Electric vehicles rely on electrochemical energy storage, typically
a Li-ion battery, for propulsion instead of on-board combustion
of liquid or gasified fuels. Due primarily to the high cost of
Li-ion batteries and limited driving range, electric vehicles still represent
a small portion of the transportation sectorElectric vehicles rely on electrochemical energy storage, typically
a Li-ion battery, for propulsion instead of on-board combustion
of liquid or gasified fuels. Due primarily to the high cost of
Li-ion batteries and limited driving range, electric vehicles still represent
a small portion of the transportation sector. Large amounts
of research efforts are being made to reduce costs and improve
energy density of electrochemical storage. Alternatively, if energy
demands on the battery were reduced, an extended driving range
could be obtained with the same battery.
It has been suggested that as high as 40% of the on-board energy
may be required for heating and cooling the cabin of the car in extreme
climates [1]. We are currently developing a low cost heating
and cooling system that provides climate conditioning of the vehicle’s
cabin through stored thermal energy rather than energy from
the Li-ion battery. The envisioned system is comprised of two separate
thermal storage units, which we refer to as the cold and hot
thermal batteries, each containing a phase change material (PCM)
capable of storing thermal energy. The need for high energy density
thermal batteries for various applications has been recentlydiscussed [2]. In our implementation, a heat transfer fluid (HTF) is
circulated between the cabin air heat exchanger and either battery
depending on operation mode. Heating is achieved through
solidification of a high temperature PCM and cooling through the
melting of a low temperature PCM. Each battery can be recharged
by circulating the HTF through an off-board charge station that
provides or extracts the heat needed for reversing the phase
change process within the batteries. Ideally, the charge station
would be designed to ensure a thermal battery charge time less
than that of the Li-ion battery charge time.
Phase change thermal storage offers effective energy density
that greatly exceeds what can be achieved by sensible storage. Latent
heats for PCMs such as erythritol (340 kJ/kg) and water
(334 kJ/kg) are comparable to the lower end of Li-ion battery energy
densities (430 kJ/kg) [3] and with application appropriate melting
temperatures, 117 C and 0 C, respectively. Sensible heat will also
add to the effective thermal energy density during the phase change
process, as significant thermal gradients form within the PCM.
When comparing thermal to electrochemical energy, one must also
consider a conversion efficiency from electrochemical to thermal,
i.e., the coefficient of performance, which will be approximately 1
for Joule heating and 1.25–2.25 for cooling [1]. Another consideration
is that electrochemical energy may be used to accomplish
either cooling or heating, which means the energy density of a given
thermal battery is effectively halved in comparison, if both hot and
cold units are on-board. On the other hand, several components are
no longer needed for the thermal battery system, e.g., the compressor,
and are not included in comparison to the Li-ion energy density.
In general, the larger the driving temperature potential between the
desired cabin temperature and the PCM melting temperature, the
less total heat transfer area, and ultimately battery volume, is
needed. However, the choice of melting temperature must be balanced
with charging and insulation considerations.
A potential issue to be considered with the use of thermal battery
air conditioning for electric vehicles is the additional weight
and volume from incorporation of the thermal batteries, which will
be partially offset by the elimination of existing heating,
ventilation, and air conditioning (HVAC) components, e.g., the
compressor, condenser, etc. Furthermore, the freed Li-ion energy
also represents a particular weight and volume that must be accounted
for. Accordingly, with both batteries on-board, we estimate
the net increase in volume and weight to be less than that
of a single thermal battery. A study investigating battery weight
impact on fuel economy found no impact on efficiency when adding
50 kg to a compact vehicle weighing 1516 kg [4]. We cannot
foresee the need for a single battery to exceed 50 kg, which would
roughly translate to 5 kW-h of stored energy, so we conclude that
the additional weight from thermal battery incorporation will not
impact fuel economy. Any net additional volume is a drawback,
although the impact is hard to quantify. Qualitatively, this analysis
suggests that volumetric energy density is potentially more critical
than gravimetric for a compact electric vehicle application. Alternatively,
hot and cold batteries could be swapped at the change
of seasons to essentially eliminate any net volume or weight increase
when accounting for the freed Li-ion energy.
The proposed phase change thermal storage system has the following
key advantages in comparison to traditional HVAC for electric
vehicles:
Heating and cooling loads are removed from the Li-ion battery.
Candidate PCMs, e.g. water and sugar alcohols, are safe and
more widely available at lower cost relative to Li-ion battery
materials.
To develop a model for the vehicular HVAC system proposed,
each battery is considered to be a shell and tube heat exchanger.
The HTF flows through a number of parallel tubes, which are surrounded
by PCM within the larger insulated shell. The problem is
complex due to the moving boundary problem within the PCM
coupled to thermally developing forced convection within the
tube. Such a unit has been studied extensively primarily for solar
water heating applications [5–17] and discussed in recent reviews
[18–21].
The most common approach to modeling is 2D (axisymmetric)
numerical solution to the heat equation with a phase change
model, e.g. the enthalpy method [22], coupled with solution to
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