Standard Test Method for Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products
Active Standard ASTM C1784 | Developed by Subcommittee: C16.30
Significance and Use
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 and , 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 (. )
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.
1.1 This test method covers the measurement of non-steady-state heat flow into or out of a flat slab specimen to determine the stored energy (that is, enthalpy) change as a function of temperature using a heat flow meter apparatus (HFMA).
1.2 In particular, this test method is intended to measure the sensible and latent heat storage capacity for products incorporating phase-change materials (PCM).
1.2.1 The storage capacity of a PCM is well defined via four parameters: specific heats of both solid and liquid phases, phase change temperature(s) and phase change enthalpy (. )
1.3 To more accurately predict thermal performance, information about the PCM products’ performance under dynamic conditions is needed to supplement the properties (thermal conductivity) measured under steady-state conditions.
NOTE 1: This test method defines a dynamic test protocol for products or composites containing PCMs. Due to the macroscopic structure of these products or composites, small specimen sizes used in conventional Differential Scanning Calorimeter (DSC) measurements, as specified in and , are not necessarily representative of the relationship between temperature and enthalpy of full-scale PCM products.
1.4 This test method is based upon the HFMA technology used for Test Method but includes modifications for specific heat and enthalpy change measurements for PCM products as outlined in this test method.
1.5 Heat flow measurements are required at both the top and bottom HFMA plates for this test method. Therefore, this test method applies only to HFMAs that are equipped with at least one heat flux transducer on each of the two plates and that have the capability for computerized data acquisition and temperature control systems. Further, the amount of energy flowing through the transducers must be measureable at all points in time. Therefore, the transducer output shall never be saturated during a test.
1.6 This test method makes a series of measurements to determine the thermal energy 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 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 energy absorbed or released by the specimen from the time of the temperature change until steady state is again achieved will be recorded. Using a series of temperature step changes, the cumulative enthalpy stored or released over a certain temperature range is determined.
1.6.1 The specific heats of the solid and liquid phases are determined from the slope of the temperature-dependant enthalpy function during sensible heating/cooling, before and after the phase change process.
1.7 Calibration of the HFMA to determine the ‘correction factors’ for the energy stored within the plate heat flux transducers and any material placed between the test specimen and the HFMA plates must be performed following . These correction factors are functions of the beginning and ending temperatures for each step, as described in .
1.8 This test method applies to PCMs and composites, products and systems incorporating PCMs, including those with PCM dispersed in or combined with a thermal insulation material, boards or membranes containing concentrated or dispersed PCM, etc. Specific examples include solid PCM composites and products, loose blended materials incorporating PCMs, and discretely contained PCM.
1.9 This test method may be used to characterize material properties, which may or may not be representative of actual conditions of use.
1.10 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.11 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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