We investigated five PCM-integrated building products using the DHFMA to generate valuable dynamic property data and to be able to develop a reliable general test procedure. The five products tested are described below:
PCM-enhanced Gypsum Board
This product is a 0.5-in.-thick gypsum board that is enhanced with approximately 25 wt% of a microencapsulated paraffinic PCM. A LaserComp FOX304 HFMA is employed to characterize the product. FOX200 can accommodate 12 in. × 12 in. and 4-in.-thick specimens with a test temperature range of −20 to 75 °C. In order to fit the product inside the instrument sample holder, the specimen was cut into a 12 in. × 12 in. dimension. Both melting and solidification (freezing) experiments were conducted to investigate the subcooling and hysteresis effects in the specimen. An earlier DSC test of the microencapsulated PCM showed phase active region of 16–26 °C for the PCM; therefore, a temperature range of 6–32 °C was considered for the DHFMA with temperature step size of 1.5 °C. A wider temperature range than the DSC test was necessary to capture the details of the phase transformation process in the specimen. To examine the repeatability of the DHFMA and the effect of temperature step on results, an additional melting cycle test with non-uniform temperature steps between 1 and 2.5 °C was performed.
Figure 1 depicts temperature-dependent volumetric heat capacity for two melting and one freezing tests. It can be seen that data points for melting and freezing tests seem to coincide. A negligible subcooling and hysteresis effects are observed—a common feature of paraffinic PCM and highly desired features for a PCM-enhanced building component. Onset of melting and freezing were found to occur at around 18 and 25 °C, respectively, with a broad phase active range of ~8 °C. The sensible heat capacities of the specimen were measured to be approximately 1.15 and 1.05 MJ m−3 K−1 when PCM was in solid and liquid states, respectively. The heat capacity is found to increase with temperature when PCM is solid, while it remains constant when PCM is melted. The heat capacity for the same temperature points are found to be very similar, verifying the repeatability of the DHFMA method.
The product comprised of a thin layer of PCM applied on one surface of a 3/8-in.-thick aerogel blanket. The specimen was prepared by Aspen Aerogel using a proprietary biobased PCM. A FOX304 instrument was used to perform DHFMA testing. DSC tests on pure PCM sample indicated a broad peak in the heat capacity; it was realized that a temperature resolution of less than 1 °C may be needed to accurately capture the sharp profile of the data. Sensitivity of heat flow transducers is usually compromised below temperature step sizes of around 1 °C, limiting the size of the minimum temperature step to be used at around 1 °C. To improve the temperature resolution, we developed a new method where we used steps of 1.5 °C to obtain ample sensitivity in the measurement; however, depending on the desired resolution, additional tests were run that were offset by an amount equal to step size divided by the resolution sought. For example, if a resolution of 0.5 °C was desired, three tests would be conducted (=1.5 °C /0.5 °C = 3) with first set at (T
0 + 1.5, T
0 + 3 …), second set at (T
0 + 0.5, T
0 + 2, T
0 + 3.5 …), and third test at (T
0 + 1, T
0 + 2.5, T
0 + 4 …), where T
0 is some arbitrary starting temperature set-point. Combining the data from these three datasets will essentially provide a resolution of 0.5 °C. Similarly, for a temperature resolution of 0.75 °C, two tests would be conducted (=1.5 °C /0.75 °C = 2) with first set at (T
0 + 1.5, T
0 + 3 …), and second set at (T
0 + 0.75, T
0 + 2.25, T
0 + 3.75 …).
Three melting and two solidification tests were performed to test the validity of the above proposed approach. Figure 1b depicts the profile produced by combining these tests. An effective resolution of 0.5 and 0.75 °C were obtained for melting and freezing, respectively, as shown in Fig. 1b. Smooth curves were produced by combining the data sets, demonstrating the validity and importance of the method in improving the accuracy of the dynamic data. As evident in Fig. 1b, finer details of the heat capacity curve can be captured by improving temperature resolution in this manner.
The onset of melting occurs at ~19.5 °C, while the peak is observed at 27.25 °C. The solidification cycle is marked by two peaks that occur at 20 and 23.77 °C, suggesting that the PCM used in the PCM–aerogel composite is an inhomogeneous mixture of two (or more) different chemical compounds with slightly different freezing temperature. Melting and solidification profiles differ significantly both in the magnitude and the shape with a sub-cooling of ~3.5 °C. One melting test was repeated, and no difference in heat capacity data was observed, suggesting good repeatability of the DHFDMA method.
Shape-stabilized PCM Sheet
This product is a paraffin derived from biological wastes that is stabilized in a polymer matrix and produced in sheet rolls of 1-mm thickness. A LaserComp FOX200 HFMA system is utilized for the characterization. FOX200 can accommodate 8 in. × 8 in. and 2-in.-thickness specimens with a test temperature range of −20 to 85 °C. To be able to fit into HFMA instrument, 20 cm × 20 cm samples were cut from the sheet roll. To improve signal-to-noise, four such samples were stacked to create a 4-mm-thick test sample.
DSC experiments suggested a need for resolution of less than 1 °C in order to accurately capture the contours of the heat capacity curve. Following the procedure described in the section “PCM−Aerogel Composite,” two tests were performed for melting and freezing each, providing an effective resolution of 0.75 °C.
The product is produced by blown mixing 20 wt% of a biobased PCM with cellulose insulation. Fox304 system was used for the testing. A 12 in. × 12 in. thin foam frame was constructed to hold blown PCM-enhanced cellulose sample. Multiple tests with uneven temperature steps were conducted and combined to achieve an effective resolution of around 0.75 °C near the peaks. A melting peak is observed at around 28.9 °C, while two peaks in freezing are observed at 22.15 and 19.5 °C. A large subcooling of almost 7 °C is found, a typical feature of biobased PCMs. Two peaks are indicative that PCM is a mixture of two (or more) pure PCMs.
This product is composed of 30 wt% blend of a uniformly dispersed ss-PCM pellets with cellulose insulation. Blown cellulose samples were prepared at Advanced Fiber Technology, Inc. (AFT) facility. The pellets were 1–3-mm long in size. A FOX200 HFMA system was used to measure volumetric heat capacity of the mixture as a function of temperature. An 8 in. × 8 in. thin foam frame was fabricated to contain blown-cellulose sample. Two tests each for melting and freezing were performed to obtain temperature resolution of 0.75 °C. Figure 1e shows that melting and freezing peaks occurs at ~27 and 25.5 °C, respectively, exhibiting a subcooling of around 1.5 °C.