As early as 2002, Japan's Daikin Corporation introduced a single-stage swing-rotor compressor of carbon dioxide heat pump water heaters. System rated power consumption 1.21kW, evaporator placed outdoors, providing 65 ~ 90 ℃ hot water, water tank capacity of 300 liters / 370 liters, the rated heat 4.5kW, under standard conditions (outdoor air temperature 16 ℃ / 12 ℃, water temperature 17 ℃, water 65 ℃) COP up to 3.72, noise 45 dB. Over the same period, Panasonic also introduced its own tank capacity of 370 liters of carbon dioxide heat pump water heaters. Jianfeng Wang and Eiji Hihara [8] of the Institute of Environmental Studies at the University of Tokyo in Japan established an address model for CO2 heat pump water heaters and contrasted them with R22 heat pump water heaters. The simulation results show that the COP of the carbon dioxide heat pump water heater is less than that of the R22 heat pump water heater; when the internal heat exchanger is equipped, it will reach the level of that of the R22 heat pump water heater, but the outlet temperature of the compressor will obviously increase; The heat will also drop significantly. Due to its simulation results without experimental verification, the model still needs to be further improved. Problems and countermeasures of carbon dioxide heat pump water heater The main problems faced by carbon dioxide heat pump water heater are to continue to improve efficiency and reduce costs. Technological progress and industrial mass production are the main driving forces for cost reduction, while efficiency gains can only depend on technological advances. For now, the main ways to improve the efficiency of carbon dioxide heat pump water heater are the following: ①new and efficient carbon dioxide compressor development and improvement. For example, a two-stage compressor represented by a two-stage rolling piston compressor developed by SANYO makes the structure more compact and the system arrangement more flexible, and the two-stage compression reduces the pressure difference, reduces the leakage and the mechanical loss and can remarkably Increasing system efficiency and compressor efficiency will be one of the future directions for compressor development (see Figure 5). In addition DENSO Japan and Daikin companies are also working hard to develop new and efficient carbon dioxide compressor. ② expansion compressor development. In the transcritical carbon dioxide cycle, reducing the loss of the expansion is an effective way to solve the efficiency. The use of the expander output power to drive the compressor to complete the compression process, the expander and compressor to form an integral expansion compressor, the expansion compressor will become a unique part of the carbon dioxide cycle. ③ high-efficiency transcritical carbon dioxide heat exchanger development. For example, Japan's Sanyo developed a new type of heat exchanger to prevent heavy ice. ④ Reasonable use of low-energy or renewable energy sources and to explore hot water supply and other heating methods of coupling. ⑤ carbon dioxide heat pump water heater advanced control methods. [Conclusion] Compared with the conventional gas water heater, the carbon dioxide heat pump water heater has the advantages of low energy consumption (generally about 30% energy saving) and environmental friendliness. Compared with the conventional heat pump water heater, the carbon dioxide heat pump water heater has the advantages of large supply hot water temperature range and hot water supply advantage. Combining a carbon dioxide heat pump with a water tank can make use of nighttime trough electricity to improve system economics. If the combination of carbon dioxide heat pump water heaters and thermal storage system, the volume of the tank can be reduced, the whole system is more compact and lightweight. Development of efficient compressors for CO2 heat pump water heaters and efficient high-pressure heat exchangers with durable and high pressure as well as a complete control strategy will be the three major focuses in the future research of CO2 heat pump water heaters. I believe that with the continuous improvement of technology, carbon dioxide heat pump water heater will be its unique advantages to become the 21st century water heater market, the most dazzling star.
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