Saturday, June 4, 2011

What percentage of our energy demand can be replaced by renewables?

I hear the question, "What percentage of our energy demand can be replaced by renewables?" There are two unquestioned assumptions that frame this question and illuminate our fossil-fuel mindset.

1. One good answer is none. "Replacement" suggests doing things the same way. We can't "replace" oil with sunshine any more than we were able to "replace" horses with high-speed 4-legged robots shaped like horses. We jettisoned horses and made devices with engines and wheels. Now we must jettison devices with engines and wheels that are 1% efficient, that weigh 2 tonnes to move 100 kg.

For example, what about biodiesel? Consider this thought exercise. Define inefficient = stupid. A car engine is 13% efficient (per RMI); the average car weighs about 4000 lbs (per DOE, DOT) and carries an average of less than 200 lbs; that's 5% efficient. So 13% (engine) * 5% (mass) = 0.65% < 1% efficient = stupid. Now how do we get biodiesel? Photosynthesis can convert 3-6% of sunshine into soybean plants. Then we take the oily portion of the plant (you can't make oil out of the stems) so even assuming that it takes zero energy to harvest and process that plant material into oil, your net efficiency is <<1% = stupid. (Using 100 gal/acre/year, I estimated that 0.05% of the sun's energy is converted to soy biodiesel. I've heard of yields as high as 600 gal/acre/year for "next-generation" biofuels. Give them the benefit of the doubt, and we're at 0.3% efficient, still <<1%. Correct me if I'm wrong.)

Now put that <<1% efficient biodiesel (stupid) into a car that is <1% efficient (stupid) and you get << 0.01% efficient. The result? Compound stupid." 

2. Another good answer is 100%. Built into the question (remember the question, "percentage of energy ... replaced by renewables") is the curious assumption that we have a choice. We don't.

Most of humanity lived within a solar budget until World War II. As near as I can tell, we have no option but to return to 100% renewables, whatever that may look like. (I'm  all ears if you think you have found something else.) With the incredible amount of knowledge and skills we have gained during the fossil fuel era, we are much more capable than our grandparents to take on the task. If we are to avoid becoming a dead  branch on the evolutionary tree, we will switch to renewables now so we can leave something for our children to work with.

It's not "practical." We will face skepticism and ridicule. But those who embrace renewables now will be the sellers in the post-oil economy, and there will be plenty of buyers who postponed the inevitable shift.

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