Transcript Presentation

Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Temperature Control
(in situ)
The temperature of VRLAB changes during operation. Heat
is evolved due to the presence of a small but greater than
zero ohmic resistance of the cell. In addition, heat can be
generated or consumed by the electrochemical and chemical
reactions taking place during discharge and charge.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
VRLA batteries evolve intensively HEAT during charge. In addition to the charge
processes in flooded lead-acid batteries, in VR extra heat is evolved due to the
oxygen recombination reaction proceeding on the negative plate. This exothermic
process is an intensive heat source: 68.32 kcal/mol. The negative plate is a good
thermal conductor and it dissipates actively heat. The electrolyte, which is a water
solution, has a large heat capacity. When the temperature of the positive plate
increases, the rate of oxygen evolution increases rapidly and a bigger portion of
oxygen recombines at the negative plate, giving rise of a further temperature rise
there. The temperature can easily exceed 100oC, the electrolyte starts boiling over
107oC, the polymer container becomes plastic and is destroyed – thermal runaway
effect.
The cell temperature can be monitored during cycling and the temperature
distribution across the plate surface can be monitored by IR thermometers.
When a VRLAB operates at elevated temperatures, the following processes are
accelerated:
gassing rate/water losses, capacity, corrosion rate, PAM degradation.
The capacity increases.
In the same time the thermal runaway danger increases, and cycle life and
recombination efficiency decrease.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Temperature changes in VRLAB under operation
VRLA batteries evolve HEAT during operation (charge / discharge) :
Discharge:
Qd = DQdECR + DQdJ
Charge:
Qch = DQchECR + DQchH2 / O2 evol + DQchCOC + DQchJ
Oxygen recombination: main contribution (exothermic – 68.32 kcal/mol).
 negative plates: good thermal conductors
 electrolyte: large heat capacity (water solution).
 cell temperature at the end of charge: > 80-90 oC.
temperature distribution along plate surface:
 a set of thermosensors
 radiation thermal detectors
When a VRLAB operates at elevated temperatures:
increases: gassing rate / water losses, capacity, corrosion rate, PAM degradation,
thermal runaway danger
decreases: cycle life, oxygen recombination efficiency
Source: Energy balance of the closed oxygen cycle and processes causing thermal runaway in valve-regulated lead-acid batteries,
D. Pavlov, Journal of Power sources 64 (1997) 131.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Calorimetric studies of VRLAB
Heat rate / mW
thermoelectric sensors
To, m, Ti
heat
battery
heat sink
bath fluid
heat rate for C/1 discharge
el. energy
D
Ch
heat
el. energy input
Q
To
C
0
C,
Ah
8.5
Eff, HR,
%
W
96.3 7.3
heat capacity: Cp = Q / m (To – Tc)
25 10.4 96.1 7.7
VRLA (16.5 Ah) – 660 J/kg.oC
NiMH (20 Ah) – 677 J/kg.oC
NiMH (6.5 Ah) – 521 J/kg.oC
Li ion 6 Ah –
795 J/kg.oC
Li polymer 4 Ah – 1012 J/kg.oC
45 13.1 98.8 2.4
heat rate: heat generated (HG) / cycle time
Energy efficiency: 1 – HG / EEI (EEO)
Source: A. Pesaran and M. Keyser, Thermal Characteristics of Selected EV and HEV Batteries, 16-th annual battery conference,
Long Beach, California, USA, January 9-12, 2001.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Temperature monitoring of UPS VRLAB during operation
Cell temperature evolution
during cycling:
A – rest
B – discharge
C – CC charge
D – CV charge
I10
3 I10
+ 7 I10
U=const
Maximum cell temperature
during CC charge as a function
of current
Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Temperature mapping of UPS VRLAB
Thermal imaging: ABS essentially transparent to the analysed infrared radiation.
116 A discharge to 1.60 VPC.
The central area of the plate is warmer than the edge parts.
The current lug is warm.
Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Temperature mapping of UPS VRLAB
Temperature changes during constant current and constant voltage charge.
CC charge – small heat evolution
CV charge – high heat evolution
Source: P. Haering, H. Giess, J. Power Sources, 95 (2001) 153.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Influence of cycling on VRLAB temperature
Cycle # 38
Cell T as a function of cycle number:
end of discharge,
end of charge
Cycle # 350
Source: http://www.ctts.nrel.gov,
M. Keyser, A. Pesaran, M. Mihalic, B.Nelson, Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles,
Presented at the 17th Electric Vehicle Symposium, Montreal. Canada, October 16-18, 2000.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Local shorts cause local temperature rise
Source: http://www.ctts.nrel.gov,
Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles, M. Keyser, A. Pesaran, M. Mihalic, B.Nelson.
Presented at the 17th Electric Vehicle Symposium, Montreal. Canada, October 16-18, 2000.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
VRLAB Complex Monitoring
The enhanced heat evolution and the operation of a closed oxygen recombination
cycle in VRLAB are the reason for developing a new complex monitoring method
for these batteries.
The current and voltage are monitored along with cell temperature and gassing
rate.
The next page presents 3 graphs:
The first one shows the current and temperature changes with time. The
temperature increases at the end of the charge, when oxygen evolution and
recombination become intensive.
In the second graph the gassing rate is plotted as a function of time along with the
current. This is possible using a gassing rate monitoring system. It comprises two
glass crucibles filled with water and a photoelectric counter. The gas leaving the cell
forms bubbles in the crucibles. The bubbles are counted by the photo electric
detector and the passing gas volume is calculated. The chemical composition of the
gas can also be monitored.
In the third graph the potentials of the positive and negative plates are shown as a
function of time. The voltage is constant, but the electrode potentials vary due to the
electrochemical reactions taking place there. Both potentials decrease with the
temperature rise. The negative plate is additively depolarised by oxygen
recombination.
Source: D. Pavlov, B. Monahov and A. Kirchev, to be published
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
VRLAB complex monitoring
95
85
75
4
65
55
2
45
8:00
9:00
10:00
Total Time
11:00
12:00
current
U = 2.65 V
Photo
detector
Current / A
30
4
15
2
gassing rate
7:00
8:00
1,5
U = 2.65 V
9:00
10:00
Total Time
11:00
0
12:00
-1,4
pos. plate potential
1,3
-1,2
1,2
-1,1
1,1
-1
neg. plate potential
1
6:00
Patm
to PC
Light
source
s
Photo detector
H2O
gas bubble
gassing rate, volume and chemical
composition of the evolved gas
-1,3
PP potential / V
1,4
-0,9
7:00
8:00
9:00 10:00 11:00 12:00
Total Time
NP potential / V
0
6:00
Pcell
Gas from
the cell
25
7:00
Polarization parameters (voltage
and current) and temperature
35
temperature
0
6:00
6
T / oC
Current / A
current
dN / dt
6
U = 2.65 V
Potential of the positive and
negative plates
Source: D. Pavlov, B. Monahov and A. Kirchev, to be published