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Performance Evaluation of a Solar Water Heating system for Industrial
Application
Keh-Chin Chang1,a, Wei-Min Lin2,b, Yi-Mei Liu1,c, Tsong-Sheng Lee1,d and Kung-Ming Chung1,e
1National Cheng Kung University, 2500 Sec. 1, Chung-Cheng S. Rd., Guiren, Tainan, Taiwan
2Tainan University of Technology, 529 Chung-Cheng Rd., Yongkang, Tainan, Taiwan
Keywords: thermal performance; large-scale solar water heater; field measurement
Abstract. The total area of solar collectors installed in Taiwan had exceeded 2 million square meters
by the end of 2010. However, there were only 98 systems in operation with area of solar collectors
installed exceeding 100 square meters from 2001 to 2010. To increase industrial awareness of solar
water heating technologies, a nursery greenhouse was chosen as the case study to evaluate its thermal
performance throughout the months of May 2010 to April 2011. The results showed that the solar
energy collected and heat loss during the night hours would affect the thermal efficiency, economic
viability and attractiveness of a SWH. This study would provide useful information for all parties
related to this market, manufacturers, potential users and policy-makers.
Introduction
Utilization of renewable energy sources is of practical importance for the socio-economic
development of a country. Thus, renewable energy is receiving increasing support due to its benefits
to the environment. Taiwan is located between latitudes 22oN and 25
oN, longitudes 120
oE and 122
oE
in the region of Tropic of Cancer. The average daily global solar radiation ranges from 3.25
kWh/m2/day in the northern areas to 4.64 kWh/m
2/day in the southern areas. It is also endowed with
annual sunshine of 2,000-2,500 hours in the southwestern regions and 1,000-1,500 hours in the
northeastern regions. Therefore, it is quite favorable for solar water heaters (SWHs) to be installed for
hot water production in both residential and commercial sectors. In view of this, the government of
Taiwan introduced several subsidy programs (1986-1991, 2000-present) for SWHs, which have
played a critical role in the dissemination of SWHs in Taiwan [1,2,3]. As shown in Fig. 1, the area of
solar collector installed (ASC) was only about 56,500 m2 in 1999 and reached the peak of 24,684 units
(ASC =114,428 m2) in 2006. Further, a general survey of users conducted by Chang et al. [4] revealed
that over 98% of SWHs had ASC 10 m2. In Taiwan, a general temperature of around 40
oC is usually
preferred for bathing in the evening and the average hot water consumption for each person is about
60 liters, which would correspond to ASC 1 m2. Therefore, most SWHs have been used in the
domestic sector (or residential systems).
Also shown in Fig. 1, government subsidy is apparently effective only at the initial stage of each
incentive program. ASC declined in the 2008-2009 period. However, the use of SWHs for domestic
hot water production represented only a small fraction of the potential applications. Karagiorgas et al.
[5] indicated that large-scale SWHs could be particularly effective in industries, such as food industry,
agro-industries, and textile manufacturing. Therefore, it is urgent to boost the local market in the
commercial sector. In Taiwan, there were only 98 systems installed with ASC 100 m2 from 2001 to
2010. These systems were mainly installed for water heating in dormitories, swimming pools,
restaurants, and manufacturing plants. According to the desk survey, high-quality installations and
maintenance (such as dust accumulation) would be the major concerns in those applications.
Exposure of solar collectors to salty air should also be addressed [6]. Thus, technical support to
system designers, installers and users is required. In this study, field measurements of a SWH for a
horticulture-nursery greenhouse were conducted to evaluate its thermal performance, which would
provide useful information for all parties related to this market, manufacturers, potential users and
policy-makers.
Year
1998 2000 2002 2004 2006 2008 2010Unit o
f sola
r co
llecto
r in
sta
lled a
nnually
0
5000
10000
15000
20000
25000
30000
Unit installed
Are
a o
f sola
r co
llecto
r in
sta
lled a
nnually
5.0e+4
1.0e+5
1.5e+5
ASC
installed, m2
Figure 1 Solar water heaters installed in Taiwan
Setup of Field Measurements
As mentioned above, there were limited SWHs installed for industrial applications in Taiwan. To
increase industrial awareness of solar water heating technologies, a nursery greenhouse of Taiwan
Sugar Corporation, located in southern Taiwan near the Tropic of Cancer (23°20'10" N, 120°22'36"
E), was chosen as the case study. The main activity of the case greenhouse is growing orchids indoor.
The ideal room temperature required for the growth of these plants is 28oC. The greenhouse and the
soil are heated by an on-floor and under-floor piping system. The hot water consumption is about 6
m3/day. Originally, the hot water was provided by a steam boiler using LPG fuel, which heated the
water in 4000-liter and 7000-liter storage tanks. SWHs were installed in 2010. As shown in Fig. 2, the
solar collectors facing south were installed on the ground next to the greenhouse at a tilt angle of 23o.
