International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Performance Analysis of Heat Pump Assisted Solar
Water Heating System
Apeksha Shandilya1, Abhay Singh2
#
Sr. Engineer, Project Department, Global Economic Advantage Pvt. Ltd., Gurgaon, India
#
Dy. Manager, Air craft division, Hindustan Aeronautics Ltd., Bangalore, India
Abstract— It is proposed to improve the performance of
solar air heater under the meteorological conditions of
Bhopal. Analytical studies done on a solar assisted heat
pump water heating system, where, flat plate solar
collectors acted as an evaporator for the refrigerant R134a.The parameters like condenser temperature,
collector area, material, thickness, absorptivity, emissivity
of absorber plate, insulation material of condenser and
water tank etc. affecting the performance of solar water
heater. when water temperature in the condenser tank
increases with time, the condensing temperature, also,
increases, and the corresponding COP and collector
efficiency values decline. With the time variation
parameters like, COP, Collector efficiency and the
spacing between absorber and bottom plate a balance in
heat transfer coefficient efficiency and pressure drop is
essential while designing a solar air heater. The results
obtained are used for the design of the system and enable
determination of compressor work, solar fraction &
auxiliary energy required for a particular application. To
ensure proper matching between the collector/evaporator
load and compressor capacity, a variable speed
compressor was also analyzed. Due to high ambient
temperature in Bhopal, evaporator can be operated at
a higher temperature, resulting in an improved
thermal performance of the system.
early solar water heating devices is given by Garg13. A solar
water heating industry in South Florida was started in 1900.
It is estimated that about 30000 to 50,000 units were installed
by 1950, but around that time their popularity began to
decline due to readily available cheap energy from fossil
fuels. Solar
water heaters employing flat plate collectors are widely use
for water heating purposes.
A. Built-in-storage type solar water heater
In which all the three functions/components i.e. collection
storage, and control are combined into a single unit. Hot
water (upto 60 ºC only)
from such water heaters has to be used during the day.
Otherwise the heat stored would be lost during the night.
1) Performance prediction of solar collector: Thermal
optimization18 of built in storage water heater must, of
course, be based on the evaluation of system performance
under diverse conditions. To facilitate engineering design of
the heater, analytical optimization relations are written and
derived from separate approximate formulations of solar
heating and cooling heat losses. Schematic diagram of typical
Built-in-storage type solar water heater shown in Fig 1.
Keywords— Solar water heater, Heat pump, Solar
collectors, Solar Energy, R-134a.
I. INTRODUCTION
Among the alternative energy sources, solar energy is
considered cheap, readily available, and nonpolluting which
can be used in domestic or industrial low temperature thermal
applications. Solar energy systems and heat pumps are,
therefore, promising means of reducing the consumption of
non renewable energy sources. To increase the evaporation
temperature, the unglazed solar collectors can act as an
evaporator to increase the thermal performance. Solar energy
to heat water has been use for many years, and the design
requirements of solar water heating equipment have been
studied for more than 100 years. Interesting description of the
ISSN: 2231-5381
Fig 1- Buit-in storage type solar water heater
Performance predictions- The instantaneous heat balance as
shown in figure may be written as:
(Radiation absorbed) = (Heat absorbed by water) + (Heat
absorbed by
container) + (Heat loss from absorber
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
This can be written as:
ITt (τα)e Ac = Ww (dTw/dt) + Wc (dtc/dt)
+ (UL + UB)Ac [(Tc – Ta) + {(dTc/dt –
dTa/dt)/2}]
Where ITt = incident solar radiation
Ww, Wc = Thermal capacities of water and container
Tw, Tc, Ta = Temperatures of water, absorber and ambient
air.For practical purposes it can be assumed (under steady
state conditions) that the water temperature is equal to the
absorber temperature, i.e. Tw = Tc and dTw / dt = dTc / dt.
Thus Eq. can be written as:
X (dTw / dt) + YTw = Z
(c) Cover plate material: The characteristic of the cover plates
through which the solar energy is transmitted are extremely
important in the functioning of a collector. The most critical
factors for the cover plate materials are the strength,
durability, non-degradability and solar energy transmittance.
The thermal and optical properties of cover plate material are
tabulated in table4.
