Economic Impact of Partial String Failure in Multistring Energy
Storage Systems Vanshika Fotedar, Sarmad Hanif, Jan Alam
File
Nam
e //
File
Dat
e //
PN
NL-
SA-#
####
PROJECT OVERVIEWPacific Northwest National Laboratory (PNNL) has
been engaged with exhaustive energystorage system and performance
testing for the past decade. With development of batterystorage
technology, the state of health and mechanics are being scrutinized
in detail. Electricutilities, and large industrial/commercial
electricity customers are integrating multi-MW scaleenergy storage
systems (ESS) for various reasons including improvement of
systemreliability and renewable energy integration. These large
scale ESSs are typicallyimplemented using multiple strings where
failure of one or more strings is not unusual. Largebattery
systems, which have multiple strings operational at the same time,
present a uniquetechnical challenge when it comes to tracking the
system’s state of health and mechanics asa whole (Crawford et al.
2020). Each individual string varies in terms of its performance
andcan fail occasionally impacting the system performance and
capacity. Shutting down an ESSentirely because it has had a partial
string failure would lead to economic losses. In order toevaluate
the accurate costs and benefits associated with a particular
battery system, it isessential to incorporate the costs associated
with battery string failure and, as a result,reduced capacity.
TECHNO-ECONOMIC IMPACT OF PARTIAL STRING FAILURE
ILLUSTRATIVE ESS CASE
RESULTS AFTER PLANNING/ADJUSTMENT
Consider a 3-string 0.75 MW/1.5MWh ESS– string 1 has a rating of
0.3 MW/0.6 MWh and string2 and 3 are rated at 0.225MW/0.45MWh. Now,
let’s assume that the forward reserve paymentrate is cleared at
$25/MW and the real-time reserve payment rate is cleared at $30/MW.
Here,Failure-to-reserve penalty = 0.3 MW x $37.5/MW= $11.25If two
of the strings failed, one with the capacity rating of 0.3 MW/0.6
MWh and another with0.225MW/0.45MWh rating,Failure-to-reserve
penalty = 0.525 MW x $37.5/MW= $19.7This indicates that the penalty
linearly increases with the fall in battery capacity.If we know the
probability of a battery string failing in a given period of time,
we can calculatethe expected penalty for a battery system over a
defined period of time.
The first order technical impact of partial string failure in an
ESS is partial loss of batterycapacity. If the ESS had service
obligations to fulfil with it’s entire capacity, it can only
partiallyaddress these obligations now. On the economics side, the
impact can vary widely dependingon the system, type of service
being provided, market rules, penalties, and so on. Thus,
theeconomic benefits may be evaluated on a case by case basis. In
this work, we evaluateeconomic losses for an illustrative battery
system, providing frequency regulation in ISO-NEmarket, due to
partial string failure.
If the battery system bids into the market despite the partial
string failure, the real-timebattery control duty cycles will need
to be updated. This poster looks at how the economiclosses are
accrued and how the real-time adjustment in battery control duty
cycles canreduce the economic impacts due to partial string
failure.
REVENUE LOSS: FREQUENCY REGULATIONIn order to calculate the loss
of revenue from providing regulation reserve due to partial
capacityloss, the cost and benefits associated with an ESS with no
string failure could be compared withthe costs and benefits
associated with an ESS which has undergone string failure. The real
timefailure to reserve is when the battery’s real time delivered
megawatts are less than the real timemegawatt obligation. In this
situation, the penalty paid by the resource is determined by
theForward Reserve Failure-to-Reserve Penalty Rate and the market
participant’s (in this case, theESS facility) Forward Reserve
Failure-to-Reserve Megawatts. The following figure lays down
theoverall net forward reserve calculation under failure to deliver
the contracted energy due to lossof capacity.
