In a heat exchanger for CO_2 heat-pump water heaters, heat transfer enhancement, especially in water-side, is required to develop a high performance system. A tube with inner dimple, which called "Smart Dimple tube (SD tube)", has been developed to increase a heat transfer coefficient without increasing a friction factor. The SD tube was applied to a new water-CO_2 heat exchanger, and contributed greatly to the improvement of COP of the heat pump unit. In this paper, the technology for high efficiency CO_2 heat pump water heater by development of the water-CO_2 heat exchanger was described.
