David Anthony’s Last Question – Can We Power the US Solely off of Solar?
On July 10, 2012, in Blog, by Neal Dikeman
By Tao Zheng, with David Anthony, an active cleantech venture capitalist, who passed away in April 2012.

The sun is the champion of all energy sources, in terms of capacity and environmental impact. The sun provides earth with 120,000 terawatt (TW) energy, compared to technical potential energy capacity of single digit TWs from other renewable sources, such as wind, geothermal, biomass and hydroelectric. More energy from the sun hits the earth in one hour than all of the energy consumed on our planet in entire year. In the last blog, we estimated that the technical potential of electricity generation from rooftop photovoltaics (PV) can take over 1/3 of U.S. electricity consumption demand. The next question is: can we power the U.S. solely by solar energy, technically? The answer will rely on development of utility-scale solar farms and energy storage solutions.

Assuming the rest 2/3 of U.S. electricity demand can be fulfilled by utility-scale PV solar farms, we can estimate how much land required to install such solar farm systems. The total U.S. electricity demand in 2009 was 3,953 TWh with 1% annual growth projection in next 25 years. Two third of U.S. electricity demand is about 2,635 TWh. The PV power density is calculated with a weight-averaged module efficiency using market share for the three most prevalent PV technologies today: crystalline silicon, cadmium telluride, and CIGS. The resulting PV power density is 13.7 MW/million ft2, assuming an average module efficiency of 18.5% in 2015. If we assume 10 hours/day and 200 days/year with sunshine, the annual available sunshine time is 2,000 hours. The total land required for solar farms to generate 2,635 TWh, can be calculated as:

Total Land Required = Total Energy Generated / PV power density / Annual available sunshine time

= 2,635,000/13.7/2000 = 96.2 ×109 ft2 = 8,937 km2 @ 100 × 100 km

Therefore, to generate energy equivalent to 2/3 of U.S. electricity demand, we need to install solar panels in a tract of land with size of 100 by 100 km, the area about 0.1% of U.S. land. Technically, to provide electricity for entire U.S. demand, we only need to cover PV-accessible residential and commercial rooftop with solar panels and install solar farms in desert area equivalent to 0.1% U.S land. In addition to rooftop and desert, there are many opportunities for installing PV on underused real estate, such as parking structure, airports, and freeway margins. PV can virtually eliminate carbon emissions from the electric power sector.

In comparison, Nathan Lewis, professor at Caltech, predicted a solar farm with land size of 400 by 400 km to generate 3 TW energy to power entire America. The represented area is about 1.7% of U.S. land size, comparable to the land devoted to the nation’s numbered highways. As shown in Figure 1, the red square represents the amount of land need for a solar farm to match the 3 TW of power demand in the U.S. Of the 3 TW energy, only 10% represents electricity demand, and the rest represents other energy needs, such as heating and automobile. Thus, Lewis’ calculation is consistent with our estimation: 10,000 km2 solar farms can generate enough electricity to fulfill 2/3 U.S. demand.

Figure 1. Solar Land Area Requirement for 3 TW Solar Energy Capacity to Power Entire U.S. Energy Demand. (Source: Prof. Nathan Lewis group at Caltech).

One of big challenges using solar to power U.S. grid is intermittency of sunlight. Solar energy is not available at night, and the variable output of solar generation causes voltage and frequency fluctuations on power network. Energy storage technology can smooth the output to meet electricity demand pattern. There are many grid energy storage technologies, from stationary battery to mechanical storage methods. Pumped hydro technology is clearly a better choice for solar energy storage, due to its high energy capacity, low cost, and public safety assurance.

For solar to have a dominant role in the electric power generation mix, in addition to power storage infrastructure, upgrading America’s transmission grid is required. In contrast to traditional electricity generation, solar power collections are distributed across numerous rooftops or centralized in utility-scale farms. Distributed solar requires grid operators to install smart grid technology to monitor power supply and demand and balance thousands of individual generators with central power plants. The current century-old transmission grid needs to be upgraded with high-voltage lines to carry electricity from remote solar farms to consumers. The American Recovery and Reinvestment Act (ARRA), signed into law by President Obama in 2009, has directed $40 billion to accelerate the grid infrastructure transformation.

The U.S. photovoltaic market has been growing quickly in recent years. In 2010, the U.S. installed 887 megawatts (MW) of grid-connected PV, representing 104% growth over the 435 MW installed in 2009. Current trends indicate that a large number of utility-scale PV power plants are in the south and southwest areas, such as in the sunny deserts of California, Nevada and Arizona. For example, the Copper Mountain Solar Facility in Boulder City, Nevada, is one of the U.S. largest solar PV plants with 48 MW capacity, as shown in Figure 2.

Figure 2. One of the U.S. Largest Solar Plants, the Copper Mountain Solar Project with 48 MW photovoltaic in Boulder City, Nevada.

Historically, solar PV deployment has been limited by economic factors, since solar energy is too expensive to compete with traditional fossil fuels, due to lack of economies of scale. However, the cheapest solar cells are now being produced for as little as 70¢ per watt. They are selling for about $1.26 per watt, with prices expected to drop to $1.17 next year. Most anticipate the price of solar module, such as thin film, will hit 50¢ per watt within four or five years. First Solar, the world’s largest maker of thin-film solar panels, has told investors that production costs will range between 52¢ and 63¢ per watt by 2014. When companies can produce solar photovoltaic modules for less than 50¢ per watt, solar energy will reach grid parity. Grid parity refers to the point at which the cost of solar electricity rivals that of traditional energy sources, such as coal, oil, or nuclear. The solar module price drop is driven by cheaper manufacturing costs, lower costs for such crucial raw materials as silicon, and rapidly improving technology. A recent study even claims solar grid parity is already here today, based on a legitimate levelized cost of energy (LCOE), calculated the cost in $/kwh. The value of LCOE is determined by the choice of discount rate, average system price, financing method, average system lifetime and degradation of energy generation over the lifetime. Figure 3 illustrates the effect of initial installed cost and energy output on the LCOE value. For a PV system with production cost at $0.5/W, the initial installed system cost will be $1.5-$2/W, after considering labor cost and module margin. If we assume energy output is 1500 kWh/kW/yr, which is reasonable in south west area in the U.S., the LCOE value in Figure 3 will fall in the range between $0.06/kWh and $0.08/kWh, the lower side of grid parity value for the U.S. residential electricity rates range.

Figure 3. LCOE contours in $/kWh for solar PV systems for energy output versus initial cost of the system for a zero interest loan, discount rate of 4.5%, degradation rate of 0.5%/yr and 30 year lifetime (Courtesy of Prof. Joshua Pearce at Queen’s University)

Based on the analysis above, it is reasonable to believe we can power the U.S. electric grid solely by solar PV, technically and economically. Thomas Edison had a great quote on solar energy: “We are like tenant farmers chopping down the fence around our house for fuel when we should be using Natures inexhaustible sources of energy — sun, wind and tide. … I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”