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).

No abstract

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Accurate knowledge of the enthalpy as a function of temperature, or the storage capacity at each temperature, is the key to design any application. Conventional methods for thermal analysis however often lack sufficient accuracy or sample size to be applied to PCM. The T-history method is a simple method to determine the storage capacity of PCM and allows the use of large sample sizes. The experimental setup and methods of data analysis have been significantly improved in recent years. In this paper, a proper methodology to verify the correct setup and data analysis method of a T-history installation using standard materials with known properties is described and tested. The implementation of the T-history method has been done at the ZAE-Bayern. Three standard materials, gallium, water and hexadecane, were measured, as well as two commercial PCM, RT27 and sodium acetate trihydrate graphite compound (SAT+G). The obtained results confirm that the T-history installation can be used to analyse different PCM.

SUMMARYThermal energy storage by latent heat allows storing high amounts of energy working in narrow margins of temperature. The use of phase change material (PCM) for the latent heat storage has been studied in different applications and it has been commercialized in containers to transport blood, products sensible to temperature, to decrease their energy demand. The use of PCM in cooling and refrigeration has been attracting a lot of interest lately, but for all applications, the properties of these materials need to be known with sufficient accuracy. Regarding heat storage, it is necessary to know the enthalpy as a function of temperature. The most widely used calorimeter is the heatflux differential scanning calorimetry (hf-DSC). The objective of this study is to investigate different methods for hf-DSC analysis, namely the dynamic method and the step method, and to test their accuracy in the determination of enthalpy-temperature relationship of PCM. For the dynamic method, a strong influence of heating/cooling rate was observed. For the step method, the resulting enthalpy-temperature relationship is independent of heating/cooling rate. Commercial PCM RT27 was chosen as sample material to avoid subcooling and kinetic effects in the test measurements. The approach introduced in this study can be used to carry out similar investigations for other classes of PCM and/or other DSC instruments.

No abstract