Experimental study of a novel downhole heat exchanger utilizing working fluids for enhanced heat exchange

This study designs and assesses a new type of shell and tube downhole heat exchanger (NDHE) equipped with organic working fluid (R32, R134a) to increase the thermal performance of the single well geothermal system. As compared to an unconfined borehole with a standard coaxial or U - tube configuration, the present NDHE has a larger heat exchanger surface area inside the confined borehole and allows for phase - change heat exchange with a low - boiling - point fluid. Lab tests did with many hot water flow rates and temp difs to see how it heats up. The result shows that the NDHE's overall heat transfer coefficient value is obtained 3000–4700 W/(m 2 ▪K) and heat extraction powers of 5–18 kW with R32, while R134a reached 1500–2700 W/(m 2 ▪K) and 2–10 kW, respectively. Compared with conventional water-based exchangers, the performance improved by about 1.5 times, particularly under higher flow and temperature conditions. A comparative evaluation highlights the advantages of R32, where latent heat and favorable flow regimes enhance heat transfer. This work contributes to bridging the gap between theoretical designs and practical implementation of compact and efficient geothermal systems. • A novel shell-and-tube downhole heat exchanger (NDHE) is proposed for confined wells. • The NDHE achieves a heat transfer coefficient 1.5 × higher than conventional models. • R32 demonstrates superior thermal performance over R134a and water. • Phase-change using organic fluids significantly enhances heat transfer at small temperature differences.