Reaching the Solar Tipping Point

Analysis: Writer Bruce Allen claims that picking the right solar options can power America's clean electricity and transportation future.
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Analysis: Writer Bruce Allen claims that picking the right solar options can power America's clean electricity and transportation future.

Over the next 10 years, we will likely see the U.S. reach a solar tipping point.

Solar energy has always been one of the cleanest and longest lasting of all energy sources. Soon, it may be one of the cheapest. The January Scientific American article, "A Solar Grand Plan" provides a glimpse of the possibilities of large-scale solar farms driving America's energy future. (Currently, solar provides less than 0.01 percent of the electricity the U.S. uses.)

Is there a more practical and compelling solution that lets us reach a tipping point more quickly?

The "Solar Grand Plan" envisions thin-film cadmium telluride photovoltaics, currently 11 percent efficient, supplemented with solar thermal farms, blanketing vast stretches of our Southwestern deserts and pumping electricity into a more efficient national high voltage, direct-current (HVDC) grid. Nighttime and cloudy-weather worries would be eased by holding electricity via compressed air storage in caverns to drive generators.

But compressed air storage is only about 75 percent efficient, and there are concerns about production scalability for the thousands of square miles of panels featuring tellurium, one of the rarest metallic elements on Earth.

A more efficient large-scale solar energy solution for the United States may be based on desert solar thermal farms with overnight heat storage and HVDC transmission. Thermal heat storage is up to 95 percent efficient and can cost-effectively convert stored heat into electricity at night and in cloudy weather for up to 16 hours.

Solar trough farms are the leading solar thermal commercial technology, with solar-concentrating towers gaining commercial traction.

Solar thermal farms would be supplemented with crystalline and thin-film photovoltaic arrays, rooftop and building-integrated photovoltaics (BIPV) capturing the best features of centralized and distributed generation.

Generating electricity that can't be delivered nationally, as renewable energy partisans are discovering, is spurring the co-development of HVDC to supercede America's antiquated power grid, which was not built with routine transcontinental movement in mind. One example of the ability of HVDC to economically deliver long-distance electricity is the giant Itaipu Dam Transmission Project in Brazil, providing 6,300 megawatts to São Paulo, South America's largest city, over a distance of 800 km (about 500 miles) since 1991.

Why Solar Thermal Farms?
The performance of modern solar thermal trough systems is highly refined compared with previous generations. They offer massive cost-effective scale with virtually no manufacturing bottlenecks, and their new reflector and heat pipe designs have reduced costs, increased durability and produce electricity more efficiently.

Costs of newer solar thermal farms are approaching $2.75 per kilowatt, significantly less than current photovoltaic-installed costs. The National Renewable Energy Laboratory estimates that these systems, with energy storage, can achieve a cost of 7 cents per kilowatt-hour within 10 years. (The average price of electricity nationally was 9.09 cents per kilowatt-hour in March.)

Today's leading solar thermal manufacturers, including Ausra, Acciona, Abengoa and Solel, are achieving in excess of 17 percent total conversion efficiency of sunlight to electricity and have the capacity to build solar thermal farms measured in square miles per year. In the near future, solar thermal manufactures will have factory capacity for solar farms covering tens to hundreds of square miles per year.

How much desert is required to power the U.S. with solar thermal farms supplemented with photovoltaics?

A modern "compact linear fresnel reflector" thermal farm requires about 2.5 acres per megawatt, about half the area required by previous solar trough or thin-film photovoltaic farms. One next-generation solar farm is the 177-megawatt Ausra plant currently being permitted in California and expected to be online in 2010.

A solar thermal study co-authored by David Mills, the chairman of Ausra, estimated the total area of solar thermal farms needed to have supplied more than 90 percent of U.S. electricity needs in 2006 to be 13,000 square miles (about the size of two Hawaiis). Dividing the land area between California, Nevada, Arizona, New Mexico, Southern Utah and West Texas would limit the area covered to about 2,000 square miles in each state.

Photovoltaics on rooftops and integrated into buildings reduce the area required for solar thermal farms and allow more energy to be directed to overnight and cloudy weather backup heat storage.

On a more human scale, the typical U.S. household uses 30 kilowatt-hours per day, equivalent to approximately 250 square feet of 19-percent efficient rooftop panels on a cloudless day.

Also offering a reduction in the solar footprint are advances in photovoltaic technology. For example, industry leader Sunpower's silicon solar cell and panel efficiencies now exceed 22 percent and 19 percent respectively.

The estimate from Mills included electricity demand for transportation if all car and truck transportation needs had been met by electric vehicles. The power of combining electric vehicles and solar electricity is that electric vehicles are about 85 percent efficient at converting electricity to motion; internal combustion vehicles are around 17 to 25 percent efficient. Manufacturing aside, electric vehicles powered by solar electricity are pollution and carbon-emission free, unlike gas-, diesel- and ethanol-fueled vehicles.

One acre of solar thermal farmland can provide the same useful electric vehicle transportation energy per year as 100 acres of switch grass converted to cellulosic ethanol.

Arriving at the Tipping Point

The dual growth of large-scale solar thermal farms and photovoltaics is on track to become the largest single source of newly installed electric-generating capacity in the U.S. in the next 10 years — the solar tipping point.

Solar photovoltaic capacity growth is expected to continue at more than 50 percent per year for the next five years. American chemical giant DuPont reiterated that statistic on Sept. 2 as it estimated its own sales into the photovoltaic industry would exceed $1 billion within five years.

Meanwhile, solar thermal capacity and installations are growing rapidly. Ausra's new automated Las Vegas solar thermal factory capacity is more than 700 megawatts per year.

Because solar thermal farm materials are based on abundant industrial materials including steel, glass and conventional electrical generating turbines, solar thermal factories can scale to beyond gigawatt capacity in only a few more years. The primary limitations are continuing incremental cost reductions, land-use permitting timelines and regional and national electric-grid-expansion agreements.

While solar thermal farms can also begin supplying nighttime and backup storage capacity, retaining existing natural gas electric power plants and integrating natural gas co-generation at solar thermal farms also can provide responsive cloudy weather capacity.

Additional but less efficient cloudy weather backup power could be provided by hydrogen electrolysis — creating hydrogen as a fuel from water — or compressed air storage.

The scale of photovoltaic capacity expansion is also accelerating. Pacific Gas and Electric in August signed contracts for the world's two largest photovoltaic farms comprising 800 MW covering 12.4 square miles in a remote valley in Central California. Optisolar will build a 550 MW solar farm using thin-film silicon photovoltaic panels, and SunPower will build a 250 MW plant using its high-efficiency crystalline-silicon PV panels.

State and local governments are also shortening the time it will take to reach the solar tipping point. California approved a law in July that allows cities and counties to provide low-interest energy loans, including for residential solar installations, with payments added to property tax bills over 20 years.

Clean solar energy may be our manifest destiny.

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