A year in green tech: Electricity from solar energy

NREL

A discussion of green technology cannot ignore the clean energy solution with greatest potential:  solar photovoltaic.  The energy from the sun that hits the earth could power the planet thousands of times over, so harnessing even a portion of its power would solve most of our energy problems.  Over the centuries, we have developed many ways to use the sun's energy to our advantage: direct heating of water (solar thermal energy capture), reflection of sunlight to provide illumination, photosynthesis in plants to produce sugars for our consumption, and direct conversion of incoming solar energy into electricity.  Even wind energy comes, at its heart, from solar energy since wind derives its energy from the pressure differences in the atmosphere, which in turn develop because of the earth's rotation and the uneven heating of the planet by the sun.  Although we can discuss many of these technologies, we first must understand the most promising in terms of energy potential and possible ease of distribution: solar PV or solar photovoltaic.

The science behind photovoltaic energy sources dates back to the 19th century.  Scientists such as Max Planck and Albert Einstein contributed to a body of work that developed an understanding of how certain materials behaved differently when bombarded with light.  Light, until then considered to behave as a wave, now had discrete packets of energy (called quanta), which under the right conditions would cause electrons in certain materials to jump away from the atom structure and move freely through the material.  This understanding of light as both wave and discrete unit (sometimes called the "wave-particle duality") forms the basis for quantum physics, and for the development of technologies based on the photoelectric and photovoltaic effects.

In standard electrical circuits, we need a potential difference to drive electron motion, then a method to channel that electron movement through the circuit we choose so that we can harness the power and use it to drive some piece of equipment.  For the better part of a century and a half, that source came from rotating magnets causing oscillating current flows, with the magnet rotation coming from a shaft that spins through the high-temperature, high-pressure introduction of steam to a series of blades surrounding the shaft.  This electricity alternates direction, and therefore led to the designation as alternating current.  As useful and profitable as alternating current power has become, it no longer holds the ubiquity in our lives it once did.  For most of the first hundred and twenty-five years we have harnessed electric power, we generated AC electricity, and made use of it in devices that rotated.  Most of us that remember grammar school science remember electric circuits that rely on the electro-chemistry of batteries to drive the current flow in a circuit.  These sort of direct current (or DC) circuits do not have the ability to deliver power over long distances in the same way that AC systems do, but they do produce consistent electricity that works well especially in digital circuitry.

This understanding of what causes the potential difference that creates electricity flow, and the manner in which our electrical infrastructure has developed, helps place the development and implementation of solar photovoltaic energy systems into context.  The Department of Energy has a useful website with much information about how specific materials produce electricity.  The basic system contains a material that absorbs sunlight, and uses the energy packets to release electrons into a conductive layer.  The material with the free electrons is paired with a layer of material that draws the free electrons.  By separating these materials, we create an electric potential that drives the electrons to flow from one material to the other.  During this trip, we channel them in the manner we desire and put them to use.

On the surface, the description suggests that by now, we should have solved this relatively simple technology.  The process of transferring the energy in a packet of light energy into electrical energy has several direct and indirect hurdles.  First, we do not have large quantities of the materials that currently make up most of the solar photovoltaic panels.  This scarcity drives up the retail price of the material, and therefore the price of the cells.  Second, the photovoltaic panels produce DC electricity, and our building infrastructure relies heavily on AC electricity.  Third, the current mix of materials, construction, and operation have a low efficiency as measured through comparing the amount of incident light energy that the solar cell (the core component of the panel) against the actual electricity generated.  Last, the use of the electricity from solar photovoltaic panels has not yet gained widespread support among grid operators.  These technicians need stability, and the thought of hundreds of thousands of electricity panels sending varying numbers of electrons with varying energy  levels into their infrastructure scares them.

Material selection and construction
The last ten years has seen great advances in the materials that produce electron movement when bombarded with light.  Currently, relatively rare elements like cadmium and hard-to-work-with materials like silicon, comprise most of the working part of the panel.  Recent research seeks to adjust the manner in which the conductive layer harnesses and moves the electrons.  Additionally, we have seen advances in ways to use more readily available materials.  We could be only a few years from such an advancement.  Research and development into solar photovoltaic systems also has produced consistent reductions in the size and weight of panels.  The length and bulkiness of the standard panel construction creates a mess of structural system, and recent advances in thin-film systems (those only a few micrometers thick) and material construction have increased the use of lighter and less material to produce the desired result.

AC versus DC
Ever since the battle for the grid was won by Westinghouse and Tesla over Thomas Edison, our method for delivering electricity over long distances has been AC.  In order to now integrate solar photovoltaic electricity into this system, we must convert it from DC into AC...a process that results in lost energy.  The devices that perform this transformation, called inverters (because they shift the oscillating wave of electricity into a steady flow, thus "inverting" part of the pattern) used to take up large amounts of space and require significant infrastructure while wasting energy.  Newer micro-inverters cover a smaller portion of each system, but also have better performance, and increase the ease of maintenance since having several smaller points of failure means more of the system stays active than when we have a single point of failure.  The next step in maximizing the performance of solar photovoltaic systems may come in the exploration of the DC grid.  As more of our systems rely on DC power (electronics, computers, communication equipment), some have considered installing microgrids of DC power, especially in large buildings and campuses.  This shift would improve the economics of solar photovoltaic, and ease transition.

Overcoming hundreds of years of infrastructure
For better or worse, we have an electric grid based on large, utility-scale electricity generators connected through large transmission lines and substations to areas of population that use electricity.  This relatively small number of generators makes it relatively ease to keep the grid stable and electricity flowing where we want it.  If every building were to have a photovoltaic array on the roof, and during period of low electricity use, all decided to send their electricity into the grid, it could (not would, but could) cause an instability that could disable the system.  Edison and others who favored a DC grid, foresaw the need for battery systems throughout the grid to help stabilize the system, and much effort (including out at Argonne National Laboratory near Chicago) has gone into the development of better, and more easily deployed battery technology to provide this stabilization.

In addition to the grid infrastructure, the physical and virtual infrastructure of our construction processes provide obstacles to solar energy deployment.  The improved material selection has certainly affected the weight of the panels, and thus lowered the cost and need for structure to support the panels.  We have extended this to now building the photovoltaic material into other building components (like windows, roofs, and wall sections) in what we have termed building-integrated photovoltaic (BIPV).  In addition, the simplification of the electrical components has lowered the complexity relative to integrating the electricity into the building systems and even into the local grid.  Even with these significant improvements, municipalities and local utilities still introduce costly permitting processes that hurt the economics of the systems through requiring unnecessary equipment and materials, and through limits on the production from the panels.

NREL

Even with many of these obstacles, the cost of solar panels has dropped significantly over the past five years, and with new advances regularly, this decline looks to continue for at least another five years.  With the potential for solar electricity in America at least as large as it is in Europe, where the number and production of solar panels dwarfs that of the US on a per capita basis, and costs declining, we approach a tipping point where new electricity installations will have to compete with the long-term benefits of solar: no commodity to purchase, low maintenance, and no moving parts.  Over the course of the year we will look at ways in which we seek to improve upon solar photovoltaic electricity systems, and create the backbone of a new energy economy with the technology.