Distributed Generation

with High

Penetration of

Renewable Energy

Sources

www.dispower.orginfo@dispower.org

Description of Vanadium Redox Flow BatteryAt the end of 2001 CESI installed a VRB manufactured by SEI (Sumitomo Electric Industries). As the system had to be connected to the battery laboratory for testing activity, it had been installed on the terrace above the lab. As a consequence, for this particu-lar installation, SEI added two sub-tanks under the stack for collecting the electrolyte falling from the cells during non func-tioning period of the system and two sub-pumps in order to force the electrolyte from the sub-tanks to the main tanks.

maximum power the main can supply is defined; if the load needs more power is the battery that helps the main. When the power of the load is lower than that the main can supply the battery can be recharged.

In any case there is the su-pervision of the controller for battery integrity. The perform-ances of VRB would be good also for power quality systems (PQS) applications in which the battery has to supply peak power for very short time.

Result realised by:

Results: The battery is under test in CESI laboratory battery.

> Preliminary tests for its characterisation

> Tests simulating peak shaving applications

> Efficiency of the converter, of the battery and of the system

> Measurement of self discharge rate

> Behaviour of the system with ohmic and inductive load, balanced and unbalanced

> Measurement of the effect of temperature on battery performances

> Measurement of internal resistance of the battery

> Efficiency variation depending on charge/discharge management procedure

> Valuation of safety aspects

Project Highlight No.06

Stack main characteristics

Construction 100 unit cells

Rated power – energy

45 kW – 2 hours

Rated DC voltage 125 V

Rated DC current 370 A

DC voltage range 100 ÷ 155 V

Electrolyte Tanks

Material Polyethylene

Capacity 4 m3

Quantity 2

AC/DC Converter

Supply 3phase 400V, 50 Hz

Rated power

70 kVA

Efficiency 90%

Operating mode

Charge: constant current, constant power, constant voltageDischarge: constant current, constant power

Energy Storage for Distributed Generation:SUMITOMO Vanadium Redox Flow Battery (VRB)

ControllerO

per

tati

on

mo

de 1 year mode (programme the

operation for the year)

Continuous operation (routine of charge, standby, discharge, standby)

Following the load operation (charge, discharge or standby operation depending on the need of power by the loads connected to the system)

Main features of VRB are summarised in the following table:

VRB installed at CESI is part of a Peak Shaving System (PSS) with an AC/DC converter. During the “following the load” mode of operation the

VRB stack

VRB tanks

MASTHEAD

The project DISPOWER is partially

funded by the European Commis-

sion, DG Research

Duration:

01.01.2002 - 31.12.2005

Contract no.

ENK5-CT-2001-00522

Co-ordinationISET e.V.

Prof. Dr. Jürgen Schmid

Königstor 59

D-34119 Kassel

Phone: +49 561 7294-0

Fraunhofer ISE

Dr. Tim Meyer

Heidenhofstr. 2

D-79110 Freiburg

Phone: +49 761 4588-0

LiabilityThe authors are solely responsible

for this publication, it does not

represent the opinion of the

European Community and the

European Community is not

responsible for any use that

might be made of date appearing

therein. Despite thorough control

all information in this brochure is

provided without guarantee. Un-

der no circumstances will liability

be assumed for loss or damage

sustained through the use of

information provided.

AuthorsCESI: Centro Elettrotecnico

Sperimentale Italiano Giacinto

Motta S.p.A.

Contact:

CESI

Antonio Buonarota

Via Rubattino 54,

I - 20134 Milano / Italy

Phone: +39 0221255269

Fax: +39 0221255626

email: buonarota@cesi.it

Date: 2003-12-04

No: 06

Technical Details:

New systems like this under study

need to be investigate both from

the point of view of performances

and in respect to their lifetime. VRB

efficiency is strongly influenced

by ambient temperature but also

managing strategy of the battery

(i.e. charging power/current and the

definition of ending discharge and

recharge criteria) are fundamental.

Still, in a flow (circulation of electro-

lyte) battery, total efficiency is strongly

dependent by the time during which

the two main pumps are running. It

is easy to understand that in a PQS

application in which the battery has

to supply energy to the load very

rarely, in general it is a mistake to take

running the pumps all the time. But in

any case the battery must be ready in

a few milliseconds to supply energy:

that means the stack has to be always

full of electrolyte and the pumps must

start very quickly.

A careful examination has been paid

to understand the effect on perform-

ances of these aspects.

Efficiency of the system

For calculating the charge/discharge

process efficiency avoiding systematic

errors it is necessary to be sure that

the initial and final state of charge of

the battery are the same. The state

of charge of SEI Redox flow battery

is measured only through the battery

voltage: the end of charge and the

end of discharge are managed by the

controller through battery voltage.

In other words it is fixed a value of

voltage for stopping the charge proc-

ess, i.e. 150 V, and another value,

i.e. 100 V, for stopping discharge

process. These values are always the

same whatever is the power used for

recharge and that used for discharge

the battery. Thus, the energy dis-

charged or charged depends strongly

on the value of current. As a conse-

quence, the real physical final charge

and discharge states are not the same.

Also the influence of the tempera-

ture on the internal resistance of the

battery (the lower the electrolyte

temperature, the higher the internal

resistance) makes worse this aspect.

Battery efficiency had been obtained

with constant discharge power from

20 to 45 kW and constant recharge

power from 20 to 60 kW according to

10 setup implemented by the manu-

facturer in the controller.

A positive message that is possible

obtain from the tests is that it is pos-

sible reaching efficiency of about 90 %

but it is necessary to implement a

procedure for the management of the

system.CESI i working on this matter

for setting up an optimised managing

procedure of the system in order to

improve the detection of the real state

of charge of the battery, thus optimis-

ing the efficiency and the exploitation

of the battery.

During a first set of tests simulating

peak shaving applications the battery

worked “on time basis” arranging

in advance the time of starting and

ending of recharge and discharge

process. Evidently with this operating

mode it is possible to take switch off

the pumps and also to take empty the

stack in order to reduce acid corrosion

of the membrane. In this condition

it is sufficient switch on the pumps a

few minutes before starting discharge

o recharge of the battery.

A second set of tests had been

conducted during which the battery

had been managed as in a real peak

shaving application supplying to the

load the part of the power that the

network is not able to supply. With

this “following the load” mode of

operation it is not more possible to

take empty the stack and stopped the

pumps.

VRB self-discharge

Another important parameter that

influences VRB efficiency and has to

take in due consideration in defining

its managing procedure is the self-

discharge. Self-discharge is a common

phenomenon in electrochemical accu-

mulators and consists in consumption

of the energy contained in the battery

without external loads connected

to it. Self-discharge losses in VRB

are very intense especially when the

pump are running but, as already

said, in certain mode of operation, the

pumps must always run. Typical self-

discharge value for lead-acid batteries

is 3 % per month. For VRB that value

is reached and exceeded in one day.

This is another reason for switch off

the pumps all the times it is possible.

VRB pictorial diagram