II. OBJECTIVE OF PRESENT WORK
As discussed above, forced circulation or pumped a solar
water heating system which is having very less efficiency due
to the poor heat exchange. In this problem a design &
analysis of solar assisted heat pump water heating system is
done with refrigerant 134-a as a working fluid. The analysis
shown in subsection. Chapter predicts thermal performance
of above type solar heater under metrological condition of
Bhopal. The influence of various operating parameters on
thermal performance has been examined and important
variables are identified.
Where,
X= Ww + Wc + (UL + UB) Ac/2
Y = (UL + UB) Ac
And
Z = ITt (τα) Ac + (UL + UB) (Ac /2) [(dTa / dt) + 2Ta]
The solution of equation gives:
Tw = Z/Y + (Tw1 – Z/Y) exp [-(Y/X)(t-t1)]
Where Tw1 is initial water temperature at time t1, when fresh
water is added. The total solar radiation recorded on the
III. EXPERIMENTAL WORK & CALCULATION
plane of the heater can be expressed as a Fourier series:
Calculation for the proposed solar assisted heat pump water
ITt = Ao + Σ (Cn cos nwt + Bn sin nwt)
heating system the input parameters are given in table 5.
These
parameters are variable and they may change
The ambient air temperature was also expressed as a Fourier series
as follows:
according
to conditions or requirement. Some other fixed
Ta = Co + (Cn cos nwt - Dn sin nwt)
parameters are given in table 6.
To make the model more realistic, the transmissivity, absorptivity product
In this system we use refrigerant R-134-a for heat
(τα)e, was also assumed to be variable and is expressed as:
transferring purposes. The schematic diagram of proposed
heat pump assisted solar water heater is shown by the figure
η = qu dt
1.
Ac (ITt dt)
Using the above equations and the measured values of total
solar radiation and ambient temperature for the test day,
hourly values of storage water temperature can be predicted.
All the above solar collector performance parameters are
tabulated in table1.
2) Materials for collector: To design and construct solar
collectors for heating and cooling purposes, knowledge of the
properties of the materials and characteristics of the various
components is necessary to predict the performance and
durability of the collector. Property data can be classified in
to three categories: thermo physical, physical and
environmental properties.
(a) Absorber plate material properties of metals used for
absorber plates shown in table2.
(b) Several thermal insulating materials which can be used to
reduce heat losses from the absorbing plate and pipes are
commonly available. The desired characteristics of an
insulating material are low thermal conductivity,stability at
high temperature (upto 200 ºC), no degassing upto around
200 ºC, self-supporting feature without tendancy to settle,
ease of application, no contribution in corrosion. Properties of
some of the insulating materials are given in below table 3
ISSN: 2231-5381
Fig2- Proposed heat pump assisted solar water heating system
Qu = AcI’ [ I(τα) – Ul (Tp –Ta)]
As the values given in table. We have the values ofAc = Collector Area = 1.5 x 2 = 3 m2
F’ = Heat removal factor = 0.9 (For nickel glass)
τα = 0.89 (for nickel glass)
Ul = Overall heat transfer coefficient = 1.3 (for glass cover)
TF = Fluid Temperature = 50 C
Ta = Ambient Temperature = 32 C
I = Instantaneous solar radiation = 800 W/m2-S
As we know, Ul = Ut +Ub
Here as Ub is very small so consider negligible,
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Qu = 3 x 0.9 [800(0.89) – 1.3(50-32)]
Qu = 2.7 [712-23.4]
Qu = 1859.22 kj/kg
B. CompressorWc = m {(p1v1)/ηc} x {n/(n-1)} [{PL /P1} {(n-i)/n} – 1]
Where, m = (Vd x N x η) / (V1 x 60)
Where,
m = mass flow rate
P1 = Inlet pressure of refrigerator = 1 bar
V1 = Specific volume of fluid (at P = 1 bar)
ηc = Compressor efficiency
n = Polytropic index = 1.3
P2 = Pure of refrigerator at outlet = 10 bar
Vd = Displacement volume
Table1
Solar collector performance parameters
Coll.
No.
