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
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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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