Vanshika Fotedar Pacific Northwest National Laboratory620 SW
12th Ave8th Floor,Portland, OR 97205(971) 940-7106
[email protected]
This work is supported by Dr. Imre Gyuk, U.S. Department of
Energy (DOE) Office of Electricityunder contract No. 57558. PNNL is
operated by Battelle Memorial Institute for the DOE undercontract
DE-AC05-76RL01830.
PROJECT OBJECTIVES 1. Develop control strategies to maximize
financial
benefits with string failure2. Evaluate the financial losses of
partial ESS string
failure and its impact on the final benefits3. Understand market
rules and repercussions of
reduced battery performance4. Validate system model results5.
Challenge in managing multiple use services
within a single optimization engine
EXPECTED LOSSESFor a battery with three strings, i, j, and k,
for which we know the probability distributionassociated with each
string failing, expected failure-to-reserve penalty can be
evaluated using:
𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑓𝑎𝑖𝑙𝑢𝑟𝑒 − 𝑡𝑜 − 𝑟𝑒𝑠𝑒𝑟𝑣𝑒 𝑝𝑒𝑛𝑎𝑙𝑡𝑦= 𝑃6× 𝐹6 + 𝑃:× 𝐹: + 𝑃;×
𝐹; + 𝑃6,:× 𝐹6,: + 𝑃:,;× 𝐹:,; + 𝑃;,:× 𝐹;,: + 𝑃6,:,;× 𝐹6,:,;
Where Pi = Probability of failure of string i, Pi,j= Probability
of failure of string i and j together Pi,j,k= Probability of
failure of string i, j, and k togetherSimilarly, Fi=
failure-to-reserve penalty when string i failsFi,j=
failure-to-reserve penalty when string i and j failFi,j,k=
failure-to-reserve penalty when string i,j, and k all fail
together
For the case described above, given probability distribution of
string failure, the expected failureto reserve penalty for a single
reserve event is calculated in the accompanying table.
Failed string Reduction in Battery Capacity (MW)
Probability of failure Failure-to-reserve penalty ($) using
(a)
1 0.3 0.3 11.3
2 0.225 0.25 8.438
3 0.225 0.20 8.438
1,2 0.525 0.15 19.688
2,3 0.450 0.10 16.875
1,3 0.525 0.10 19.688
1,2,3 0.750 0.5 28.125Expected Failure-to-
Reserve Penalty35.453
Assuming the ESS in this example placed half of its rated power
(0.375 MW), on average, for providing reserves to the market, where
the average settled reserve price was 10 $/MW, the annual earnings
would amount to 10 * 24 * 0.365 * 365 = 32,850 $, where due to
partial string failure the net revenue = 32,850-12,592 = 20,258,
which is 38% less than the estimated revenue.
ACKNOWLEDGEMENT
Scenarios Planning/Real-Time Adjustment
Capacity Payment ($)
Mismatch Penalty ($)
Revenue ($)
S1 No/No 324 297 27S2 No/Yes 320 105 215S3 Yes/No 287 215 72S4
Yes/Yes 297 57 240
Different ESS performance scenarios were tested to see the
impact of planned duty cycle and real-time adjustment on the
economic losses. In the associated table, Scenario S1 represents a
case where the duty cycle is planned without considering string
failure and no real time adjustmentsare made. The mismatch penalty
($) gives the penalty owing to mismatch between powerrequested and
power provided. The other scenarios represent different
combinations ofconsidering planning and real time adjustment. The
results in the table highlight the fact thatconsidering potential
string failures in the planning stage and performing real time
adjustmentenhanced revenue from regulation service. With both
planning and real time adjustment, therevenue is enhanced by the
highest amount ($27 to $240) as one would expect.
CHALLENGES AND NEXT STEPS1. Stakeholders from the industry will
need to
consider the planning and real-time adjustment upgrades in order
to extract the highest economic benefits.
2. Most ESS use off-the-shelf control systems to determine duty
cycles. It may not be easy to integrate duty cycle adjustment
strategies with these control systems
3. Additional costs may be incurred owing to string failure and
battery maintenance
