Battery Thermal Management System
Battery Thermal Management System And cooling methods
SUBMITTED BY: - ME-A B3 Grp-2
Upendra Dani (55)
Vishwajeet Desai (64)
Nivedita Deshmukh (66)
Swanand Deshmukh (69)
Saurabh Dhirde (76)
UNDER THE GUIDANCE OF:
PROF. (DR.) D.B Hulwan
Battery Thermal Management System
Battery cooling is directly
proportional to the heat generated inside them; it is critical to understand
where the heat is coming from. A thermodynamic energy balance was used to
generate a formula for the heat generated within a battery. He considers four
processes that have an impact on this balance. The first is the electrical
power generated within the battery, and the second is reversible reactions and
the entropic heating that results from them. The reaction in a typical
Lithium-ion battery is shown below. The square represents the lithium-ion
battery's empty site [4].
πΏππΆππ2
+ ∎
πΆ6
⇆
∎πΆππ2
+ πΏππΆ6
The third
process is the heat produced by the mixing as the concentration of the battery
changes as the reaction progresses. The heat dissipated from the phase changes
of the materials is the final process in the energy balance. In most
publications, the Bernoulli equation is simplified and presented as:
π
= πΌ(π
− π)
− πΌ(π
ππ
ππ)
The heat of mixing and phase
change are ignored in this phrase. The first term represents the overpotentials
and ohmic losses that occur during charge transfer at the interface. The second
term is the reaction's reversible entropic heat.
Heat Generated and C-rate.
C-rate is a relative measure of
battery performance. For example, a C-rate of one indicates that the cell is
operating at full capacity, a C-rate of 0.5 indicates that the cell is
operating at half capacity, and so on.
As the amount of current drawn
from the battery increases, C-rate increases proportionally to the heat
generated in the battery. As Q (Irreversible)= I^2R, it generates more heat,
with an exponential increase as current drawn increases. There is also a negative
relationship between C- Rate and temperature. Lowering the temperature of the
cell increases its internal resistance, preventing it from operating at its
maximum C-Rate and resulting in performance loss.
Thermal management impact on
battery performance
The performance of a lithium-ion cell is highly
dependent on its temperature. The optimal operating temperature for lithium
batteries is 15-35 °C. Operators outside of this range
will have a negative impact on battery performance and lifetime. The main
consequences of incorrect battery temperatures are discussed here.
1.
Degrading performance
High cell temperatures increase the
internal resistance of the cell, reducing output power. Furthermore, higher
temperatures will result in a greater loss of cycle performance. Cycle loss is
the loss of cell capacity when it is cycled (e.g., charged then discharged). In
comparison, cells that operate at higher temperatures have a greater capacity
loss after each cycle.
2.
Temperature distribution
More heat will be generated in battery packs as
their size and charge/discharge rate increase. If this heat is not properly
dissipated, it will build up inside the battery packs. Furthermore, convective
heat transfer is greater at the pack's outer surfaces. As a result, the
temperature distribution inside the battery packs will be uneven. As previously
discussed, the performance of a cell is highly dependent on its temperature. This
means that temperature inequity will result in capacity variability between
cells. This will result in a vicious cycle in which the cells with the proper
temperature must deliver more power to compensate for the low performing cells,
which will result in an increase in cell temperature. Furthermore, because
lithium cells have a low tolerance for overcharging, the overall charging
capacity of a battery pack is limited to the lowest performing cell.
3.
Thermal Runaway
When the temperature of the cell exceeds a
certain threshold, a series of undesirable exothermic reactions occur, raising
the temperature even higher. This chain reaction will continue, resulting in a
phenomenon known as thermal runaway. If a thermal runaway is not properly
managed, the large amount of heat and gas produced can result in fire and
explosion. Thermal runaway can happen for a variety of reasons, including high
temperature, overcharge, short-circuiting, and nail penetration. The focus of
this review has been on thermally caused incidents. At around 90oC, thermal runaway begins.
Thermal Analysis Performed-
We Performed an analysis on
30 Li-ion Samsung 18650 cells at ambient temperature 25 deg C, and heat disceptation
rate of 3.375 joules/sec (Referred from a research article.), and inlet air
speed of a cooling fan i.e. 4m/s from fan data sheet. And at outlet keeping
atmospheric pressure. The geometry selected is like a tube structure with inlet
and exit on either sides with cells with 2.5 mm spacing in between them for air
passage.
