How many renewable energy facilities covering how much area are required to meet the electrical energy demand of the United States? The following will identify some critical issues along with a possible solution, while demonstrating that renewable energy resource installations could be available to meet the required demand, should sufficient will be exerted to actually install them.

One of the major dilemmas facing the widespread implementation of renewable energy resources is resolution of how to distribute the newly installed resources. The existing grid is predicated on the use of very large centralized generation sources, like dams and power plants, while most renewable energy resources, like solar photovoltaic and wind, are conducive for distributed generation.

Existing large generators produce a great amount of power per unit area. Renewable sources require much more land area for a comparable power production. A major benefit of this conundrum could be the installation of a large number of small generation sources at existing sites -- houses, businesses, ranches and farms -- with no requirements to install additional distribution capacity. The downside is how to plan for the transfer of energy from where it is generated to where it is needed when the renewable energy generators are not firm.

This is exacerbated by the financial consideration that nonrenewable generators are generally most efficient and cost effective when operated at full capacity. A model for solving the problem, or more accurately, debugging the solution, is to initiate the widespread use of renewable energy generation in rural areas. Although the largest markets will be in urban areas, virtually all problems could be resolved first on a small scale by implementation in rural areas. Rural areas have a small fraction of the total population; however, this fraction is highly independent and well skilled in solving-problems.

Unfortunately, this is also the segment of the population that has the least disposable income to invest in anything, much less energy with a long term payback. Thus, some sort of cash flow assistance will be required, noting tax credits are of little benefit to those whose income is not sufficient to pay much in taxes.

In order to understand the magnitude of the task, one must consider how much electrical energy is going to be required to be converted from nonrenewable to renewable sources. The United States consumes ~ 7.5 trillion-kW per year, not including fossil fuel energy consumption for transportation, heating, and other uses.

There are four primary sources of renewable energy generators in operation today; three sources can be used for large commercial (e.g., light industry or towns) generation and two sources that are primarily for residential use. Depending on size of the installation, two of the sources can be in either category.

  • Geothermal, usually available in ~ 5 MW increments for commercial use. Note, geothermal is generally considered a firm source, so would be preferred for ease of compatibility with existing distribution systems.
  • Wind, usually available in 1-5 MW sources for commercial use and 1-25 kW sources for residential use.
  • Photovoltaic (PV), usually available in 100-200 kW for commercial use and 1-10 kW for residential use.
  • Hydro, usually 1-3 kW for residential use; commercial use is generally not considered renewable since dams and reservoirs that fill up are needed for larger hydro units.The following is a rather arbitrary assignment of expected capacities from the various generator types, small hydro is not included for convenience and lack of data on how many streams are available:

    • Geothermal, 20 billion-kWh/year.
    • Wind, 20 billion-kWh/year
    • Photovoltaic (PV), 20 billion-kWh/year
    For geothermal, assume each 5 MW module operates 24 hours per day for 300 days per year (allowing time for maintenance and any possible variation in steam flow). Each module then provides 36 Mega-kWh/year. Thus, approximately 560 modules would be required. Note, most geothermal locations in the U.S. would support the use of either larger modules or multiple modules; therefore, the total number of needed geothermal locations is much less than 500, e.g., ~100.

    For wind, assume that 19 billion-kWh/year are produced by commercial size windmills and the rest with residential. Assume that each 1 MW windmill operates 8 hours per day for 300 days per year (allowing time for maintenance and variations in wind velocity and duration). Each windmill then provides 2.4 Mega-kWh/year. Thus, approximately commercial 8,000 windmills are needed. If each windmill occupies ~ 1 square mile, then ~ 8,000 square miles are needed, noting that almost all the land near a windmill can be used for ranching or farming purposes. Use of newer larger wind turbines would produce more energy in the same land area.

    This represents a small fraction of the land under cultivation in the western United States, where much of the wind resources are. For residential windmills, assume that each 3 kW windmill operates 6 hours per day for 250 days per year (allowing for conversion efficiency, time for maintenance, and variations in wind velocity and duration). Each windmill then provides 4,500 kWh/year.

    Thus, approximately residential 225,000 windmills are required, which is significantly lower than the total number of farms, ranches, and rural residences in the U.S.. With larger residential windmills, especially for farms and ranches, not so many windmills would be required. A 25 to 50 kW windmill is much more appropriate for farm or ranch use, noting some farms that use well irrigation would need several windmills, e.g., a large windmill at each corner of the quarter section containing a pivot.

    For residential PV, assume a panel conversion efficiency of 10 percent from the nominal solar radiance of 1000 W/m2size="1">. Assume a DC to AC conversion efficiency of 60 percent and operation for 5 hours per day for 300 days per year (allowing for variations for systems installed at a wide variety of locations). Thus each m2size="1"> of solar panel area will produce 90 kWh/year. In order to generate, 20 billion-kWh/year, there needs to be ~225,000,000 m2size="1"> of photovoltaic panels. This is about 140,625 square miles of solar PV panels, about the size of the state of New Mexico. Assuming that the majority, say 150,000,000 m2size="1"> are directly used on single family dwellings, with the availability of 75 m2size="1"> per dwelling (still allowing room for solar hot water heating), then only 2,000,000 homes are necessary.

    The remaining PV generation would come from commercial facilities, the panel conversion efficiency is 12 percent and the DC to AC conversion efficiency is 80 percent, assuming a capacity of 200 kW operating 6 hours per day for 300 days per year (allowing for variations for systems installed at a wide variety of locations) each location would generate 360,000 kWh/year. There would need to be at least 18,500 such installations, with each installation having about 2,100 m2size="1"> of PV panels (a little over 1.25 square miles or 700 acres).

    All of these estimations are just that, estimations; however, the numbers clearly show that renewable energy resources can provide all of the electrical energy needs of the U.S. As renewable energy resources are installed, no new fossil fuel power plants need be built. Eventually, all fossil fuel plants can be allowed to retire, starting with the least efficient first. The transition cannot be smooth, since both nonrenewable and renewable energy sources are only available in discrete units; however, by starting with implementation in rural areas the methods and techniques can be fully developed, which will ease large scale implementation in urban areas.

    Reference http://www.eia.doe.gov/cneaf/electricity/epav1/elecprod.html for information
    on USA electrical energy production and usage.
    David Sweetman
    P.O. Box 189
    Dyer, NV 89010
    775-572-3359
    E-mail David