Fuel Comparision - Practical & Enviromental
Fuels not only have different environmental impacts, they are have different energy densities, and require different devices to utilize them. Because hydrocarbon fuels (including fossil fuels and biomass), contain a very large amount of energy in an easily portable form that is easily utilized, they have historically been the fuel of choice for heating, generating electric and transportation. Nuclear fuels contain much more energy but require large sophisticated devices to utilize them--they cannot be used directly in heating or transportation (unless of course your vehicle is the size of an aircraft carrier). For aircraft, hydrocarbon fuels are the only fuels that are currently practical. While there is no direct comparison, in practical terms, electricity has much lower energy density than other sources.
Energy companies like to tout the benefits of their particular energy source, but in reality easy source has its tradeoffs. Here is a summary of the sources.
Fossil Fuels
Oil, gas and coal are considered fossil fuels because they have been underground for millions of years. When they are burned, the carbon that enters the atmosphere has not been part of the carbon cycle for all those years, resulting in a level of greenhouse gases that has probably not existed for all those years.
Heating Oil: it is an easily used, very energy dense source (between 140,000 & 150,000 Btu/gallon for fuel oil) with many environmental and political problems. Based on recent estimates the US holds somewhere around 5% of the worlds proven reserves of oil, and must of the rest is in unstable and/or hostile territories. Environmental problems include water and land contamination near the drill site (including contaminating the water table), spills during transportation, air pollution from incomplete burning and from impurities, and increased carbon dioxide in the atmosphere. Needless to say, there is significant controversy around easy of these effects, but given the potential size of the negative effects, and the obvious issue that oil is a finite resource, it would make sense to migrate toward using much less oil. (Note: determining how finite the supply of oil is a complex question that involves estimates, probability and the use of technology which may increase the environmental effects of drilling. In particular a greater supply estimate also means a higher price per barrel. While common practice used to be that proven reserves were listed as those with a 90% confidence, lower confidence levels are now often used, and every estimate should be read with suspicion. For a more complete discussion, see: http://dieoff.org/page140.htm or Scientific American, Mar 1998 for an in-depth article on cheap oil.)
Countrywide, oil accounts for only a small percentage of heating fuel use and a very small percentage of electric generation (mostly confined to emergency and peak load generation).
Natural Gas: occurs in similar geologic formations as oil (and coal), but is formed under different conditions. Like oil, extracting all that is underground is a challenge once the natural pressure is reduced, and so drillers use various technologies to force the remaining gas out. The latest technique, fracking, has come under much scrutiny for the potential for groundwater contamination, but has also created a boom in natural gas because fracking is able to get gas out of geological formations that were previously considered useless because the hydrocarbons in them were not in the standard gas/oil form and are bound more tightly into the surrounding rock than traditional wells. The downside, is that it also takes more energy to get anything of of the well, ie its no longer the case that you drill a hole and the gas (or oil) comes out on its own.
Compared to other fuels, burning natural gas emits less carbon per Btu of heat released. For many years, gas companies got people to convert to natural gas due to its lower cost, but recent price spikes have narrowed the gap, and price instability has made fuel choice on the basis of cost difficult, if not impossible.
The environmental impact of natural gas is less publicized than that of oil, but is still quite significant, including local environmental disturbances due to pipelines and well pads, and local pollution of groundwater.
A hundred cubic feet of gas (1 ccf) contains between 1.008 & 1.030 therm, with a typical value being about 1.025 or 102,500 Btu, so 1 cubic foot contains 1025 Btu.
Propane:: is a derivative of oil, so the price depends on the price of a barrel of oil. In general propane more expensive than oil or gas, and has a similar environmental impact as oil.
A cubic foot of propane contains about 2500 Btu. A gallon (which the unit you often buy it in) is about 91,000Btu.
Coal: is the most abundant fossil fuel in the US, and also produces the highest carbon emissions, the most air pollution (because of a generally higher quantity of impurities, especially sulfur, which causes acid rain, and mercury), and the largest land disturbance (including removing entire mountaintops). Coal used to be a common heating fuel, but is now used largely only for electric generation.
While "clean coal" was a hot topic five years ago, it has faded as many questioned how clean it really was and the price of natural gas went so low that many power plants converted from coal to gas, partially due to high pressure to clean up emissions from older coal power plants.
There is also ongoing research into carbon sequestration, which would benefit coal because it emits the most carbon. The problems so far have been with both financial expense, energy cost of the sequester, and the question of whether the sequestered carbon really stays sequestered.
1 Lb of coal is between 8,000 and 13,000 Btu, depending on the kind of coal.
Gasoline: contains 125,00 Btu/gal, or about 18,000 Btu/Lb, which is way it and its relatives are the fuel of choice in transportation: it is energy dense, easy to transport, and relatively safe (as long as you don't let it get to vapor!).
