There is much talk recently about the extent to which we can replace our dependence on fossil fuels with energy derived from renewable resources. While renewable energy holds great promise, there are some limitations to renewable forms of energy (as there are with anything) which are very widely proclaimed and generally used as “proof” that they can never solve all our problems. Obviously, we do not expect, nor desire, any single source of energy to be our sole and complete source; that would be foolish and dangerous. The more interesting question is how much of our demand the various forms can supply, and at what point in time will we reach the limit for each form. However, before we can begin to answer that question it is necessary first to establish what our expectations are.
The energy that we are interested in comes in two basic types, work and heat. Work is the more valuable type and is most familiar to us as either electricity or the action of an engine upon some machine, for example our car. We should further distinguish between stationary and mobile applications of energy since this is critical to the way the energy needs to be stored or delivered. The table below indicates a typical energy form for each category:
This article will look at the various forms of renewable energy which are currently available and the limitations, both to application and to extent, for each of the various forms. Many emerging technologies are also in various stages of implementation, but for our purposes we will not consider those. The leading forms of renewable energy at the present time are wind generated electricity and biofuels. However, solar photovoltaic, thermal solar, and free flowing hydroelectric power are rapidly gaining in popularity although they still lag far behind in construction activity.
The next table shows the energy categories again, this time indicating which form of renewable energy can be expected to fill its demand.
The importance of distinguishing between these categories is due to the relative value of energy in each category. For example, Mobile Work has a cost of approximately $100/MMBTU, Stationary Work has a cost of about $25/MMBTU, Stationary Heat has a cost of about $10/MMBTU, and Mobile Heat has a cost of $0 / MMBTU. It is obvious that one would desire to use an energy form in its most valuable application, leading to the conclusion that it would be foolish, for example, to use ethanol or biodiesel for a stationary heating application when it should be used for a transportation fuel.
Wind
Wind is currently the second largest source of new electric generating capacity being installed in the United States. The AWEA reports that in 2007 more than 5,200 MW of new wind capacity was installed, compared to about 9,000 MW of gas fired capacity which is currently the leading source of new capacity. It is conceivable that within a few years wind energy could overtake natural gas to become the leading new source. The EIA projects that by 2030 347 GW of new capacity will need to be built to account for load growth and the retirement of old plants, and while they predict that it will be supplied principally by coal and natural gas, I predict it will be supplied by wind and natural gas. The famous study from Minnesota concluded that as much as 20% of our electrical energy consumption could be supplied by wind without any major modifications to our T&D infrastructure.
If wind energy development increases by only 5%/year then by 2030 approximately 245 GW of wind capacity will be installed and will be providing about 15% of our electrical energy. On a national level, wind energy could be providing 20% of our consumption by the year 2036. In most areas of the country though there is not sufficient wind resources to achieve 20% of total generation. However, in the Great Plains much more than this 20% will be easily achievable. Of course, this 20% value ignores everything else that is likely to occur during that time such as T&D system expansions, deployment of new technology, adoption of load controlling schemes, and population shifts all of which should be favorable to increased wind generation. The Minnesota study was based on the assumption that no changes would be made to the existing infrastructure, the wind generation needed to be accommodated by the current configuration.
However, increasing the geographic size of the local grid, interconnecting with other grids, and installing storage or peaking capacity will increase the quantity of wind energy which can be incorporated. These changes are taking place now because they are desirable for other reasons as well. AEP has announced plans to install 25 MW of sodium sulfide battery storage by 2010 and 1000 MW by 2020 for the purpose of supporting weak areas in their system, facilitating more wind generation, and postponing other T&D expansion projects. There are several large transmission projects underway, and more being proposed which will allow for greater utilization of wind power. Furthermore, nearly all the hydrocarbon fueled power plants built in the last decade are based on gas turbines, which can be quickly and easily modulated to accommodate fluctuations in wind generation.
If we do actually reach the limit of wind energy that can be safely incorporated, there are even more tricks up our sleeves we can pull out. DOE is currently funding studies which explore the possibility of using excess electrical power to make hydrogen and then to react the hydrogen with coal to make natural gas. This scheme then allows the electrical power to be stored and transported as natural gas, which avoids many of the constraints in the electrical system. Alternatively, the load on the system can be regulated to match the generation, rather than the current approach of regulating generation to meet load. Of course, regulating load is nothing new and there is much activity underway right now to increase the amount of loads which can be remotely controlled.
A sample of some of the developments recently reported in various news sources include: Expanding use of off-peak ice storage for building cooling; electric cars capable of supplying power into the grid during peak periods while recharging during off-peak periods; Time-of-use pricing is now available in several large metropolitan areas; Standards for remotely switching off major loads are being adopted by many utilities and appliance manufacturers; and recently California has considered mandatory remotely controlled building thermostats. Thus, I do not foresee interconnectibility as ever being the limiting factor for wind energy development.
Bioliquids
In this article I am going to divide biofuels into the categories of “bioliquid,” meaning a liquid fuel such as ethanol or biodiesel and “biomass,” which is any solid material. This distinction must be made because the two forms are used for different applications and are subject to different influences. The largest form of bioliquid currently in use in the U.S. is ethanol, principally as blended with gasoline as an octane booster, oxygenate, and volume extender. Biodiesel is rapidly growing as well and it's use also is as a volume extender, to improve emissions, and enhance performance. Both of these fuels are currently made from agricultural commodities and consequently their supply will be limited to the "excess" supply of crops.
An estimated 3-4 billion bushels/year of grain can be diverted into ethanol production before grain prices rise to the level where the incremental cost of ethanol production exceeds its value. From this quantity of grain, approximately 8 billion gallons/year of ethanol can be produced and that is about 6.5% of our present gasoline consumption. At the present time we produce approximately 6.5 billion gallons/year of ethanol from about 3.2 billion bushel/year of corn, and our production capacity is increasing by 0.6% / year. So we are already near our maximum economic production capacity for ethanol from grain. Further increases in production will need to be supplied by other feedstocks such as sugarcane, sugarbeets, or cellulosic materials.
Biodiesel is commonly produced from vegetable oils. Current biodiesel production in the U.S. is 450 million gallons/year which is 0.8% of the distillate oil consumed. The current feedstock for biodiesel is various oilseeds which, like ethanol, must be diverted from the food supply. The "excess" supply of oilseeds seems to have already been consumed since the US is currently a net importer of vegetable oil. There probably is some potential to plant additional acreage into oilseeds, but that will allow for only marginal increases in production. Therefore, the limit to biodiesel production has already been reached, increases in production must now come from imported oil or at the expense of another commodity.
Part II of this article will continue with a discussion of energy from biomass, solar, and ocean and river currents.