Manufacturer
& remarks
Absorber
Material
1
NASA/ Honey
Well
MSFC
NASA/ Honey
Well
NASA/ Honey
Well
(Mylar
honeycomb)
NASA/ Honey
Well
PPG
Owens
(evacuated)
Solaron(data
furnished by
manufacturer)
Aluminium
2
3
4
5
6
7
8
Absorber
Surface
Coating
Black
Nickel
Black Nickel
Black Nickel
Transparent
cover
FR
UL
ταε
ταε
Ε
ταε
2glass
0.94
0.56
0.74
0.95
0.07
0.78
2 tedlar
1glass
0.94
0.90
0.69
1.3
0.56
0.89
0.73
0.97
0.1
0.97
0.77
0.92
Aluminium
Black Nicke
Black Nickel
2glass
0.96
0.57
0.77
0.97
--
0.79
Aluminium
Black Nickel
2glass
0.93
0.80
0.76
0.97
0.97
0.78
Aluminium
Glass
Black Nickel
Selective
surface
Black Paint
2glass
1glass
0.85
0.75
1.1
0.20
0.73
0.72
0.8
--
0.95
0.07
0.77
0.9
2glass
0.67
0.77
0.73
--
---
--
Aluminium
Aluminium
Steel
Table 2
Properties of metals
Material
Density(kg/m3)
Specific heat(kj/kgºC)
Thermal conductivity (W/m ºC )
Aluminium
Iron
Steel
Copper
Brass (70/30)
Silver
Tin
Zinc, pure
2707
7897
7833
8954
8522
10524
7304
7144
0.996
0.452
0.465
0.383
0.385
0.234
0.226
0.384
204
73
54
386
111
419
64
112
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Table 3
Properties of insulating materials
S.
N
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Name of Material
Crown white wool
Crown bonded 150
Spintex 300 industrial
Glass Wool
Calcium silicate
Expanded polystyrene
ISO Cynurate
Phenotherm
Thermocole
Polyurethane foam
Cellular foam
PIPE SECTIONS
Rocklloyd
Isoloyd
Thermocole foam
ISSN: 2231-5381
Thermal
conductivity
0.034
0.066
0.975
0.044
0.07
0.017
0.020
0.029
0.035
0.016
0.093
0.075
0.021
0.035
0.017
Density
48
48
48
48
251.6
32
32
32
16
32
400
48
32
16
32
Out
gassing
No
Yes
No
No
No
Yes
No
Yes
Yes
Yes
Yes
No
No
No
No
Saging
Yes
No
No
Yes
No
No
No
No
No
No
No
No
No
No
No
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Colour
Change
No
Yes
No
Yes
No
No
No
No
No
No
No
No
No
No
No
Remarks
Good but expensive
Not good
Good, reasonable cost
Good
Good, But component system
become very heavy
Not good
Under testing
Not good
Not good
Not good
Not good
Good
Good
Good
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Table 4
Thermal & Optical properties of cover plate materials
Material
Glass
FiberglassReinforced
polyster(sunlight)
Acrylic (plexiglass)
Polycarbonate (lexan)
Polyttrafluproethylene
(Teflon)
Polyvinyl Fluoroide (Tedlar)
Polyster (mylar)
Polyvinylidene Fluoroid
Index if
refrection
Normal
incident shortwave
transmittance
λ=0.425μm
Normal
incident long
wave
transmittance
Λ=2.5-40 µm
Thickness
(m)
Density
Specific
heat
Thermal
Capacity
1.518
1.540
1.490
1.586
1.343
1.460
0.840
0.870
0.900
0.840
0.960
0.920
0.020
0.076
0.020
0.020
0.256
0.207
3.175*10-3
6.350*10-3
3.175*10-3
3.175*10-3
5.080*10-3
1.076*10-3
2.489*103
1.399*103
1.189*103
1.199*103
2.1448*103
1.373*103
0.754*103
1.465*103
1.465*103
1.193*103
1.172*103
1.256*103
1.659
3.61
1.534
1.260
0.036
0.049
1.640
0.870
0.178
1.270*10-3
1.394*103
1.046*103
0.051
1.413
1.500
0.930
0.920
0.230
0.810
1.016*10-3
1.016*10-3
1.770*103
0.910*103
1.256*103
2.302*103
0.063
0.059
N = Speed of motor (variable)
ηv = volumetric efficiency
ηv = 1 + C – C (PL/P1)1/n
Here,
C = V1 /V2
If value is not given then we know,
V1 = 0.03 V2,
ηv = 1 + 0.03 – 0.03 (10/1)1/1.3 = 85%
