interesting article in nytimes (replicated for those who dont have a signon)
assume average house would want/need a storage capacity for 10kw or 20kw
Wind Drives Growing Use of Batteries
By MATTHEW L. WALD
The rapid growth of wind farms, whose output is hard to schedule reliably or
predict, has the nation's electricity providers scrambling to develop energy
to ensure stability and improve profits.
As the wind installations multiply, companies have found themselves dumping
late at night, adjusting the blades so they do not catch the wind, because there
demand for the power. And grid operators, accustomed to meeting demand by
supplies, are now struggling to maintain stability as supplies fluctuate.
On the cutting edge of a potential solution is Hawaii, where state officials
percent of energy needs to be met by renewable sources like the wind, sun or
by 2030. A major problem is that it is impossible for generators on the islands
export surpluses to neighboring companies or to import power when the wind
On Maui, for example, wind generating capacity over all will soon be equal to
one-fourth of the island's peak demand. But peak wind and peak demand times do
coincide, raising questions about how Hawaii can reach its 70 percent goal. For
the best option seems to be storage batteries.
In New York and California, companies are exploring electrical storage that is
enough to allow for "arbitrage," or buying power at a low price, such as in the
of the night, and selling it hours later at a higher price. In the Midwest, a
is demonstrating storage technology that can go from charge to discharge and
several times a minute, or even within a second, bracing the grid against the
vicissitudes of wind and sun and transmission failure. And in Texas, companies
looking at ways of stabilizing voltage through battery storage in places served
just one transmission line.
Renewable goals can be met, many in the industry insist. But if the energy
intermittent, "you can't do that without batteries of some sort," said Peter
spokesman for the Hawaiian Electric Company.
His company has agreed to buy electricity from a wind farm on the northern shore
Oahu, where the Boston-based power company First Wind has just broken ground.
The spot is one of Hawaii's best wind sites, Mr. Rosegg said, but the supply is
and erratic. What is more, it is at the farthest point on the island from the
main load center, Honolulu, and does not even lie on its high-voltage
So the 30-megawatt wind farm, which will have enough power to run about 30 Super
Wal-Marts, will have Xtreme Power of Austin, Tex., install a 15-megawatt battery.
Computers will work to keep the battery exactly half-charged most hours of the
said Carlos J. Coe, Xtreme Power's chief executive. If the wind suddenly gets
or falls off, the batteries will smooth out the flow so that the grid sees only
gradual increase or decrease, no more than one megawatt per minute at some hours
The Hawaii installation is designed to succeed at a crucial but obscure
frequency regulation. The alternating-current power system has to run at a
cycles per second, and the battery system can give and take power on a micro
changing directions from charge to discharge or vice versa within that 60th of a
second, to keep the pace steady.
The battery system can also be used for arbitrage, storing energy at times when
are low and delivering it when prices are high. It can hold 10 megawatt-hours,
is as much energy as a 30-megawatt wind farm will produce in 20 minutes if it is
running at full capacity. That is not much time, but it is huge in terms of
Neither First Wind nor Xtreme Power would say what the project cost, but
disclosed figures put the project in the range of $30 million, with about $0
for the battery. The Energy Department has provided a $17 million loan
Across the country, it is proving hard to predict the cost and the value of
storage to consumers. The electricity stored in off-peak hours could be quite
cost, and prices at peak hours could be quite high. If the reliance on renewable
energy reduces the need to burn coal and natural gas, that would yield an
Mr. Coe estimated the battery system's round-trip efficiency - that is, the
electricity the batteries could deliver per megawatt-hour stored in them - at
percent. If that figure is borne out, it would be a significant advance from the
largest form of energy storage now in general use, pumped hydropower, whose
is put at 70 to 85 percent.
At a pumped hydro plant, off-peak electricity is used to pump water from a
at a low elevation to one at a higher one. At hours of peak demand the water
back down through a turbine, creating electricity.
Electric companies are using other strategies for storage and frequency
Stephentown, N.Y., near Albany, a Massachusetts company, Beacon Power, is
bank of 200 one-ton flywheels that will store energy from the grid on a
moment-to-moment basis to keep the alternating current system at a strict 60
Atop each flywheel is a device that can be a motor at one moment and a generator
next, either taking energy and storing it in the flywheel or vice versa. The
Department provided a $3 million loan guarantee to assist in the $9 million
The Energy Department is also supporting storage projects that rely on
Surplus electricity is used to pump air into an underground cavity; when the
electricity is needed, the air is injected into a gas turbine generator. In
acts as a turbocharger that runs on wind energy captured the previous night,
of natural gas burned at a peak hour.
The department is contributing to two projects explored by PSEG Global, an
of Public Service Electric and Gas, based in New Jersey. It plans to provide $0
million of the $25 million estimated price of a 150-megawatt project envisaged
upstate New York, perhaps at an abandoned salt mine, and $5 million toward a
million, 300-megawatt project to be built in Northern California.
Both will be used to store power made in off-peak periods and deliver it in peak
times, when prices are higher, said Paul H. Rosengren, a spokesman for P.S.E.G.
In Presidio, Tex., American Electric Power and MidAmerican Energy Holdings have
completed a four-megawatt battery system that is not tied to any particular wind
but is intended to improve reliability in the town, served by only one major
transmission line. American Electric Power already has smaller batteries working
Ohio and Indiana to provide more stability in its distribution systems there.
OK, let's do a little "napkin math" to see if this makes any sense. (You are
free to make your own assumptions, but let's start with mine)
First assumption: 50% DOD (More than that will shorten the life of the battery)
So we can store .6 KWH/cycle per battery.
Second assumption: One cycle per day
Third assumption: Battery life = 3 years = 1095 cycles @ 50% DOD (A more
expensive battery will last longer.)
Over its lifespan our battery will store a total of ..6 X 1095 = 657 KWH
Divide $00.00 battery cost by 657 to get storage cost per KWH 100.00/657= 15.2
cents/KWH for STORAGE, you need to add the actual cost of the energy plus the
other costs for the storage system to get your full storage cost.
I don't know about you, but that does not sound like anything that would make
financial sense for a home that has access to the grid. My cost for grid power
is far less than that.
On Thu, 29 Jul 2010 14:46:43 -0400, "vaughn"
IIRC, Nick Pine demonstrated that deeper discharges resulted in the
highest amount of lifetime kWhs. He used data from Trojan, for L16s I
think. It wasn't a big difference though.
I've seen folks with poorly maintained T105s go beyond 5. The usual
assumption for normal maintenance and correct loading is 5 years for
105s, 10 years for L16s, and 20 for industrial quality.
That looks about right, but don't forget to allow for the fact that
with most battery-based power setups, not all the energy makes a trip
through the batteries. At my own place, some days it's virtually none.
For a few it's 100%. Anything over about 2kW generally comes from the
batteries. When welding during daytime while simultaneous loads are
invariably present, it tends to be more than half the power from the
batteries. Quick estimate of the average overall - perhaps a quarter
of the total consumption takes the battery route. Tracking and wind
power helps considerably.
Quick and dirty for 1kW solar, 2kW inverter, 48V string of L16s...
say, $k total, and 5kWh per day production. Call it an average 20
year life for most of the setup except the batteries = 17 cents per
kWh for generation, and an additional 11 for batteries. Some folks are
already paying more than that for grid energy, without counting the
hookup cost which can be substantial even if it might be buried in the
home purchase price. Add in the generally ignored external costs which
are put off onto future generations, and a very good case can be made
for home power.