|The fossil fuel advocates are coming 'round to the notion that renewables will have to be a part of the energy mix in the future. For example, just Thursday, the head honcho at Shell Oil said his peers should stop their fight to put down renewables and join the fight against climate change. However, he also misstated the potential for solar to become the dominant source of energy, saying fossil fuels need to play a part. He's wrong and so are many who misanalyze the potential for solar, such as Gail Tverberg.
The fossil fuel advocates use a measure they call “Energy Return On Energy Invested”—variously abbreviated EROEI, EROI or ERoEI. On the measure solar appears to come up very short of what's needed to become the dominant energy source.
The problem is that EROI simply does not apply to renewable energy. It does apply to fossil fuels. The reason is simple: When a unit of energy from fossil fuels is burned, it produces a fixed amount of energy. Once the fuel is consumed, that's it, there's no more energy which can be produced from that amount of fuel. On the other hand, for renewables, the amount of energy which can be extracted from the environment is essentially infinite—for example, the sun will last many billions of years producing harvestable energy. The mechanism we use to extract energy from this infinite pool—a solar array—is the producer of energy output. A theoretical mechanism which would last forever would have an infinite EROI. In practice, of course, the mechanism has a finite life, but we can approximate the value of solar using the “Energy Payback Time” — (EPBT). This is the time it takes for the solar array to produce exactly as much energy as that energy required to build the array in the first place. Read more about this topic in Investing in Solar Electricity: What's the Payback? and How Solar Power Could Slay the Fossil Fuel Empire.
Today's solar arrays are warranted to produce 95% of their rated output over a 25-year lifetime. Studies have shown that, depending upon the type of solar cell technology used, the EPBT at which the solar array produces as much energy as it took to build the solar array ranges from 0.7 to 4 years. Thus, the most inefficient solar array would produce 6¼ times as much energy as it took to create it over its 25-year warranty. The most efficient array would produce output of almost 36 times over its 25-year warranty period. Consequently, if we want to compare EROI for solar arrays with EROI for fossil fuels, we get a large range of values for solar, from 6 to 36. This is why looking just at EROI for solar, which varies wildly, is a bad way to evaluate solar versus fossil fuels.
Solar arrays actually last longer than 25 years. As time goes by, the solar arrays produced are likely to last much longer. Thus, the EROI of solar will move much higher than the current maximum of 36 because the EPBT remains at least constant for a particular technology (in reality even EPBT for solar is dropping due to technological advances and that causes EROI to continue climbing). It's estimated that by the year 2020, new solar arrays will last 50 years or longer. The implied EROI for the most efficient array is likely to produce 100 times or more as much energy as it took to build the array with no end in sight to the rise in EROI.
So, the next time someone starts claiming that solar will never become a dominant source of energy because its EROI is inferior, just remember that solar's implied EROI is expanding due to better technology. At the same time, the EROI of fossil fuels is on a downward trajectory (this is something even the fossil fuel advocates admit is the case due to the fact that the easiest-to-extract fossil fuels have already been extracted).