|Fig. 1: Solar Panels. (Source: Wikimedia Commons)|
There has been cyclical talk of an energy crisis for a while now. There was a push for renewable energy in the 1970's and this crisis occurs whenever oil prices rise. [1,2] The fact of the matter though is that this is merely a nuisance, and not at all a crisis but a mere foreshadowing of what is to come. As can be seen in Fig. 3, at current rates of consumption, the fossil fuels that run the world's economy are being consumed rapidly and will be gone within a century or two.  While some may think that new discoveries will allow us to keep consuming fossil fuels for a long time, they take "millions of years to form".  Eventually, the ride will end. Some predict that this won't occur until the 24th century.  When the last of the fossil fuels are all but consumed, synthetic gasoline will probably still be around, owing to the fact that it is the most energy dense substance possible and that the majority of the world's infrastructure has already been designed to run on it.  It will still be quite valuable, if only as a means for air travel.  The question that remains then, is what will this new energy source be that we use to power our cities and run our trains and appliances? The answer is: Solar.
"In a free market, the forces of supply and demand generally push the price toward its equilibrium level, the price at which quantity supplied and quantity demanded are equal."  It stands to reason then, that while demand for fossil fuels increases, production will not be able to keep up with demand, and prices will rise. Whether this will occur gradually, smoothing the transition for all and allowing for the adaptation of technology, or occur abruptly near the end resulting in skyrocketing prices and an energy crisis that the world has never imagined, remains to be seen. The Tipping Point, or "... moment of critical mass" will be the time in which the cost of solar energy per kWh is far below the cost of fossil fuels (coal, oil, and natural gas) per kWh. 
|Fig. 2: Ivanpah Solar Power Facility with all three towers under load. Taken from I-15. (Source: Wikimedia Commons)|
The cost of solar power has fallen from $0.21/kWh in 2010 to $0.11/kkWh in 2013.  In Lancaster, California, the city reduced the cost of electricity for their schools by 35% by buying and installing 32,094 solar panels.  In 2011, 1,855 megawatts of photovoltaics [PVs] were installed, up from 877 megawatts in 2010.  Some companies have even started offering group discounts to their employees on PVs, "Homeowners paying an average of $147 a month for electricity would instead pay an average of $97 a month over 12 years if they financed the entire system, after which the payments would go to zero".  In Tasmania, dairy farmers are projected to save 10-15% on power by using solar water heaters. 
Silicon, the primary element in many photovoltaic cells, is made from sand.  "The electronics industry takes sand ... and transforms it into large single crystals of silicon that are most chemically pure and structurally perfect crystals on Earth."  Silicon is one of the most plentiful compounds on the Earth's crust.  The scale and magnitude of solar power plants is therefore not constrained by the lack of abundance of materials. It wouldn't be unimaginable to turn the entire Sahara desert into a solar power plant (how to store and transport that energy is another matter).
The amount of solar insolation (number of hours of radiation at 1kW/m2 per day) in Phoenix, Arizona is 6 kWh/m2/day.  The total land area of Arizona is 113,508 square miles, or 293,982,909,042 square meters.  If the entire state were to be covered with solar panels with an efficiency of 20%, it would create 353 TWh of electricity in a day. That is the equivalent of 207,540.274 thousand barrels of oil each and every single day or 75,752,631 thousand barrels of oil each year. The world's current consumption of oil is only 33,335,815 thousand barrels of oil.  Obviously the transportation and storage of solar power of that magnitude would be a herculean task and very impractical. The main point in this example is to illustrate the amount of solar energy being beamed down to the surface of the earth. It's a lot. The high cost of fossil fuels (once they are all but gone) would also make it more cost- effective to build high-speed magnetic levitation trains across the United States to replace air travel. Something that today has the prohibitive price tag of $100 billion dollars for a route just from Washington, D.C. to New York City. 
There is also another way to generate electricity that does not rely on solar cells. Solar thermal energy uses solar radiation to heat liquids and produce steam, turning a gas turbine generating electricity.  Concentrating solar thermal power (CSP) plants existing in the world today are the solar electric generation systems (SEGS) in California's Mojave Desert and the PS10 plant in Spain. The levelized energy cost of the SEGS project is currently about $0.12-14/kWh, higher than the current market rate for energy which averaged $0.1092 in August 2014. 
|Fig. 3: Oil, Coal, and Natural Gas projections by year, based on published reserve estimates and current rates of consumption, assuming no future change in consumption or discovery of new resources. |
The main challenge to solar power in this future devoid of fossil fuels is going to be the price of competing energy sources, mainly wind and nuclear energy. The adaptation of nuclear energy will depend on whether political capital will be enough to prevent the construction of nuclear power plants. South Korea today has 23 nuclear reactors and is looking to build 13 more by 2030 despite an increase in cancer rates in the population surrounding some nuclear power plants.  On the other hand, wind is predicted to supply 19% of global energy by 2030.  Whether or not the world shifts to the use of these alternative types of energy will depend on whether or not CO2 emissions becomes important politically.
Storage for solar energy will be also be a challenge. A solar power plant cannot generate 24 hours a day, seven days a week. It will need to be able to store massive amounts of electricity in order to sufficiently power the needs of its customers over a 24 hour period. There are at least several methods of storing energy. It is possible to store it as heat in a liquid, gravitational potential energy (pumped-water storage) or electrical potential energy (batteries).
Storage today costs between $100-1,000 per kilowatt-hour (unlevelized). [23,24] Pumped water storage can cost $100/kWh while sodium ion flow batteries cost $400/kWh "... with less than 1 percent of pumped hydro's capacity." 
These numbers will need to fall dramatically, or prices of traditional fossil fuels will need to rise drastically, before these become viable solutions. In Nevada, Tesla is building "... the world's largest lithium-ion battery factory... which would produce enough batteries to fuel 500,000 cars annually by the end of the decade."  That type of scale, is sure to help to bring costs down.
There is already one tiny island that has already made the change to using purely renewables and storage. El Hierro is a remote Spanish island near the coast of Africa. The island has five windmills producing 11.5 megawatts. Excess power will be stored "... by pumping water more than 700 meters, or 2,300 feet, up into the crater of a sealed-off, long-extinct volcano." The total maximum capacity of it is 500,000 cubic meters. El Hierro is projected to save about 40,000 barrels of oil a year. 
It's impossible to know with absolute certainty what the future will hold. The only thing that can be done is to look at all the parts of the puzzle and anticipate what the possibilities could be. Whatever will happen, it will most be certainly be an interesting time to be alive.
© Oscar Galvan-Lopez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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