LBNL Report Number
A phase change material product distributed by Stasis Group, designed to reduce the cooling energy requirements of multi-story commercial buildings, was tested in a side-by-side comparison at FLEXLAB® (LBNL, 2018), a calibrated test facility for low energy building technology solutions. The product was installed according to Stasis recommendations in the ceiling plenum of a 600 ft2 cell representative of an office space, while an identical cell was operated in the same conditions, except for the night-time pre-cooling sequence and without the product, to gather baseline data. The cells were operated to replicate summer conditions of a candidate office building for retrofit. For the tests conducted at FLEXLAB, two Phase Change Material (PCM) compositions were tested, with the second product having an upgraded composition for thermal performance and upgraded packaging that was used to meet with fire safety requirements for a plenum application.
Two rounds of tests were performed, which were designed to collect data for both core (designated as Round 1) and perimeter (Round 2) office space conditions. Following the analysis of the data collected during Round 1, the experimental design was modified to better represent the thermal conditions of a typical office space in relation to the PCM performance. A new product was also brought in for testing in Round 2, which had an improved operational temperature range and packaging properties. In Round 1, multiple tests were conducted to evaluate the performance of the product under different conditions of internal loads, temperature setpoints, pre-cooling strategies and temperature control strategies. In Round 2, the internal conditions were kept constant, while the test conditions only differ in the amount of PCM installed in the test cell plenum and the length of the nighttime pre-cooling sequence.
The performance benefits of PCM are generally two-fold: the material may reduce peak cooling energy use during daytime hours (a peak demand energy use benefit) and may allow for energy savings with pre-cooling at night through night flush HVAC controls when night time outside air temperatures are lower, and fan energy use for cooling is more efficient than the use of daytime compressor-based cooling. The focus of this research project was mainly on quantifying the energy savings potential of the latter case although some effects on peak reduction were noted. Between the cell with PCM and nighttime precooling and the reference cell without PCM or precooling, a reduction in cooling load during occupied hours was noted between 7.6 kWh (or 22% of occupied hours cooling load) and 12.5 kWh (or 33% of occupied hours cooling load). Future work can further evaluate the peak cooling load reduction benefits of this product in more detail.
In Round 1, the results for the original product tested showed limited potential for shifting cooling load between occupied hours and nighttime, when compared to a baseline office that uses the same pre-cooling strategy without PCM. A significant shift in cooling load can be observed when the baseline cell does not use any pre-cooling strategies. That shift could be attributed to the thermal mass of the envelope of the cell or from the sensible and latent heat of the PCM, but the sources of the shift are not easily distinguishable.
The test conditions that were developed during the project proposal stage were investigated and we discovered by going in the test cell during different stages of the cycle and physically testing the material, that the PCM product was not performing at the freeze and melt conditions expected. Further tests were conducted to investigate the discrepancies between performance and expectations. A combination of factors was identified as the source of the limited energy savings and those factors were helpful in determining the potential issues that could hinder the performance of the PCM when installed in real buildings. This work was used to design product improvements and the re-design of the second round of experiments. Overall, the results of the Round 1 testing provided valuable insights into experimental design conditions to help discern the PCM’s performance, as well as the desired thermal performance of the PCM product, which informed the development of the improved PCM product.
In Round 2, the combination of PCM and nighttime pre-cooling was shown to provide load shifting, which can in part be attributed to the PCM. This load shifting from daytime to nighttime can be used to reduce the HVAC energy use, by replacing mechanically cooled return air with outside air at night, and the daily cost of energy, assuming lower rates at night (not evaluated in this report). Compared to the baseline case, which did not include a pre-cooling strategy, the PCM plus pre-cooling strategy resulted in 12-18% daily HVAC energy savings.
This report highlights the results of Round 2, which reflect the performance of the modified PCM product when operated in suitable conditions. The results of Round 1, along with an analysis of the experimental data to identify the factors that prevented the product from performing as desired are presented in Appendix B.
The following table gives an overview of the results obtained during the Round 2 experiments. The results are given as the daily cooling load (including pre-cooling) and as the HVAC energy, which includes fans and cooling energy, assuming a cooling efficiency of 1 kW/RT (i.e. 1 kW of electricity is needed to produce 1 refrigerant ton – or 3.51 kW of cooling) and assuming that pre-cooling is done with 100% outside air, which corresponds to the optimal condition for cooling energy savings. In practice, the chiller plant efficiency will vary depending on several factors, including outside air temperature, however the use of a varying efficiency was outside of the scope of this study, and should be the focus of future work. In Table 1 ‘Ref’ and ‘Test’ refers to the reference cell (without PCM) and the test cell (with PCM). Positive savings means that the test cell has a lower HVAC consumption than the reference cell. Overall, the PCM product used in Round 2 demonstrated a savings ranging from 12% to 18% depending on the amount of installed PCM, and the length of time the test cell used night time outside air-based pre-cooling.