Development of Ejector Refrigeration Cycle Technology for CO2 Heat Pump Water Heater


As an environmental protection measure, public attention is now focusing on improving the energy savings of residential water heating, which accounts for about one-third of total household energy consumption.
One practical solution is to promote expanded use of "EcoCute," a high efficiency heat pump water heater using a natural refrigerant (CO2).
As part of our efforts to increase the popularity of EcoCute, we have developed the world's first variable ejector refrigeration cycle technology for CO2 refrigerants.
This technology enables more efficiently designed, larger-capacity water heating systems that can supply hot water for floor heating and other purposes as well as standard hot water systems.

2.Contents of techniques
In a conventional refrigeration cycle, the refrigerant loses part of its kinetic energy in the expansion valve since energy is wasted as heat in generating a vortex in the decompression-expansion process.
Among its physical properties, CO2 refrigerant loses larger amounts of energy compared with usual fluorocarbon refrigerants.
We devised a refrigeration cycle that effectively recovers the wasted energy, and thus improves coefficient of performance (COP).

An ejector is used in place of a conventional expansion valve, and an accumulator located downstream from the ejector, it should be possible to realize a refrigeration cycle in which liquid refrigerant can continuously be supplied to an evaporator.  The ejector itself consists of a nozzle, mixing unit, and diffuser (Fig. 1).
The refrigerant flowing into the nozzle (drive flow) is decompressed at the nozzle to draw the refrigerant from the evaporator (intake flow).
The drive flow and intake flow are mixed until they are homogenized in the mixing unit, reducing the flow velocity and increasing the pressure of the mixture.
The mixture further reduces velocity when it passes through the diffuser with its larger passage, further increasing the pressure.  The new ejector refrigeration cycle thus uses the kinetic energy of the refrigerant to increase the pressure, thereby supplementing the power of the compressor.

The following are key points for ensuring the ejector refrigeration cycle of a heat pump water heater under a wide range of operating conditions (atmospheric temperature, feed water temperature, hot water temperature, etc.).

The first point is to maintain a heating capacity that will ensure the required discharge temperature of the compressed refrigerant, the basic function of a heat pump water heater.
One of the features of the ejector refrigeration cycle is that the heating capacity tends to drop because the refrigerant discharge temperature decreases as the refrigerant pressure rises.
Increasing the refrigerant pressure at the discharge port is one way to maintain the required discharge temperature.  However, this measure will impair the result of ejector refrigeration cycle.
To secure the required discharge temperature by absorbing heat into the refrigerant, we combined the features of an external heat exchanger and internal heat exchanger by positioning them downstream from the accumulator (Fig. 2).
The external heat exchanger absorbs heat in the air whereas the internal heat exchanger exchanges heat with the high-side pressure refrigerant.
The external heat exchanger functions most effectively when the feed water temperature is low, whereas the internal heat exchanger is more effective for high temperature feed water.

The second point is to maintain a high COP.  Optimal control of high-side pressure is essential for CO2 refrigerant cycle, as has already been introduced in conventional CO2 heat pump water heater.
For the new ejector refrigeration cycle, the high-side pressure should also be optimally controlled to realize a high energy without decline energy recovery efficiency of ejector. (ejector efficiency).
The ejector efficiency is influenced mainly by its nozzle.  An ejector nozzle includes a "throat" and an "exit."  The throat adjusts the refrigerant flow rate to assure the required heating capacity and maintains a high-side pressure for high COP, while the exit has a large effect on ejector efficiency.
We have developed a unique flow restriction mechanism that optimally controls the cross-sectional areas of the throat and exit simultaneously (Fig.3).  The ejector is designed so that its flow path configuration is best suited to the physical properties of CO2 refrigerant.

The new ejector design discussed above has expanded the range of applications of the ejector refrigeration cycle, which previously was only available in a restricted operating range, to heat pump water heater that have to operate all year round.

Alongside our development of the new variable ejector refrigeration cycle technology, we also improved the compressor and CO2-water heat exchanger designs.  As a result, we succeeded in realizing a new CO2 heat pump water heater with 30% heating capacity and a COP approximately 20% higher than conventional type.


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