Last week news came from the Borexino collaboration that an accurate solar neutrino flux measurement had been made. A fantastically difficult observation that not only confirms the basic model of how solar fusion works, but also gives a window into the near real-time production of energy in the Sun. Although light only takes about 500 seconds to travel from the Sun's surface to the Earth, it takes photons about 100,000 years to work their way from the core to the surface. Borexino allows the comparision of power generated by the Sun 500 seconds ago with what we observe when we look at light that gives us a sense of what was happening in the solar core 100,000 years ago. The numbers, to the accuracy of the observation, are the same.
Our star is remarkably stable. Over large amounts of time (hundreds of millions of years) its output has changed, but the evolution has been slow. It has been burning about five billion years and should last about that much longer before something really dramatic happens. The process that produces more than 99% of the Sun's energy is extremely slow - so slow that the energy density of the core of the Sun is about a quarter the energy density of human metabolism - closer to that of a lizard.
What makes the Sun useful to us is its size. The output somewhat boggles the mind - about half a ten billionth of its output makes it to Earth. That works out to about 174 petawatts of power to our upper atmosphere. About a third is reflected back into space, but the rest is absorbed by the oceans, land masses and our atmosphere. It drives the weather and water cycle, keeps the average temperature of the planet reasonably stable.1
A petawatt is a thousand terawatts which is a thousand gigawatts. A gigawatt is roughly the output of a large nuclear power plant and very roughly enough to power a million homes. A terawatt is the largest unit of power we usually invoke when talking about human power requirements. The current power usage by all of humanity is about 18 terawatts - working out to a bit more than 2,500 watts for every person on the planet.2 That's about three and a third horsepower for everyone. Americans use more like 13,000 watts but some in the developing world are closer to a hundred watts - roughly the power delivered by food to the average person.
Most of this power comes from burning hydrocarbons like coal, natural gas and petroleum - the so-called fossil fuels. Before the industrial revolution wood was a primary fuel for most people with a bit of peat and coal along with wind for long distance transportation. The industrial revolution created a large demand for fueld and shifted the focus to fossil fuels.
Most of our power ultimately comes from the Sun. The chemical bonds in hydrocarbon fuels were created by photosynthesis and represent a beautiful way of storing solar energy for later use. Photosynthesis is an inefficient mechanism for converting and storing solar energy, Most plants have an efficiency on the order of a half percent, so a good deal of collector area is required. Farms are solar power plants with storage and we can talk about the size of collector required to support a person, city or all of humanity.
Wood was convenient for many populations, but was easy to over harvest - that pesky half percent efficiency gets in the way. The industrial revolution focused on hydrocarbons that had accumulated over multi-million year periods. The evolution of fuel use is fascinating - for the US3
The population of the US increased dramatically during the period so consider the per capita total primary energy relative to 2010.
The interesting point is that per capita energy use has only increased by a factor of three since the beginning of the industrial revolution in the US. We're extracting much more work from our energy sources - efficiency has improved dramatically.
Fossil fuels have some great properties - particularly for transportation - but there are limits. They were once easy to mine (some, like coal, still are) Their creation was enormously inefficient - less than a thousandth of a percent of the product of photosynthesis is transformed into coal, natural gas and petroleum. As such they are a finite resource that is becoming increasingly difficult to mine and there is the additional problem burning them is inefficient, produces several forms of dangerous pollution, and dumps an enormous amount of sequestered carbon into the atmosphere.
Changing energy sources is a slow process. Ultimately solar energy is likely, but the path requires decades and will be messy. We're stuck with fossil fuels for a few decades in one form or another and that brings up the question of which ones are 'better' and why ... fuel for the next post.
1 Unfortunately not stable enough these days. Accelerated global warming may seem relatively small, but the rate of change is astonishing - 1,000 to 10,000 times greater than what has happened in the past 50 million years or so.
2 I like to talk about power usage rather than energy. Power is just the rate at which energy is 'used' - most people are conceptually more comfortable with power than energy. The 18 TW figure is for primary energy sources.
3 A collection of sources, mostly the EIA and expressed in exajoules of energy.
This is an attempt at making a stuffed bread. It came out well - I'm guessing at the measurements as it was thrown together, so consider this a starting point and taste along the way.
Sweet Potato Stuffed Bread
° about a pound of sweet potatoes
° 1/3 cup finely chopped scallions, white and green bits
° 1/3 cup finely chopped cilantro
° 1/4 plus a tbl olive oil
° 1 tsp kosher salt
° 2 tbl all-purpose flour
° about 2 tbl water
° a half dozen six inch flour tortills (I cheat:)
° plain full yogurt
° oven to 425°F
° roast the sweet potatoes 'til tender, cut them in half and scoop them into a bowl - get rid of the skins
° add about a tbl of olive oil, scallions, cilantro, and salt and mash until mixed
° in a small dish mix the flour and water to make a paste
° lay a tortilla on a flat suface and plop about a sixth of the sweet potato mixture onto the center spreading it out to within a half inch of the border.
° paint the half of the edge with the paste, fold the tortilla in half and seal tightly
° put the remaining olive oil in a sauté pan over medium high heat (don't burn the oil!) and brown a stuffed bread or two until golden on both sides
° serve with some plain yogurt