Battery thermal management
system (BTMS)
Inappropriate
battery temperature has a negative impact on battery performance, lifetime, and
safety. As a result, a BTMS is required for each battery system. A BTMS's
primary responsibility is to keep the batteries in the optimal temperature
range and to maintain an even temperature distribution in the battery pack. Following
that, other factors such as weight, size, reliability, and cost must be
considered based on the application of the battery packs.
The battery is the most important component of
EVs, as it goes through several charge and discharge cycles in an external
environment over the course of its life. Because the performance of lithium-ion
batteries is closely related to temperature, it is critical to understand how
heat is generated inside the battery. Heat generation within Li-ion batteries
is a complex process that necessitates an understanding of how the rate of
electrochemical reaction varies with time and temperature, as well as how
current flows in the battery.
Liquid
Cooling System
Because liquid cooling has a higher
heat conductivity and heat capacity, it performs very well. It has its own
advantages, such as ease of assembly and a compact structure. Liquid cooling
aids in keeping the battery pack at the proper temperature. Increases in
coolant flow rate or plate thickness can improve average temperature and
temperature uniformity. Any fluid's cooling performance is determined by its
thermal conductivity and viscosity. Water has the highest specific heat, but it
cannot be used alone and must be mixed with glycol. Glycol and water mixture is
a cheap and well-known cooling fluid. This mixture contains 50% glycol, 45%
water, and 5% additives, which may include antifreeze, corrosion inhibitor,
dye, and antioxidant. Glycol has a high
specific heat capacity and excellent heat transfer properties. Indirect cooling
is provided by water-glycol systems. The glycol is pumped through the pipes
that surround the battery. The supply pumps are used to deliver this
water-glycol mixture. Heat transfer is achieved in BTMS using liquid cooling by
installing discrete tubing around the battery cells with a jacket around the
battery cells that places the heated liquid or cooled plate to the battery cell
surface or by submerging the cells in a dielectric fluid.
There are two kinds of glycol that can
be used:
1. Ethylene Glycol (EG): This is an
antifreeze used in the cooling of automobile engines.
2. Propylene Glycol (PG): PG has the
same benefit as EG. Furthermore, PG is thought to be non-toxic.
Battery cooling is divided into two
types:
1. Passive cooling
2. Active cooling based on the control
strategies.
In passive cooling, the coolant is
cooled using air via a parallel flow heat exchanger, whereas in active cooling,
the coolant is cooled using refrigerant via an internal heat exchanger. A
radiator serves as the heat sink for cooling in passive cooling. The heat
transfer fluid is circulated by the pumps within a closed system in passive
liquid cooling. The circulating fluid will absorb heat from the battery pack
and dissipate it through the radiator.
Fig.No 10: Active liquid cooling system
There are two loops in active cooling.
The lower loop is referred to as the secondary loop, while the upper loop is
referred to as the primary loop. The primary loop is analogous to the loop in a
passive cooling system, in which the heat transfer fluid is circulated by a
pump. The air conditioning loop is the secondary loop in active cooling. In
this case, instead of being a radiator, the upper heat exchanger acts as an
evaporator for cooling and connects both loops. When the heating operation
begins, the 4-way valve is activated, and the upper heat exchanger begins to
function as a condenser, while the lower heat exchanger functions as an
evaporator.
[1]
Indirect liquid cooling (TESLA & Chevrolet)
Figure 11-Tesla and Chevrolet cooling
Water is used as one of the most
efficient coolants in a variety of industrial applications. The main issue with
directly cooling batteries with water is the potential for short-circuiting. To
prevent electrical conduction with the cells while maintaining high thermal
conductivities, indirect methods are used. Adding an electrical resistance will
also increase thermal resistance, but if controlled, it will have little effect
on cooling. In their vehicles, EV manufacturers such as GM and Tesla use
indirect cooling. Figure 7 shows how GM uses cold plates between each prismatic
cell. The cold plates are thin and have
several microchannels running through them. Figure 5 shows Tesla's use of wavy
tubes running between cylindrical cells. To fill the space between the cells
and cooling channels, a thermally conductive but electrically isolating
material was used. Although the wavy tubes appear ineffective due to the small
heat transfer contact area, they are safer mechanically and electrically. All
coolant connections are made outside of the battery enclosure, removing
potential leak points.