Vd = Displacement volume
Vd = (π/4) D2 x L
Vd = (π/4) (0.035)2 x 0.026
Vd = 0.025 m3
m = (0.025 x 1000 x 0.85) / (0.001043 x 60)
m = 340 Kg/s
Wc = m (P1V1/ ηc) x {(n)/ (n-1)} [(p2/P1)(n-1)/n -1]
Wc = 340 x (0.001043 x 1/0.85) x (1.3/0.3) [(10)(0.3/1.3) – 1]
Wc = 1.26 KW
3.1.3 CondenserQc = Qu + Wc
Qc = 1.859 + 1.26
Qc = 3.119 KJ/Kg
COP (System) = Qc / Wc = (3.119) / (1.26)
= 2.47 ≈ 3
3.1.4 Collector Efficiencyηcoll = Qu / (AcI) = (1859.2) / (3 x 800)
ηcoll = 77%
To find out condenser temperature we will use the following
relation,
Qc = [ ao + a1 (Tc – Tw) + a2 (Tc – Tw)2 ] x 100
Where,(ao, a1, a2 are coefficients)
ao = 0.2225
a1 = 0.4838
a2 = 0.024
Tw = 32C m2
ISSN: 2231-5381
Qc = 3.119 Kj/Kg
By putting these values we get,
Tc = 58C
Table 5
Input parameters
Collectors
Area
Absorber Plate
Tube
Insulation
Case
Compressor
Condenser/ Water
tank
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Area (Each Collector)
1.5 m2
Material
Copper
Thickness
1.0 mm
Surface Treatment
Black
absorptivity,
90%
emissivity,
0.9
Material
Copper
Outer diameter
9.52mm
inner diameter
8mm
spacing,
100mm
Material,
Polyurethane
Thickness
50mm
Material
Aluminum Sheet
Bore
0.035 mm
Stroke
0.026mm
Number Of cylinder
01
Size
250 Liter
Insulation Material
Polyurethane
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Thickness
50mm
Graph4.3
Table 6
Some other useful parameters
Latitude
Collector Azimuth
Collector tilt angle
Ambient Temperature
Initial Water Temperature in
the Tank
Specific Heat of Water
Wind Speed
Time Step
Heat loss coefficient of tank
Coefficients
23 degree 15 minutes
Zero degree
10 degree
32 degree
30 degree
Variation of collector efficiency with compressor speed
0.8
0.7
C o l l e c to r e ffic i e n c y
0.6
4130 J kg -1 K-1
3ms-1
5, 60 min
0.36 Wm-2K-1
a0 = 0.2225,
a1 = 0.4838,
a2 = 0.024
0.5
Collector efficiency(Ac = 3m2)
0.4
collector efficiency (Ac = 4m2)
0.3
collector efficiency (Ac = 5m2)
0.2
0.1
0
900
Graph 4.1
Variation of COP with time for different speed of the compressor (solar
COP
radiation 600 Wm
-2
10
9
8
7
6
5
4
3
2
1
0
RPM=900
RPM=960
1020
1200
1500
Graph 4.4
Variation of collector fluid temperature with Compressor
speed
35
30
25
Collector fluid
temp(collector area=3m2)
20
15
Collector fluid
temp(collector area=4m2)
10
Collector fluid
temp(collector area=5m2)
5
0
RPM=1020
900
RPM=1500
960
1020
1200
1500
Compressor speed (rpm)
Graph 4.5
0
60
120
180
240
Variation of collector efficiency with time for different storage
volume
Time, min
Graph 4.2
Collector
efficiency(Storage
volume=150 lt)
0.8
10
9
8
7
6
Collector Area, 3m2
5
4
3
2
1
0
Collector Area, 1.5 m2
0.7
C o ll ecto r effi ci en cy
Effect of Compressor speed on COP with collector area as a
parameter
CO P
960
Compressor speed (rpm)
C o l l e c to r flu i d te m p e r a tu r e ( 0 C )
IV. RESULT
A series of calculations conducted under the meteorological
conditions of Bhopal and these results are presented in this
section.
We changed the values of variable parameter with respect to
other parameter and get the predicted values for the proposed
heat pump assisted solar water heater.
The following graphs show the relations between two or more
parameters.
