Journal of Power and Energy Engineering, 2013, 1, 73-76 Published Online October 2013 (
Copyright © 2013 SciRes. JPEE
The Characteristics of the Evaporator/Evaporator for
Direct Expansion Solar Assisted Heat Pump System
Mingyan Zhu, Huanrong Xie, Biao Zhang, Xin Guan
Institute of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, China.
Received October 2013
Direct expansion solar assisted heat pump (DX-SAHP) technology is developed by combining solar energy heat utiliza-
tion with heat pump energy saving technology. The experimental researches of the DX-SAHP hot water system are
conducted in this paper, and overall performance of DX-SAHP is analyzed with three different structures of collec-
tors/evaporators, namely a bare-plate collector, a glass-plate collector and double collectors/evaporators (a bare-plate
collector and a glass-plate collector). The influence factors and overall performance are studied, which show that the
overall performance of the system is mainly influenced by solar irradiation intensity an d the collector area. Comparing
with glass-plate collector in similar conditions, bare-plate collector system COP is higher. While increasing collector
area is conducive to improve the system COP, but will reduce the collector efficiency and increase the workload of the
compres s o r by c omparing the ba re -plate collector with double-plate collecto rs.
Keywords: Direct Expansion Solar Assisted Heat Pump; Collector/Evaporator ; Experimental Research
1. Introduction
Energy sources are widely utilized in many aspects with
a high-speed human society development period. Solar
energy is known as the renewable and “free” energy
source, without doubt, it is the best choice to be a heat
source of heat pump like the air source. In order to im-
prove the heat pump COP, the idea of combining the heat
pump with solar energy application system has been
proposed and developed by many researchers around the
world, which is the direct expansion solar assisted heat
pump(DX-SAHP), the solar collector and the heat pump
evaporator are installed into a single unit (collector/eva-
porator), where the refrigerant is directly evaporated in
the solar collector-evaporator by absorbing the solar
energy (and/or ambient air energy) to undergo a phase
transition from liquid to vapor. The DX-SAHP concept
was first proposed by Sporm and Ambrose in 1955 [1].
Following their w ork, many theoretical and experimental
studies (thermodynamic analysis, numerical simulation
etc.) have been rep orted. Such as the United Sta tes Sporm
S.K, Chaturvedi, M.P.O Dell, Australian G.L. Morrison,
Hawlader, Krakok and Lin in Japan, Huang BJ etc [1-8].
Chaturvedi conducted a theoretical analysis for several
types of collectors/evaporators in 1979 and found that
using the inexpensive bare-plate collector can have a
high COP and collector efficiency, whether the capacity
of the collector and the compressor capacity match or not,
which directly affect system performance, in order to
achieve higher COP and the collector efficiency, the col-
lector/evaporation temperature should keep greater than
ambient temperature at a range of 5˚C - 10˚C [3,4]. In
order to solve the reliability problems, Shanghai Jiaotong
University Guo Junjie studied two different evaporation
areas and found that systems reliability has improv ed by
increasing the evaporation area in low-temperature con-
ditions [9].
2. Experimental System Device Introduction
In order to investigate inf luence of collectors/ev aporators
on thermal performance of DX-SAHP, we set up the
DX-SAHP experimental equipment, which is shown in
Figure 1. It mainly contains the solar energy collector/
evaporator, compressor, condenser, heat storage water
tank, gas-liquid separator, filter drier and thermal expan-
sion valve parts, and the structural parameters of the
main components are as follows: 1) Solar collector/eva-
porator: Sampux “PYT/L2.0-3” tablets of finned tube
type collector/evaporator, two pieces of collectors/eva-
porators (one is a bare-plate collector, the other is a
glass-plate collector), both the collectors area are 2 m2,
the valid heating area 1.87 m2; 2) Compressor: NJ6226Z
type Hermetic reciprocating compressor, the rated power
of 735 W; 3) condenser:BL14-20Dtype plate heat
exchanger with heat transfer area of 0 .35 m2; 4) h eat sto-
The Characteristics of the Evaporator/Evaporator for Direct Expansion Solar Assisted Heat Pump System
Copyright © 2013 SciRes. JPEE
1, 2—Collector/evaporator; 3—Gas-liquid separator; 4—Compressor; 5—Condenser; 6—High pressure accumulator 7—Thermal expansion valve;
8—Circulating water pump; 9—Heat storage water tank.
