hybrid VRF, flammable refrigerant, finless, bifurcated tube, heat exchanger
Variable refrigerant flow (VRF) system can achieve higher efficiency by modulating the flow of refrigerant according to individual area’s cooling and heating load than traditional central air conditioning unit. A water-based hybrid VRF system is a combination of traditional VRF system and water chiller system. Instead of using refrigerant inside building, hybrid VRF system utilizes water (or any heat transfer fluid) coils as indoor units. Compared with traditional VRF system, it has the advantages of reduced refrigerant charge, wider selection of refrigerants including flammability and lower maintenance cost since there is no concern for indoor refrigerant leakage . A field test of hybrid VRF system by Takenake in 2017 shows a slightly COP degradation around 10% as compared with conventional VRF system. One major reason is the water-side heat transfer coefficient of indoor coils is lower than that of two-phase refrigerant. Therefore, indoor coil design is crucial for hybrid VRF system performance. Meanwhile, we proposed a bifurcated bare-tube heat exchanger (bBTHX) in 2017 using air and water for its working fluids, which has 30% smaller total pumping power and 80% smaller package- and material-volume than current state-of-the-art microchannel heat exchanger when delivering the same capacity. The key feature of this design is the addition of bifurcation that enables 3D flow mixing on air-side and boundary layer redevelopment on water-side, which enhance heat transfer. This design also allows the potential of fast onsite customized indoor unit manufacturing using 3D printing technology. We applied the newly proposed bBTHX as indoor coils of hybrid VRF system to (1) reduce indoor coil size; (2) increase system performance and (3) lower logistics cost. In current study, we simulated the traditional VRF system as baseline and the hybrid VRF system with R-410A using detailed steady state numerical model developed by Winkler et al. in 2008. Indoor coils are tested with 4 mm conventional fin-and-tube heat exchanger (baseline) and bBTHX. The bBTHX was designed and optimized using multi-scale approximation assisted optimization method developed by Abdelaziz in 2009. Results show that the bBTHX has 60% less total pumping power, 65% smaller volume and 70% smaller package- and material-volume than those of traditional fin-and-tube heat exchanger when delivering the same capacity and similar system COP. Moreover, flammable refrigerants were also investigated to further explore the benefits of utilizing this heat exchanger. Simulation results show that the system charge of hybrid VRF system with R-290 and R-600a are 28% and 27%, respectively lower than that of R-410A hybrid VRF system. Overall, the bBTHX shows a potential applicability as indoor coils for water-based VRF systems with less refrigerant charge and flexibility of using flammable refrigerants.