[2]
Direct liquid cooling (Immersion) (Koenigsegg’s Regera and Aston Martin’s
Valkyrie):
Direct cooling, also known as
immersion cooling, cools the cell uniformly across its entire surface. This
reduces hot/cold spots in the cell and improves cell performance. A dielectric
coolant with low viscosity and high thermal conductivity and thermal capacity
should be used for direct cooling. Immersion cooling is becoming more popular
in data centre servers and power electronics. The use of immersion for BTMS is
still uncommon in the mass-produced EV market. This is most likely due to
financial and safety concerns. As of today, immersion cooling for batteries has
only been used for concept high-performance EVs and EV racing, according to the
authors. 3M (Minnesota Mining and
Manufacturing Company) manufactures dielectric liquid for cooling purposes. 3M
Novec 7200 Engineered Fluid, for example, is used by Xing Mobility for Electric
Racing. The fluid has a boiling point of 76oC, which keeps the cells from
overheating and causing thermal runaway. For the cells that will be submerged
in liquid, they use modular containers.
Rimac, a manufacturer of electric
hyper cars, is another company that uses immersion cooling technology. For
extreme conditions, they partially submerge the cells in liquid. Other car
manufacturers, including Koenigsegg's Regera and Aston Martin's Valkyrie, use
their battery pack technology. As of now, immersion cooling for BTMS has been
used only for concept high-performance EVs due to their high-power
requirements, which can only be met by cooling. More improvements in leakage
proofing the battery pack and lowering the cost of the dielectric liquids are
required for this method to reach the mass market.
Figure 12-Xing battery module with immersion cooling
Air cooling (Toyota
Prius and Nissan Leaf)-
Air is the
most common method of cooling and has been widely used in a variety of
industries. Air may not appear to be a good cooling medium due to its low heat
capacity and thermal conductivity. However, because of its simplicity and low
cost, it remains an appealing cooling solution. Toyota Prius and Nissan Leaf
are two well-known examples. Toyota
Prius Air Cooled Battery Pack Cooling can be accomplished using both natural
convection (passive cooling) and forced convection (Active cooling). Natural convection
is only appropriate for low-density batteries, and blowers/fans are typically
used to increase the convection coefficient.
Figure 13-Toyota Prius battery
As a result, cell temperatures at
the pack outlet rise, resulting in an uneven temperature distribution. As a
result, extra precautions must be taken to ensure uniformity, such as
increasing the coolant medium speed, creating turbulence in the flow, and
optimising the positioning of each cell. Wang et al. investigated various
cylindrical cell arrangements and fan positioning. The best cooling performance
was found when the fan was placed on top of the module, and the most desired
arrangement in terms of cooling effect and cost was when the cells were
arranged side by side in a square pattern.
Phase Change Materials (PCM):
PCM is a material that
accumulates or emits heat by transitioning from one state to another at a
specific temperature. This has been widely used in many industries, including
civil and energy engineering, because it can absorb and release large amounts
of heat through phase change processes while consuming no energy. As the energy
density and load of batteries have increased in recent years, BTMS has used a
powerful cooling system that employs numerous channels of liquid cooling loops.
However, such a system has the disadvantage of increasing the system's
complexity and the compressor's power consumption. The PCM cooling system is
one way to mitigate these disadvantages; the cells are directly attached to the
PCM, which is generally a solid material block machined or moulded so that the
cells can be inserted. There are also two plates on the top and bottom, or
right and left, sides of the PCM to dissipate the heat absorbed by the PCM.
Heat is generated in the cells when the battery is charged or discharged, and this
heat is transferred to the PCM, which is in direct contact with the cells due
to the conduction phenomenon depending on the temperature difference absorbs
heat as sensible heat first, then as latent heat until the end of the phase
change process at a constant temperature, when it finally reaches the melting
point as the temperature gradually rises. This means that it can withstand the
battery's high thermal load without experiencing abnormal temperature rises or
significant temperature unevenness.
When only PCM is used as BTMS,
however, it is difficult to operate continuously if the PCM is completely
melted due to hot weather or continuous charge/discharge cycles of the battery.
As a result, additional cooling systems that dissipate the heat generated by
the PCM are critical. Although increasing the PCM mass delays the phase change
completion time, it will not be able to reduce energy consumption, which is
important when using PCM, because it increases the weight and reduces EV
performance. As a result, the proper PCM mass must be determined. Another
important consideration in the application of PCM to BTMS is the selection of
the appropriate PCM. Within the battery's operating range, the proper PCM
should have high latent heat, high heat capacity, high thermal conductivity,
and phase change temperature. Furthermore, the PCM should be chemically stable
and nontoxic, with little to no sub-cooling effect during the freezing process.
Very detailed information..!!
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