The global energy budget

Alternative, renewable, and sustainable energy is on my mind a lot these days. An awful lot. While I was at the Materials Research Society‘s Fall Conference in late November, I was fortunate enough to hear Dr. Stephen Chu‘s plenary address on renewable energy and climate change. One of the things that struck me about his lecture was his utterly upbeat attitude towards the problem and his optimism that we would be past this latest energy crisis and into an era of sustainable energy.

As energy goes, there are only a few ultimate sources. You can harness energy from solar radiation, from decaying isotopes, from the residual heat of the earth, and from gravity via the tides. Obviously, fossil fuels and biomass energy sources are merely ways of capturing and storing solar energy. Many of the proponents of solar power, whether that is photovoltaic or solar thermal power, claim that solar will ultimately be the most efficient and cheapest source of power to harness. I’m inclined to believe this assertion personally, although I do know that there are many interesting geothermal and tidal power projects under development.

As I have been doing my own study and research into this area, I thought about the limits of the problem. At our current state of technology, or really at any given state of technology, there is a finite amount of energy that can be harnessed. But even if you assume there is an evolutionary growth in efficiency of capture, there has to be some limit, some budget that you cannot exceed without a true step-change in the technology available for energy capture and storage.

So where is the limit here? For solar energy, there is clearly a finite amount of solar radiation available to the planet. Earth subtends a vanishingly small solid angle in the solar system; we cannot capture even the barest fraction of the Sun’s output. The assertions I allude to above can essentially be rephrased as “the Sun sends enough energy to Earth to provide our civilization with enough power to grow for the indefinite future.” I want to test that assertion and find out what the limit is.

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