Detailed specifications of the system are shown in Table 1. There are solar collectors (ASC = 2.2 m2
each) installed in five rows and 40 evacuated tube, which are 1500 mm long with the inner and outer
diameter of 37 mm and 47 mm, respectively. Each solar collector is integrated with a 200-liter water
storage tank. The water heated by the solar collectors circulates in a closed loop. The hot water
leaving the solar storage tanks is fed to the original storage tanks where auxiliary heating of the water
is provided by the steam boiler.
To investigate the actual operation conditions of the system, several monitoring devices were
installed. As shown in Fig. 3, the precision spectral pyranometer (Li-Cor, Inc., Li-200SA) is set up
near the system to determine the incident solar radiation values. Two flow meters (Shin Yuan
Precision Machinery Co., Ltd., Model HTL20-S1-F0-A) are located in the cold water supply line to
the hot water storage tank (hot water consumption) and in the circulation line from the bottom of the
storage tank to the inlet of the collectors (circulation flow rate), respectively. There are 13 platinum
resistance thermometers (Izuder Enterprise Co., Ltd, Model Pt 100) installed to monitor the local
water temperature. The data from monitoring devices are sampled every 10 seconds by a data
acquisition system (ICP DAS CO., LTD., Model ET-7017 and ET-7015) and transmitted
synchronously to the host computer at the Energy Research Center, National Cheng Kung University,
through the Internet.
Figure 2 Solar water heating system at Wu-Shu-Lin Nursery, Taiwan Sugar Corporation
T
T10
T6
F1
F2
Pump
T5 T2
T12
T8
T7
Solar meter Temp. sensor
T1
T11
T9
T13
T4
21 1
21 1
1
1
1
T3
F Flow meter
Figure 3 Schematic drawing of monitoring devices
Table 1: Arrangement of solar water heating system for greenhouse of orchid garden
System Specifications
Type of collectors Evacuated tube
No. of collectors 40
Total collector Area 88 m2
Water storage tanks (each) 200 liters
Main water storage tanks 4,000 liters; 7,000 liters
Auxiliary heater LPG steam boiler
*designed temperature of main water storage tank: 85oC
*function of the system: sterilization of the soil in summer and the greenhouse in winter
Thermal Performance of a SWH
For a solar collector, the Chinese National Standards (CNS 15165-1-K8031-1) is in compliance with
the existing international standards. The standard specifies an outdoor test method to determine
steady-state and quasi-steady-state thermal performance of solar collectors, in which FR() and
FRUL are the slope (useful energy collected) and intercept of collector efficiency curve (heat loss),
respectively, under natural solar radiance ( 800 W/m2). Furthermore, the thermal efficiency of a
SWH (CNS 12558-B7277, η 0.5) is given as the ratio of useful heat absorbed by a SWH to the
total amount of solar energy incident on solar collectors over the test period. The standard specifies an
outdoor test method. The test conditions specify the daily solar radiation per square meter ( 7 MJ/m2),
average wind speed (< 4 m/s), fluid initial temperature Ti and ambient air temperature Ta. For the
present test case, η would be dependent on hot water consumption pattern, as shown in Fig. 4, where
state B represents the periods during which hot water is used. Then the thermal efficiency η is
calculated by the following formula.
Figure 4 Hot water consumption pattern
Q = QA1 + QB1 + QA2 + QB2 + QA3 + QB3 +QA4
= MCp[(T2-T1)+ (T3-T2) + (T4-T3) + (T5-T4) + (T6-T5) + (T7-T6) + (T8-T7)]
mB1Cp(To1-Ti1) + mB2Cp(To2-Ti2) + mB3Cp(To3-Ti3)
= MCp(T8-T1) + mB1Cp(To1-Ti1) + mB2Cp(To2-Ti2) + mB3Cp(To3-Ti3) (1)
and
η = Q/(AgG) (2)
where
Ag: effective area of solar collector, m2
Cp: specific heat, MJ/(kgoC)
G: daily solar radiation per square meter, MJ/m2
M: water mass within water storage tanks, liter
m: water mass flow
Q: thermal energy collected, MJ
T: temperature at the measurement location
Ti: inlet temperature in hot water storage tank, oC
To: outlet temperature in hot water storage tank, oC
Results and Discussion
The solar radiation and ambient temperature for a selected partly cloudy day (8/28/2010) are plotted in
Fig. 5. As can be seen, the maximum ambient temperature and the maximum solar radiation during
the day were 34oC and 965 W/m
2, respectively. The averages ambient temperature and solar radiation
on the solar collector from 9:00 to 14:00 were 31.4oC and 737.2 W/m
2, respectively. Furthermore, the
measured data of the inlet and outlet temperature of the collectors are shown in Fig. 6. As can be seen,
the rises in water temperature were associated with solar radiation during the day and hot water
consumption pattern. The peak inlet and outlet temperature of collectors were 32.3 o
C and 54.2 o
C,
A1 B1 A2 B2 B3 A3 A4
6:00 18:00
T1 T2 T3 T4 T5 T6 T7 T8
respectively. The maximum outlet temperature corresponded to the temperature rise of about 23oC in
comparison with the ambient temperature. In addition, hot water consumption was mostly
concentrated in periods, say 8:30-11:00 and 13:30-16:00.