Figure 1. Schematic diagram of the direct expansion type of the solar heat pump hot water system.
rage water tank pressure type 304 stainless steel tank
design, capacity of 150 L; 5) Throttling device:TN 2
type thermal expansion valve, the system refrigerant is
the pollution-free R134a; 6) Circulating water pump:
HRS20/11-Z” type pump, the rated input power of 120
W, the rated head of 7.5 m and the rated flow of 0.55
In the direct expansion type solar heat pump system,
the operating temperature of the collector and the refri-
gerant evaporation temperature keeps consistent and in
low temperature range, which can obtain higher collec-
tion efficiency and avoid the solar energy heat pump af-
fected by the shortage of time and weather, the refrige-
rant is directly evaporated in the solar collector-evapo-
rator by absorbing the incident solar energy (and/or am-
bient air energy) to undergo a phase transition from liq-
uid to vapor, then the vaporized refrigerant passes through
the compressor and finally pumped into the condenser,
where it gets condensed by water as cooling medium
through a refrigerant-to-water heat exchanger out of the
water tank.
3. Running Performance of DX-SAHP
The performance of DX-SAH P is greatly influenced by
the environment parameter, in order to study it, we use
three kinds of collectors/evaporators operation mode
respectively, a set of experimental data are selected to
analyze the time-dependent performance of the DX-
SAHP system in different meteorological conditions. The
collectors/evaporators are a glass-plate collector (Ac = 2
m2), a bare-plate collector (Ac = 2 m2) and double-plate
collectors (Ac = 4 m2), where Ac is the collector area.
The solar irradiance ranges 143.12 W/m2 - 664.6 W/m2
and the system COP is 2.49 - 3.47. The following is the
comparative analysis on the collectors in similar condi-
From the data shown in Figures 2 and 3, with the
glass-plate collector (Ac = 2 m2) and a bare-plate collec-
tor (Ac = 2 m2), respectively, the DX-SAHP is affected
by external environment (solar irradiance, ambient tem-
perature, wind speed, etc.), during the experimental pe-
riod, the average value of the solar radiation and ambient
temperature are 602 W/m2, 30.2˚C and 604 W/m2, 29.4˚C
respectively, the COP of the glass-plate collector opera-
tion mode is 2.69 and the other is 3.25. The average val-
ue of the solar radiation is almost alike, however, the
instantaneous solar radiation varied with the effect of
cloud, thus unstable system operation thereby affecting
the heating power and compressor power consumption, it
was found that the system COP increases or reduces with
the solar irradiance increases or decreases at the begin-
ning of the system’ running, then, the system operation is
stable gradually with time, namely, although the system
is still affected by the instantaneous solar radiation,
which is not with a high fluctuation.
Firstly, the water temperature difference maintains a
constant basically, secondly, the compressor power con-
sumption rises all the time, which are the reasons that the
COP of the bare-plate collector has been declining. Even
though the compressor power consumption and the water
temperature difference are all higher than the glass-plate
The Characteristics of the Evaporator/Evaporator for Direct Expansion Solar Assisted Heat Pum p Syste m
Copyright © 2013 SciRes. JPEE
Figure 2. System COP and solar irradiance with different
operation mode.
Figure 3. Compressor power consumption and water tem-
perature difference with different operation mode.
collector, however, the COP of the bare-plate collector is
higher than that of the glass-plate collector along the
running process, by comparing the rise rate of the water
temperature difference with the rise rate of the compres-
sor power consumption, we can find that the proportion
of the bare -plate collector is higher.
As for the experimental data for the bare-plate collec-
tor/evaporator during the test, the average value of the
solar radiation, the average ambient temperature and the
average wind velocity are 602 W/m2, 29.1˚C and 1.0 m/s
respectively; And corresponding to the DX-SAHP sys-
tem with double plate collector/evaporator, which is
664.6 W/m2, 22.5˚C and 0.8 m/s.
It can be noted from data shown in Figure 4, double
collectors operational sys tem COP is higher than that of
the bare-plate Collector operational mode in the process
of system operation, by calculation, the former is 3.47
and the latter is 3.26; while the collection efficiency of
double collector is constant lower than that of the bare-
plate collection, when the external environmental para-
meters are under similar condition. With the increase of
Figure 4. Comparison with the collection efficiency and COP
between bare-plate collector and double-plate collectors.
heating area, the COP increases while the efficiency de-
creases oppositely. Firstly, this is because that the col-
lector area increases, the solar irradiation absorbed by the
evaporator increases, leading the refrigerant evaporation
temperature increases, finally improving the COP of DX-
SAHP; as the evaporation temperature increases, the heat
absorbed from the atmosphere in evaporator increased,
whereas the efficien cy of the collector is decreased. While
the collector area further increases, the rate of increase of
the COP gradually decreases, and this is because when
the collector area increases to a certain extentthe further
increase in heat absorption, the superheat degree and com-
pressor suction superheat degree increase, causing the
exhaust temperature overtops and the compressor power
consumption increases, at last the system performance
Under the similar external environment, as the heating
process progresses, the comparison with the exhaust tem-
perature of compressor and hot water temperature be-
tween bare-plate collector and double-plate collector is
shown in the Figure 5. Of course, we can note that the
larger the collector area is, the faster the rise rate of hot
water temperature is and the shorter the time for making
hot water is. In addition, the exhaust temperature of com-
pressor rises, which is owing to the increasing of collec-
tor area, then the solar irradiation absorbed by the eva-
porator increases, thus the refrigerant temperature en-
hances, under the condition of the throttle valve opening
degree invariable, the degree of superheat of the evapo-
rator outlet increases, leading to the compressor exhaust
temperature rise eventually.
