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Re: Pumped storage



Laurent's original message concerned using gravitational PE to store solar
energy. Many have commented on pumped storage as currently used, but what
about using it as originally stated, to store solar energy? While possible,
there are several concerns here. One is that most of the good natural sites
are already being used for hydro, hydro/pumped facilities and the best sites
for collecting solar energy may have no natural terrain suitable for pumped
storage. If that is the case, then you need to think about building
artificial reservoirs. A large electrical power plant runs 1000-3000
Megawatts in capacity. Work out the size of a reservoir that could provide
1000 MW (for up to 12 hours). The total energy capacity would be 12 x 10^6
kWh--43 x 10^12 J. Let's drop the water 20 m and make the reservoir 20 m
deep. Each kg of water then picks up about 200 J in falling. For
simplicity lets assume 100% efficiency in converting this to electrical
energy. We then need to store .215 x 10^12 kg of water daily. At 1000
kg/m^3 we have .215 x 10^9 m^3. At 20 m deep the surface area of the
reservoir has to be about 1.1 x 10^7 m^2 or 3.3 km on a side--10 km^2 built
up 20 m and then walls (all probably earthen) built up another 20 meters
around the whole thing). Large but within engineering capabilities--assume
this is situated next to a river or else you need a second lower reservoir
as well. Assuming the Solar power output of this setup was 2000 MW (to
supply users during the day and pump water to the reservoir as well) and
actual delivered energy at 50% of capacity (current numbers are more like
40%) then you'd only need about _800_ such 'plants' to supply the current
U.S. demand for electricity with solar/pumped storage! {That's 8000 km^2 or
reservoirs--but to be land efficient, mount the solar collectors over the
top of the reservoirs.} However, if we are to phase out fossil fuels
(either because supplies dwindle or global warming legislation demands such)
then much more than the 16% of current energy use must become electrical and
from renewable resources.

Of course the efficiency of energy conversion isn't 100% so the reservoir
much be larger (anyone know a reasonable conversion % for hydro plants--PE
to KE to Electrical Energy?), but we would never try to supply a very large
percentage of our energy (electrical or total) this way. However, a few
calculations like this can show the tremendous problems associated with
turning even small percentages of our energy production over to renewable
resources in light of the fact that we currently get over 90% of our total
energy from the fossil fuels. It's all a matter of scale.

(You can tell what kind of course I'm teaching this semester--and the fact
that labs don't start until next week ;-)

Rick



----- Original Message -----
From: "Laurent Hodges" <lhodges@IASTATE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, January 18, 2001 12:51 PM
Subject: Electrons


A state energy official, not a scientist, once asked me, obviously
puzzled,
this question: He knew that the transmission lines carried electrons into
your house, but he wondered why there was no problem with the electrons
piling up in the house, or at least the last house on the line. I started
to explain that first of all, the electric wires were double wires, and he
struck his forehead and said, "Of course, the second wire is to carry them
back."

Another state energy official, thinking about the problem of storing
energy
(like solar electric energy collected during daytime hours), once came up
with this idea, which really excited him: Use the excess to lift heavy
weights (like steel bars) into the air, and retrieve their (gravitational
potential) energy by lowering them again. I did a back-of-the-envelope
calculation, which you might like to reproduce, to show him what would be
entailed in storing, say, one million Btu (approximately one billion
joules), what a house might use for heating on a cold night. He gave up
on
the idea.

Laurent Hodges