United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/053 April 1992
» EPA Project Summary
A Performance Evaluation of a
Variable Speed, Mixed
Refrigerant Heat Pump
P.I. Rothfleisch and D.A. Didion
The performance of an innovative
heat pump, equipped with a distillation
column to shift the composition of a
zeotropic refrigerant mixture, was evalu-
ated. The results of U.S. Department
of Energy (DOE) rating tests and sea-
sonal energy calcuations are reported
with the main cycle refrigerant
compostions. No composition shifting
of the circulating refrigerant mixture
was observed. To demonstrate the po-
tential value of composition shifting,
an ideal vapor compression cycle com-
puter program was used to predict what
the system performance would have
been had the composition shifted. Sea-
sonal energy usage calculations based
on the computer predictions demon-
strated that the effect of compostion
shifting on the heating seasonal per-
formance factor (HSPF) was very small,
increasing slightly with climate zone.
However, the savings in auxiliary heat
were found to be substantial. In the;
cooling mode, computer predictions;
showed pure Refrigerant-22 (R-22) to
have a seasonal energy efficiency ratio
(SEER) approximately 2% higher than
a mixture of 20% R-13B1 and 80% R-22
by weight.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of thes
same title (see Project Report ordering)
information at back).
Introduction
Even though heat pumps are highly ef-
ficient, they find limited use in colder cli-
mates because of their reduced heating
capacity at low outdoor temperatures. As
the outdoor temperature falls, the suction
pressure and the suction temperature also
fall, causing both the suction vapor spe-
cific volume and the compression ratio to
increase. The system heating capacity is
thereby reduced, and the compressor work
input is increased. The building heat load,
on the other hand, increases as the out-
door temperature falls. These relation-
ships are shown in Figure 1.
The outdoor temperature at which the
building load equals the system capacity
is called the balance point. If the outdoor
temperature is below the balance point,
the system capacity will be insufficient to
satisfy the heating needs of the structure.
In order to maintain the indoor tempera-
ture, the system capacity will have to be
supplemented by an auxiliary energy
source.
The auxiliary energy required for an en-
tire heating season is the difference be-
tween the seasonal building load and the
seasonal heat pump output below the bal-
ance point. The auxiliary energy is usu-
ally supplied by electric resistance heat-
ing, which has a coefficient of performance
(COP) of 1. Since the COP of a heat
pump is virtually always greater than 1,
the HSPF can be increased by reducing
the auxiliary heat required.
To reduce the amount of auxiliary heat
required, the heat pump capacity must be
Printed on Recycled Paper
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I
Balance
Point
Outdoor Temperature
Flguro 1. BuMng load and building system capacity versus outdoor temperature for a single speed heat pump operating with a pure refrigerant.
increased to match the building load as
the outdoor temperature falls. The only
commercially available heat pumps ca-
pable of matching system capacity to the
building load have been those which vary
the volumetric capacity of the compressor
by using multi- or variable-speed motors.
The system capacity can also be con-
trolled by varying the composition of a
zeotropic refrigerant mixture. The capac-
ity increase that can be achieved by chang-
ing the composition of a refrigerant mix-
ture has been demonstrated; however, the
complexities involved with providing con-
tinuous composition control have limited
its commercial applications.
In this project, the performance of an
innovative new heat pump, equipped with
a distillation column to shift the composi-
tion of a zeotropic refrigerant mixture, was
evaluated. The unit is charged with a
zeoJropic refrigerant mixture of 80% R-22
and 20% R-13B1 by weight. The distilla-
tion column is intended to optimize the
composition of the circulating refrigerant
for different operating conditions. In the
cooling mode, the column should sepa-
rate and store the more volatile R-13 B1
component. In this way, the system takes
advantage of the lower operating pres-
sures and higher COP of pure R-22. In
the heating mode, the capacity is increased
to match the building load by shifting the
refrigerant composition toward greater per-
centages of R-13B1. The properties of
the resulting refrigerant mixture are much
better suited to the low temperature heat-
ing application, than those of pure R-22.
The unit is also equipped with a variable
speed compressor which gives the sys-
tem an additional method of capacity con-
trol.
There were two primary purposes for
conducting this study: (1) to determine the
extent to which the distillation column can
control the composition of the zeotropic
refrigerant mixture, and (2) to demonstrate
that controlling the composition of a
zeotropic refrigerant mixture can increase
the HSPF and reduce the seasonal auxil-
iary energy requirement. This study was
conducted utilizing a ductless split heat
pump because its unique design incorpo-
rates both a variable speed compressor
and a zeotropic refrigerant mixture to vary
the capacity. While once commercially
available, this design is no longer avail-
able because one of the refrigerants used
(R-13B1) has a very high ozone depletion
potential.
Results
The distillation column was expected to
achieve the greatest degree of composi-
tion shifting that is practically possible in a
residential heat pump. However, the unit
did not demonstrate composition shifting
in any of the tests.
The.economy of instrumentation that
was utilized to ensure a fair performance
evaluation made it difficult to determine
why no composition shifting occurred.
However, one possible reason could be
that excessive liquid refrigerant subcooling
could have prevented the refrigerant from
flashing in the capillary tubes leading to
the distillation column and rendered it in-
active. Another possibility which could
prevent rectification is that the expansion
valve was set with the resistance too low,
causing most of the refrigerant to bypass
the distillation unit. There was no way to
determine this since the expansion valve
was hermetically sealed and electronically
driven.
Since the distillation column proved in-
effective in controlling the refrigerant com-
position, the test shed little light on the
value of composition control. Alternatively,
a vapor compression cycle computer simu-
lation program was used to simulate the
system performance as if it would have
shifted composition. Computer calcula-
tions were conducted for all the test heat
source and sink temperatures with a re-
frigerant composition of 20% R-13B1 and
another set of calculations were made with
pure R-22 refrigerant as the cycle working
fluid. Capacity, efficiency, and auxiliary
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heat requirement predictions were pre- gible, but the resistance heat saved is point of auxiliary energy savings. If the
dieted using these data, assuming that significant, depending on the region for test heat pump system is modified, addi-
ideal cycles can be used to predict rela- which the calculations were made. tional experiments can be conducted, com-
tive changes in system performance. The Although this system failed to produce paring variable speed operations and
calculations show that the increase in any composition shifting, the concept of zeotropic mixture composition shifting as
HSPF with composition shifting is negli- rectification is still valid from the stand- a means of capacity control.
•ku.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40233
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P. Rothfleisch andD. Didion are with the U.S. Dept. of Commerce, National Institute
of Standards and Technology, Gaithersburg, MD 20899.
Robert V. Hendriks is the EPA Project Officer (see below).
The complete report, entitled"A Performance Evaluation of a Variable Speed, Mixed
Refrigerant Heat Pump," (Order No. PB92-143 759/AS; Cost: $19.00; subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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EPA/600/SR-92/053
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