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Subject: RE: [emix] Power storage strategies
David, Thanks for getting an
informative debate going. I assume that you are
suggesting that storage can be generically modeled as a device with a MWH
capacity, a power ratio for both charge and discharge of say 4 MW per MWH (4 to
1 ratio) and a current state of charge (% of the MWH energy capacity) with some
updating of these parameters as necessary. Further, I assume you are suggesting that this information
be used by other parties ( and possibly the owners ) to dispatch the storage. However you have not mentioned how third
parties would be charged or contract for the use of the storage. Keeping with your idea to
keep the storage model simple. we would need at least also specify a round trip
efficiency of storage devices since this efficiency ( MWH Out / MWH in) can vary
between 50% and over 90% for various storage technologies. Additionally, some compressed air energy
storage devices (CAES) also require natural gas as an input energy source in
addition to electric energy. ( Note: round trip efficiency is also a function
of state of charge and rate of charge and discharge, but let's say we ignore
that for simplicity.) A fixed power ratio is
also problematic for many storage devices.
Many batteries have asymmetric charge and discharge ratios, so that we
would need to specify different ratios for charging and discharging. Additionally, many batteries are able to
charge or discharge at high rates for short time periods or when they are not
near full or not near empty and then at much lower rates on a sustained basis. Another critical
parameter is response ramp rate.
Some devices such as batteries and flywheels have an almost instant
response whereas pumped hydro and CAES have a much slower response, limiting
their value for frequency regulation. Battery life is also an
issue. A battery typically might be
able to discharge a fixed number of MWH over its life depending somewhat on how
charging and discharging is done.
So charging and discharge for small economic benefit must be avoided to
save the battery for situations where such use has high value. What information we provide
about storage also depends on what side of the plane of control (energy
services interface) we might be on.
On the storage device side of the interface, the physical models that you
suggest may be useful, however the need to over simplify is less. On the inter domain side
of the interface communicating even a simplified storage model to other parties
and then figuring out how to dispatch that storage in coordination with
generation and load is challenging.
US ISOs are currently working on tariffs and software to allow limited
energy devices such as flywheels and batteries with 15 to 30 min of storage to
participate in frequency regulation markets. It is a significant software and market
design challenge to recognize the limitations of storage (which vary by device
type) in comparison to generation while at the same time given storage the
benefit to the system of the much faster response of storage in providing
regulation services. And most have
not yet fully implemented the economic dispatch of deeper storage devices into
their economic dispatch and locational pricing models. If avoiding over
complication by engineers and striving for simplicity is a goal, then I recommend
the pure simplicity of Transactional Energy outside of the plane of control of
specific devices. What is done
inside the plane of control is another matter, where the specifics of each
device are much easier to accommodate. With Transactional Energy
a storage owner can make or accept an offer to buy MWH at a given rate and at
given low price at night or when the wind is blowing hard. The amount and price will depend on many
factors such those we have discussed above. The storage owner can also make or
accept an offer to sell energy at a higher price in the afternoon or when the
wind is not blowing. A party could
perhaps simultaneously enter into a transaction to sell in the morning and buy
in the afternoon from the storage owner.
This is real simplicity and it is the way we buy and sell almost
everything else in our life.. Perhaps as both an
economist and an engineer, I can revise
your statement " Never under estimate an
engineer's ability to add complexity! to say, "Never underestimate
the ability of an economist's market to make simple what an engineer can make
complex!" Ed Edward G. Cazalet, Ph.D. 101 First Street, Suite 552 Los Altos, CA 94022 650-949-5274 cell: 408-621-2772 From: Phil Davis [mailto:pddcoo@gmail.com] Actually, GE announced
such a system last week and is hiring 400 people in Atlanta to staff the new
business. It's a substation level product. Also, I have spoken personally
with people at Hitachi and Samsung who are testing a 1 MW battery. Such a
battery from another vendor is in test operation behind PJM's main offices. So
local here takes on a new meaning depending on whether it is truly behind the
customer meter, or behind the distribution grid meters (substations and the
like), or on a transmission system. Theoretically, batteries of this size
could replace generators used for voltage or frequency support. Phil Davis From: David RR Webber (XML) [mailto:david@drrw.info] Toby, It occurs to me that
local storage can potentially play a role here - depending on its efficiency of
course. One can anticipate that future technology will offer higher %
there - especially if market forces drive that equation. Therefore - a future
system could offset power surges by drawing on locally stored resources that
were captured during off-peak or excess capacity. In fact such a system
may notify suppliers that they can "push" excess power to local
storage at some pre-determined cost point - and of course also need to indicate
that the storage facility is at a certain % level, or if empty - accept units
at a higher cost rate. DW
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