From: dietz@cs.rochester.edu (Paul Dietz)
Newsgroups: rec.arts.sf.science
Subject: Re: Hydro energy storage systems
Date: 27 Aug 92 14:58:31 GMT
Organization: Computer Science Department University of Rochester

In article <1992Aug27.063914.4425@bb1t.monsanto.com> bjgaed@bb1t.monsanto.com writes:

 >And more to the point, is this a good idea?  Obviously it's a good 
 >idea for the power companies.  They get to sell more electricity 
 >without more investment in capacity.  While this is clearly an 
 >economic advantage, I don't think it is advantageous from the 
 >standpoint of overall efficiency, conservation of resources, 
 >environmental impact, etc.
 >
 >Now, I'm only guessing at these numbers, but if the thermal efficiency 
 >of the original generating system is 35%, and the in-out efficiency of 
 >the pumped storage system is 60%, as someone suggested, and we lose 
 >10% of the electricity each direction from transmission losses, we 
 >have an overall efficiency of .35 * .90 * .60 * .90 = 17% of the 
 >energy finally reaching the consumer.  
 >
 >Now, unless we all decide to have mini-fast-breeder reactors in our 
 >basements to heat our homes in the future there will always be 
 >generation and distribution losses, but can't we come up with a better 
 >system than pumped-storage?  What's the most efficient 
 >electricity-storage system?  Water electrolysis/hydrogen & oxygen 
 >storage/fuel cells?  


This is all terribly muddleheaded.  First of all, conserving energy is
not an end in itself, unless you have some sort of quasireligious
fixation.  The goal is to minimize costs.

Having said that, energy storage can reduce environmental impacts.  In
the absence of energy storage, a utility must maintain generators that
are used with low duty cycle.  These are typically low efficiency
units (it would not make sense to make them more efficient, as they
don't run all that often, and the extra cost would be amortized over
fewer kWh).  Alternately, the utility must discard output from the
baseload units sized to handle the peaks.

A related use is in maintaining reliability and quality of a
electrical distribution network.  Again, utilities maintain extra
capacity -- called a "spinning reserve" -- to help compensate for
sudden fluctuations in demand or supply.  Fossil-fuel based reserves
are typically gas turbines kept running at low power, which wastes
considerable fuel.  Some energy storage schemes are well suited to
supplying a spinning reserve (batteries, hydro, SMES, for example).
Some storage systems (SMES, batteries) are also sources of megaVARs,
and so can be used to control reactive power flows in AC networks.

Hydrogen is a rather inefficient means of storing electricity (most
hydrogen used in industry comes from reformed fossil fuels, not
electrolysis.)  I refer you to Annual Review of Energy, 1989.  For
systems sized to 5 to 10 hour discharge periods (appropriate for
diurnal load leveling):

	System		Efficiency (%)		Cost ($/kW)

	Batteries	  70-85			600-1100
	Pumped Hydro	   70			700-900
	Compressed Air	  70-85			500-600
	Flywheels	   70			1600-3200
	Hydrogen	  50-60			> 1000
	Magnetic	   90			1000-1800

For typical US electricity costs, advanced batteries are projected to
be cheapest for < 4 hours discharge time; compressed air systems are
cheaper for longer discharge times.  SMES (magnetic) storage is most
efficient, but its capital cost is too high.

 >My original point was that these projects are possibly the right idea 
 >but at the wrong time.  Conservation and efficiency are a better 
 >investment right now for an industrial society like the U.S., IMO.

Well, electricity is still going to be sold, and loads will still be
uneven.  So conservation and load leveling via energy storage are not
mutually exclusive.  Given that the vast majority of electricity
consumed in this country is fossil fuel based, and that conservation
appears to, at best, be able to keep up with demand growth, limiting
oneself to a single ideologically pure strategy seems unwise.  Note
that electricity demand in the US, unlike energy demand in general,
has increased nearly monotonically throughout the "energy crisis".

	Paul F. Dietz
	dietz@cs.rochester.edu