Biofuels
Ethanol: contains 85,000 Btu/gal, which is substantially less than gasoline, and why you get lower gas mileage in your car when the gasoline contains 10% ethanol. Ethanol made from corn uses almost approximately the same amount of petroleum to make it as the energy you get out of it. More promising are cellulosic ethanol, which uses lower value biomass, and possibly ethanol or biodiesel from algae. In all cases (except possibly algae) the amount of land needed to produce a significant quantity of fuel is quite large: probably larger than the world can afford. Until the processes to produce biofuels are more fully developed, it will be difficult to make any environmental assessment, other than to state the obvious fact that producing biofuels on the scale needed to replace fossil fuels will require a very large surface area. Obviously this area could be much reduced by increasing fuel efficiency.
Essentially biofuels are very inefficient solar collectors (plants convert between 1-2% of the energy they receive, with a theoretical maximum of 3-6%), but make sense only because the resulting fuel has a high energy density.
Wood: although plants are inefficient at gathering solar energy, the advantage of wood is that the forest provides habitat and other valuable quantities in addition to being a source of wood, wood does not add to carbon to the atmosphere (or rather, it doesn't add carbon beyond what was already there 500 years ago, but of course it does actually add carbon), and it can be harvested sustainable. Of course to be environmentally benign, you need a lot of land so as to keep the wood harvest always at a small percentage of the overall land, so wood is not at all practical for most people: at least not in a world with nearly 7 billion people, but is a very good choice for rural people with enough acreage for a low-impact sustainable harvest.
New efficient stoves are much cleaner and more efficient than older ones, and always more efficient than open fireplaces which are terribly inefficient. They still produce emissions, especially particulates. The overall environmental impact of wood burning is probably limited by the limited supply of wood.
Wood contains between 4500 to 7000/Lb, with the heat content being heavily dependent on how dry the wood is. For wet wood, the energy used to evaporate the water content reduces the overall heat output.
Garbage: some municipalities incinerate their garbage, and energy can be generated off it (this is after all the glass, metal and other non-combustible material has been removed). From the perspective of resource use, it is not clear that recycling the paper, composting the remaining non-recyclable organic (including food) waste, and recycling most of the waste plastic isn't the better way to go.
Other Fuels
Nuclear power: is currently used almost exclusively for electric generation (discussed fully below). Although touted as a carbon neutral fuel, in fact the fuel mining, fuel processing and waste elimination processes are use fossil fuel (this is presumably not a requirement, but is the status quo.) Politically the biggest problem with nuclear power is the issue of waste disposal, and in particular the public's general "not in my backyard" sentiment. Depending on your safety viewpoint, it may be possibly to safely store glassified waste in most states, or it may not be safe to store them in any known geological formation. The issue of weapons proliferation, including lower grade nuclear waste makes any storage of waste subject to scrutiny, and transporting it likewise carries some risk factor.
Nuclear fuel is, by some measures, more than 90% waste even when it is first put in the reactor, with a fuel mix around 95% non-fissionable U238, and only 5% fissionable U235. Breeder reactors work by converting this excess U238 to Plutonium. In either case, the fuel gets slowly polluted with fission by-products. In standard reactors the entire mess, which consists of a lot of U238, some Plutonium and related byproducts, and a significant quantity of highly radioactive fission byproducts, is disposed of (which currently means it sits in cooling ponds on site for ten or so years, then sits in dry storage on site). Breeders are designed to convert a much larger percentage of the U238 to Plutonium, all of which can then be fed back into the reactor, which is done by reprocessing the spent fuel to remove the highly radioactive fission byproducts, which would be subject to the same storage issues standard reactors face, but a much smaller volume and much shorter lifetimes. Electricity in France is almost entirely nuclear, and it mostly done with breeder reactors. Reprocessing by nature gathers up a lot of plutonium, which is weapons grade fuel, and so adds an additional risk of nuclear proliferation to the normally added risks of the accidental release of radioactivity into the environment due to mechanical failure of the reprocessing plant.
The threat of climate change, and the general pollution problems with fossil fuel plants looked like it was about to create a renaissance for nuclear power, but then the Fukishima disaster happened, and at this point, the public is back being very negative about nuclear power.
At this point no reactor burns nuclear fuel until it reaches a benign state.
Hydrogen: is not really a fuel because it does not naturally occur--it has to be manufactured. There is currently not any especially efficient ways to make hydrogen, and as a fuel, its energy density is quite low unless it is compressed to high pressure. The most promising use of hydrogen is in fuel cells, which produce both heat and electric, making the conversion nearly 100% efficient both are used. The conversion to electric is current between 40 & 60%, so if the waste heat isn't used, the efficiency goes way down. Also if the conversion efficiency of electrolysis is factored in, the overall conversion efficiency to electric goes down to 30-50% (the electrolysis of water by itself has an efficiency of 50-80%).
Currently large fuel cell units operating a high temperatures are more efficient than smaller ones, but the technology is new enough that it is difficult to say. Until the efficiency of electrolysis is high under typical commercial conditions, and the efficiency of the fuel cell is high enough, and the overall cost is low enough, other methods will probably win out.
Electric: like hydrogen, electric does not natural occur, but must be generated. If generated by renewable sources, it can be relatively environmentally benign, but even those sources have some environmental impact.
