Download Thermal characterization of phase change material

 Thermal characterization of phase change
material using the t-history method
Thanks to its precision, reproducibility and wide acceptance by industry
and academia, differential scanning calorimetry will continue to be the
main method for characterizing and benchmarking PCMs. But the simple
and inexpensive t-history method, which uses significantly greater sample
volumes, can provide important data on supercooling.
By Luke Haun, M.E.
Phase change materials (PCMs), due to their inherent large storage capacity, have
been used across a multitude of industries for energy conservation and storage.
Applications range from cooling vests and warming blankets to temperaturecontrol packaging and thermal energy storage tanks. Choosing the correct
material for each application is necessary to ensure optimum design and
performance. The defining characteristics of an ideal material are its thermal
properties: latent heat, specific heat, thermal conductivity, and melt and
solidification temperatures.
Differential scanning calorimetry (DSC) is the leading method for determining a
phase change material's melt and solidification points. This technique uses a
small sample of the PCM, roughly 10µL to 40µL. The sample is subjected to a
temperature profile alongside a well-documented reference material, usually
water. As a constant heating rate is applied to both samples in a differential
scanning calorimeter, the voltage signals are measured to obtain a curve like the
one below. The specific heat is a simple algebraic calculation using voltage and
mass values, and the latent heat is calculated as an integration of signal over a
temperature interval.
© 2015 Entropy Solutions, Inc. 151 Cheshire Lane N., Suite 400, Plymouth, MN 55441 USA P: +952-­‐941-­‐0306 | F: +952-­‐944-­‐6893 | www.puretemp.com Several factors can influence the results of the measurement, including heating
rate and sample size. While a skilled user of the equipment is able to decipher the
effects of heating rates, the effect of sample size produces some inaccuracies that
prove to be problematic when designing a system on a larger scale. The onset of
solidification (nucleation) of a material starts at a single point and then
propagates until the entire sample is solid, assuming the ambient temperature is
lower than the solidification temperature. Due to the extremely small sample
size, many materials experience a degree of supercooling; that is, the material is
capable of temporarily being in a liquid state at a temperature lower than its
normal solidification temperature. Solidification is a statistical process governed
by the volume of the material; the smaller the volume, the lesser the chance of
solidification. Therefore the solidification temperature measured and indicated
on a DSC is often significantly lower than that experienced in applications when
larger volumes are used.
Another calorimetric method is useful in such situations. The “t-history method,”
developed by Zhang et al.1, uses significantly greater sample volumes, between 15
ml to 30 ml. That’s nearly 1,000 times the volume used in a DSC measurement.
The possibility of supercooling is eliminated, and all characteristics measured by
t-history will be seen in an application or process using the PCM.
© 2015 Entropy Solutions, Inc. 151 Cheshire Lane N., Suite 400, Plymouth, MN 55441 USA P: +952-­‐941-­‐0306 | F: +952-­‐944-­‐6893 | www.puretemp.com While no commercial instrument is available for t-history, the setup is extremely
simple and the materials are readily available in most laboratories, at far less
than the cost of a $250,000 differential scanning calorimeter. The PCM and the
reference material are placed in long glass tubes (to prevent axial heat flux) and
put in an environmental chamber. The temperature is monitored in the axial and
radial center. A typical t-history curve and experimental setup are shown below.
After the data has been obtained, the enthalpy curve is calculated using the
equations developed by Zhang and modified for improvements by H. Hong et
al.2 The latent heat [kJ/kg] is calculated as follows:
With sub/super scripts
© 2015 Entropy Solutions, Inc. 151 Cheshire Lane N., Suite 400, Plymouth, MN 55441 USA P: +952-­‐941-­‐0306 | F: +952-­‐944-­‐6893 | www.puretemp.com A2 and A'2 are the integrated area between the curves as indicated below. Note
that the modified t-history method takes into account the supercooling found in
many phase change materials and clearly defines the end of solidification with an
inflection point (b, in the first graphic).
Several laboratories have compared enthalpy values between DSC and t-history
and have found that the results coincide well. H. Hong et al.2 found that the heat
of fusion measured by t-history was within 4 percent of that measured by DSC.
DSC will continue to be the main method for characterizing and benchmarking
PCMs, thanks to its precision, reproducibility and widespread acceptance by
industry and academia. For many engineering applications, however, the degree
of supercooling must be well documented with a realistic volume. Collecting
more calorimetric data via the t-history test can be highly beneficial for designing
and augmenting systems that use phase change materials.
About the author
Luke Haun graduated from University of Minnesota’s Institute of Technology
with a B.S. in biomedical engineering, specializing in biotransport phenomena
and heat transfer. After joining Entropy Solutions, he began working with phase
change material and helped design the award-winning Greenbox temperaturecontrol shipping system. He is now focused on application engineering and
research for the thermal energy storage market.
© 2015 Entropy Solutions, Inc. 151 Cheshire Lane N., Suite 400, Plymouth, MN 55441 USA P: +952-­‐941-­‐0306 | F: +952-­‐944-­‐6893 | www.puretemp.com References
Yinping, Zhang, and Jiang Yi. "A simple method, the-history method, of
determining the heat of fusion, specific heat and thermal conductivity of phasechange materials." Measurement Science and Technology 10.3 (1999): 201.
1
Hong, Hiki, Sun Kuk Kim, and Yong-Shik Kim. "Accuracy improvement of Thistory method for measuring heat of fusion of various materials." International
Journal of Refrigeration 27.4 (2004): 360-366.
2
© 2015 Entropy Solutions, Inc. 151 Cheshire Lane N., Suite 400, Plymouth, MN 55441 USA P: +952-­‐941-­‐0306 | F: +952-­‐944-­‐6893 | www.puretemp.com