30% of Japan’s energy comes from nuclear reactors, at least it did until March 11th, 2011 when an earthquake lead to a nuclear disaster at the Fukushima Daiichi power plant. 11 others shut down that day. Within a year, the countries remaining nuclear power plants all shut down in order to make upgrades and undergo inspections. At the time Peter Fairley’s MIT Technology Review article was written in 2014, Japan was facing its “third summer in a row without use of the nuclear reactors that had delivered almost 30 percent of its electricity” (Fairley, 2014, 28) leaving a gap to be filled. Solar looked to be filling that gap.
Japan more than doubled its solar generating capacity to 9.6 gigawatts of photovoltaics. That’s just over 3% of the overall energy capacity in Japan, a far cry from the 30% provided by nuclear. Even still, when the nuclear reactors do come back online, the clean power generated by these will combine to be a greater amount than they were before.
What’s interesting is how this development came about. On one hand, you can credit the gap left by nuclear which forced reliance onto imported fuel sources and raised costs which created both a vacuum of opportunity and provided fiscal motivation to explore other options at home with the best ratio of energy return on investment. On the other hand, there was even greater fiscal motivation in the way of feed-in tariffs established by then Japanese Prime Minister Naoto Kan resulting in the installation of facilities that “promised industrial-scale solar facilities 40 yen (35 cents) per kilowatt-hour generated for 20 years.” (Fairley, 2014, 30), seeing a full return on investment in 10 years and predicting substantial profits for the next decade.
Interestingly, this isn’t Japan’s first surge in solar power. The first came as a result of another energy crisis, that of the 1970s. As an island nation, Japan was desperate to find an energy source at home, and by “2001 total solar-power output in Japan was 500 times higher than it had been a decade earlier—a decade in which U.S. solar generation edged up by a meager 15 percent.” (Fairley, 2014, 32)
This technology is one that I’ve only ever had a basic understanding of until just recently, and the way it works is truly fascinating. Electrons associated with an atom may be described in terms of energy levels. Silicon, which most solar cells are made of, contains 14 electrons with 4 in its outer shell. This outer shell is the most reactive and has the highest energy level. Although there are 4 electrons in this region of the atom, their energy levels differ, differentiating the electrons into different sublevels. In the case of silicon, the 4 outermost, highest-energy electrons are split between sublevels referred to as 3s and 3p.
When a large number of silicon atoms come together to form a solid, the electrons of the atoms begin to react with each other resulting in variable energy levels in their outer bands. The thing is, these interactions actually result in the formation of even more sublevels within individual silicon atoms. 3p, for example, splits into now 3 separate sublevels, as does a lower energy sublevel referred to as 2p. A silicon atom is a solid at ground state that will fill all levels and sublevels except for the last 2 of 3p and is known as the conduction band. The space in between the conduction band and the new outermost valence band is known as the energy gap and is a forbidden zone with a width corresponding to a specific amount of energy depending on the type of atom.
This is where sunlight comes in. When a photon strikes the silicon solid with enough energy, it can transfer energy to an electron in the valence band causing it to jump across the forbidden zone into the conduction band. This, in turn, leaves a vacancy in the valence band. A hole, if you will. This hole effectively skews the balance of electrons (negatively charged particles) and protons (positively charged particles) resulting now in an atom with a positive charge. In order for this to happen, “the photon must have energy greater than the energy gap” (Dunlap, 2019, 314) which in silicon is 1.1 eV. This energy amount happens to correspond with a wavelength of 1130 nm which is handily provided by most of the solar spectrum. Those ejected electrons essentially become free-agents, able to move through the aggregate solid silicon material, and this is simultaneously happening across the entire material. This movement of electrons is by definition an electrical current, thereby providing energy.
The problem is, the hole left behind is still a hole and a positively charged one at that. Those negatively charged free-agent electrons and the positive gaps left behind will eventually attract one another, seeking the neutrality and balance that is a core principle of chemistry, effectively shutting down any electrical generation in a process called recombination.
Creating a reliable, functional photovoltaic cell requires eliminating as much opportunity for recombination as possible. If we incorporate the inclusion of at least a small amount of an element like phosphorus whose atoms contain 5 electrons in their outer shell, it creates just enough imbalance for the generation of free-floating electrons which remain free-floating because there simply isn’t an open hole for them to fill. This is called a negative type (n-type) semiconductor, also known as a type of doped semiconductor.
There are also positive-type (p-type) semiconductors, however, these are constructed using elements such as aluminum or boron with 3 valence electrons, creating a receptacle.
Functioning photovoltaic cells are made from combinations of n-type and p-type semiconductors. The holes left behind by the ejection of electrons from photon interaction are filled at junction points between the p- and n- types resulting in something called the depletion region—the thing being depleted being the holes.
As the holes are filled, the negative charge strengthens, producing a barrier-like electric field. When an electron inside a hole gets hit by a photon, the energy causes the electron to be ejected which gets pushed towards the n-type side, allowing for another to move in resulting in a consistent electrical current.
Most photovoltaic cells available commercially “have efficiencies for converting energy from solar radiation into electrical energy of 12 to 18%” (Dunlap, 2019, 316), and Japan has already shown they can effectively implement vast solar arrays that work efficiently and have a marked return on investment in 10 years or less. Again, it’s ultimately a question of economics above all else. A larger switch to renewables such as solar or wind will require a reengineering of the grid. While a substantial undertaking, it “is not necessarily more costly than the path back to nuclear that the current government and the utilities are charting.” (Fairley, 2014, 35), especially when taking into account the level of risks associated nuclear. The cost of insurance and upgrades could double the cost of nuclear energy making it a fiscal gamble in comparison to Japan’s already promising solar potential.
References
Dunlap, R. A. (2019). Sustainable Energy, SI Edition (Second ed.). Cengage Learning.
Fairley, P. (2014, December 18). Can Japan Recapture Its Solar Power? MIT Technology Review, VOL 118(1), 28-35. https://www.technologyreview.com/2014/12/18/169847/can-japan-recapture-its-solar-power/
