Analysis of a twisted double-pipe heat exchanger with lobed cross-section as a novel heat storage unit for solar collectors using phase-change material

In this paper, a twisted double-pipe heat exchanger containing phase change material is designed and analyzed as an energy storage unit in solar collectors. Since solar heat exchangers cannot store energy, a study of such devices is essential. The governing equations solving and numerical simulation were performed by Ansys 19 software in a 3D model based on enthalpy–porosity method and PRESTO! scheme. The effects of several parameters like pipe's position, twisting, and cross-section on the PCM melting in the storage unit have been investigated, which was unprecedented. The outcomes illustrate that vertical heat exchangers are less efficient and have 15.3% more melting time than horizontal ones. And, 3-lobed cross-sections comparing to circle cross-sections enhance the PCM melting by 6%. Moreover, a double-pipe helical coil or twisted pipe heat exchanger with a 3-lobed cross-section decreases the melting time. The reduction for the helical coil is 10%, while it is 14.6% for twisted pipe heat exchangers. Finally, we examined the twisted inner pipe with 2, 3, 4, and 5-lobed cross-sections. The achievements demonstrated that the 3-lobed twisted cross-section has the highest heat transfer surface and the melting time for 3-lobed is 1.5%, 5%, and 6% less than 2-lobed, 4-lobed, and 5-lobed.