Component Enhancement and Performance Evaluation of a Rotary Heat Exchanger Utilizing Phase-Change Material
Authors:   , Kurtz Jason Andrew , Sun Ying 
Published: July 2021

This research focused on several aspects of the development of a dry cooled rotary heat exchanger that utilizes encapsulated phase change material (EPCM). The heat exchanger prototype was designed with the intent to offer an alternative to current air-cooled condensers (ACC's) with a more efficient dry cooling alternative. The motivation was to provide power production efficiency comparable to that of wet cooling towers while still using dry cooling technology. This is achieved by taking advantage of the high latent heat capacity of solid-liquid phase change and by using the short-term thermal storage capabilities of paraffin-based PCM to reject heat to the atmosphere at the melting temperature of the PCM. The first goal of this research was to improve upon the method by which heated water is dispersed across the EPCM. This involved updating the design of the mechanism that creates a pattern of water jets that contact the PCM, which is encapsulated within thin-walled flat HDPE tubes that are arranged as 7 layers of 15 concentric rings. The spraying device is a trapezoidal spray box that receives heated water at the top side of the box and sprays through precisely located holes aligned in a radial fashion so that they are directed to the top of the EPCM rings on the bottom plate of the box. The updated design increased the number of holes in the sprayer plate from 128 to 450, decreased the hole diameter from 1/8 in to 1/16 in, and the holes were made vertical in the plate so that the water jets exited perpendicularly from the plate. Although these changes resulted in an increased minimum allowable water flow rate from 3.9 · 10-3 gpm to 7.5 · 10-3 gpm, the minimum allowable water temperature decreased from 46 °C to 44 °C, which yields a lower initial temperature difference (ITD) between the atmospheric and water temperature. The second goal of this research was to develop a method of manufacturing the EPCM tubes in such a way that both improved the quality and increased the production rate of the tubes. Previous methods involved manually melting and drawing PCM through HDPE tubes by using a pneumatic hand pump. This manufacturing method created porous PCM within the HDPE tubes and thus resulted in less available PCM within the tubes to provide a thermal storage medium from the hot water to the atmosphere while the heat exchanger was operating. This porosity was evident in the ~10% decrease in usable tube length at the end of the PCM tubes after the PCM melted and solidified. New methods were introduced to decrease the porosity of the tubes, which was reflected in the lack of change in useable PCM tube length upon the melting and solidification. A concept is proposed for future manufacturing of PCM tubes that would involve the simultaneous filling of 16 HDPE tubes with PCM through the use of a vacuum chamber and this method would be ideal for manufacturing PCM tubes for the current prototype and especially for future designs that may require greater lengths of tubing. Finally, an evaluation of the coefficient of performance (COP) was conducted to evaluate and verify the heat exchanger design performance and to use as a guide in the future steps of the development of the heat exchanger. The COP was calculated to be 964 with an uncertainty of ± 286. Further analysis of the data revealed that a region of the collected data was likely inaccurate and a possible influences on the data could have been PCM leakage into the water circulation system, humidity of the surrounding air, and sensors requiring recalibration.