The Characteristics of the Evaporator/Evaporator for Direct Expansion Solar Assisted Heat Pump System

doi:10.4236/jpee.2013.15012

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Journal of Power and Energy Engineering, 2013, 1, 73-76

http://dx.doi.org/10.4236/jpee.2013.15012 Published Online October 2013 (http://www.scirp.org/journal/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.

Email: [email protected]

Received October 2013

ABSTRACT

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 system’s 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: “BL14-20D” type 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

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

m3/h;

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-

tions.

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

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 extent，the 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

deteriorates.

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

400

420

440

460

480

500

520

540

560

580

600

620

640

660

680

700

720

740 solar irradiation(glass-plate collector)

solar irradiation(bare-plate collector)

COP(glass-plate collector)

COP(bare-plate collector)

Time( min)

Solar irradiation(w/

㎡

)

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

COP

010 20 30 40 50 60 70 80 90100

600

650

700

750

800

850

900

950

1000

1050

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)

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

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)

Time(min)

COP

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

collection efficiency

The Characteristics of the Evaporator/Evaporator for Direct Expansion Solar Assisted Heat Pump System

Figure 5. The comparison with the exhaust temperature of

compressor and hot water temperature between bare-plate

collector an d dou ble-plate collectors.

ically.

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

compressor;

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.

REFERENCES

[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.

286-297.

[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. http://dx.doi.org/10.1115/1.3266330

[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.

http://dx.doi.org/10.1016/0038-092X(94)90631-9

[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.

http://dx.doi.org/10.1016/S1359-4311(00)00105-8

[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.

http://dx.doi.org/10.1016/S0038-092X(01)00076-7

[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)

Time(min)

Temperat ur e(

℃

)

Tem perature(