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Energy Storage using Microencapsulated Phase Change Materials
James Howard and Patrick Walsh
Stokes Research Institute, University of Limerick, Ireland
james.howard@ul.ie
Abstract
This paper investigates laminar heat transfer
characteristic of two-phase microencapsulated phase
change material (MPCM) suspension flows within minichannels
under a constant wall heat flux boundary.
Capsules containing paraffin are examined and found
to be well suited for electronics cooling applications
using liquid cold plate technologies. In particular, it is
shown that the large thermal capacity of MPCM
slurries around the phase change temperature can lead
towards greater isothermality of isoflux systems, a
characteristic of significant interest to
telecommunication, laser and biomedical applications.
Introduction
Phase Change Materials (PCMs) have been widely
used in thermal energy storage applications. They offer
the ability to absorb and store large quantities of
thermal energy through the endothermic melt process.
The high latent heat of fusion and considerable energy
densities associated with such materials have made
them very desirable and these thermal properties have
led to many direct applications in a range of fields.
Investigations have found important uses in solar
energy storage to reduce peak demands on systems, El-
Sebaii et al. [1] and subsequent incorporation in
numerous building materials, such as wallboard and
under floor heating, Farid et al. [2] However,
significant potential lies in the field of MPCM slurry
flows, allowing store energy to be easily transported.
Thermo Physical Properties
The primary aspect of the MPCM slurry is the latent
heat of fusion associated with the phase change process.
This can be quantified using a DSC and for simplified
modeling, the latent heat is absorbed as a specific heat
capacity enhancement. This results in a step change in
the apparent heat capacity over the melt temperature
range as demonstrated in Figure 1.
Figure 1: Modified enthalpy model over phase change
region
Analytical Model
23
Heat transfer for single phase flows in circular
geometries is described using the Graetz solution. This
is characterized by the Nusselt number which in based
on the inverse of the temperature difference between
the wall and bulk mean of the fluid. Theoretical wall
temperatures can be determined based on the modified
bulk temp from the enthalpy model shown in Figure 1.
Results
Experimental wall temperatures determined from the
Infrared Thermography are plotted versus the
predictions using the Graetz solution. An example using
30.2% mass particle concentration is represented in
Figure 2. Further tests were carried out investigating
various scenarios, i.e. constant heat flux or constant
flow rate. Different particle concentrations were also
investigated, from 5.03 – 30.2%.
Temperature [°C]
64
59
54
49
44
39
34
29
24
19
Twall (mean) Twall (theory) Tfluid (theory) T1 T2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
Distance from Thermal Entrance, x [m]
Figure 2: Experimental and Theoretical Wall
Temperatures plotted against distance downstream (Re
= 3.6 and q″ = 11077.8 W/m 2 )
Discussion
From the experimental conditions examined, there is
very agreement in the temperatures until the wall value
reaches the onset melt temperature. Downstream of this
experimental values deviate from predictions, resulting
in and augmentation followed by a degradation in heat
transfer. This is before the temperatures collapse once
more, when the particles have changed from solid to
liquid phase. Compared with single phase flows,
MPCM slurries can provide temperature reductions up
to 33%.
References
[1] El-Sebaii, AA, Al-Ghamdi, AA, Al-Hazmi, FS, et al.,
2009, “Thermal performance of a single basin solar still with
PCM as a storage medium”, Appl Energy; 86 (7–8):1187–95
[2] Farid, M.M., Khudhair, A.M., Razack, S.A.K., Hallaj,
A.A., 2004, “A review on phase change energy storage:
materials and applications”, Energy Convers. Manage. 45
1597–1615