By ASHLEY E. ORGAN, Assistant Editor Alternative energy continues to be a high-priority focus in the search for substitutes to existing resources. According to “Nine Challenges of Alternative Energy,” an essay by David Fridley, staff scientist at the Lawrence Berkeley National Laboratory, as part of the Post Carbon Reader series, there are several factors that could influence the development of renewables. In my blog, Is alternative energy that easy?, I provided Fridley’s first three challenges. Here are the next three: 4. Material Input Requirements It is generally assumed that the input to an alternative energy process is money. According to Fridley, it actually is the type and volume of resources and energy needed that can limit the scalability and affect the cost and feasibility of an alternative, particularly in processes that rely on advanced technologies manufactured with rare-earth elements such as fuel cells, solar-photovoltaic technology, and advanced batteries. “Expressing the costs of alternative energy only in monetary terms obscures potential limits arising from the requirements for resources and energy inputs,” Fridley says. In addition, the volume of resources and energy demanded for production of alternative energy production has so far been accommodated easily because it constitutes only a small fraction of total energy production. With large-scale expansion, this might not be the case, Fridley says. Thin-film solar, for example, currently uses indium because of its versatile properties; however indium also is used as a component of flat-screen monitors. According to a 2007 study, known resources of indium will last 13 years if consumed at current rates. “Can greatly increased demand for these resources be accommodated?” Fridley asks. 5. Intermittency The exploitation of large stores of fossil fuels is the result of millions of years of intermittent sunlight concentrated into a continuously extractable source of energy. On the other hand, alternative energies such as solar and wind power produce only intermittently when the wind blows or the sun shines. The capacity factor in electric power generation is one indication of intermittency challenges for alternative energy sources. The current US electricity system is dominated by large baseload coal- and nuclear-power generation. According to Fridley, the integration of energy forms such as solar and wind is seen as a matter of expanding transmission capacity and grid interconnections to extend the area over which the variations are felt. “This approach in effect relies on strengthening and expanding the large centralized energy production and distribution model that has characterized the fossil-fuel era, but may not necessarily be suitable for a future of renewable energy generation,” he says. Fridley suggests that storage – the development of technologies and approaches that can store energy generated during periods of good wind and sun for use at other times – is the key to tempering the intermittency impact. 6. Energy Density This challenge refers to the amount of energy contained in a unit of an energy form that can be expressed in the amount of energy per unit of mass (weight) or in the amount of energy per unit of volume, according to Fridley. For example, in the 17th and 18th centuries, coal provided twice as much energy as wood for the same weight of material. In the early 20th century, petroleum-powered ships replaced coal-powered ships because petroleum possesses nearly twice the energy density of coal which allowed ships to travel farther without refueling. According to Fridley, many alternative energies and storage technologies are characterized by low energy densities, and their deployment will result in higher levels of resource consumption. A more dense energy form will require less land for deployment and many alternative energies are far less energy dense than fossil fuels. Large-scale deployment of renewable resources could, therefore, result in considerable land costs. For more information on the Post Carbon Reader and the essay by David Fridley, visit