In physics, radiant energy is the energy of electromagnetic and gravitational radiation. The term “radiant energy” is most commonly used in radiometry, solar energy, heating, and lighting. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radiant flux for time. Radiant heat transfer is very important in the power industry because it is one of the most important ways to transfer thermal energy. It does not need a medium, such as air or metal, to take place. Any material that has a temperature above absolute zero gives off some radiant energy. Most energy of this type is in the infra-red region of the electromagnetic spectrum, although some of it is in the visible region.

The radiant heat transfer rate from a body (e.g.,, a black body) to its surroundings is proportional to the fourth power of the absolute temperature. It can be expressed by the following equation:

q =  εσT4

where σ is a fundamental physical constant called the Stefan–Boltzmann constant, equal to 5.6697×10-8 W/m2K4. This relationship is called the Stefan–Boltzmann law. The emissivity, ε, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.0. It can be seen and radiation heat transfer is important at very high temperatures and in a vacuum.

Two bodies that radiate toward each other have a net heat flux between them. The net flow rate of heat between them is given by:

Q = εσA1-2(T41 −T42)  [J/s]

q =  εσ(T41 −T42) [J/m2s]

The area factor A1-2, is the area viewed by body 2 of body 1, and can become fairly difficult to calculate.

References:
Reactor Physics and Thermal Hydraulics:
1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
5. Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
6. Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
7. Moran Michal J., Shapiro Howard N. Fundamentals of Engineering Thermodynamics, Fifth Edition, John Wiley & Sons, 2006, ISBN: 978-0-470-03037-0
8. Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2, and 3. June 1992.

Energy