Updated: Apr 22, 2020
By Connor Sanborn, Co-Founder of SunFlower LLC
Considering the brief existence of the human species on Earth, we’ve managed to take some of the most basic life processes and mimic them extremely effectively. Take your average stalk of corn, converting only 1-2% of the sunlight hitting it into stored energy (Gust, 1996), it seems to grow pretty well on its own. And given the energy demands of your average piece of corn, that’s just fine. Corn isn’t out there starting businesses or going to visit its family in Minnesota; it simply stands there and absorbs light, content with growing ever-taller, staying in the same place.
Humans, meanwhile, require significant amounts of power over extended periods of time. This, by definition, means lots of energy. To suit this growing need, we’re creating chemically-designed surfaces well on their way to approaching 50% light conversion efficiency (Bellini, 2020). The processes of harvesting solar energy for biological lifeforms vs. human technologies have different means to the same end: capture the energy transferred by waves of light. Comparing the two types of energy capture first requires an understanding of why these light harvest systems do what they do.
Photosynthesis (When an Organism Eats Sunlight and CO2, Producing Sugar)
If we decided to take a field trip inside of a nearby plant’s leaves one sunny afternoon — shrinking down to the size of the molecules that make us up (and everything around us) — we’d probably be surprised at the degree of theme park-style excitement occurring all around. When an organism absorbs sunlight and stores its light energy as sugar (called photosynthesis), it’s taking parts of the sunlight (visible and not) and using its energy to split water molecules (H2O) into hydrogen ions (H+) and individual oxygen atoms (O). Hydrogen is used to help store the light energy as chemical energy within carbon compounds like sugar and new oxygen molecules (O2) are released into the atmosphere as a byproduct.
Figure 1 The small and exciting world allowing humans to breathe on Earth is run by three key players: sunlight (input energy), carbon dioxide (source of carbon for energy storage) and water (source of hydrogen as an energy carrier). This process is capable of and optimized for capturing and storing energy for plants and other photosynthetic organisms, even on cloudy days (Photosynthesis, 2020).
That process is great news for humans, because we animals need O2 to breathe. Meanwhile, this plant’s microscopic adventure park showcases a complex network of cellular machinery moving about and doing its thing, turning normal old carbon dioxide gas (CO2) into useful sugars like glucose (C6H12O6) kept for later on — think of a maple tree storing sap for the wintertime. This is the plant’s daily meal, whether you’re talking about an individual algae in the sea or a massive white pine tree.
Photovoltaics (Light Hits a Surface, Producing Electrical Energy)
Humans — always intrigued and inspired by the genius of nature — one-upped this process by producing the world’s first photovoltaic cell inside of Bell Labs in 1954 (The History of Solar, 2001), just in time for the space race! This ‘cell’ was really a piece of super-refined crystalline silicon (Si) reaching approximately 4% light conversion efficiency — a massive achievement at the time. The invention marked the emergence of a new human talent: harnessing electrical energy directly from the Sun (photovoltaics). In a less complex process (compared with photosynthesis), this technology takes advantage of an electrical current produced when electrons are knocked loose from Si atoms by incoming carriers of sunlight (photons). This is called the ‘photoelectric effect’ and plants, indeed, also utilize this natural phenomenon in their capture of the Sun’s energy.
Figure 2 An illustration of the photoelectric effect as utilized by a solar PV panel of the type commonly seen on people’s homes (Voudoukis, 2018). Units of incoming sunlight allow some electrons in silicon atoms to escape their ‘holes’ and flow through metal conductors on the panel in an electrical current to power the ‘load’, a common light bulb.
As manufacturing techniques and scientific knowledge improved in the past seven decades, these solar PV cells have become vastly more efficient at harvesting light energy. In 2020, quality residential solar panels typically reach around 20% light conversion efficiency. Some very gifted folks over at the National Renewable Energy Laboratory (NREL) have just unveiled an experimental stacked (multi-junction) silicon solar cell reaching 47.1% efficiency (Bellini, 2020). Get ready for everything to be covered in solar.
A Comparison of Purpose and Function
While nice to compare the light-capturing abilities of a plant to that of a modern human technology, it doesn’t highlight the fundamental difference in why these surfaces are capturing light energy in the first place. Your average houseplant has one central goal — obtain sunlight to turn CO2 into sugar for food (energy) and structural building blocks for growth (like cellulose or starch) later on. On the other hand, a solar panel has one different purpose — generate a stable flow of electricity (electrons) to be used there or sent somewhere else immediately. Plants use their talents to absorb sunlight and store its energy. Solar panels create electricity on the spot, absorbing sunlight to move its energy.
Implications of Differences in Efficiency
If trees could walk, they’d have bigger issues on their limbs (or maybe branches are tree hands) because mechanical motion requires great expenditures of energy. Due to their sedentary and passive nature, trees and plants of all types need very little energy to get by compared with an animal that actively moves and changes locations throughout its daily life. This fact, along with the plant strategy of storing the Sun’s energy first, means that it gets by just fine on a 1-2% photosynthetic efficiency. As for humans — not so much; we need higher efficiencies, we need more energy to move our heavy objects around, including ourselves.
A Moment to Reflect
Organisms that absorb sunlight for food (including cyanobacteria and algae in the oceans) are utilizing the same source of energy as the blue or black solar panels that you see on rooftops and in people’s yards — these creatures are simply doing something different with the solar energy they encounter. Figure 3 below shows a good example of how human technology is coming ever-closer to replicating and mimicking the natural functions of biological systems, though nature’s design knowhow and R&D experience is a bit over a billion years ahead of ours in the evolutionary race (Timeline of Photosynthesis on Earth, 2008). Next time you sit comfortably in the shadow of a tree underneath the afternoon sun, able to read your book yet escape the worst of the UV rays, remember that it’s only possible because of the leaves overhead — designed to capture sunlight and release breathable oxygen, enabling us to survive.
Figure 3 A look at the differences between the processes of natural and artificial photosynthesis (Artificial Photosynthesis, 2009), a process theorized to have massive potential for the creation of biofuels and bioenergy in the future. Separate from photovoltaics, this new human technology attempts to mimic natural processes in plants by reducing CO2 from the ambient air to store chemical energy.
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