Some Renewable Energy Calculations
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.
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, e.g., dams, power plants; while most renewable energy, e.g., photovoltaic, wind, is very conducive for distributed generation.
The existing very large generators are large in the sense of the amount of power they produce 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, e.g., houses, businesses, ranches, farms with no requirements to install additional distribution capacity. The downside is how to plan for the transfer of energy from where generated to where needed when the renewable energy generators are not firm, i.e., the amount generated is neither constant nor predictable. 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 eventually the largest market will be in urban areas, virtually all problems could be resolved on a small scale by implementation in rural areas first. 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 ~ 4.1 trillion-kWh per year (4.1x1012 kWh/year), note this does not include fossil fuel energy consumption for transportation, heating, and other uses. Since a significant fraction is required for industrial use, which requires large concentrated sources, e.g., existing power plant dams will still be operational for 100 or more years depending on location, one could then reasonably expect the want to generate ~ 3 trillion-kWh per year with renewable sources.
There are 5 primary sources of renewable energy generators in operation today; four sources can be used for large commercial (e.g., utility scale, light industry, or towns) generation and three sources that are primarily for residential use. Depending on size of the installation, two of the sources can be in either category.
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 (basically an assumption, the total production will be small compared to the other sources):
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 13,900 modules would be required. Note, most geothermal locations in the USA would support the use of either larger modules or multiple modules; therefore, the total number of needed geothermal locations is probably less than 1,000.
For wind, assume that 0.99 trillion-kWh/year are produced by commercial size wind generators and the rest with residential. Assume that each 5 MW wind generator operates 12 hours per day for 300 days per year (allowing time for maintenance and variations in wind velocity and duration). Each wind generator then provides 18 Mega-kWh/year. Thus, approximately commercial 55,000 wind generators are needed. If each wind generator occupies ~ 1 square mile, then ~ 55,000 square miles are needed, noting that almost all the land near a wind generator can be used for ranching or farming purposes. This represents a small fraction of the land under cultivation in the western USA, where much of the wind resources are. Also, wind generators can be place off-shore.
For residential wind generators, assume that each 15 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 wind generator then provides 22.5 kilo-kWh/year. Thus, approximately residential 450,000 wind generators are required, which is significantly lower than the total number of small businesses, farms, ranches, and rural residences in the USA. 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 wind generators or larger, e.g., 150-200 kW wind generators.
For residential PV, assume a module conversion efficiency of 15% from the nominal solar radiance of 1000 W/m2. Assume a DC to AC conversion efficiency of 85% and operation for 6 hours per day for 300 days per year (allowing for variations for systems installed at a wide variety of locations). Thus each m2 of solar module area will produce 230 kWh/year. In order to generate, 0.5 trillion-kWh/year, there needs to be ~2,175,000,000 m2 of PV modules. This is about 840 square miles of solar PV modules, smaller than most Western state counties. Assuming that the majority, say 1,500,000,000 m2, are directly used on single family dwellings, with the availability of 75 m2 per dwelling (still allowing room for solar hot water heating collectors on the south facing roof), then ~20,000,000 homes are necessary. The remaining PV generation (~ 0.16 trillion-kWh/year) would come from commercial PV facilities. Assuming a capacity of 200 kW operating 8 hours per day (use of at least ground mount single-axis trackers) for 320 days per year (allowing for variations for systems installed at a wide variety of locations) each location would generate 512,000 kWh/year. There would need to be at least 312,500 such installations, with each installation having about 1,570 m2 of PV modules and assuming an area efficiency of 10% (for trackers and mounting) so occupying ~ 4 acres.
For solar thermal generation, assume each 100 MW of power capacity requires 1000 acres, including all support structures. Since solar thermal requires significant water usage for cooling (up to 1000 acre-feet/year per 100 MW), not all locations are suitable. Assuming a 500 MW plant produces 8 hours per day for 300 days per year, each location produces 1.2 billion-kWh/year. For the 1 trillion-kWh/year, then ~850 plants, occupying 4.25 million acres or 6,640 square miles (the size of larger Western state counties)
All of these estimations are just that, estimations; however, the numbers clearly show that renewable energy resources can provide the majority of the electrical energy needs of the USA. 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.