Tracking energy

by Barbara Urbano, Department of Electrical and Electronical Engineering, University College Cork 

Energy can neither be created nor destroyed, but luckily, it can be transformed [1]. But where does the energy on Earth come from and how does this energy reach humans so they can use it? This blog skips some billion years of formation of solar systems and planets, and lots of complicated energy-transforming steps. Instead, it begins with our Earth nowadays, including the solar system it is circulating in. Furthermore, because this is UV-SINTEC, the path of energy we will follow in this blog is not going the direct way from the sun to plants but rather makes a detour via greenhouses and LEDs instead.

The most significant energy source for planet Earth is the sun [2]. Solar cells can capture solar energy directly and convert it into electrical energy [3]. Alternatively, photosynthesis in plants converts solar energy into chemical energy, which in some cases can be stored for millions of years in coal. By burning coal the chemical energy is converted into heat energy, which can be transformed into kinetic energy (steam), then mechanical energy (rotating turbines), and finally, electrical energy [4]. Energy losses occur at each energy transformation step, for example in the form of heat given to the surroundings [1]. Other routes for the transformation of solar energy into electrical energy include the exploitation of wind using turbines, of rain using hydroelectric power plants, or of plant growth using biopower plants [5].

The generated electrical energy can supply greenhouses with the required electricity. A crucial part of plant growth in greenhouses is artificial light to ensure that plants can perform photosynthesis. Let's look closer at light-emitting diodes, also known as LEDs, and how they transform electrical energy into light energy. A diode is a semiconductor. As the name reveals, such a semiconductor's peculiarity is that it is conductive on some occasions and isolating in others. Let's have a closer look at semiconductors and their band structure. According to the band theory, semiconductors feature a valance band, which is the highest energy level still occupied with electrons. The conduction band is the lowest unoccupied energy level. The semiconductor's energy gap between these two levels is between 0.1 eV and 4 eV, whereas the bands overlap in the case of a metal, and the energy gap is larger in the case of an insulator. As you might have recognised, light from UV to IR also has energy below 4 eV. Indeed, the bandgap of an LED defines the wavelength of the emitted light. As the bandgap can be adjusted by combining semiconductors or by doping the semiconductors with additional materials, different wavelengths of light can be generated [6].

However, a semiconductor alone does not build a whole light-emitting diode. A pn-junction would be one solution to create a device that emits light. Here, the n-doped side features more electrons, whereas the p-doped part features more empty slots in the valance band. Applying a forward current on the pn-junction, meaning connecting the plus pole to the p-doped material and the minus pole to the n-doped material, results in an electron flow from the n-doped conduction band recombining at the junction of the materials with the free slots on the valance band of the p-doped material. This recombination of electrons and holes releases the energy between these two bands. The energy E can be emitted as light in the form of a photon with the respective wavelength λ, related with , with Planck's constant h and speed of light c. Because the energy gap of the material defines the photon's wavelength, LEDs emit light in a very narrow spectrum. Hence, it is possible to build different light spectra by combining LEDs with varying energy gaps. Another advantage of LEDs is the tunable intensity. Increasing the current flowing through the diode results in an equal increase of the light emitted by the diode. Furthermore, high-power LEDs, which are necessary for greenhouses, require less power, develop less heat, and feature a longer service life than other artificial light sources used for plant growth [6].

The light energy emitted by the LED now reaches the plants, where the plant's photosynthetic processes use water, carbon dioxide, and the generated light energy to create oxygen and chemical energy in the form of glucose and other high energy chemicals [8]. As photosynthesis is critical for us in UV-SINTEC, I could go into more detail here; however, I am an electrical engineer, so I won't! Humans and animals can now consume and use the plants' chemical energy in the form of fat, protein, or carbohydrates [8].

Looking back at the long way the energy had to travel to finally reach its destination, we at UV-SINTEC study and research the opportunities of different wavelengths of ultraviolet light, used to supplement the visible light of artificial light sources for plant growth. Our goals are to build energy-efficient LED lamps with a high amount of spectral and energy uniformity to evaluate the influence of different spectra on the plants. Possible benefits investigated are increasing critical nutritional components or secondary metabolites in crops, their absolute bioavailability, and thus the human diet.

References

[1] F. A. Farret and M. G. Simões (2006) "Principles of Thermodynamics". In: Integration of Alternative Sources of Energy, IEEE,  pp. 28-56.

[2] M. K. Hubbert (1971) "THE ENERGY RESOURCES OF THE EARTH". Scientific American, 225 (3): 60-73.

[3] S. Botti and J. Vidal (2013) "Energy Generation: Solar Energy". In: Computational Approaches to Energy Materials, R. Catlow, A. Sokol and A. Walsh (eds), John Wiley & Sons, pp. 29-70.

[4] Wikipedia, "Coal-fired power station", https://en.wikipedia.org/wiki/Coal-fired_power_station. Accessed March 22, 2022.

[5] R.P. Walker and A. Swift (2015) "Overview of Wind Energy and Other Sources of Electricity". In: Wind Energy Essentials: Societal, Economic, and Environmental Impacts, John Wiley & Sons, pp. 48-70.

[6] S. Dutta Gupta and A. Agarwal (2017) "Artificial Lighting System for Plant Growth and Development: Chronological Advancement, Working Principles, and Comparative Assessment". In: Light Emitting Diodes for Agriculture, S. Dutta Gupta (ed.), Springer, Singapore, pp. 1-25.

[7] Halbleiter.org, "Semiconductor Technologyfrom A to Z", https://www.halbleiter.org/en/fundamentals/conductors-insulators-semiconductors/. Accessed March 23, 2022.

[8] Wikipedia, "photosynthesis", https://en.wikipedia.org/wiki/Photosynthesis. Accessed March 22, 2022.