Enthalpy of Phase Change Materials as a Function of Temperature: Required Accuracy and Suitable Measurement Methods

Abstract

PREFACE Materials used for thermal insulation and storage, along with other construction and building envelope components, are subjected to transient thermal conditions which can include dynamically changing temperature, moisture content, surface heat transfer, specific heat, etc. In addition, most building design and energy-related standards are based on a steady-state criterion (R-values using the apparent thermal conductivity measurements). This mismatch between the steady-state principles used in design and code requirements and the dynamic operation of buildings can result in lower thermal efficiency than achievable or higher cost (due to addition of more insulation than required). This mismatch can also lead to a gross underestimation of the performance of materials that store energy under cyclic temperature conditions, for example phase change materials (PCM). Although some experimental methods for transient analysis of building envelopes have been developed, there are no standardized testing procedures available to quantitatively characterize materials and systems under dynamic conditions. Data on dynamic material characteristics are needed to improve thermal design and analysis, whole-building simulations, and energy code-related work. This led to the development of a proposed ASTM Standard Test Method for characterizing PCM products under dynamic conditions. A series of measurements are needed to determine the enthalpy storage of a test specimen over a temperature range. First, both HFMA plates are held at the same constant temperature until steady state is achieved. Steady state is defined by the reduction in the amount of energy entering the specimen from both plates, or vice-versa, to a very small and nearly constant value. Next, both plate temperatures are changed by identical amounts and held at the new temperature until steady state is again achieved. The enthalpy absorbed or released by the specimen from the time of the temperature change until steady state is reached at the new plate temperatures is recorded. Using a series of temperature step changes of 1.5 ± 0.5°C, the cumulative enthalpy stored or released over a certain temperature range is determined. The specific heats of the solid and liquid phases are determined from the slope of the sensible enthalpy storage as a function of temperature, above and below the phase change temperature range. The proposed test method requires the measurement temperature range to begin at least 10°C below the phase change temperature range and continue till 10°C above. Since the phase change temperatures may not be known a priori, preliminary tests with coarser temperature step changes are useful in estimating the required measurement temperature range. Complete details of the test method can be found in the draft standard test method document (attached as Appendix A).

Review of solid–liquid phase change materials and their encapsulation technologies

“…Eva Günther et al [94] compared the T-history method with two types of DSC measurement modes: dynamic mode and isothermal step mode. They found out that the isothermal step mode offers higher precision than dynamic mode, but the T-history method is more suitable for heterogeneous PCMs.…”

A new calorimetric technique for phase change materials and its application to alkane-based PCMs

“…Whereas DSC experiments typically are performed at 10 K min À1 (although lower rates are recommended for experiments with PCMs [1,16,17,40]), ASC experiments are typically performed at a few K h À1 away from the transitions. But this disadvantage is offset by the higher resolution of the ASC data and the fact that a complete understanding of a sample from DSC experiments requires in principle runs at several rates.…”

“…In our view, while a DSC gives a useful first picture of a phase transition, high-quality data cannot be obtained with it. This is realised by researchers in the field of PCMs [15,16], and careful DSC use [17] and improved analysis of DSC data [18,19] have been proposed.…”

“…Among the examples, we first note the possibility of running a DSC in an isothermal (step) mode [1,16], operating much like a relaxation calorimeter [20][21][22][23]. A common method is the so-called T-history method [1,16,[24][25][26], in which a PCM sample is studied in conditions that mimic the operational use of PCMs: its response to a changing environment is recorded. A relatively large PCM sample (for example 20 ml [16] as compared to a DSC sample of few ll) is placed inside a temperature-controlled chamber together with a reference sample.…”

“…A common method is the so-called T-history method [1,16,[24][25][26], in which a PCM sample is studied in conditions that mimic the operational use of PCMs: its response to a changing environment is recorded. A relatively large PCM sample (for example 20 ml [16] as compared to a DSC sample of few ll) is placed inside a temperature-controlled chamber together with a reference sample. The temperature of the chamber is then suddenly changed, and the temperature response of sample and reference is recorded.…”