As of 2013, the proportion of electric generated from each source is (taken from the energy information administration. The values are rounded to whole percent increments. Locally, the generation mix varies significantly: in the northwest hydro can account for 80% or more of the mix, while in other places coal may account for 80% or more of the mix.
- Coal 39%
- Gas 27%
- Nuclear 19%
- Hydro 6%
- Other renewables 6%
- Other 3%
Electric is generally considered a poor choice for heating fuel because the generation efficiency at the plant is typically around 40% (for fossil fuel plants), and then there is an additional 10% loss in the electrical distribution system. For vehicles, the tradeoff is much tougher, since the typical combustion efficiency of a car is around 22%. Looking at the generation mix, and considering the 10-20 years it will take to replace fossil fuels in the electric generation mix even with a fairly aggressive approach, it would seem like society would be better off focusing on ultra-high mileage vehicles rather than plug-in hybrids and electric vehicles. (This shouldn't stop enterprising individuals from putting up their own PV and powering their PHEV or electric car!)
Renewable energy sources are currently under rapid development, and in some cases beginning to come under environmental scrutiny. The biggest issue with renewables is their intermittent nature (the wind doesn't always blow, the sun doesn't always shine), but large scale deployment and an advance countrywide electrical grid can alleviate the problem to some degree, since the wind is always blowing somewhere. Another alternative is to store excess electric for use during calm, dark periods, but this has its own set of problems (in particular finding a technology whose round-trip efficiency is reasonable).
One situation where renewables is a clear environmental win is in supplying electric for peak cooling loads--in this case the output of a PV system is high during the typical period of maximum cooling demand. Likewise, for cooling loads, rather than storing power, you can store cool in the form of ice made at night, and melted for cool during the day. These examples show that any analysis that doesn't look at the complexities of the electric load will likely make converting to renewables look more difficult.
There is much debate about the best renewable technology, and whether it is best deployed in a more local, distributed manner, or in centralized areas. Some technologies (PV) lend themselves better to being distributed, while others have economy of scale issues.
The following is a brief rundown of each technology:
Wind - the largest complaint about wind is that its ugly, while the biggest controversy is that the turbines are bird killers. The latter is being addressed by building very large (300') towers that spin more slowly. Wind farms take up large quantities of land, and although most animals ignore the towers, some (like prairie chickens) won't go near them. Wind farms also require service roads, that are traveled by service vehicles- a situation that tends to be detrimental to wildlife. In at least one case an environmental group (the Nature Conservancy) has produced an area wide map of critical habitat that wind companies can use to avoid conflict). In general the largest areas of wind resources are in mountain passes, and across the plains states.
Small scale wind is growing in popularity, and can be more cost effective than PV. Urban building codes will often prohibit it, due to the required height of the tower (30' above any obstructions like trees or houses), although vertical axis generators may solve this. The general issue is that turbulence reduces the generators efficiency, and puts stress on it. Building top wind generators have the same turbulence issues.
Hydro - currently accounts for the majority of renewable electric. Controversy around dams is not new, with destroyed riparian habitat and blocking fish passage (in particular salmon), being a large issue. In areas where the river historically carried large quantities of slit, the reservoir will eventually fill will silt and render it useless.
Geothermal - exploits heat leaking from the mantel to the surface. Although there are some naturally occurring geothermal sites (hot springs), this technology is generally accomplished by pumping water deep underground. The environmental concern is that whenever surface water is exposed to rock deep down, it tends to pick up dissolved minerals, which tend more toward the toxic side than the benign side, although clearly it depends on the rock.
Wave/Tidal - this is a newer technology that has only been used on a small scale. Some devices appear to be likely environmentally benign, but only time will tell. The big advantage of waves and tidal energy is that its is highly reliable. Since much of the population of the country lives near the coasts, it is also near where it is used.
Concentrating PV (CSP) - this system uses mirrors to focus sunlight on a fluid and heats it hot enough to generate electric via steam. Currently the power plants are quite large and only located in the sunniest desert regions. The is an alternative technology using sterling cycle generators which can be used at a much smaller scale. The large CSP plants take up acres and render the land nearly useless as habitat. Although it is common to consider the desert as wasteland, increasingly people are recognizing that it is a valuable functioning habit that needs protection as well as any other, and so covering the entire desert southwest with CSP plants is probably not an environmentally good idea, but putting up some of them certainly is reasonable.
Two new plants have come on line recently, one of which stores excess heat from late afternoon so that it can continue to operate for up to 6 hours after dark, a technique that reduces much of the "variability" of solar power, especially given that loads drop dramatically after around 10pm.
PV - is the most expensive renewable energy, although the cost has been steadily coming down as producers have improved their manufacturing. The two common concerns with PV is that the contents are toxic chemicals, and that it takes more energy to make them than you get out. The latter has been proven wrong many times (its takes only a 1-2 years to pay back the energy). Most common PV cells contain few toxic chemicals, but newer technologies do. The simple solution will to be recycle the panels at the end of their lives.
One innovative solution to the high initial cost of PV is to have someone else pay the upfront cost, and for the consumer to agree to buy the electric at a fix rate for the useful life of the system. This has become a very popular approach in some areas.