United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/SR-95/148 October 1995
4>EPA Project Summary
A Simple Method of Composition
Shifting with a Distillation
Column for a Heat Pump
Employing a Zeotropic
Refrigerant Mixture
Peter I. Rothfleisch
The report presents a simplified
method of controlling heat pump ca-
pacity by shifting the composition of a
zeotropic refrigerant mixture with a dis-
tillation column. Simplicity is achieved
by incorporating the distillation column
into the typical suction accumulator
used in residential heat pumps. An ex-
perimental system employing this con-
cept has been evaluated in the
laboratory for two hydrofluorocarbon
(HFC) refrigerant mixtures—HFC-32/
HFC-134a (30%/70%) and HFC 32/HFC-
125/HFC-134a (23%/25%/52%). For the
binary mixture, the circulating refriger-
ant composition was shifted to HFC-
32/HFC-134a (54%/46%). For the ternary
mixture, the circulating refrigerant com-
position was shifted to HFC-32/HFC-
125/HFC-134a (36%/36%/28%). Seasonal
calculations have shown that these
composition shifts reduce the seasonal
resistance heat requirement by up to
5% compared to a conventional heat
pump. Additionally, the instantaneous
peak energy requirement of the dwell-
ing has been reduced relative to a con-
ventional heat pump by 6-9%
depending on the climate region. The
distillation insert should be capable of
producing greater composition shifts
after further optimization of the insert
and improved integration with the heat
pump system. For the ternary mixture,
it is expected that the insert will be
capable of producing a circulating re-
frigerant composition composed en-
tirely of HFC-32/HFC-125.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Overview
The heating capacity of a single-speed,
air-to-air residential heat pump is directly
proportional to the outdoor temperature.
As the outdoor temperature falls, the sys-
tem heating capacity is reduced and the
compressor work input is increased, mak-
ing the system less efficient. The building
heat load requirement, on the other hand,
increases as outdoor temperature falls.
For most residential systems, as outdoor
temperatures get lower, a temperature is
reached (called the balance point) below
which the output of the heat pump must
be augmented with an auxiliary energy
source to meet the heat requirement of
the heated space.
The auxiliary energy is usually supplied
by electric resistance heating, which has
an energy efficiency much lower than a
heat pump. Therefore, the heating sea-
sonal performance factor (HSPF) of a heat
pump can be increased by reducing the
dependence on auxiliary electrical heating
during cold weather and modulating the
heat pump capacity to meet the building
energy load. The increase in HSPF can
be significant for colder climates, and the
reduction in peak load to the electric utility
can be beneficial.
To reduce the amount of auxiliary heat
required, the heat pump capacity must be
increased to match the building load as
the outdoor temperature falls. The only
commercially available heat pumps ca-
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pable of matching system capacity to build-
ing load have been those which vary the
volumetric capacity of the compressor. The
most common ways of accomplishing this
are by using two-speed compressor mo-
tors and variable-speed compressor mo-
tors with frequency inverters, both of which
require considerable additional cost. Re-
cent interest in the use of zeotropic refrig-
erant mixtures, prompted by the phaseout
of the use of chlorofluorocarbon-based re-
frigerants, has provided an opportunity for
an additional unique method of varying
heat pump heating capacity by varying
the composition of the zeotropic mixture.
Specifically, by controlling mixture compo-
sition the thermodynamic properties of the
refrigerant mixture can be altered to in-
crease system capacity as the outdoor
temperature drops. Using this technique,
the heat pump would operate on the origi-
nal refrigerant mixture until the outdoor
temperature falls to the balance point and
then shift the mixture composition to in-
crease capacity.
Methods and systems for varying the
capacity of a heat pump by controlling the
composition of a zeotropic mixture may
be classified as either an active or pas-
sive system. An active system uses a
distillation column, whereas a passive sys-
tem uses a more simple accumulator. Gen-
erally, the methods that use accumulators
are simple to implement but require a
large refrigerant charge. Additionally, the
magnitude of the composition change is
limited by thermal equilibrium properties
and storage space in the accumulator.
Conversely, the column methods can
achieve much larger composition changes
at the cost of increased system complex-
ity. Many proposed active systems have
been too complex and costly to be ap-
plied in the competitive residential heat
pump market.
This work presents a simplified method
of controlling heat pump capacity by shift-
ing the composition of a zeotropic refrig-
erant mixture with a distillation column.
Simplicity is achieved by incorporating the
distillation column into the typical suction
accumulator used in residential heat
pumps. An experimental system employ-
ing this concept has been evaluated in
the laboratory for two hydrofluorocarbon
(HFC) refrigerant mixtures—HFC-32/HFC-
134a (30%/70%) and HFC-32/HFC-125/
HFC-134a (23%/25%/52%). For the bi-
nary mixture, the circulating refrigerant
composition was shifted to HFC-32/HFC-
134a (54%/46%). For the ternary mixture,
the circulating refrigerant composition was
shifted to HFC-32/HFC-125/HFC-134a
(36%/36%/28%). Seasonal calculations
have shown that these composition shifts
reduce the seasonal resistance heat re-
quirement by up to 5% compared to a
conventional heat pump. Additionally, the
instantaneous peak energy requirement
of the dwelling has been reduced relative
to a conventional heat pump by 6-9%
depending on the climate region. The dis-
tillation insert should be capable of pro-
ducing greater composition shifts after
further optimization of the insert and im-
proved integration with the heat pump sys-
tem. For the ternary mixture, it is expected
that the insert will be capable of produc-
ing a circulating refrigerant composition
composed entirely of HFC-32/HFC-125.
Peter I. Rothfleisch is with the National Institute of Standards and Technology's
Building and Fire Research Laboratory, Gaithersburg, MD 20899.
Robert V. Hendriks is the EPA Project Officer (see below).
The complete report, entitled "A Simple Method of Composition Shifting with a
Distillation Column for a Heat Pump Employing a Zeotropic Refrigerant Mixture,"
(Order No. PB95-255821; Cost: $17.50, 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
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management
Research Laboratory (G-72)
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use
$300
EPA/600/SR-95/148
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