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Development of Ejector Refrigeration Cycle Technology for CO2
Heat Pump Water Heater
Susumu Kawamura, DENSO CORPORATION, Takeshi Sakai, DENSO CORPORATION,
Yoshitaka Akiyama, DENSO CORPORATION, Ken Yamamoto, DENSO CORPORATION,
Hirotsugu Takeuchi, DENSO CORPORATION
1.Outline
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.
3.Conclusion
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. | 
