One question which made it's way past the "front line" of SEM students that are mostly fielding this interest was "How efficient is tidal power compared to other alternative energy sources?"
It’s not a very well-thought out question, and probably a “canned” question thought up by the teacher and passed on unthinkingly by the student.
Here’s my answer, which I told SEM student Andrew B to first forward to the student uncut, then think about it.
I added, as a kind of humorous aside, "This is as deep as I can get ;)"
I wasn't entirely joking. Other scientists might be concerned with the "meaning of life, the universe and everything," but I'm mostly interested in this nexus between economics, renewable energy, and climate change.
"Your question requires a better definition of efficiency. Do you mean thermodynamic or economic efficiency?I might have added, "Become a scientist or engineer and find a cheap, decentralized and efficient way to convert solar energy to an energy-dense, safe, liquid fuel that runs in existing engine types, and we can not only solve climate change and make you incredibly rich, but also make dozens of dictators around the world obsolete, strengthen democracy worldwide, and save millions of lives."
Renewable energy is ambient energy. Ambient energy is abundant in sun, wind, waves, tides, biological organisms, and the internal heat of the earth. Questions of thermodynamic efficiency don’t really apply to an energy source so abundant. When we look at the thermodynamic efficiency of fossil energy, then efficiency is important. A hybrid car might be able to convert 45% of the energy in gasoline to useful transportation, and this is better than 35% thermodynamic efficiency, which is what a regular car might get. But if you could cheaply extract even 0.1% of the total of terrestrial ambient energy, which is about 85,000 terawatts a year (tW/y), that would be several times more than humans would ever need. We currently use 15 tW/y, so do the math.
Economic efficiency is a more important question. We live in a society of (mostly) free market economic systems. If it costs 5 cents a kW to convert, say, wind energy to electricity, and 3.5 cents a kW to convert fossil gas energy to electricity, then a free market for electricity production will choose the fossil gas energy. But from the point of view of climate change and the longevity of gas supply, the wind energy is far more efficient.
This is why we need a national climate policy, and possibly a carbon tax: because free markets choose inefficient outcomes when there are significant external effects such as climate change.
This is also why, in the renewable energy industry, we generally think of efficiency in terms of leveled cost per unit energy, not in terms of thermodynamic efficiency."
So maybe it wasn't such a bad question after all.