0.6
Collector
efficiency(Storage volume
= 250 lt)
0.5
0.4
Collector
efficiency(Storage volume
= 300 lt)
0.3
0.2
Collector efficiency
(Storage volume = 400
lt)(Sheet3!$A$1
0.1
0
0
60
120
180
240
Time (seconds)
900
960
1020
1200
1500
Graph 4.6
Speed of the Compressor, RPM
ISSN: 2231-5381
Collector
efficiency(Storage volume
= 500 lt)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 6- June2013
Variation of COP with Time for different storage volume
12
10
COP(Storage volume = 150 lt)
8
CO P
COP(Storage volume = 250 lt)
6
COP(Storage volume = 300 lt)
COP(Storage volume = 400 lt)
4
COP(Storage volume = 500 lt)
2
0
0
60
120
180
also analyzed.Due to high ambient temperature in
Bhopal, evaporator can be operated at a higher
temperature, resulting in an improved thermal performance
of the system. Results show that, when water temperature in
the condenser tank increases with time, the condensing
temperature, also, increases, and the corresponding COP
and collector efficiency values decline. Average values of
COP ranged from about 4 to 9 and solar collector
efficiency was found to vary between 40% and 75% for
water temperatures in the condenser tank varying between
30°C and 50°C.
240
VI. REFERENCES
Time (seconds)
Graph 4.7
Effect of storage volume on thermal energy output by the collector and
condenser
60
T em p e r a tu r e ( 0 C )
50
Collector fluid temperature (0C)
40
30
Condensing Temperature
(0C)+Sheet2!$A$1
20
Water (0C)
10
0
150
250
300
400
500
Storage volume (m3)
Graph 4.8
Variation of COP with solar radiation for different collector area
9
8
[1] Kreider, T.F.; and Kreith, F. 1981. Solar Energy Hand book. McGraw-Hill,
New York,NY, USA.
[2] Duffie, J.A. and Beckman, W.A.(1980),solar engineering of thermal
processes, john Wiley & Son’s, New York.
[3] Chiou, J.,E1-Wakil, M. M., and Duffle, J.A.(1965), “A slit and expanded
aluminum foil matrix solar collector” , Solar Energy,9, 73-80.
[4] Hamid, Y.H. and Beckman, W.A.(1971),” Performance of Air cooled
radiatively heated screen matrices”, Trans.ASME, J. of Engineering for power,
221-224.
[5] Sorour, M.M., and Hassab, MA.(1986),”A screen type solar water heater”,
Proc.8th International Heat Transfer Conference, San Francisco,6,3097-3103.
[6] H.P. Garg, Solar Water Heating Systems, D. Reidel, Holland, 1986
[7] F. A. Brooks, “Solar energy and its uses for heating water in California”,
Bull Calif Agric. Exp. Sta, No. 602, 1936.
[8] S. Chandra, “ Domestic Water Heating” , Solar Energy Technology
Handbook, Edited by W.C. Dickinson and P.N. Cheremisinoff, Marcel Dekkar,
Inc. New York.1980.
[9] Liu, Yeh-Di., Diaz, L.A., and Suryanarayan , N.V., “Heat transfer
enhancement in the water heating flat plate solar collectors,” ASME Trans., 106,
pp.358-363, (1984).
[10] Singh, P. (1978) “Cheap packed bed absorbers for the solar air hweaters”,
proc. International Solar energy Society, New Delhi, 29-,900-904.
[11] SP Sukhatme, Solar Energy, Tata McGraw Hill, New Delhi 1984.
[12] WM Kays, Convective Heat and Mass Transfer, Tata McGraw Hill, New
York (1966).
[13] Solar Energy Fundamentals and Application, HP Garg
and J prakash, Tata Mc Graw Hill Publishing Company
Ltd., 2002
CO P
7
6
COP (Ac=1.5m2)
5
COP(Ac=3m2)
4
COP(Ac=4m2)
3
COP(Ac=5m2)
2
1
0
300
400
500
600
700
Solar Radiation (w/m 2)
V. CONCLUSIONS
Analytical studies were performed on a solar assisted heat
pump water heating system, where, flat plate solar collectors
acted as an evaporator for the refrigerant R-134a. The system
was analyzed under meteorological conditions of Bhopal. The
results obtained are used for the design of the system and
enable determination of compressor work, solar fraction &
auxiliary energy required for a particular application. To
ensure proper matching between the collector/evaporator load
and compressor capacity, a variable speed compressor was
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