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Envisioning the US with 69% solar electricity

What would the United States look like 69% of today’s electricity were generated from solar technologies, photovoltaic and concentrated solar power? A recent paper in Energy Policy (see 1 below: Fthenakis, Mason, and Zweibel., 2009) proposes this type of vision of generating 69% of US electricity from solar by 2050 (using also large quantities of energy storage technologies such as molten salts and compressed air energy storage). The authors note that 640,000 km2 of available land area exists in the Southwest US that can be used to for solar power stations (see link on National Renewable Energy Laboratory website for resource maps, but without land restrictions – NREL Solar Maps). This area is 48% of the total area of the included states of California, Nevada, Arizona, and New Mexico – which in total cover 1,320,000 km2.

In 2008 approximately 4,100 TWh of electricity were generated in the US. Thus, 69% is approximately 2,840 TWh. If we assume that installed solar systems convert sunlight (at an average of 6.4 kWh/m2/day for the Southwestern US) at 8% efficiency from sunlight to delivered electricity to the consumer, then approximately 2.4% of the available sunlight on the 640,000 km2 would be needed for solar systems. Note that current photovoltaic (PV) and concentrated solar power (CSP) systems can have solar-to-electric conversion efficiencies from 7%-40%, where the high end of the efficiency range is by laboratory multi-junction PV cells under concentrating lenses.

As stated by Fthenakis at al. (2009) their projected 2050 solar electricity generation (which is more than 2,870 TWh due to assumed increases in generation each year) would occur from an installed capacity of approximately 5.5 TW of solar generation – 1.5 TW from solar CSP and 4.0 TW from PV. For a 500 MW solar plant the authors estimate 10.6 km2 of land is needed (at 14% efficiency). Using this ratio of land for all solar installations projects the 5.5 TW of capacity would need approximately 110,000 km2 of land, or 17% of the available 640,000 km2.

Only 2.4% of the sunlight hitting the land needs to be converted to electricity, but 17% of the 640,000 km2 of land would be required for power plants. The reason is that the conversion efficiencies are calculated assuming that the PV panels are tilted toward the sun at an angle equal to the latitude of installation. This causes a PV panel to shade the next PV panel just to the North if it is placed too closely. Thus, there needs to be some spacing between panels and mirrors for CSP systems. If the solar power plants were equally distributed spatially throughout the Southwestern US (which is a practical impossibility and strategically undesirable) one could likely not walk on any path from southern California and Nevada throughout Arizona and New Mexico without seeing a solar power plant. Solar power plants would essentially be pervasive throughout the Southwestern United States.

This large scale of solar power pervasiveness brings to mind the question of whether there are enough materials to manufacture all of the solar panels and mirrors. A paper from this year in Environmental Science and Technology by Wadia, Alivisatos, and Kammen takes a first-order look at this question for the various semiconductor materials that can be used to create the functional component of the PV panels. They point out that the known reserves of the inorganic photovoltaics materials in ore deposits are sufficient for creating a broad variety of PV panels to generate the 17,000 TWh of electricity in the world today. The other necessary materials associated with circuitry (silver, copper, etc.) and mounting (aluminum) would also need to be available. Because there are many competing products for these resources (e.g. indium for flat panel displays) and some elements are only secondarily mined substances – meaning they are essentially byproducts associated with mining for other primary materials – we will only find out the future to what uses these elements are applied. It might also be interesting to think about how much energy (and power) would be required to mine all of necessary materials …

Perhaps we should use our fossil fuels to mine the fossil minerals and elements that are needed to stop using fossil fuels. Sounds like a chicken or the egg problem. We may not know which came first, but eventually, we’ll learn which one is last.

1. Fthenakis, V.; Mason, J.E.; and Zweibel, K. The technical, geographical, and economic feasibility for solar energy to supply the energy needs of the US. Energy Policy. 2009, 37, 387-399, doi:10.1016/j.enpol.2008.08.011.

About Carey King

Dr. Carey W King performs interdisciplinary research related to how energy systems interact within the economy and environment as well as how our policy and social systems can make decisions and tradeoffs among these often competing factors. The past performance of our energy systems is no guarantee of future returns, yet we must understand the development of past energy systems. Carey’s research goals center on rigorous interpretations of the past to determine the most probable future energy pathways. Carey is a Research Scientist and Assistant Director at the Energy Institute at The University of Texas at Austin, and appointed also at the the Center for International Energy and Environmental Policy within the Jackson School of Geosciences and Business, Government, and Society Department of the McCombs School of Business. Visit his website at: and follow on Twitter @CareyWKing
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  1. Thermoplastic material is available, representing 30% of cost of all others and can be used to “plaster” the Mojave Desert with CSP parabolic trough solar fields, by 2015 at efficiency of 29%.
    By utilizing Twin Parabolic Collectors, only 3.7 acre/MWe is needed.
    If integrating 3 storage tanks with dual HTF, fostering 18 hours of storage, only 1.8 acre/MWe is needed.
    When implementing solar field with storage, the capacity factor is up to 57%.
    (Any natural gas-fired plant will not be permitted beyond the threshold of 49% capacity factor, thus solar with storage will pass fossil fuel fired.

  2. VULVOX Inc’s breakthrough collectors will be able to generate more than double the amount of power generated by current solar power tower plants! The Abengoa solar power tower in Spain is currently the worlds biggest. If it is retrofitted with VULVOX hybrid solar energy collectors it will generate 46.5 megawatts, not 20 as it does currently.
    The dual solar thermal-photovoltaic system will wrest approximately twice as much power from an area as regular solar thermal or photovoltaic energy systems. Our technology can be used to retrofit existing parabolic troughs and solar power towers to increase their efficiency. Besides applications at utility scale solar power plants that are contributing electricity to the California power grid, they will also have an important advantage in the upcoming industry of rooftop solar power. Apartment buildings, offices and industrial buildings all have flat roofs that can accommodate our solar power systems and the greater efficiency of dual thermal-photovoltaic energy generation systems will make it cost competitive with other generation systems. Between 2009 and 2012 it is expected that the amount of installed CSP solar thermal power in the United States will grow 14 times!
    The Vulvox collectors will not depend upon complicated advances in quantum or solid state physics. Our novel combination photovoltaic-solar thermal collectors will achieve the unprecedented efficiencies predicted here by means of relatively simple modifications to solar energy equipment; modifications that can be developed at a moderate cost.
    The Vulvox solar system will generate higher power levels than competing parabolic troughs and solar power towers, while retaining all of the storage capabilities of solar thermal power.
    Besides the inherent efficiency advantages of this collection system, we are sure we can add other modifications that will increase energy collection and electricity generation beyond those efficiencies. Modifications to increase the heat flow rate of the thermal collectors are an example.
    Every time a photovoltaic panel system is upgraded and increases in power we can substitute it for a lower power panels used in previous “builds.” and the higher power panels can be used in upgraded combination PV-solar thermal collectors with higher efficiency. Also, every time solar thermal systems are upgraded they can be combined with the latest practical photovoltaic collectors to keep the next generation combination systems cost competitive and to keep their efficiency higher than all other collection systems.
    Contact us for more information.

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