The bottom line: We have more options than problems.
A number of people made some really interesting comments on my recent CleanTechnica article, “Mark Z. Jacobson & The 100% Renewable Energy Naysayers.” Some of these had to do with the problem of getting the last 20% of our energy from renewable sources, after we got to the 80% mark.
The problem is that some applications seem to need combustion, and they seem to make that last 20% daunting. Two examples are manufacturing cement and powering large passenger aircraft. I decided to make up a list of sources of renewable energy that might have some bearing on the discussion.
Renewable fuels for heat and transportation can be divided into two categories. One consists those that contain carbon, and this can be subdivided into those that are derived from natural sources and those that are synthesized. The other category is fuels that are carbon-free.
We should note that it is not necessarily true that a fuel that contains carbon will contribute to climate change or pollution. Similarly, it is untrue that carbon-free fuels will certainly not pollute. Also, please remember that the list presented here is incomplete. (I am including general references to Wikipedia articles, in addition to references to more specific sources, for the readers’ convenience. They are indicated as such.)
Natural Carbon-Based Fuels
Biomass can be purpose-cut wood, waste from other wood products, straw, or other products, including even municipal waste. Its use may be very polluting or very clean. Its production may take a lot of energy and its use release a lot of carbon dioxide, or it may be carbon neutral. Possibly the least polluting way to use biomass is for generating gases, including wood gas, biogas, and similar products.
Biogas can be made using anaerobic digesters. The source materials can be just about all organic materials, including agricultural waste, waste wood, food waste, and municipal waste. The United States Department of Agriculture says we could have 11,000 electricity-producing anaerobic digesters on farms, but that figure represents a fraction of what could be used for processing municipal waste. They really can be used almost anywhere. There are about 5 million households in China that use anaerobic digesters to process their waste into fertilizer, with the gas produced used to cook food.
The National Renewable Energy Laboratory (NREL) says we could replace 40% of our natural gas with biogas, if we are aggressive. My suspicion is that we will be aggressive because entirely separate from a need for energy is a need to deal with the waste that can produce biogas.
Another approach is gasification, which can use biomass as fuel for any gas-powered engine, whether it is an ordinary car engine or a turbine. The first systems using wood gas seem to have been invented in the 1830s. The technology was widely used in Europe during the Second World War, when systems were put on ordinary road vehicles because gasoline was not available. There were about 600,000 cars and trucks running on firewood, according to some estimates. Wood gas is also currently used for producing electricity. It is arguably cleaner-burning than natural gas, it has low cost, and it can come from local sources. (Wikipedia: Wood Gas)
The Fischer-Tropsch process, which is over 90 years old, mean that wood gas and biogas can be used for creating liquid fuels. The resulting fuels have very little sulfur and are less polluting than fossil fuels. Using this process, it is possible to get the advantages of wood gas without altering engines or burners. These liquid fuels are often more expensive than low-cost petrochemicals, but, as we will see, they are not our only alternatives for renewable replacements for gasoline, home heating oil, or diesel oil. (Wikipedia: Fischer-Tropsch process)
Algal biofuel is considered a potentially low-cost, low-pollution, and low-carbon possibility. The US Defense Department is researching algal biofuel extensively. It has concluded that 1,000 gallons of oil can be extracted per acre per year, about 3 times the productivity of corn ethanol. The cost has been higher than fossil oils, but it has been declining. (Wikipedia: Algal biofuel)
Biodiesel can be manufactured out of any biological oil or fat, from soybean oil to lard. It can be made from used oil, which would otherwise be a problem for treatment plants or landfills, and which we would have to pay to get rid of. It is in some ways better than the fossil fuel it replaces. It is kinder to engines and does not pollute as badly. It costs a bit more, but many people believe it is worth the difference because it is considered carbon neutral. (Wikipedia: Biodiesel)
Methanol can be made from biological sources, including wood. We would suggest not using it as a fuel because of its toxicity. It does irreversible damage to the optic nerves from exposure to minute quantities, making it dangerous to breathe the fumes, with problems getting worse with increasing exposure. It is of use in the manufacture of biodiesel, but if you are using it, we recommend caution. (Wikipedia: Methanol fuel)
Ethanol is created in large quantities from corn, in the United States, and is a major transportation fuel in Brazil, where it is made from cane sugar, with the bagasse being used for heat and power. Its use is very controversial in the USA, partly because the energy it provides is only 10% to 20% greater than what was required to create it, making it at best only slightly better than gasoline as a carbon polluter. It can also damage engines if too much is blended into the fuel. (Wikipedia: Ethanol fuel, section on efficiency of common crops)
It also gets the largest single subsidy we have for renewable energy. There are crops that are thought better alternatives than corn for making ethanol, and switchgrass is one.
Butanol is a fuel that is very similar to gasoline. It has an octane rating of 87 and can be blended with gasoline in any proportion to fuel cars without damaging auto engine parts as ethanol does.
In October of 2013, ITRI, the largest research and development laboratory in Taiwan, introduced a system for manufacture of butanol it called “ButyFix,” which it claimed sequestered more carbon than the product released in manufacture and use combined. It also claimed that this solution could be made from any cellulose source, including wood, straw, or algae, and sold for $2 per gallon at the pump, making it highly competitive with fossil fuels. ITRI also said that any ethanol-producing plant could be easily converted to use the process. In April of 2016, ButyFix received a gold Edison Award in the alternative energy category.
