Today, solar power and lunar power. The University of
Houston's College of Engineering presents this series
about the machines that make our civilization run, and
the people whose ingenuity created them.
Renewable energy is a term worth
looking at. We can use direct solar energy to heat water
or our homes. We can focus solar energy to supply steam
boilers. We can build engines to take energy from the
warm surface of the ocean and discharge it into the cold
water below. We use solar cells to convert solar energy
to electricity. Nature often does the work of converting
heat into usable power for us, by moving water and
wind around the planet. Thus windmills and hydroelectric
plants deliver solar power.
We call such energy renewable only because the sun
replaces what we use. No energy is truly renewable.
Sooner or later, every source runs down, even the sun.
It's just in comparison with our brief human lives, that
it might as well be inexhaustible.
Another nearly inexhaustible source of energy is
planetary kinetic energy. Every day the moon's
gravitational pull lifts countless tons of water up into,
say, the East River or the Bay of Fundy. When that water
flows back out to sea, its energy dissipates and, if we
don't use it, it's simply spent.
So where did that energy come from? It was taken from the
moon's and Earth's kinetic energy. The tides slow us
down. Each year on Earth is about half-a-second longer
than the one before it. But, like the flow of solar
energy, that consumption is present whether we claim it
We might claim it in many ways. The most obvious is to
catch sea water at high tide, then run it out through a
turbine during low tide.
Still, every large energy system comes at a price. Fossil
fuels soil the air. The problem of nuclear waste haunts
us. Hydroelectric dams create huge reservoirs and pose a
threat when they grow old. Modern windmills kill
endangered birds as well as consuming real estate. So
what price do we pay for tidal power?
It has the potential for picking up about a fiftieth of
our energy consumption. That could be a great help, but
never more than a help. Tidal power systems are
potentially big and expen-sive, and they pose poorly
understood threats to ocean ecosystems.
Tidal power (what I like to call lunar energy) is
not new. Tidal mills were built in the eighteenth century
when their major competition were windmills and water
wheels. The tidal mills largely vanished once we had
cheap steam engines.
I have a 1921 book on Tidal Power that describes
systems we might use today. It discusses tidal
variability and intermittency and treats the delicate
business of placing tidal plants.
Power use in 1921 was only a thirtieth of today's
consumption. Even then, tidal power clearly couldn't have
been enough to replace the use of coal. Yet tidal power
was going to waste then, as it goes to waste today. The
author of this old book might as well be writing for us
when he sums up the situation by quoting Cecil Rhodes'
dying sigh, "So little done, so much to do."
I'm John Lienhard, at the University of Houston, where
we're interested in the way inventive minds work.