Sola
r ra
dia
tion,
W/m
2
0
200
400
600
800
1000
1200
Time, hour
4 6 8 10 12 14 16 18 20
Am
bie
nt
tem
pe
ratu
re,
oC
20
25
30
35
Figure 5 Variations of solar radiation and ambient temperature
Te
mp
era
ture
, oC
10
20
30
40
50
60
70Inlet temperature
Outlet temperature
Time, hour
4 6 8 10 12 14 16 18 20
Flo
w r
ate
, L
/s
0.0
0.5
1.0
Figure 6 Inlet (T1) and outlet (T13) temperature of solar collectors; Hot water consumption
The monthly average η during the day (6:00-18:00) is shown in Table 2. The maximum solar energy
collected and the maximum average daily solar radiation were 621.2 MJ/day and 17.6 MJ/(m2day) in
August, respectively, which corresponded to the maximum η of 40%. Moreover, according to the
monitoring results, the solar storage tanks were insufficiently insulated, causing a large heat loss
from the tanks to the environment during the night hours (18:00-6:00). As shown in Table 3, the heat
loss could be as high as 175.1 MJ/day, resulting in a decreased η. Therefore, extra insulation on tanks
is required. Furthermore, economic consideration is one of the major factors in dissemination of
SWHs. The thermal efficiency of a SWH with the daily solar radiation should be taken into account to
evaluate the real energy or economic savings. As shown in Fig. 7, the monitoring results indicated
that calculated using solar energy collected between 6:00 and 18:00 is less than that of the national
standard, in which should be greater than 0.5. However, would range from 0.3 to 0.4 once the
daily solar radiation exceeds 10 MJ/(m2day) and decrease with lower daily solar radiation,
particularly for the days of less than 5 MJ/ (m2day), resulting in an even lower .
Table 2 Thermal efficiency (6:00-18:00)
month/year
Solar energy
collected
[MJ/day]
Average daily
solar radiation
[MJ/m2day]
Thermal
efficiency
(monthly)
June, 2010 464.1 14.0 0.38
July, 2010 556.3 14.7 0.43
August, 2010 621.2 17.6 0.40
September, 2010 538.5 15.8 0.39
October, 2010 483.1 15.6 0.35
November, 2010 458.7 14.5 0.36
December, 2010 518.2 16.6 0.35
January, 2011 384.9 12.3 0.35
February, 2011 514.7 17.6 0.33
March, 2011 427.8 13.4 0.36
April, 2011 462.7 17.5 0.30
Table 3 Thermal efficiency (00:00-24:00)
Heat loss
[MJ/day]
Thermal efficiency
(monthly)
June, 2010 88.4 0.30
July, 2010 90.5 0.36
August, 2010 99.3 0.34
September, 2010 97.5 0.32
October, 2010 110.8 0.27
November, 2010 93.7 0.29
December, 2010 79.9 0.30
January, 2011 96.1 0.27
February, 2011 119.9 0.26
March, 2011 101.6 0.28
April, 2011 175.1 0.19
Conclusions
Field measurements of a SWH for industrial application were conducted. According to the monitoring
data, the thermal efficiency was less than 0.5. The system also faced significant heat loss during the
night hours. Extra insulation on tanks is required in order to obtain better thermal performance. The
data also revealed that the thermal efficiency of the SWH system is associated with the daily solar
radiation. Once the daily solar radiation is less than 10 MJ/(m2day), the thermal efficiency decreases
significantly. In future studies, simulation and more experimental data are needed to optimize the
collector parameters.
Daily solar radiation, W/m2
0 5 10 15 20 25
Th
erm
al e
ffic
ien
cy,
0.0
0.2
0.4
0.6
Figure 7 Thermal efficiency (6:00-18:00; 2010/5/22-2011/4/30)
Acknowledgements
The study is under the financial support of Bureau of Energy, Ministry of Economic Affairs
References
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