Performance analysis of a cascade PCM heat exchanger and two-phase closed thermosiphon: A case study of geothermal district heating system

In this paper, a geothermal energy extraction system has been designed based on a two-phase closed thermosiphon, cascade phase change material heat exchanger, and an organic Rankin cycle. The proposed system is thermodynamically analyzed for supplying power and heat to a selected hotel in Sarein city in Iran. In the first step, the optimum diameter of the two-phase closed thermosiphon, according to the average yearly energy consumption rate (3100kW), is obtained. A two-level cascade heat exchanger with a storage medium of phase change materials is used to supply the building's energy consumptions. For the summertime, only the first level of the heat exchanger is operating to drive the ORC unit with two turbines for power generation. The second turbine output power is transmitted to the upper grid. On the other hand, in the wintertime, since the heating demands dominated, besides the first level of the heat exchanger, the second level of the heat exchanger is operated to supply the heating demand. Based on the thermodynamic analysis, employing phase change materials in the heat exchanger is an effective method for reducing overall exergy destruction, and consequently, increasing system efficiency. The energy and exergy efficiency is more favorable in terms of the application of PCM in the heat exchanger.