Syngas is a synthetic replacement for natural gas and biogas. Though it can come from fossil fuels, it can also come from biologic sources. Believe it or not, it can even be made from carbon dioxide captured at the stacks of plants burning carbon-based fuels, or extracted from lakes or the ocean, by using the Sabatier reaction, which was discovered about 1910. It can even be made from the air. Alternatively, syngas can be made from carbon monoxide, using the Fischer-Tropsch process, invented in 1925.
Research has continued up to the present, and improvements to these reactions now exist, requiring comparatively little energy for the production. Though some processes and fuel sources produce syngas with only about half the energy density of natural gas, it can be purified to make its energy density higher. It can also be made into other fuels, including replacements for oil, gasoline, or plastics. (Wikipedia: Syngas, Power to Gas, and Alternative fuel.)
Since the feedstock for syngas based on carbon dioxide can be just lake or sea water, the cost of syngas is closely related to the cost of the electricity used to make it. In 2010, when the low demand costs of wind power could be as low as 0.71¢/kWh, syngas could compete with fossil fuels when oil cost $55 per barrel. As it happens, the cost of wind power has dropped so low that long-term power purchase agreements average 2¢/kWh (see: 2015 Wind Technologies Market Report and Lazard’s Levelized Cost of Energy Analysis), and in low-demand times, the cost actually can fall into negative territory. This implies very inexpensive gas fuel can be produced where a lot of wind power is available.
Synthetic gasoline and aviation fuel can also be made from carbon dioxide found in the ocean to make synthetic gas, and that could potentially be used to make a liquid fuel cost-effectively. For this reason, it is being studied by the Navy. A study published in 2010 says, “The U.S. Navy estimates that 100 megawatts of electricity can produce 41,000 gallons of jet fuel per day and shipboard production from nuclear power would cost about $6 per gallon.” If you follow the numbers in the spreadsheets in the study, it is clear that $4.32 of that $6 is a pay-down on the cost of a nuclear reactor. This implies that if the energy is supplied on land at low-demand times, the cost of aviation fuel might be well below $2 per gallon. (I saw a Navy slideshow presented in 2012 in which it was stated that the cost on land from wind power at low-demand rates would be $1 per gallon, but unfortunately I cannot document how that amount was calculated.)
Compressed air is being put forward as a power source for both electricity and transportation. Though it has been used to power cars and other vehicles, it is being considered for large-scale storage for electric power. A 3,000 MW wind farm proposed for Wyoming plans to have energy storage using compressed air in Utah, as the electricity flows to Los Angeles. (Wikipedia: Compressed air energy storage)
Liquid nitrogen boils at the low temperature of -195.79° C (-320° F). It can be maintained without pressure in an insulated tank for periods of days to months by allowing it to boil slowly, cooling what remains. This is acceptable environmentally, because the atmosphere is 80% nitrogen anyway. It has been suggested as a transportation fuel for years, but it is regarded as impractical for many reasons. (Wikipedia: Liquid nitrogen vehicle)
Hydrogen has great potential as a fuel for a number of reasons, but also has some important drawbacks.
One method for manufacturing hydrogen is electrolysis of water, which releases only oxygen as a by-product. If this is done during times of low electricity demand, while the wind powers wind turbines, hydrogen can actually be produced at no marginal cost, or even negative cost. Burning hydrogen produces water, using the same amount of oxygen that was released when the hydrogen was made. This makes its use potentially environmentally neutral.
One big drawback for storing hydrogen is that its molecules are so small that they can fit between the atoms in steel, so they leak out of storage by passing right through the sides of the containers. This is a problem for long-term storage, but does not happen quickly enough to be of much concern for short-term use. (Wikipedia: Hydrogen fuel)
Hydrogen is not energy dense. It can, however, be mixed with other gases, such as syngas or biogas, to improve the mixture. (Wikipedia: HCNG)
Ammonia has been used successfully as a fuel for transportation. What we are talking about here is not the ammonium hydroxide that you can buy in stores, which many people call “ammonia.” This is anhydrous ammonia, a gas that can be liquefied and held in a tank. Unlike ammonium hydroxide, ammonia will burn in air.
During the Second World War, shortage of gasoline and diesel oil in German-controlled areas of Europe pushed the Belgians to look into alternate fuels for their buses. By fitting the vehicles with appropriate fuel tanks and making the necessary alterations to the engines, they were able to use ammonia as a replacement for diesel oil and gasoline.
The production and consumption of ammonia is similar to those of hydrogen, as the products of combustion are identical to the sources for the fuel. In the case of ammonia, hydrogen is manufactured, with oxygen as a by-product, and then it is combined chemically with atmospheric nitrogen, using heat, pressure, and a catalyst. The first system for doing this is about a century old, but new techniques are more efficient.
Ammonia has its drawbacks, including low energy density and toxicity, though the products of combustion are quite safe. When the ammonia is burned, the products of combustion are nitrogen and water, the same products it was made from. (Wikipedia: Ammonia, section on use as a fuel)
There are doubtless other ways of dealing with those last dribbles of fossil fuels we need to get rid of, and with research, more may appear. I apologize for any I might have missed.
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