When evaluating energy technologies – whether conventional fossil fuels or alternative energy – one thing that I pay close attention to is the Energy Return on Energy Invested (EROEI). While there are legitimate criticisms of the methodology, it can serve as a useful tool for comparing and contrasting various alternatives.
To give a flavor for why this is, consider an example. Let’s say society as a whole produces 50 million barrels of oil equivalents (could be oil, nuclear, wind, solar, biofuels, or a combination). Consider a couple of energy options. Option A has an EROEI of 10/1 (Energy Output/Energy Input). Option B has an EROEI of 2/1. Option A has to consume 5 million barrels to produce 50, for a net of 45. This net is what would be left for powering transportation, heating homes, and fueling industry. Option B, however, requires an input of 25 million barrels, so the net from the initial 50 is only 25 million.
The implications of this are that as EROEI falls, society must produce a lot more energy just to stand still. Even if total energy produced is constant, a falling EROEI means that there is less net energy left over after the energy input bills are paid. And because the easy energy is produced first, as time goes by this is in fact what happens: EROEI declines, and then it takes more time, effort, and money invested across society to keep things running. (Or, as EROEI declines energy efficiency must increase at such a rate that what is lost from the decline is made up from increased efficiency).
That’s a very basic introduction to EROEI. For a much more detailed look, see Understanding EROEI. In that essay I look at a number of examples, and explain how the EROEI of Brazilian sugarcane ethanol is probably much less than the 8/1 that is generally claimed, but that model still works well because a large portion of the energy inputs are waste biomass left over from sugarcane processing.
Over the past few years, I have seen a lot of speculation about the EROEI of tar sands (also known by the more marketable term, ‘oil sands’). I had seen estimates ranging from as low as 1.5/1 up to 4 or 5/1. My own suspicion has been that the number was higher than that, and I once did a back of the envelope based on some industry energy usage numbers that put the number at about 8/1 (for just the oil production step).
But now I have a much better number, thanks to a recent discussion at The Oil Drum. A reader linked to the following story:
This is the best reference I have ever seen for the EROEI of tar sands. Here are the important bits:
Oilsands Review: How much energy do you consume for every barrel of oil you produce?
Marcel Coutu: About 1.5 gigajoules (1.5 MCF of natural gas equivalent) per barrel. That’s higher than 0.8 MCF, the number I mentioned earlier; that refers to purchased energy. The total energy we consume in our operations includes energy we generate as a by-product to our upgrading processes. It is largely electrical energy, in which we are more than self-sufficient.
We produce a lot of waste gas from our processes, and use that to fire gas turbines. We also have a lot of waste heat from our operations, and we raise steam with that heat and put that steam into steam turbines. This makes our operations more efficient.
So, what we have is that some of the energy that is used is produced by the process. This is the accounting that results in an 8/1 energy return for sugarcane ethanol. By sugarcane accounting the EROEI of tar sands is about 5.8 million BTUs (the value of a barrel of oil)/0.8 million BTUs (the approximate energy content of 0.8 MCF that was externally purchased), or 7.25. By true EROEI accounting – which includes the internally consumed energy as an input – the EROEI would be 5.8/1.5 = 3.9.
Of course then the oil has to be refined. For a light, sweet oil such as the output of a syncrude unit, that step is going to be 12/1 or better. Putting the two steps together, I calculate that I need to spend 1.5 million BTUs to produce the oil, and another 5.8/12 = 0.5 million BTUs to refine it to gasoline and diesel. Total process is then 5.8 million BTUs/2 = 2.9/1 for the production and refining processes. Conventional light, sweet oil is around 6/1 for the entire process of oil in the ground to gasoline in the tank.
Let’s look at one more example to understand the implications. Let’s say we want 10 gallons of gasoline equivalent for our car. We need to solve two equations: Net Energy = Energy Output – Energy Input; and EROEI = Energy Output/Energy Input. If we combine equations and solve, we find that for light, sweet oil at a 6/1 EROEI, the total energy that must be produced is 12 gallons of gasoline equivalent. Two gallons of gasoline equivalent were consumed in the process of producing the 12 gallons, netting 10 gallons for the end user.
If we wanted to produce gasoline out of tar sands at a 2.9/1 total ratio, then 15.3 gallons of gasoline equivalent must be produced. 5.3 gallons would be consumed in the process, netting 10 to the driver. What I conclude from that is the tar sands is more than 2.5 times as energy intensive to refine to gasoline than is conventional oil.
While I don’t know what the ‘real’ EROEI is of sugarcane ethanol, it is probably in the vicinity of tar sands. However, as stated the big difference is that the bulk of those energy inputs are waste biomass, which dramatically boosts the sustainability of that option. Sugarcane ethanol – even if it has a lower energy return than tar sands – far exceeds tar sands in the sustainability category. This is one of the weaknesses of EROEI accounting; accounting for energy inputs from diverse sources – some more sustainable than others.