storage; by localizing the heat emitted from a laser heated tip (i.e., plasmonic antenna) to a small area of 50 nm (beyond the diffraction limit). For microscopy, near-field thermal radiation has been utilized to realize a new category of scanning probe microscope; thermal radiation scanning tunneling microscope (TRSTM).
The strong dependence of near-field thermal radiation intensity with both the separation distance and temperature-dependent material properties proposes near-field thermal radiation for heat modulation applications, such as radiative cooling of microstructures, thermal rectification, thermal logic and memory operations.
Engineers develop chip that converts wasted heat to usable energy
It’s estimated that as much as two-thirds of energy consumed in the U.S. each year is wasted as heat. Take for example, car engines, laptop computers, cell phones, even refrigerators, that heat up with overuse. Imagine if you could capture the heat they generate and turn it into more energy.
University of Utah mechanical engineering associate professor Mathieu Francoeur has discovered a way to produce more electricity from heat than thought possible by creating a silicon chip, also known as a “device,” that converts more thermal radiation into electricity. His findings were published in the paper, A Near-Field Radiative Heat Transfer Device, in the newest issue of Nature Nanotechnology.
Researchers have previously determined that there is a theoretical “blackbody limit” to how much energy can be produced from thermal radiation (heat). But Francoeur and his team have demonstrated that they can go well beyond the blackbody limit and produce more energy if they create a device that uses two silicon surfaces very close together. The team produced a 5mm-by-5mm chip (about the size of an eraser head) of two silicon wafers with a nanoscopic gap between them only 100 nanometers thick, or a thousandth the thickness of a human hair. While the chip was in a vacuum, they heated one surface and cooled another surface, which created a heat flux that can generate electricity. The concept of creating energy in this manner is not unique, but Francoeur and his team have discovered a way to fit the two silicon surfaces uniformly close together at a microscopic scale without touching each other. The closer they are to each other, the more electricity they can generate.
“Nobody can emit more radiation than the blackbody limit,” he said. “But when we go to the nanoscale, you can.”
In the future, Francoeur envisions that such technology could be used to not only cool down portable devices like laptops and smartphones but also to channel that heat into more battery life, possibly as much as 50% more. A laptop with a six-hour charge could jump to nine hours, for example.
The chips could be used to improve the efficiency of solar panels by increasing the amount of electricity from the sun’s heat or in automobiles to take the heat from the engine to help power the electrical systems. They could also be designed to fit in implantable medical devices such as a pacemaker that would not require replaceable batteries.
Another benefit is such technology can help improve the life of computer processors by keeping them cool and reducing wear and tear, and it will save more energy otherwise used for fans to cool the processors. It also could help improve the environment, Francoeur argued.
“You put the heat back into the system as electricity,” he said. “Right now, we’re just dumping it into the atmosphere. It’s heating up your room, for example, and then you use your AC to cool your room, which wastes more energy.”