Energy Quiz: What renewable fuel delivers the most net energy?
Surprise! Humble firewood yields the highest energy return on energy invested
Firewood as a residential heating fuel is rarely mentioned in energy policy discussions. When discussed at all, the conversation usually centers around how to restrict wood burning because of the pollution created by users of bad equipment and bad fuel. But considering its many advantages, a better strategy would be to promote the ways its smoke emissions can be reduced.
Most economists, as well as financial pundits in the mainstream media, focus on the money cost of energy. But the energy costs of energy can provide useful insights into environmental impacts and the underlying reasons for the money cost. For this reason, the energy return on energy invested (EROEI) should be included in any review of the quality, impacts and appropriateness of various energy sources.
EROEI analysis aims to answer this question: What is the ratio of the amount of usable energy acquired from a particular energy resource to the amount of energy expended to obtain that energy resource? Some commentators use the term EROI, or energy return on investment, but this term causes confusion because it is often interpreted as the energy return on financial investment, which is an entirely different issue. So despite the fact that it is a hard acronym to say quickly, EROEI provides greater clarity. EROEI is also sometimes referred to as net energy, a simpler term that can also be used.
Theoretically, it is not worth mining, pumping or processing an energy source that has a net energy ratio of 1:1 or less because just as much energy is consumed in production as would be available for consumption. However, there are cases in which companies can make money by developing very low EROEI resources because the process yields a higher quality, more valuable energy commodity than the energy resources consumed to produce it.
The analysis can become impossibly complicated if one tried to account for all energy expenditures to energy production. For example, when a factory producing solar-electric panels is being analysed, should inputs include just the energy consumed within the factory, or should it also include the energy embedded in the infrastructure like the road, sewers and water pipes that service it? What about the people who work there and their homes, cars and food? Then there is the aluminium that the frames are made of, including the mining of bauxite, smelting, shipping and labor and all the related energy inputs. After all, if any of these energy inputs is removed, there can be no more solar panels produced. Because of the complexities of EROEI analysis, there continues to be debate about how it should be done and how far back in the energy supply chain it is necessary to go.
The study of EROEI is fairly new because until recently energy supply was made easy by fossil fuels. In the 1950s you could drill a hole on level ground in Texas, Saudi Arabia or Alberta and get a gusher. (Jed Clampett, the Beverly Hillbilly, famously struck oil with a rifle bullet while out hunting) Those gushers were like free energy, with an EROEI of 100:1 or so. But gushers like that are in the distant past, along with such high EROEIs. Now that most oil is produced off shore and other hostile environments, or from tar sands or from newly-discovered deposits at depths of up to 35,000 feet below the surface, the EROEI of oil production is now rarely as high as 20:1, and for new oil fields much less. The steady decline in oil’s EROEI at least partly explains its stubbornly high price, averaging around $70 per barrel over the past five years after being in the $20 to $30 range for the previous twenty years.
Regardless of how one chooses to calculate EROEI, it is hard to deny that the energy cost of energy is going up fast. It makes sense, then, to investigate all sources of energy looking for those with the lowest net energy costs. Here is a sample EROEI analysis for fuelwood.
- the energy cost of labor is not included
- hardwood fuel: 28 million btu/cord
- 1 gallon of gasoline: 125,000 btu
- average round trip for fuel delivery: 60 miles
- fuel consumption of pick up truck: 15 mpg
- two round trips per cord: 8 gallons
- chainsaw fuel per cord: 0.5 gallon
- log splitter fuel per cord: 1 gallon
- energy input: 9.5 gallons x 125,000 = 1,188,000 btu
- therefore: 28 million / 1,188,000 = EROEI of 24:1
A person cutting firewood from their own property or moving it only a short distance could achieve an EROEI of around 40:1. If, in the example above, the fuel was softwood like spruce or poplar, the EROEI would be much lower, around 14:1 because of the lower energy content of the fuel.
An EROEI for firewood of between 14:1 and 40:1 is only meaningful when compared to the net energy achieved with other sources. Charles Hall, a professor in the Faculty of Environmental and Forest Biology at the State University of New York, is one of the foremost EROEI researchers. Below is a graph produced by Hall and his colleague John Day that was published recently in American Scientist. The tan colored parts of the bars represent the range of EROEI resulting from different source quality and processes. The figures along the x axis show the exajoules of energy supplied in the U.S. by each of the sources.
Wood, in the form of natural firewood, compares favorably with all other energy sources in the amount of net energy realized after processing and transportation. Only coal and some forms of hydro electric power deliver a higher return. For comparison, the EROEI of wood pellets has been calculated to be around 13:1.
A high EROEI means greater price stability over time for fuelwood because the net energy yield is less affected by inputs than other fuels. Price stability is not likely for the fossil fuels, however, because, as the easily accessible deposits are consumed and the energy inputs to produce them rises, their retail price is likely to rise too.
The energy returned on the energy invested is not necessarily the most important thing to consider when comparing energy sources. The present and long-term availability, price and environmental impacts of production and consumption of an energy commodity can be more important. But EROEI analysis is a valuable tool that provides insights that no other form of analysis can offer.
In the case of firewood, EROEI analysis reveals one reason why it is a popular choice among middle and lower income families who live in forested areas. The labor inputs to firewood production are significant, but the energy inputs, and therefore money cost, tend to be low. Since many users report that they enjoy being outdoors to process trees into firewood, even the high labor inputs provide a significant health benefit. The analysis also shows that if the smoke emissions from wood heating can be reduced to manageable levels, its other attractive attributes make it a high value resource for home heating in the future.
Like other renewable energy resources, firewood has some significant drawbacks and limitations. The smoke pollution problem must be tackled to make it an acceptable fuel for home heating in populated areas. The low bulk density of firewood means that it must be used close to where the trees are harvested to keep transportation impacts reasonably low.
At a time when energy sources of all types are under scrutiny because of their cost, environmental impacts and security of supply, firewood should not be overlooked, or worse, unduly restricted.