Standard Test Method for Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products

Importancia y uso:

5.1 Materials used in building envelopes to enhance energy efficiency, including PCM products used for thermal insulation, thermal control, and thermal storage, are subjected to transient thermal environments, including transient or cyclic boundary temperature conditions. This test method is intended to enable meaningful PCM product classification, as steady-state thermal conductivity alone is not sufficient to characterize PCMs.

Note 3: This test method defines a dynamic test protocol for complex products or composites containing PCMs. Due to the macroscopic structure of these products or composites, conventional measurements using a Differential Scanning Calorimeter (DSC) as specified in E793 and E967, which use very small specimens, are not necessarily representative of the relationship between temperature and enthalpy of full-scale PCM products due to the specimen size limitation.

5.2 Dynamic measurements of the thermal performance of PCM products shall only be performed by qualified personnel with understanding of heat transfer and error propagation. Familiarity with the configuration of both the apparatus and the product is necessary.

5.3 This test method focuses on testing PCM products used in engineering applications, including in building envelopes to enhance the thermal performance of insulation systems.

5.3.1 Applications of PCM in building envelopes take multiple forms, including: dispersed in, or otherwise combined with, a thermal insulation material; a separate object implemented in the building envelope as boards or membranes containing concentrated PCM that operates in conjunction with a thermal insulation material. Both of these forms enhance the performance of the structure when exposed to dynamic, that is, fluctuating, boundary temperature conditions.

5.3.2 PCMs can be studied in a variety of forms: as the original “pure” PCM; as a composite containing PCM and other embedded materials to enhance thermal performance; as a product containing PCM or composite (such as micro- or macro-encapsulated PCM); or as a system, comprising arrays or assemblies of PCM products.

5.4 This test method describes a method of using a heat flow meter apparatus to determine key properties of PCM products, which are listed below. Engineers, architects, modelers, and others require these properties to accurately predict the in-situ performance of the products (2).

5.5 The objective is generally to conduct a test under temperature conditions that will induce a phase transition (for example, melting or freezing) in the PCM product during the course of the test.

5.6 Determination of thermal storage properties is the objective of this test method, and key properties of interest include the following:

5.6.1 PCM Active Range, that is the temperature interval over which the phase transitions occur, for both melting and freezing of the PCM product or composites containing PCMs.

5.6.2 Specific heat of the fully melted and fully frozen product, defined outside the PCM Active Range.

5.6.3 Enthalpy as a function of temperature, h(T).

5.6.4 Enthalpy plot—a histogram or table that indicates the change in enthalpy associated with incremental temperature changes that span the tested temperature range.

5.6.5 Enthalpy changes associated with phase transitions during the PCM melting and freezing processes in materials and composites containing PCMs.

5.7 PCM products often possess characteristics that complicate measurement and analysis of phase transitions during a test. Following are some of the known issues with PCMs:

5.7.1 Imprecise PCM Active Range—PCMs in general do not have precise melting or freezing temperatures, and the entire active temperature range, from the beginning to the end of phase transitions, must be determined.

Note 4: The onset of freezing will not necessarily coincide with the end of melting. Therefore, the freeze and melt enthalpy curves must be independently defined to determine the PCM Active Range.

5.7.2 Multiple Phase Transitions—Many PCMs exhibit a solid-solid transition with significant latent heat effects at temperatures near the melting transition.

5.7.3 Sub-cooling—Occurs when the specimen cools below its nominal freezing temperature before it actually begins to freeze, thus exhibiting an unusual enthalpy-temperature curve. Solid-liquid and solid-solid phase changes are often dependent on heating and cooling rate.

5.7.4 Hysteresis—Occurs when a specimen heated from one temperature to another, and then returned to the original temperature, absorbs more (or less) heat at any particular temperature during the heating stage than it releases during cooling.

5.8 The properties measured are determined by fundamental thermophysical properties of the constituent materials of the product, and are thus inherent to the PCM product. The desired thermal performance enhancement, however, will depend strongly on the particular environment, climate, and mode of the actual engineering application of the PCM.

Subcomité:

C16.30

Volúmen:

04.06

Número ICS:

17.200.10 (Heat. Calorimetry)

Palabras clave:

energy storage; latent heat; phase change material;