To sum up, increasing collector area is conducive to
improving the system COP, but will reduce the collector
efficiency and increase the workload of the compressor,
thus increasing the system cost and installation space.
Therefore, the solar Collector/evaporator is required to
carry out a reasonable selection technically and econom-
-10 010 20 30 40 50 60 70 80 90100
740 solar irradiation(glass-plate collector)
solar irradiation(bare-plate collector)
COP(glass-plate collector)
COP(bare-plate collector)
Time( min)
Solar irradiation(w/
010 20 30 40 50 60 70 80 90100
1100 compressor power consumption (glass-plate collector)
compressor power consumption (bare-plate collector)
water temperature difference(glass-plate collector)
water temperature difference(bare-plate collector)
Time( min)
Com pressor power consumption(W)
W ater temperature difference(
010 20 30 40 50 60 70
CO P(bare-plate collector)
CO P(double-plate collector)
collection efficiency(bare-plate collector)
collection efficiency(double-plate collector)
collection efficiency
The Characteristics of the Evaporator/Evaporator for Direct Expansion Solar Assisted Heat Pump System
Copyright © 2013 SciRes. JPEE
Figure 5. The comparison with the exhaust temperature of
compressor and hot water temperature between bare-plate
collector an d dou ble-plate collectors.
4. Conclusions
1) The overall performance of DX-SAHP is researched
with three different structures of collector s /evapor a tors;
2) Under the similar external conditions, in compari-
son with the glass-plate collector, the bare-plate collector
system COP is higher with better irradiation and higher
environment temperature; increasing collector area is con-
ducive to improving the system COP, but will reduce the
collector efficiency and increase the workload of the
3) Adopt bare-plate collector at summer with better ir-
radiation and higher environment temperature, in winter,
at the opposite case, using the glass-plate collector, whe-
reas the circulation water temperature is not reached the
requirement, double-plate collectors are needed;
4) If the irradiance is higher, the water temperature
rise rate will accelerate, the COP will increase and the
evaporation pressure (temperature) will rise, but the com-
pressor suction superheat and power will increase, which
impact the performance, so the superheat degree control
in a certain range is key to improve the direct expansion
type solar heat pump system performance.
[1] P. Sporm and E. R. Ambrose, “The Hea t Pump and Solar
Energy,” Proceedings of the World Symposium on Ap-
plied Solar Energy.
[2] S. I. C. Chaturvedi, A. S. Roberts and V. Mei, “Solar
Collector as Heat Pump Evaporator,” Proceedings of 13th
Intersociety Energy Conversion Conference, 2000, pp.
[3] S. K. Chaturvedi and A. S. Roberts, “Analysis of Two-
Phase Flow Solar Collectors with Application to Heat
Pumps,” Journal of Solar Energy Engineering, Vol. 104,
1982, pp. 359-365.
[4] M. P. O’Dell, J. W. Mitchell and W. A. Beckman , “Solar
Heat Pump Systems with Refrigerant-Filled Collectors,”
Trans ASHRAE, Vol. 89, 1983, pp. 519-525.
[5] G. L. Morrison, “Simulation of Packaged Solar Heat-
Pump Water Heater,” Solar Energy, Vol. 53, No. 3, 1994,
pp. 249-257.
[6] M. N. A. Hawlader, S. K. Chou and M. Z. Ullah, “The
Performance of a Solar Assisted Heat Pump Water Heat-
ing Syst em,” Applied T hermal Enginee ring, Vol. 21, 2001,
pp. 1049-1065.
[7] K. L. Krakow and S. A. Lin, “Solar Source Heat Pump
with Refrigerant-Cooled Solar Collectors for Cold Cli-
mates,” Trans ASHRAE, Vol. 88, 1982, pp. 417-439.
[8] B. J. Huang and J. P. Chyng, “Performance Characteristic
of Int egral Type Solar-Assisted Heat Pump,” Solar Ener-
gy, Vol. 71, 2001, pp. 403-414.
[9] J. J. Guo, J. Y. Wu, M. L. Jiang and R. Z. Wang, “Air
Source Heat Pump Water Heater Evaporator Matching
Characteristics Research,” The Chinese Society of Engi-
neering Thermop hysics. Engineering T hermodynamics and
Energy Conversion Conference, 2006, pp. 668-672.
05 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
exhaust temperature(bare-plate collector)
exhaust temperature(double-plate collector)
hot water temperature(bare-plate collector)
hot water temperature(double-plate collector)
Temperat ur e(
Tem perature(