Energy
In physics, energy is the currency that allows for work. Energy exists in different forms that can be categorized under potential energy or kinetic energy. The former is stored energy while the latter is the energy of movement. Different textbooks classify subgroups of energy slightly differently; however, the forms of energy according to the U.S. Energy Information Administration are chemical, mechanical, nuclear, gravitational, radiant, thermal, motion, sound, and electrical.
One of the most critical aspects of energy is that it abides by the Law of Conservation of Energy, which states that energy cannot be created nor destroyed. This means that energy must be transformed between the various types to be used efficiently. For example, humans use chemical energy in food for the motion energy of arms and legs.
For the purpose of transforming energy, both living organisms and artificial systems have evolved naturally or by man to harness energy. In particular, this article discusses two systems that capture and convert radiant energy (light) into more practical energy.
Radiant energy
Radiant energy has been a consistent source of energy for the Earth for as long as the Sun has been around. As a result, the most primitive living systems such as bacteria and plants have adapted to be powered by sunlight in a process notoriously known as photosynthesis. While the capturing of light for energy has been used in living organisms for at least three billion years, the recent man-made solar panel of the 19th century employs atomic interactions that nearly mimic that of plants. In fact, these systems are so similar to each other that it is worth discussing.
Plants and the photoelectric effect
The first step to converting light into other forms of energy is the excitation of electrons. Both plants and solar panels depend on this energized electron state to proceed to the next steps of the energy cycle.
In plants, chlorophyll is a molecule that absorbs light. When light, in the form of photons, strikes the chlorophyll molecule, one of the molecule’s electrons rises to a higher electron shell, which excites the electron, giving it more potential energy. This excitation of electrons by light is called the photoelectric effect. Because this excited electron is unstable, it typically drops down to its original state, giving off heat or fluorescent color. The diagram below shows the photoelectric effect if one chlorophyll molecule was isolated.
However, the plant contains the strict organization of many chlorophyll molecules, proteins, and other smaller molecules that form photosystems, or light-harvesting complexes.* The purpose of a photosystem is to preserve the energy of an excited electron by bouncing the electron through the chain of chlorophyll until the electron reaches a molecule that can capture the energy of that electron. The diagram below is a photosystem within the plant. (To reduce complexity and focus on comparison, the details of the plant cell have been omitted.)
This is the first step of the plant photosystem; however, solar panels also involve the excitation of electrons. The only difference is that solar panels involve the photovoltaic effect. In the photovoltaic effect, light energy is directly converted into electricity, while the photoelectric effect does not create electricity. As a result, one of the main differences between the two systems is that plants convert light into chemical energy (in the molecule ATP), while solar panels convert light into electrical energy.
A typical solar panel is composed of solar cells, which are the smallest working units that can convert light into electricity. Their function is similar to that of a photosystem; however, their structure is made of rigidly layered conductive and semi-conductive metals and an anti-reflective coating.
Solar panel structure
The base layer of a solar cell is a conductive metal plate (conductive means electricity readily passes through it). A silicon layer is placed on top of this metal plate, which is a semi-conductive layer that is divided into two sublayers: silicon boron, and silicon phosphorus. These two sublayers are separated by the p-n junction, a vital component for the movement of electrons. An anti-reflective coating and conductive metal fingers lie above to optimally capture light and electron energy respectively.
The most important part in comparing the plant photosystem and the solar cell is what happens to the electrons within the silicon layer when light strikes the solar cell. When light strikes the solar cell, an electron is excited, leaving a hole in the electron shell. As in the photoelectric effect, the electron has a tendency to drop back into that hole; however, the p-n junction and silicon layers force the electron to move upward and away from the hole (diagram on the left).
Despite being forcefully separated from the hole by the atomic arrangement of silicon and the p-n junction, the excited electron still wants to “rejoin” with the hole. To achieve this, the solar cell contains a wire that leads the electron back to the hole (diagram on the right). Doing so, the electron’s energy as it moves through the wire forms an electric circuit and generates electricity.
Essential similarity:
- Both the plant photosystem and solar cell use light energy to excite electrons
- The excited electron energy is then captured through the movement between chlorophyll or metal wire to reach a final destination.
- Both systems attempt to absorb as much light as possible, with plants having many units of chlorophyll and solar cells having anti-reflective coating.
- Light is converted into another form of energy, either chemical or electrical.
But which system uses light more efficiently? and can the properties of both the plant and solar panel combine into a single piece of technology?
First, it is important to note that the value of comparing plants and solar panels is that upon determining which is a more efficient energy source, it is also possible to determine how that efficiency can be maximized. For instance, how can plants be modified to produce more chemical energy in the presence of sunlight or how can solar panels can be improved to produce more voltage per solar cell?
Efficiency
These questions are critical because a greater diversity of energy systems available to humans allows for increased stability when one source declines. The problem with energy transfer is that while some of the energy is used more efficiently, much is lost as heat, such as when an excited electron clashes back down without passing through the chain of chlorophyll or metal wire. Also, the more stages and sub-transfers of energy within a system, the more heat is given off at each stage.
Since both chlorophyll and silicon are involved in the absorption and structure of these light energy transfer systems, can they replace each other? Can silicon extraction be lessened by using plant chlorophyll instead? According to market research company Mordor Intelligence, the Global Silicone Market size produced 2.86 million tons of silicone in 2023, with predicted silicone extraction increasing to 3.68 million tons by 2028. To reduce silicon harvesting, can solar energy systems be refined?
Chlorophyll solar panel?
One attempt at using chlorophyll in solar cells to generate electricity has been by the partnership between the Ivanovo State University of Chemistry and Technology and the Institute for Physics of Microstructures of the Russian Academy of Sciences located in the Russian Federation. While the results are largely unsuccessful, fluctuating data reveals that chlorophyll-type dyes may mimic the photovoltaic effect. However, the methods need to be refined and more research is required.
