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We need only look at the goals of various renewable portfolio standards (RPS) in the United States to see this. As of September 2008, more than 30 states have "state-mandated" RPS goals of obtaining from 10 to over 25 percent of their electricity from renewable sources. The deadlines range from less than two years to nearly 17 years from now (Figure 1). For states to meet these goals in the mandated timeframes, the solar industry must think "gigawatts" for its power plants, not "kilowatts," and it needs to employ technologies that will make utility-scale solar cost-competitive with fossil fuels.

At such a large scale, there are two prime hurdles to the widespread adoption of solar power. The first hurdle is the solar industry's reliance on costly or rare materials for its systems; the second is the expense involved in siting a plant. Although these challenges are relevant to other sectors of the power industry, they are particularly onerous for the solar industry.

It's a Material World

Financial outlay for materials is one issue that casts a shadow over solar's large-scale prospects. Most solar technologies still depend primarily on materials that are heavy, expensive to produce or not readily available. For example, consider steel and aluminum: many commercial solar solutions need up to 1,000 pounds of steel for support structures, tubes, towers and so on, and approximately 100 pounds of aluminum for systems such as solar mirrors, support structures and heat sinks. Increased worldwide demand for steel has sent prices soaring over the past year, while aluminum stands at $1.20 per pound as of this writing. Solar designs that minimize the use of these materials will have a competitive advantage.

The price of solar cells -- the devices that transform sunlight into electricity -- is also a consideration when looking for ways to save money. Solar cells are generally the most expensive components of solar energy systems. The average installed cost of conventional, one-sun flat plate solar technologies is about $8 per watt. New technologies are being developed that could cut these costs drastically. For instance, multi-junction (III-V) cells capture more solar energy than ordinary "one sun" silicon-based solar cells and are over 40 percent more efficient. Thin film is another option solar technologies are exploring. As for the ordinary silicon-based cell, some technologies are beginning to use thinner silicon wafers for their cells: using less of this expensive element can only improve cost-competitiveness.

Some solar designs use concentrated photovoltaic (CPV) technologies that leverage lenses or reflectors to concentrate sunlight onto highly efficient solar cells, such as multi-junction cells; this dramatically slashes (sometimes hundreds of times over) the amount of traditional solar cell area needed to produce electricity and cuts overall system costs. CPV systems are typically priced around $4 per watt, compared to the $8 per watt of silicon flat panels.

Staying Grounded

Rooftop solar systems alone can't meet the energy challenges set out by state RPS. For instance, 100,000 rooftops, each topped with a typical 2-kilowatt residential solar-power system, would deliver a total of 200 megawatts -- about as much as one large utility-scale solar plant. We must rely on megawatt- and gigawatt-scale solar power plants instead, and deal cost-effectively with siting issues. Most of the proposed utility-scale solar power plants today are looking to build in areas rich in "solar resources," such as the U.S. Southwest (Figure 2). In fact, the Bureau of Land Management has received more than 130 proposals from solar companies to build on a total of nearly one million acres of public land.

A number of siting issues can drive up costs. First, there is construction. For most solar technologies, sites must be leveled and scraped -- a direct result of the heavy systems that need to be put in place and supported. Any solar technology that minimizes the extensive renovation and destruction of the landscape will diminish costs and the impact on the environment. Solar system designs that forgo the use of heavy materials can avoid large foundations and footings. Some solar technologies are moving in this direction with lightweight solar collectors supported by poles and wires.

Another siting consideration is water. In desert areas much coveted for solar power production, water is a scarce -- and expensive -- commodity. Some solar installations, such as solar power towers and solar parabolic troughs, use upwards of 750 gallons to produce a single megawatt-hour (MWh), surpassing the water use of coal plants (about 670 gallons per MWh). Solar technology that is thrifty with water will generally be effective in lowering overheads while also preserving a precious resource. For instance, dish Stirling, solar flat panel photovoltaic (PV) and solar concentrating PV technologies all consume less than 5 gallons of water per MWh.

Finally, just as in real estate, the solar industry must think "location, location, location." Siting plants closer to load centers curtails the need for additional transmission infrastructure and the additional construction expense of installing transmission lines. However, public pressure and perception can narrow choices of possible plant locations. NIMBY (Not in My Back Yard) adherents will often push for power plants that (on one hand) are distanced from populated areas and (on the other hand) avoid sensitive ecological areas. Solar technologies that are environmentally friendly will likely face less opposition to siting proposals.

All Renewables Have a Place on the Stage Of course, solar is not the only option for fulfilling state RPS requirements. The targets are so large that all renewable utility-scale technologies need to step up in a big way. Solar has the advantage in that the sun reaches all corners of the globe.

Conversely, there are only so many places where the wind blows, the tides rise and the rivers run under conditions favorable for generating utility-scale electricity at a reasonable price. In some regions, many of the best locations for harnessing such renewable power have already been developed.

But solar's star is still ascendant. There are approaches, technologies and designs that, once put in place, can curb plant construction and siting costs as well as material costs. The result: clean, renewable, large-scale energy that major utilities will be able to obtain at prices commensurate with those of fossil fuels.

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