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The state of residential solar power | Ars Technica


The state of residential solar power
Choosing solar in a vacuum now makes sense—but location and bureaucracy still loom IRL.
LEE PHILLIPS -  2/5/2017, 8:00 AM
Don't panic, but we will need to generate approximately 15TW of usable energy from renewable (carbon-neutral) sources by 2050 in order to stabilize the atmospheric CO2 concentration. And purely in terms of available energy, solar power has the greatest potential for meeting this requirement.
Solar is "probably the only long-term supply-side energy solution that is both large enough and acceptable enough to sustain the planet's long term requirements," according to Richard Perez, senior research associate at the Atmospheric Sciences Research Center at SUNY-Albany. Perez' analysis includes geothermal, wind, all other significant renewable sources, nuclear fission, and all forms of fossil fuels.
So while wind, hydropower, and geothermal extraction may work well on a local or regional scale in certain areas, today the potential of solar exceeds any other renewable energy source by several orders of magnitude. It's simply the only contender, besides nuclear power, for a global solution to supply civilization with the massive amount of energy it demands.
On average, the power from the Sun striking the Earth's surface is 175 W/m2. If we assume that 10 percent of this incident solar energy could be converted to electricity, supplying the energy used by the United States would require covering roughly two percent of the land in the US with solar cells—that's roughly the area of North Dakota. Since this is about 30 times our available roof space, supplying the grid with electricity from the Sun means building large solar farms.
However, that doesn't diminish the usefulness of some panels on your roof. If you own your home, you have the potential to make your own electricity. You can reduce or eliminate your dependence on the power company—maybe even sell your surplus power back to it, reducing your costs further, or perhaps even turning a profit.
Given the recent change of federal leadership, it's likely a time of great uncertainty for large US solar initiatives. But individual organizations, businesses, and even citizens can still make decisions for themselves about embracing solar to a greater extent. To get a better idea about the current state of residential-scale solar power in the United States, Ars has been looking at the practicalities, the economics, and the experiences of some people who have recently turned their houses into tiny electrical generating stations. Hopefully, even if you live in a basement apartment, you might find the findings... illuminating.
Better than ever
As children, many of us have been fascinated by solar-powered calculators and watches. A few of us may even have received science kits with tiny motors attached to palm-sized solar cells. Generating electricity from light seems magical. Why can't we run the world this way?
One of the main historical obstacles to a solar-fueled civilization has been the low efficiency and high cost of photovoltaic (PV) cells—the wafers that directly convert photons to electricity. Their efficiency, or, more formally, photovoltaic conversion efficiency, is the ratio of the electric power produced by a solar cell to the power of the sunlight striking its surface.

Enlarge / One of the first photovoltaic cells, demonstrated by General Electric.
Public domain

These actually have a long history. The first solar cell was invented in 1883 by Charles Fritts, who imagined his solar cells competing with Thomas Edison's growing network of coal-burning power plants. However, his cells' one-percent efficiency made this grand vision an impossible dream.
By 1954, Bell Labs demonstrated a PV panel to the public by hooking it up to a toy Ferris wheel and a radio transmitter. This device was six-percent efficient, which was a remarkable advance over previous solar cells. It was also a true "panel," with several individual cells connected together to form a solar "battery." Although it was still too expensive for widespread adoption, The New York Times was impressed, proclaiming that it "may mark the beginning of a new era, leading eventually to the realization of one of mankind's most cherished dreams—the harnessing of the almost limitless energy of the Sun for the uses of civilization."

During the '50s and '60s, research on silicon solar cells continued. Small cells began to appear during this period in some toys and consumer devices. By the middle of the decade, efficiency had doubled, but cost was still very high, especially compared with the low price of electricity at the time. A one-watt solar cell would set an early adopter back $300, while power plants were being built at a cost of 50¢ per watt.
By the end of the decade, however, PV cells would prove themselves worthy as a power source for the then-secret embryonic fleet of satellites. The Navy, initially skeptical, was won over whenthe conventional battery on the first satellite died in a matter of days. Its solar array kept it alive for years.
The high-grade solar cells used in satellites and spacecraft, although expensive, account fora small fraction of the cost of these systems, and the relatively low cost of fuel and terrestrial power during the '50s and '60s provided little pressure in the direction of reduced costs. Nevertheless, by the early '70s, solar cells using cheaper materials had been developed to reduce the cost to $20 per watt. This, combined with the energy crisis starting in 1973, created a renewed interest in solar power for Earthly purposes.
The technology was still not ready for mass adoption, however: efficiency was still only in the neighborhood of 10 percent. Additionally, it remained far too expensive.
Today, we are experiencing an acceleration of interest in solar energy, both at the residential level and on larger scales. This is due to several factors coming together: a significant decrease in cost; increases in efficiency of solar cells; an encouraging regulatory and taxation environment; widespread concern over climate change; and significant entrepreneurial innovation.
This new surge of interest comes on top of solar power's exponential growth over the past 20 years. Prospects look good for that growth continuing. In at least 30 countries, including parts of the United States, energy from rooftop solar power is now cheaper than energy from the grid—a comparison that does not factor in subsidies for solar panels.
Another factor that has helped modern adoption is that many firms market alternatives to the conventional roof-attached panel. Some companies sell more aesthetically pleasing offerings such as solar cells in the form of roof shingles (the result is a shiny roof rather than a roof with panels jutting off). At least one firm offers prefabricated "tiny houses" that are designed to appeal to several types of customers, including a segment that seeks to minimize its carbon footprint. Some of those products come with integrated solar roofs and an option to be completely off-grid.
Elon Musk has recently claimed that his "solar roof" is cheaper to install than a conventional roof, even without taking into consideration its ability to generate electricity. Such a reality would make a solar roof a no-risk choice for new construction.
Finally, homeowners are not limited to their roof surfaces. The same installation techniques can be applied to the roofs of carports and other auxiliary structures, and PV panels can even be erected in fields and backyards.

Enlarge / Remote, low power PV applications are possible—like this water monitoring station in Virginia.
By Lee Phillips, CC BY-NC-SA 4.0

Although we're focusing on individual residential solar power, we should mention the growing phenomenon of intermediate-scale PV installations. These are bigger than residential but still far smaller than utility-sized solar generating stations. In a recent drive through rural Maryland and Virginia, we noticed the occasional plot of land planted with rows of solar panels rather than filled with cornfields and cow pastures. And in fact, "solar farms" are growing in popularity as a way for communities to gain some of the benefits of solar power without requiring each individual to invest in a separate rooftop system.
The sight of them interspersed with crops made one irresistible impression. This is just the latest way to exploit the abundant, free energy of the Sun—converting it to electrical power rather than to sugar through photosynthesis.

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