ES6: Mechanics of Energy Storage and Conversion—Batteries, Thermoelectrics and Fuel Cells
Last Day of Spring MRS

A Note on Photovoltaics

A topic that remains hot, both in the materials science community and the nation, is photovoltaics.  For the past several decades, there has been a movement to find alternatives to conventional energy production, which has typically been dominated by fossil fuels such as oil and coal.  This search was accelerated in the early 2000's when the price of oil surged, making alternative energy options more economically favorable.  The search gained even more urgency due to concerns of the environmental and climate impact of combustion-based fuels and the release of greenhouse gasses.  The search for clean energy has resulted in many alternatives; however, the one with the most potential is generally considered to be solar power.

The sun drops a about kilowatt of clean, usable energy for every square meter of the earth's surface (while the sun is shining, that is).  This is the main motivation behind the surge of interest in solar power conversion.  However, a key thing to note is that there are two main ways of converting the energy in sunlight into useful electrical or mechanical energy.  The first method is called "solar thermal", and it involves capturing sunlight as heat, and then converting that heat to electricity.  This is the typical scheme for the impressive "Solar Power Towers", massive towers surrounded by mirrors that concentrate sunlight onto a receiver on top.  The concentration is crucial in this system because the efficiency of the heat engine in the tower increases as temperature increases, as per the carnot cycle.  

The other way of producing electricity from sunlight is by a direct photon-to-electron conversion using photovoltaic devices.  These devices, commonly called "solar cells", are typically made of semiconductors, with silicon photovoltaics dominating the market.  They are increasingly more common to see on residential and commercial rooftops, and are also being used in power-plant scale installations.  If they are increasing so much in size and scale, then the question is this: Why are we still so dependent on fossil fuels?  Why do photovoltaics and solar thermal systems still only compose a small fraction of our national energy production?  At the end of the day, it all comes down to cost; if solar energy production was cheaper for my parents than simply connecting to the grid, I can assure you, they would install a bunch of solar cells on their house.  So why are they still expensive?  As you can imagine, there are several reasons, some physical, some economic, and some political.  

Unfortunately the answer to this question is longer than I can reasonably put into a single blog post, but the one thing that I will confidently say is this: the cost of the photovoltaics is not the primary problem.  The photovoltaics manufacturing industry has done a great job scaling up their production and decreasing the cost of producing the polycrystalline silicon wafers that are usually used for solar panels.  If you look at the chart below produced by NREL, you can also generally see that the efficiencies of the cells are not the problem.  The efficiencies of practically all cell types have been increasing for decades.  Another big shoutout to the perovskite researchers, who are producing a new photovoltaic architecture that may soon replace silicon as the standard material.  I would argue that the primary place that still needs innovation and cost-reduction is in storage.  Power is only produced when the sun in shining, which is for less than half of a day.  Solar power cannot replace conventional power plants yet because there is no inexpensive way of producing continuous, stable power, regardless of the time of day.  If I were to invest in a particular industry right now, it would be in large-scale power storage.  If we can figure this problem out, I believe solar power would seamlessly replace fossil fuels as our primary energy production method.  For now, this is not the reality, but as materials researchers, it is a reality that we can actually contribute to through our work.  





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