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This report describes the design of a direct-contact heat exchanger (DCHEX) to be used for thermal energy storage at the National Institute of Standards and Technology’s Net-Zero Energy Residential Test Facility (NZERTF). The heat exchanger was designed for heat exchange between a phase-change material (PCM) and refrigerant. The design required the selection of a PCM that would be immiscible with the refrigerant and have a freezing point temperature of approximately of 285 K. A key facet of the investigation was the construction of a test apparatus to observe the flow between the refrigerant and the freezing PCM to ensure a well distributed flow and one where the refrigerant could flow past the frozen PCM. The final part of the project presents a design for direct-contact heat exchange between octanoic acid (continuous phase) and the dispersed phase R410A. Twin DCHEXs were designed to house a vertical column of PCM and to distribute the refrigerant flow throughout the PCM. The height of the PCM column was determined with measurements from the literature that were re-correlated to predict the droplet heat transfer coefficient and the required height of the PCM column. A disengagement section was designed to prevent entrainment of PCM droplets into the exiting refrigerant vapor during the PCM charging. Basic equations are provided so that the design may be modified for a different PCM mass, refrigerant flow rate, temperature difference and operating fluids. An overall drawing of the DCHEX with placement of orifice holes, mist eliminator, and piping was given. The energy storage capacity of the DCHEX is approximately 191 MJ and has the potential of providing a 33 % energy savings in cooling the NZERTF.

Design of a direct-contact thermal energy storage heat exchanger for the NIST net-zero residential test facility