This article is the third in a series outlining technological and economic obstacles to the widespread use of solar power.
From the standpoint of a photovoltaic (PV) owner/user/purchaser, efficiency boils down to the question of how large the solar collector array has to be to get the job done.
If the efficiency is low, larger (or more) collectors are required. If the efficiency is higher, fewer or smaller solar collectors will be required.
What, exactly, is efficiency? Simply, it is the ratio of the electrical power produced (in watts) to the input solar power (sunlight, but expressed also in watts) striking the solar collector, the ratio usually being expressed as a percentage.
Most photovoltaic devices (solar cells and PV cells) that are manufactured in quantities large enough to be considered for use in producing electricity have efficiencies in the range of 10 percent. That is, just one-tenth of the power in the sunlight becomes electrical power.
Inefficient Conversion
In bright noontime sunlight, a one-square-meter surface facing the sun is exposed to somewhat less than 1,000 watts of solar power in the form of light. At 10 percent efficiency, a PV collector of that size will produce about 100 watts of electricity.
To produce the 1,200 watts required for a hair dryer requires the full output of 12 square meters of PV collector. Of course, the output will be lower when the sun is low in the sky (because the sunlight is passing through much more atmosphere), not directly facing the collector, or blocked by clouds. At night, there will be no production whatsoever.
Overall, you will be lucky to produce a year-round average of 20 watts from a single square meter of PV collector.
Forget about politics and economics. The reasons PV cell efficiency is so low are physical, involving the solar spectrum and properties of materials.
PV Power Minuscule
All of our sources of electricity–batteries, generators (turned by hydropower, steam engines, gas turbines, or your automobile engine), and solar cells–energize electrons. Our lamps, motors, TVs, computers, and all other electrical devices extract that energy.
There is always an energy source (water behind a dam, coal, wind, natural gas, petroleum, nuclear fission, sunlight, etc.), a mechanism for energizing electrons (generator, PV cell, etc), and a method of delivery (wires).
The goal of photovoltaics is to produce lots of power, and to do it cheaply and reliably. The Carter administration predicted we would get 1.4 trillion kilowatt hours (kWh) annually from PV, equivalent to an astounding 160 billion watts of around-the-clock power, by 2020.
The American Physical Society (APS) studied the problem of photovoltaics and concluded, “It is unlikely that photovoltaics will contribute more than about 1 percent of the U.S. electrical energy produced near the end of the [20th] century.”
As it happened, solar electric systems produced a whopping 0.013 percent of our electricity in 2000, only 1 percent of the upper limit expected by the APS and a microscopic fraction of the Carter administration’s estimate for 2020.
Plenty of Money Available
It was not politics, the low price of coal, the low price of petroleum, or a lack of research money that made PV fail to meet the lofty goals envisioned during the oil crises of the 1970s.
The amount of money that could be made by manufacturing cheap, reliable, highly efficient solar cells is absolutely stupendous. The reward for picking the right combination of ingredients–chosen from that most democratic of documents, the Periodic Table of the Elements–and turning out square mile after square mile of PV arrays would be enough to make Bill Gates look like a pauper.
Hence there was and is a lot of research money available from both public and private sources, yet PV remains expensive and inefficient. After decades of intense research, large-scale PV makes sense only in places that have no other source of electricity.
Photovoltaic Mechanisms
Let us take a look at the mechanism of PV cells. Typical cells are made of a single wafer of silicon with two adjacent areas containing tiny amounts of impurities to make what is called p-type and n-type semiconductors.
A photon (a “particle” of light) strikes the p-type semiconductor and energizes the electron, sending it into the adjacent n-type semiconductor, leaving a hole behind. A natural electric field in the p-n junction between the two types of semiconductor keeps the electron from directly going back to refill the hole, and allows that electron to pass through the external circuit, doing something useful. Electrons return to the p region through an external circuit where they accomplish something useful as the process as a whole creates electricity.
However, the voltage produced by a single PV cell can’t even run a household light bulb.
In the next, and final, installment of this series, we will examine some remaining practical problems with widespread photovoltaic power.
Howard Hayden ([email protected]) is professor emeritus of physics at the University of Connecticut and adjunct professor at Colorado State University at Pueblo. He writes a monthly energy newsletter available for $35 annual subscription at The Energy Advocate, PO Box 7595, Pueblo West CO 81007.
